95 YJ crank, no start

This is the FSM page on your PCM 2.5:
F1 blown kills dead PCM memory storage so that only live errors are seen keyon. see why?
be a huge shame running any jeep with F1 blown. and will not store live DTC found.
F1 also kills dead FP relay and ASD both
if you have spark past 3 seconds, F1 is working
page 14-31, FSM page states (free and now you have it)
The auto shutdown (ASD) relay is energized by the
PCM for approximately three seconds.

this is the 3 second rule I mentioned
knowing this 1 rule we must test fuel pressure and spark both for at least over 3 seconds, 10 is a good choice.
to see if spark or fuel fails. or both.

in effect the PCM monitors A142 and lets it overload (may) for 3 seconds and if does it drops the ASD killing both spark and fuel.(on purpose)

this 3second rule is for key on pump test, heard to spin for 3 seconds, this is called the PRIME mode, before the crank.
the pump is active cranking and running both, full time. done.

the pump has many tests not just 1.
even 75psi shunt test
if shunt fails and only 6volts makes it to the pump, the shunt test fails. (or cranking)
41psi keyon,3 times it must be , (assumes pump check ball leaks a tad as mine does)
the highest pressure (normal) as at cranking, 41psi. then drops
then running idle pressure is 31. psi. FPR does that.drop.

the cranking voltage is about 11vdc. at the battery and near that at the pump, may be 10v. there at pump pins actual, IDK. the wiring is small gauge (AWG)
key off voltages is 12.6vdc rested , this is SOC state of charge spec.

if spark ends at 4 seconds 1,good, 2good, 3good 4 fails spark.
we know the ASD dropped off line!!!! killing spark dead and so is fuel.

for 2 reasons. voltage at the battery is way way to low.
or A142 voltage tests failed. (o2 shorted is like slam dunk guess but is easy to prove unplugged O2)
i guess A142 fails at 8v or 6v IDK, but means overload. on the bus.(or car battery is a wreck) my battery fail at 5year marks,
for the reasons in the manual stated.

this post is to understand PCM timing, rules. 3 second rule is super important to know and PSI cranked and more.
most tests are 2 man tests .
man1 cranks
man2 measures
lacking long test leads or even long hose fuel pressure gauge.

my last 95 car (not this one yet) I had a dash mounted electronic fuel pressure meter. (my work)
The PCM can't tell me this but the meter sure can.
and a real scan tool.

F1 is also called (modern name) Ignition off drain IOD.
we pull the fuse and the PCM resets.forgets all DCTs stored.
F1 is hot all the time.
the PCM memory (OID) is hot all the time. unless F1 blows. (or car battery pulled or battery cells dead open)

A code 12 may mean the fuse is blown or battery is over loaded (weak)
CODE 12 each time keyed on (called live code errors)
if the battery goes weak driving is can store CODe 12 in memory but if f1 is blown only live codes are resented
this is a key point to understand on all cars MPI oBD1 or OBD2.

but on this unique car F1 blown kills spark and fuel both.
and kills dead the DTC memory pin, a triple kill in 95s/
so code 12s are a bad fuse or weak battery
1 or the other.
page 8w-11-6 ,to pcm pin 3. memory pin

this error is common as folks never keep a dead car charged and code 12, weak battery pops
the FSM tells you in first EFI chapter, to keep the battery fully charged or tests will fail hard. ( chaos)

so got spark for 4 to 10 seconds??
in my 5speed if the engine stalls my rear wheels keep the dead engine spinning and the dash tachometer stays high and slows)
on a slush pump tranny , I am not sure this works.(here)

the front pump on the tranny must not go dead at the stall or tacho goes dead too. and Im not sure in this case.

my buddy has 1964 pontiac GTO with A/T box that has a rear tranny pump too, and the box still works coasting fast. engine dead too.
he gets engine compression braking too , stalled.
A/t boxes are a tricky lot.

on mine the tacho does NOT go dead moving say 50mph in 4 gear (1:1 ratio) and the engine is still spinning called compression braking (as truckers do) Jake brakes.
and if the CKP and CMP are not dead, the tacho still works on my now dead still spinning engine, PCM not dead, fuses not blown.
a huge clue there.
But will show lots of errors DTC on the fly. stalled. like this.
now I will re-post my ASD test photo.

if the engine does not start and stay running over 3 seconds (the ASD rule)
and spark and fuel is lost both , that is ASD doing that.,
to prove ASD (means PCM commanded) is going deEnergized (ASD relay ) after 3 seconds
we use a jumper wire to slot 24 ASD cavity pin 24 per this way

IF ASD does not drop then we work spark dead or fuel dead. spark is first if dead always.
the PCM cuts both relays when the PCM is not happy via wire yellow K51

if pin 24 goes dead 0v, at or after 3+ seconds, the ASD did drop (more specifically PCM did it)
then we fix that. (why is PCM not happy)
it is that simple my tests.
I find a super small wire, say 3 foot long. and strip both ends back 1/2" or so
with ASD relay pulled, I insert the wire to cav,24 this is ASD OUTPUT PIN.
Then I put the ASD relay back crusing my bare wire in to pin 24.
person 1 cranks, (dead engine now) cranks for 10 full seconds, for sure 5 at least.

person 2, watches the meter sees it go to 11 to 12voltes, less is A142 overload
at the 3 second mark in real time cranked the PCM sees A142 , cavity 24 is too low it cuts the ASD relay dead. at pin K51 yellow wire
this means you get to find the short(or overload) on the A142 wire, pulling all parts on the feed until the ASD monitor is happy.
This is one way to find it and not have to guess 10 parts that can fail and a bunch of old wire.

I check all fuses. not ABS , I have none
and no A/C fuses mine is custom now.
check all fuses, related to the PCM and spark and fuel and all sensors to the PCM.
per the FSM schematics.
fuse in cab and PDC infront of the main battery.
make sure no fuses are blown . is best on any car made, failing.
fuse are what stop wiring fires that might reach fuel lines so is a serious matter, fuses.
never us oversized Amps fuse EVER.
I use the meter ground at engine metal bare.
the side if the engine has this (right side ,drivers perspective USA) has huge ground wire bolt stud, keep clean and tight or all hope ends
the green arrow in my drawing corrects the blunder error there not showing PIN 57 monitor.
the monitor pin is super important there.


Last edited:
I took the wiring diagram and went at it Saturday. Fortunately, I had a friend that's well versed in electrical stuff to help.
Cleaned all the body grounds and battery terminals/ connections, just to play it safe. took power distribution box apart so that we could access outgoing wires and make sure that if something was showing that it was getting power, that it was going out as well. Checked ASD relay, cofirmed relay was good. confirmed that it was getting power when key on... which it was. Then checked to make sure that when ASD got power, that fuel relay also gets power...which it does.

This was key, as then I knew that the fuel pump circuit was powered, it just wasn't making it to the pump. Then checked the car side of the plug at the fuel pump. 12 volts w key on.... plug fuel pump back in, turn key on... pump kicks on for 3 seconds. Turn key to start... Jeep starts.

Long story short... The plug on the car side of the fuel pump was bad. Looked brand new, tested positive for 12 volts every time I checked, but when plugged in, it only connected w fuel pump plug "sometimes".
After it started, we found that if the plug was wiggled, it would die. If held in one position it would stay running fine, but if we pushed on it, would cut out. I ended up cutting both plugs off and put connectors and shrink wrap on the wires.... Have put 120 miles on it since Saturday, running like a top.

Thanks for the advice all.... Glad to figure this one out.

  • Like
Reactions: ytradio
SOLVED, now we know the FP relay closes and sends pump DC power to the rear, for over 3 seconds cranked. making the job 10x more easy to find.
a 2 man test. (or super long meter test leads 1man) fails at the tank power pins.
so C401 failed, as most cars this old do by any other name. called A141 power feed.
no power to the pump for CORROSION
in fact connectors C105,c127, c401 ,s112 G104 can fail this path, G104 is ground if ground fails so does fuel level gauge (a great clue there)
this is very common on all cars. and 10x worse in the salt belt


Last edited: