Non starting after new stat and core flush through

jpjeep26uk

YJ Enthusiast
Joined
Nov 12, 2022
Messages
163
Location
UK
Ok, here's the deal.
Early 1989 2.5l 4cyl TBT LHD here in the UK. I believe she's quite possibly the rarest YJ of her kind in the UK according to people who know about this stuff.
That's not a good thing. Not on this island anyway.

Other than a back fire issue or two she's never let me down despite my tinkering.

Really fun drive home 40 miles a couple of weeks ago, Jeep was great.
Next day, sun was out so
1. Put a new stat and housing on.
2. Flushed the heater core and pipes.
3. Put it all together again.
4. Gave the bunch of filthy wires down under the battery a good scrub.
5. Learned this is the ignition relay/ solenoid. (GND, SOL, BATT and IGN)
6. Drove the car around the block to face her uphill on my drive to burp the system.
7. Car cut out.
8. Nothing on key turn, totally dead.
9. No lights no stereo, no dash lights, no dome light, no crank nothing. Silent.
10. Checked battery- full.
11. Checked tank - half full.
12. Checked Batt connections - good and tight.
13. Checked every connector - all connected.
14. no start, nothing, basically dead.
15. Checked fuses under the dash - all good.
16. Swapped fuel relay on left side of battery.
17. no start- nothing. silent.
18. Figured I might have shorted something out when cleaning wires off/ to the ignition relay block.
19, Blow dried the engine bay with a hi powered blower.
20. Jiggled the wires under the battery and noticed one had broken off.
21. Fixed that with a new spade and reattached to the ignition starter block.
22. PROGRESS!! Electrics come back and heater blower running, lights, wipers, stereo, dome, everything.
23. Car cranks! No start. Healthy crank - not starting.
23. Replaced the fuel relay on left of battery with brand new one.
24. No joy.
25. Dropped the tank to test the pump/ sender wires = wires showing correct voltages.
26. Checked fuel filter - seems good.
27. Checked fuel lines - all good.
28. Rechecked the fuses, connections etc = all good.
29. Called a Jeep mechanic/enthusiast to take a look (one of only two in a 50 mile radius who will touch my Jeep- welcome to the UK)
30. Sparks are not firing on crank.
31. Can smell petrol from block with plug removed - assuming pump is working.
32. Checks everything same as I did including crank position sensor, alternator, ignition pack, plugs, cables, wiring etc.
33. No obvious reasons for good crank but no spark.
34. I order a new Ignition Relay, new Dist cap, new rotor, new better quality ignition cables.
35, Took off the Ignition relay and cleaned it up, filthy and corroded.
36, Put it back on, hooked up the wires which had also been cleaned and metal brightened.
37. Cranks better, wont fire up.
38. Decided to replace the wires (found out four of them are fusible links) with better connectors.
39. IDIOT! forgot to unplug the neg cable, wires shorted and blew up the relay, sparks and smoke. IDIOT!
40. Braided ground cable from block to firewall starts smoking, singes the thin metal tube above it, nothing serous.
41. Unplugged the battery and
42. Obviously Jeep is officially dead.
42. Cleaned and wire brushed every ground point and nut/ bolt and washer.
43. Took Ignition pack off the firewall, cleaned it and brightened up all metal parts.
44. Coil is less than a year old but cleaned it anyways.
45. Took the big black box firewall connector apart and cleaned it up and took the connects down to metal but still kept most of the black dielectric grease to aid in connectivity etc.
46. Checked all fuses.
47. Installed brand new fuel pump relays.
48. Attached brand new Live batt connector and brightened up the live cable to remove green corrosion, reconnected.
49. Put new stronger connects on the ends of the fusible links, cleaned the ends to bright metal, fit new plastic covers and heat shrunk them.
50. Parts arrived from Germany!
51, Installed and very carefully rewired the Ignition relay in and attached to firewall.
52, Installed new Dizzy cap, rotor and proper ignition leads.
53. Lights all working - the Jeep turned over like it was a new car, really fast, healthy crank.
54. STILL WONT FIRE UP. AAAAAARRRRRGGGGGHHHHH!
55. Swearing, cursing, kicking things, throwing a fit in the garage, plugged in guitar and wailed, kicked the bins, kicked the other car….
56. Drank tea. Calmed down.
57. Checked fuses again – all good.
58, Checked battery – all good
59. Read how to check continuity of wires – thinking it’s a busted but invisible fusible link.
60. Didn’t understand the instructions.
61, Came here for help.

Help.
I’ve tried everything in my humble arsenal to get this bloody YJ running again. She just won’t fire up. Everything looks and sounds like its all wired and connected properly, I can hear a quiet wooshy sound from near the TBT when I turn the Key so I’m assuming that must be fuel being pumped to the Throttle Body.
It seems like there is no literal ignition of that fuel.
Living in the UK there are only about 3 known companies who know something about YJ’s and it’s gonna cost me a small fortune to get it towed and lord knows how long or how much to fix. I think it’s just something stupid like a fried link or perhaps the signal from the ignition relay isn’t getting to the coil pack to ignite the fuel by the plug?
I don’t know how to test all that and I have sneaky suspicion of one of you folks was to take a look, in under 2 minutes you’d say ‘ it’s that thing there mate’ and rig up a workaround and she’d roar into life hahaha.
I buy the busted bit or repair ait and we’re off on the road once more.
What say you folks…..what have I missed???
Thanks for you patience in reading..... thought I'd list everything I've done.
G
 
Go through your under dash/hood fuse panels...Even the low amps are connected to many circuits that control the important stuff....
Look closely at #10...

maxresdefault.jpg


9b669f62ec9e9c762ff9937fde1d0de26d3ac02d868f84761c.jpg
 
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step 23b is wrong and spark is first the EFI brain called PCM cuts all fueling if spark is detected by IT is bad in any way shape or form to prevent, crash and burn. DOT laws here dictate that. act. The programming does that deep inside the PCM.
PCM/ECM/ECU etc.
once fuses are all good , in cab and in the engine bay
all fuseable links too , I think 3 there,


never fix old cars lacking the FSM. after all it is free. and has full schematics in the car using car code 80 only. 80= wrangler.
spark is dead
but chapter 8D page 42 shows testing spark directly at the HV coil and the connector to the primary conn, snaps on and loves to fail.rusted.



I will assume the engine and all EFI goodies are stock not , huge modifications and swaps, no photos of you engine presented.

I do not know how to go DTC errors on a renix, sold mine, 1995, 88 to 99 owned and the bought a 99.
renix does not store code only life ones, (or the reverse I forget)
I have scan tool that works but is costly to buy. for 88 renix.


this car has ICM model for spark. 89
at fire wall near battery. chapter 8w-106 (page 106)
this is TBI fueled, TBI means throttle body injected with 1 injector, (my two 88 cherrokee had it and the wild RENIX french ECU)
when it cranks, try to press 5% throttle ,to hide a dead IAC valve (idle air valve) mine stuck ever 10,000 miles for 250,000 miles owned. I have cure.
using dry lube gun oil spray, that looks like teflon sprayed the tip when dry.
the engine needs air to start if the spark tips drip fuel bingo this ,
no air no start and will want to flood. RENIX horrors well know.
the silly spark coil with the inane horse shoe , spark coil 2 connector , thing fails. they never last this long so , there you are. replace it if it looks
old, looks bent, warped or the contacts are rusty junk.
the fSM is free if you google for it. 1989 jeep wrangle factory service manual (not fake ,but real, no chilton stuff wanted)

test 30 was no spark !
so its a no spark engine.
and that takes work to find on any RENIX french ECU.
the coil needs power. 12vdc on the lug that is for 12vdc.
the horse shoe spark coil connector loves to fail, replace it if it looks old is a best Idea. warped or rusty.

the manual is hard to follow then as they mix or 2 or 3 engines in one book. code 80 2.5 L is it. all else is wrong. (some fuse pages are in common)
the 02 sensor if it shorts out, its heater as it loves to do will blow up power fuses to the ICM

ICM pin A is 12vdc key ohn.< B= ground,black.
C= tachometer out.
d = orange is. (not sure me its function)
the other wire ICM is coil lead to distributor.so it say. the ICM drives the coil if ICM fails there can not be any spark.
right rear of engine right side, ground large must be good or EFI goes dead
spark test at the spark plugs all 4 dead we check spark at the HV spark coil next the same way.
this ICM and coil both need power and all grounds relate good.
power in electronics means power and ground both or there and good, or gets no flow of current if either is failing, (rust happens)
I do not like RENIX in fact from 1999 ive tried hard to forget its horrors, for sure the DTC limits are bad to the bone.



if 35years old it warps and all 4 pins rust, 2 here and 2 on the coil brass turns green and fails
horse shoe clip spark.jpg





coil.JPG


these are excerpts from the FSM.
Renix may still be good. so must be the distributor, that pulses it.
 
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warning on the CKP the crank sensor if removed and put back it is then destroyed in 1second flat cranked.
the CKP were set with rubber pad gaping device , of you failed to do that it will be wrecked
some engines, for A/T and M/T are not the same ,the new one with the stick on pad on the end tells you it must be gaped, if CKP has 2 mount round holes it is not gaped.
best i can till yours is not like that.

ok found this.
the ICM is only $28
if the input called ECU signal is dead, check for dead CKP off the flywheel, crank pos. sensor.

icm.JPG
 
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now we know the 2nd B is really B2, ECU input signal to fire the coil.
B1 must be 0vdc, (meter neg lead to battery minus lug) 0vdc to B1. here.
CKP may be bad. if ICM is good and all wiring good and coil not bad and if has horse shoe connector and is bad.
 
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Hi all.
Tonnes of useful intel to absorb haha! I got a busy weekend coming up testing out all the above.
I'll report back with my findings and hopefully get closer to a starting car. Absolutely baffled!!


Before and after of the Starter Relay. Just for reference and some engine shots as well. ( ignore cover colour its the same jeep but no recent pics of the engine bay)

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IMG_20240206_131111.jpg


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1000003486.jpg


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IMG-20221121-WA0007.jpg


IMG-20230129-WA0000.jpg
 
nice photos, !! and jeep.
the ICM with ECU they make spark
fuses all good
must have 12vdc and ground to ICM

the crank sensor must be good to the ECU
and then the ECU (Renix) sends spark pluses to the ICM I guess the ECU controls spark advance here.
I see you have the better HV spark coil , no horse shoe junk there.
how cool the original jack on fender,

ICM voltmeter to conn1 , A to B pin must be 12vdc cranking, of reads like 6vdc the resistor wire is blown out.
 
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warning on the CKP the crank sensor if removed and put back it is then destroyed in 1second flat cranked.
the CKP were set with rubber pad gaping device , of you failed to do that it will be wrecked
some engines, for A/T and M/T are not the same ,the new one with the stick on pad on the end tells you it must be gaped, if CKP has 2 mount round holes it is not gaped.
best i can till yours is not like that.

ok found this.
the ICM is only $28
if the input called ECU signal is dead, check for dead CKP off the flywheel, crank pos. sensor.

View attachment 122490
I'll take some pics of the CPS. I still have the older one somewhere. I'll be getting under the Jeep tomorrow weather permitting. I'll double check it is fitted correctly and connected correctly. I tested the ignition module to the dist cap and there is no spark at all. Could be a busted/ ignition module. I tested the volts going into it and it's showing a light on my tester on A1. Will do more in the morning and take pics and post my progress.
 
nice photos, !! and jeep.
the ICM with ECU they make spark
fuses all good
must have 12vdc and ground to ICM

the crank sensor must be good to the ECU
and then the ECU (Renix) sends spark pluses to the ICM I guess the ECU controls spark advance here.
I see you have the better HV spark coil , no horse shoe junk there.
how cool the original jack on fender,

ICM voltmeter to conn1 , A to B pin must be 12vdc cranking, of reads like 6vdc the resistor wire is blown out.
thanks, I try to look after her. It's bloody hard to do in the UK. Extremely limited help and terrible for parts. I'll check everything tomorrow, take some snaps and report back. I managed to test the coil to dist cap and there is no spark at all. Great cranking but zero spark. There is power showing on one of the cables on the incoming connector but nothing going out by the looks of it. ....the mystery thickens!!! Need to test the coil and ignition module for continuity or being busted. Cheers.
 
Ok, with the key in on notch two but not starting up, I've tested the current coming in on the coil module connector and it's 12v on the A1 pin.
Plugged it into the module and tested the positive terminal on the module itself and its 12v.
I think that's a good thing?
Seems like current is getting to the module to start it.
Then I tested the ECU connector and with the key in the same notch it shows 0.06 volts.
Not sure what that means really?

Took the coil off the module and tested ted for continuity and it's coming up with '1' on each combination of multimeter / terminal
Not too sure about that either haha.

I checked the CPS and it seems fine from a fitting perspective and the connectors are clean and snug so nothing physically wrong
Not sure how to test that item with a multimeter or light / power on / power off etc.

Going out to see if I can find an ignition module and coil and see if that's the culprit.
I'm also wondering if I fried the ECU.
They are a nightmare to remove in these things I've been told.

They are also about $500-$700 to buy over here. Not including shipping so I bloody well hope it's not that !!!

More to come in the next bulletin, now back to you in the studio....

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IMG_20240210_085922.jpg


IMG_20240210_081149.jpg


IMG_20240210_081637.jpg
 
that is a fixed CPS (new name CKP) there, it is Pnp bolt on.
the FSM has errors, it does now cover the spark coil. at all , 8w pages. only the 4.2, is covered, fully.
that test lamp is no good if it is LED.
only a voltmeter tells the truth.
key on tests and cranking , the ICM power pin must have power.
ICM socket 1
A to B, check voltage with meter leads A to B. ( this will find if power or ground to the ICM is bad.
the LED may work much better , at Socket 2, from Socket1 B to socket 2B, the LED may be fast enough to see it blink fast, with the firing signal., cranking,
I use scope to see S2B flash. but and LED is fast, so may blink fast. cranked.
if S2B blinks and the spark coil is dead the ICM is bad. (assumes S1A to S1B has power cranking)
I am not sure how the B+ relay works. to the PCM
it is a latching relay and most latch up cranking, once key turns on , I presume it latches up, and allows PCM to say with full power.
Rockauto ships world wide, I think.
make sure the HV coil primary shows like 10 ohms
and the HV PIN lug (secondary) from hug lug to ,small MINUS lug is like 10,000 ohms. if either fail for infinity or 0 homes the spark coil is bad.
primary is tiny pin - to pin + tiny.
if S2B is dead, the cps/ckp is dead.

the CPS is not a hall sensor it has no power.
it is just a 2 pin coil pickup, sensor. and the signal is very weak. the connections must be perfect or it fails.
it just may be the ground to all of the above are bad, (loss,rusty,or too greasy )


rock

 
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Thanks Dorf. I'll do all that above. Raining here so gotta pause.
Does seem to be pointing at the ICM based on my humble tests.
Will dig deeper per above and see what what. Cheers.
 
jeep this year calls CPS a speed sensor (RPM) speed Is MPH called VSS today. CPS is call CKP now, SAE and ISO rules. for names, I am not arguing names
only to read relic manuals what they meant then, is really funny today.
the CPS (non hall) uses a ground shield cable if ground to the shield is lost, CPS tiny singles fail cranking, slow. the HALL sensors ended this pain later.
it is a design weak point, CPS 2 wire sensors. and it must work or will never start the engine..
the 2.5 has no CMP cam sensor in the DIZZY like the 4.0L has.

my new 1988 jeep xj, 4.0 , then, the CPS failed fast, (recalled by jeep) then the alternator failed at 15k,(bad rear china bearings, used by Delco)
I had a 88 MJ jeep too, same time. (yes I love trouble):mad:
the CPS is real important here, the newer ones from SMP may be better, may have better outputs cranking for sure if the main battery is weak the crank spins slower and the CPS output is weaker. (keep the battery fully charged at al times on any dead engine and testing it)
or CPS NTK (high altitude)

I'd make sure every inch of the CPS wiring is perfect on this engine.
then P+ relay works
and S2B1 pulses. (ECM out) the pulses ICM with a pure 5Vdc square wave Chapter 8D-8 page stated.
then DC power is good to ICM
and coil is good.
and all grounds related here good.

test for spark out of the spark coil only , at first , until that works first. do not test spark plugs for spark yet , that is last.0
the HV induction spark coil must work first, goal #1 using a HV spark wire to the HV spark tower, terminal and spark plug on end grounded to the engine.
keep in mind cars this old 2 problems related can be here.

S2B1 leading edge charges the primary coil , (called dwell in the olden days) and then when the S2B1 goes to zero volts the coil drive is released (ICM does)
and HV happens, spark. happens.

that is all there is on the 2.5L, there is no CMP sensor. like the other 2 or more engines have in the same book.
page 8D -10 see how the complex teeth work on the flywheel rim tone wheel. 3 differnent teeth types, that must not be damaged.
every 90degr;. of crank angle, the last tooth is huge and allows the super WeAk cmp TOP FIRE OFF. ( YES COMPLEX THEN)
8d IS FSM CHAPTER SPARK,IGNITION 2.5: PAGE 9 TO 23. IT SAYS DO NOT CLEAN THE DIZZY ROTOR TIP. (AND WHY)
THE SCHEMATICS IN THE , ARE MIXED, INterspersed, madly. making the job harder.
8w-83 is the beginning pages for this engine.
watch out for the bad ground it mentions in the FSM about ICM ground going to the bloody, engine tipstick tube screw clamp, wow. did grounds. :oops:
ground to right rear lug of engine, must be good. page 95
the 2.5L coil page 8w-96 is missing sheet 15, and in the place is 4.2L, a huge error in 1988/98 FSM both are wrong. both books. I have.
the CPS page too is missing in the schematic but seen on the ECU pin connector pages. 2 pins CPS. Jeep must hate the 2.5L
the other engines in the book are covered fully, but not 2.5, in this regard.

make sure +5vdc is seen at TPS and MAP (ECU created 5vd this is)
if dead 5v, the PCM is offline, and or B+ relay dead or its fuses blown. fuses are always first. skip none or go way off track. fast.

ECU pin 11 and 28 is CPS 2 wires.
ECU pin 27 is fire the coil signal from ECU to ICM,. it is a 0 to 5vdc square wave, if dead the TPS/and MAP must have 5vdc or the B+ latch relay is dead
or fuses to PCM or B+ relay are blown
that is it for all tests basic spark only 2.5L diagnosis.
and cheers to you and all folks working wranglers.
 
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fuses (plug in) all good cab and engine bay.
and those painful fusible links, color coded wires. they all must be good. every one, but not the glass heaters.
8w-3 shows up to 6 fusible links. (like old Fords had) I even replace some with real fuses. and new box to contain them.
so links good, all, and all gounds to ECU., ICM .
B+ relay latches up and stays so until key is tuned off.


1707571632572.png
 
I found this mess is under fuel. chapters 14. and helps.
the CPS is there
the ICM too
the ground i think is the dip stick tube mount,
the coil wiring in my book is not ever told. only vaguely here.
PCM pin 27 must plus 0 to 5v. or the ICM will be OFFLine (inop)
all ECu grounds must be good.
pin 3 ecu, is ign key run turned on key. 12vdc to ICM and ECU.
pin 27 to ICM pulse 0v 5v 0v 5v as you crank, if not the PCM is not seeing the CPS< assumes B+ good and all grounds and power pins seen here are good.
if 5vdc happens the ECU is NOT DEAD.,, means has power) the ECU creates 5vdc from 12vdc pin 8 or 3.
the purpose of B+ is so if engine stalls on the fly, we can let the cars momentom restart it in gear. B+ only ends key off.

1980schem2.jpg
 
Last edited:
that is a fixed CPS (new name CKP) there, it is Pnp bolt on.
the FSM has errors, it does now cover the spark coil. at all , 8w pages. only the 4.2, is covered, fully.
that test lamp is no good if it is LED.
only a voltmeter tells the truth.
key on tests and cranking , the ICM power pin must have power.
ICM socket 1
A to B, check voltage with meter leads A to B. ( this will find if power or ground to the ICM is bad.
the LED may work much better , at Socket 2, from Socket1 B to socket 2B, the LED may be fast enough to see it blink fast, with the firing signal., cranking,
I use scope to see S2B flash. but and LED is fast, so may blink fast. cranked.
if S2B blinks and the spark coil is dead the ICM is bad. (assumes S1A to S1B has power cranking)
I am not sure how the B+ relay works. to the PCM
it is a latching relay and most latch up cranking, once key turns on , I presume it latches up, and allows PCM to say with full power.
Rockauto ships world wide, I think.
make sure the HV coil primary shows like 10 ohms
and the HV PIN lug (secondary) from hug lug to ,small MINUS lug is like 10,000 ohms. if either fail for infinity or 0 homes the spark coil is bad.
primary is tiny pin - to pin + tiny.
if S2B is dead, the cps/ckp is dead.

the CPS is not a hall sensor it has no power.
it is just a 2 pin coil pickup, sensor. and the signal is very weak. the connections must be perfect or it fails.
it just may be the ground to all of the above are bad, (loss,rusty,or too greasy )


rock


that is a fixed CPS (new name CKP) there, it is Pnp bolt on.
the FSM has errors, it does now cover the spark coil. at all , 8w pages. only the 4.2, is covered, fully.
that test lamp is no good if it is LED.
only a voltmeter tells the truth.
key on tests and cranking , the ICM power pin must have power.
ICM socket 1
A to B, check voltage with meter leads A to B. ( this will find if power or ground to the ICM is bad.
the LED may work much better , at Socket 2, from Socket1 B to socket 2B, the LED may be fast enough to see it blink fast, with the firing signal., cranking,
I use scope to see S2B flash. but and LED is fast, so may blink fast. cranked.
if S2B blinks and the spark coil is dead the ICM is bad. (assumes S1A to S1B has power cranking)
I am not sure how the B+ relay works. to the PCM
it is a latching relay and most latch up cranking, once key turns on , I presume it latches up, and allows PCM to say with full power.
Rockauto ships world wide, I think.
make sure the HV coil primary shows like 10 ohms
and the HV PIN lug (secondary) from hug lug to ,small MINUS lug is like 10,000 ohms. if either fail for infinity or 0 homes the spark coil is bad.
primary is tiny pin - to pin + tiny.
if S2B is dead, the cps/ckp is dead.

the CPS is not a hall sensor it has no power.
it is just a 2 pin coil pickup, sensor. and the signal is very weak. the connections must be perfect or it fails.
it just may be the ground to all of the above are bad, (loss,rusty,or too greasy )


rock

Hi,
Ok, A to B switched on gave 11.9 volts.
Socket B1 to 2B read 0.20 switched on.

A to B cranking gave 9.9v
B1 to 2B gave .20v cranking.

Is that any help??
Thanks.
 
Go through your under dash/hood fuse panels...Even the low amps are connected to many circuits that control the important stuff....
Look closely at #10...

View attachment 122485

View attachment 122486
Ok, checked all fuses, all are good - (visually anyway :) )
jeep this year calls CPS a speed sensor (RPM) speed Is MPH called VSS today. CPS is call CKP now, SAE and ISO rules. for names, I am not arguing names
only to read relic manuals what they meant then, is really funny today.
the CPS (non hall) uses a ground shield cable if ground to the shield is lost, CPS tiny singles fail cranking, slow. the HALL sensors ended this pain later.
it is a design weak point, CPS 2 wire sensors. and it must work or will never start the engine..
the 2.5 has no CMP cam sensor in the DIZZY like the 4.0L has.

my new 1988 jeep xj, 4.0 , then, the CPS failed fast, (recalled by jeep) then the alternator failed at 15k,(bad rear china bearings, used by Delco)
I had a 88 MJ jeep too, same time. (yes I love trouble):mad:
the CPS is real important here, the newer ones from SMP may be better, may have better outputs cranking for sure if the main battery is weak the crank spins slower and the CPS output is weaker. (keep the battery fully charged at al times on any dead engine and testing it)
or CPS NTK (high altitude)

I'd make sure every inch of the CPS wiring is perfect on this engine.
then P+ relay works
and S2B1 pulses. (ECM out) the pulses ICM with a pure 5Vdc square wave Chapter 8D-8 page stated.
then DC power is good to ICM
and coil is good.
and all grounds related here good.

test for spark out of the spark coil only , at first , until that works first. do not test spark plugs for spark yet , that is last.0
the HV induction spark coil must work first, goal #1 using a HV spark wire to the HV spark tower, terminal and spark plug on end grounded to the engine.
keep in mind cars this old 2 problems related can be here.

S2B1 leading edge charges the primary coil , (called dwell in the olden days) and then when the S2B1 goes to zero volts the coil drive is released (ICM does)
and HV happens, spark. happens.

that is all there is on the 2.5L, there is no CMP sensor. like the other 2 or more engines have in the same book.
page 8D -10 see how the complex teeth work on the flywheel rim tone wheel. 3 differnent teeth types, that must not be damaged.
every 90degr;. of crank angle, the last tooth is huge and allows the super WeAk cmp TOP FIRE OFF. ( YES COMPLEX THEN)
8d IS FSM CHAPTER SPARK,IGNITION 2.5: PAGE 9 TO 23. IT SAYS DO NOT CLEAN THE DIZZY ROTOR TIP. (AND WHY)
THE SCHEMATICS IN THE , ARE MIXED, INterspersed, madly. making the job harder.
8w-83 is the beginning pages for this engine.
watch out for the bad ground it mentions in the FSM about ICM ground going to the bloody, engine tipstick tube screw clamp, wow. did grounds. :oops:
ground to right rear lug of engine, must be good. page 95
the 2.5L coil page 8w-96 is missing sheet 15, and in the place is 4.2L, a huge error in 1988/98 FSM both are wrong. both books. I have.
the CPS page too is missing in the schematic but seen on the ECU pin connector pages. 2 pins CPS. Jeep must hate the 2.5L
the other engines in the book are covered fully, but not 2.5, in this regard.

make sure +5vdc is seen at TPS and MAP (ECU created 5vd this is)
if dead 5v, the PCM is offline, and or B+ relay dead or its fuses blown. fuses are always first. skip none or go way off track. fast.

ECU pin 11 and 28 is CPS 2 wires.
ECU pin 27 is fire the coil signal from ECU to ICM,. it is a 0 to 5vdc square wave, if dead the TPS/and MAP must have 5vdc or the B+ latch relay is dead
or fuses to PCM or B+ relay are blown
that is it for all tests basic spark only 2.5L diagnosis.
and cheers to you and all folks working wranglers.
Wow, how do you folks know all this stuff? I am humbled by everyone's help and totally embarrassed at my own lack of YJ know-how!!
Right, tomorrow, Sunday, I am going to go through the above very carefully and make notes for every thing covered above and by other posters and record every voltage.
I am going to find the culprit if it kills me.
By the way what or where is the 'B+ latch relay' ?? Battery Positive ?
 
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cars in general 1980 to present and for sure 80 to 96
had vastly different spark systems, and hard to track lacking a matched FSM book, so do not worry this., at all we can do the tests
keeping mind my 88 was sold in 1999 a very long time ago and had to refresh my memory.
first those readings

Ok, A to B switched on gave 11.9 volts.
Socket B1 to 2B read 0.20 switched on.

A to B cranking gave 9.9v
B1 to 2B gave .20v cranking.

A yellow to B blk is power and ground a tad low but ok. 12.6vdc is normal full charged battery but what does A to B do cranking, 11vdc is ok 10 is low limit
below is bad.
you get 10v(9.9) so battery is weak. it need to be kept charged and must be a good battery I cant hear the starter spin fast or slow, so no judge me.
B1 to 2B
well b1 black is ground
2B is ECU sparking signal. (and spark timing ) .2 is not possible to evaluate. ever meter on earth made read 5v pulse differently, and LED test lamp is best or scope you dont have.
A 5vdc LED will flash on 2b(orange) to ground (LED plain, needs resistor to do that. a 150 ohm resistor and LED in series as a test lamp
anode of LED must go to pin 2b. or it can never flash.
This LED
will allow a human to see the 5vdc lamp flash. in the blink of an eye fast. LED are fast. so works. ( a scope shows the actual square wave) easy as pie.
the B+ relay. latching.
the schematic shows
B+ has 4 pins (relay) PK,BK,PK,RD
pk is pink and RD is red and BK is black
as you can see C8 is fusible link that must not be blow out.
and is battery + from the starter.
the RD wire goes to relay
the ECU wakes up. and and pin 1 on the relay goes hot. and make ECU fully work, latched.



b+.JPG

the relay is here, per this page 2.

b+1.JPG
 
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FYI old ECU go dead below 10vdc battery some can run at 9. but most can not this old. (a general statement)
the ecu internal regulator goes dead at 10v. but..
the FSM starter test pages claims 9.5vdc is the lowest voltage cranking, most cars do 11vdc, for sure a tiny 2.5L with 550 CCA battery.
yours is 9.9v.
the starter uses 100 to 200 amps max.
the one way to see this failure, that is the 5vdc pin in the MAP sensor is dead. key on. ( loss of power to ECU or its grounds or voltage is too low)

newer cars can to to 8v. with far better regulators inside the ECU. (called LDO,low drop out regs) none existed in 1989, not yet.
why is there 11.9vdc there not cranking,, my guess is cranked the battery flat or battery is bad.
also starters do short out inside (rotor coils or field), even partially shorted, and use way too much current. and overloads the battery. easy.
I must say if the starters sounds good, it is.
the starter solenoid goes berserk at 8.v chatters like mad., so is moot point there. we have 9.5.
if battery shows 12.6vdc across the terms now, the wiring is bad in the car and easy to find using voltage drop tests, ask how. (dmm voltmeter simple tests)
 
warning on the CKP the crank sensor if removed and put back it is then destroyed in 1second flat cranked.
the CKP were set with rubber pad gaping device , of you failed to do that it will be wrecked
some engines, for A/T and M/T are not the same ,the new one with the stick on pad on the end tells you it must be gaped, if CKP has 2 mount round holes it is not gaped.
best i can till yours is not like that.

ok found this.
the ICM is only $28
if the input called ECU signal is dead, check for dead CKP off the flywheel, crank pos. sensor.

View attachment 122490

Hi Dorf
I pulled and checked the CPS/ CKDS and holy moly, the sensor is only 5 months old and is in pretty good condition, but the hole it goes into was filthy.
Took the sensor off and cleaned it up a little, didn't really need it. Then took the rubber surround / door off the crank case and wow, clogged with grease, dirt, what ever.
There is a thin plastic film with a split in it, I guess to stop crap entering the case, so I replaced that, scrubbed the plastic ' door' and refit the whole lot. Still didn't spark but at least I know its clean!!!
I'm testing all fusible links tomorrow morning and the diag blocks for current / breaks/ shorts.

Thanks for the amazing advice and directions so far man, really useful.