89 YJ: help with engine temp gauge issue


New Member
May 12, 2023
Scotts Mills Oregon
Here are symptoms: Engine temp gauge stays at 100 degrees (bottom) regardless of coolant temp.
Here is what I've found so far. If I ground the coolant temp sending unit wire, and turn key, gauge doesn't peg as it does on videos I've found on youtube on diagnosing issues with temp gauge. However, when I turn the key (with sending unit wire grounded OR attached to sending unit), the needle jumps just a tiny bit...I'd say less than 1/16th of an inch....but I can definitely see the slight movement.

My assumptions/questions: Since the gauge doesn't peg when grounding sending unit wire, I assume the gauge itself could be bad. However, it's possible the wiring could be bad as well...that is to say, no continuity between the sending unit wire and gauge cluster. What is the best way to solve this issue, without going straight to parts cannon approach? Maybe check continuity from cluster plug pin and sending unit wire? What does it mean that the needle jumps a little when key turned to on position? Does this confirm the "wiring" to the gauge is most likely OK, suggesting that the gauge itself needs replacement? If pointing towards faulty gauge, Is it possible that the gauge could be restored, cleaned, etc., to resolve? Any and all help you all could provide would be greatly appreciated!

Thanks folks!
Dash gauge reads wrong,. so the the problem is simple , the themostat is bad , or the gauge lies. (and you know that so...
100F !?
well if the ground test fails, do so at the actual gauge. (3 pins there power, input and ground)

do know that gauge needs power first. we'd check that first at the gauge directly
do now the Gauge has flex PC and that is raw copper traces and we clean those first. at the gauge screws first. never seen one ever not corroded me.

I bet using a pencil eraser on the pads fixes it.

see the problem here, took me 30min to fix it, eraser in hand.

cluster center.jpg
and this.
89 schematic, 8w-40-x
the engine has 2 temp senders , 1 for PCM and for the gauge ,page 8E-2 in service manual code 80/.81 car= wranger
the 100f ohms is, 1365 ohms (sender 1 pin sensor) << fixed typo i made sorry.
220f is 93 ohms
260f is 55 ohms,
grounding out some gauges over heats them, and it is a thermal type gauge for sure. but the XJ same book says to do that, (ground out)
pin1 is the input ,temp sender to the large connector
at the gauge, screws off x3 , clean all pads and even consider washers new, and not rusty.

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Thanks Dorf! I have new bulbs coming for instrument cluster, so I'll be pulling cluster again (recently pulled to replace faulty oil pressure gauge) and will clean and test as you suggest! Hopefully gauge is still functional, otherwise will replace.
i made the 1 wire sensor graph here.
i added the 180 data point, using log math
the head temp sensors there must not use pipe dope or tefon tape or it fails. brass to iron head connection it is.
or use only dielectric sil. grease.
if good the Flex PC in the gauge set is cooper oxide corroded. as they all seem to do. (rag tops do that)
the sensor is like only 10 to 20% accurate, it really is toy grade, and good enough.
jeep did not disclose the 180f data, and is normal temp, how strange is that.???? so I did it. this magic here, a log graph of log, thermistor device.
making it look linear. cute as bugs here and fun to do. Ln(x) is natural logs.
gageF-ohms  YES.JPG
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the book says the gauge is magnetic balanced gauge. most of the time never fail ,as did therm bi-metal gauges loved to do but is not here.
I think the balanced means (it compensates from alternator 13-15vdc changes. making the needle steady , at those events.
with this data we can find true cause easy.
Cleaned gauge and ribbon contacts as suggested. Not only does my temp gauge now work perfectly...but I now have a gas gauge for the first time since I bought the YJ! Thought I was going to have to pull tank and replace the guts. So happy to report this issue solved! Thank you, thank you! I should also point out my contacts did not even appear to have much corrosion at all....but it was enough to cause problems.
no bath in fuel ,is that not wonderful>? so happy for yah.
this panel made new, did not use plated flex.
back in mid 80s I did PCB layout, and gold flash on PCB was like $5 extra after a solder mask used.
even tin plated is better. raw copper is crazy. copper corrodes easy. even in FOGGY days./
my sons old Dodge Neon did the same thing.

the pink pearl to the rescue.
rock on