Possible faulty temp gauge? Hear me out on this...

Ok... I know the overheating issue has been beaten. I spent over an hour searching through posts and I still dont know exactly what route Im going to take. Sorry if this gets long winded...please stick with me. Here is the issue.

Had to replace the head on my 94 4.0 awhile back. While apart, I replaced the rad due to a small leak. I also replaced the water pump due to a bearing issue. Brand new temp sensor as well ( the one in the head). As of late, my temp gauge will rise up almost to the red when Im stopped. Its there for a few seconds, then will drop back down to around 210. This drop DOES NOT occur due to the electric fan. Just magically drops. Yesterday I replaced the thermostat, coolant temp sensor (in the thermostat housing), and the fan clutch. Still doing it.

Some things I have noticed. Even when my gauge reads almost to the red, the elec fan does not come on, and nothing really boils over. The elec fan works under all other circumstances. When I switch on headlights, driving lights, heater blower, and step on the brakes, the gauge moves UP almost 1/8". The other weird thing is it will drop from almost in the red, to 210 in just a few seconds. I dont even see how its even physically possible for it to drop that much in that short of time. Unless my rad is making ice water.

So... am I crazy to think that the gauge is messed up? Or could this be the "new" sending unit? Im thinking about installing an aux temp gauge to see what it does. Ive got the elec fan wired to a switch already...but dotn think I should have to use it during normal operation.

Thanks for reading my novel and for any input. Goin wheeling this saturday...so this is something I want a firm grasp on by then.

Justin

I'm looking at a '91 FSM -- should apply to '94.

According to the FSM -- The line that runs from the temp sender, at the back of the engine, to the guage is also tied into ignition switch, parking brake switch, brake warning switch(on proportioning valve).

It's a bit of a long shot but, could the switch on the proportioning valve be causing this ??

Try unpluging it.

Hmm... I didnt realize it was tied in with so many other things. That kind of supports my thought that there might be either a wiring fault or a gauge issue. I definetly get a good amount of movement from the needle when I flick on other accessories. I might try unplugging that sensor...thanks for the suggestion and the insight into the wiring. i have to find an FSM someplace. Hanes sucks. Anyone else think this idea might be possible? Or atleast a breakdown in the wiring?


Justin

So I understand a little better...the air charge temp sender on the intake manifold at the rear of the engine is actually what moves the gauge and not the thermostat temp sender? Or do they work together?

Thanks,
Mark 91 XJ 126k

Have you tried cleaning the pins on the connector that plugs into the temperature sensor? It could simply be due to bad contact.
I had something similar happen on my 99.

MAP

Re: Possible faulty temp Gage? Hear me out on this...

Could be electrical. Could also be the wrong gasket in the thermostat, a plugged up heater valve, air trapped in the system or a slow thermostat. Possibly some silicon, messing with the thermostat.
The open system isn't as sensitive to air as the closed, but it's still possible to get an air blockage on the top radiator hose. Try squeezing the top hose a half dozen times. Try with the heater on and off and see what happens. What gasket did you use? A universal gasket won't hardly work (partially blocks the bypass and makes the thermostat slow to function). Check and make sure the head gasket isn't still pushing some gases into the cooling system. Messy, but running the motor with the cap off, will push some coolant out, but will eventually stabilize and if there is hot gases being pushed into the cooling system, they coolant will often foam and bubble some.

Mestopin said:

So I understand a little better...the air charge temp sender on the intake manifold at the rear of the engine is actually what moves the gauge and not the thermostat temp sender? Or do they work together?

Thanks,
Mark 91 XJ 126k

Click to expand...

There are two coolant temperature sensors. One is used for the guage or warning indicator -- mounted on the head at the back. The other is mounted on the water pump and feeds the ECU solely.

The air temp sensor is a separate item.

Thanks for the replies fellas. MAP: Good idea about the connectors. I have not cleaned them, and that is the first thing on my list for today.

If that doesnt help, Im going to hook up a autometer gauge with its own sender to see what that says.

Here is some funny info. Today when I got home, the gauge was halfway between the 210 mark and the red. I turned the jeep off, and turned the key back on and it went right to 210. I flashed my high beams and the needled moved up. I so kept flashing them, and before too long the needle was in the red. The pic below shows how far it went just from flashing the lights.



Im pretty convinced something is goofy in the wiring. Guess Ill find out when I hook up the autometer gauge. Any other ideas? Keep em comin.

Justin

A higher temp at the guage is caused by a lower resistance at the sensor.

Unless the heep is normally running hot, this would likely represent a weak short/insullation damage more than a dirty contact.

Try willing wires under the dash, also the ones coming out of the steering column.

:big shrug:

It's normal for the temp to go up for a while after you turn the engine off.
It will go up wither you flash your lights or not.

Right... But imagine the engine is running and the temp is like it is in the pic. Now...shut off the engine, and start it right back up. Now the gauge is steady at 210. Whether you leave the engine on...or just leave the key in the on position with the motor off, the gauge will rise alittle every time you flick the lights or hit the horn or the heater etc. Does this still sound normal?

I was fighting with senders tonight. The one for the aftermarket gauge was too long. So thats a no go. The new one I bought doesnt work. I think its the one for the idiot light in the wrong box. Great. The resistance of the aftermarket sender, and the one that was sort of working is off by about .40 ohms... so I dont think the aftermarket gauge will work correctly off the stock sender. Anyone have any thoughts on that? Back at it tomorrow when I get home. I have to figure something out by the end of tomorrow so I can go wheeling saturday without worrying about heat.

Justin

Use a good digital volt meter connected to the gauge fuse. Look for a voltage change as you turn the the key off and on. You could be seeing the start of the ingnition switch failing. The yellow wire feeds the gauge fuse and coil and Alt.

First be certain that the system is completely filled. A 94 has an open system, so check the radiator after it has sat overnight and be sure the rad is full right to the neck, and the overflow bottle is about 1/3 to 1/2 full. Air pockets in the system can cause faulty readings by exposing the sender to steam rather than liquid. This might explain the rise followed by a sudden drop not associated with the fan kicking in.

Eagle: Yeah the system being full was something I concentrated on today. Since I had the temp sender out... I made sure there was fluid all the way back at the hole...and that the rad was full. Tomorrow Im going to check that again. Air pockets were my first concern. I figured after the amount of miles I put on it, as well as using the heater on and off that the air was all out. But...I know how the air can be stubborn sometimes. So Im going to give it a try tomorrow first thing. Then the next thing Im going to do is run to the dealer for a sender. I just wnat to make sure that is working good...try to eliminate the cheap simple things all together. If that fails to work...then Ill go from there. I am goin to go crazy with my multimeter tomorrow as well. Trying to check as many wires and circuits as I can. Gotta be something screwy.

Justin

Could it be a problem with a bad ground in the system? The guage shouldn't move when you activate another electrical device. As far as the t-stat is concerned grab an OEM one while at the dealer. Kinda spendy but it comes with the correct gasket and the fancy air bleed hole with check ball. X-2 on the ignition switch.

Ugh... ok now Im getting frustrated. I went to the dealer and got a temp sender. Installed it... now the gauge wont go above 155... What the hell. I know that after driving around on back roads for 15 mintues that it should be hotter than 155. Im wondering if I mucked up my gauge even more when I was trying to use that other sender that didnt work.

I did install a dealer thermostat and gasket. Just 4 days ago. Didnt want to take any chances with anything else. But now Im just stumped. I was going to put the gauge from the 87 I have in the 94... but since the senders are different I didnt think that would do any good.

Does anyone know the easiest way to find the wires for the gauge? In other words... which wires in the harness coming from the cluster handle the power, ground and sender signal for the temp gauge. Maybe I can make new connections and get 'er working again?

Justin

Try connecting the wire on the temp sender to ground on your engine, tell me what the gauge reads.
Then connect it to ground on your battery and see what it reads.

You mean touch the sender wire to ground? then touch it to the battery? What am I looking for the gauge to do?

Im wondering if I can use some sort of resistance or voltage value at the coolant temp sensor to determine what temp it sees... Im going to do some searching for a few minutes. Thanks for all the help.

Justin

Unplug the sensor and touch the wire to the head, the gauge should read full scale (HOT). If it don't the touch it to the battery ground. If it goes full scale then, your engine ground is bad. It it goes full scale when grounded to the engine head the sender is bad new or not. If it never goes full scale then the gauge is bad.

Ok... I took off the wire to the sender. Got nothing on the gauge. Touched the sender wire to the head... got full scale hot. Touched it to the batt. ground... got full scale hot. I even re-pulled the sender, took off all the teflon tape, and sealed it with just a dab of copper rtv. Still only reads 150 or so on the gauge. I can see where you would say bad sender whether new or not... but three bad senders in a row? Can my luck really be that bad? Even with one sender coming from the dealer... Errrrr... Im pretty much convinced that Im not actually over heating. Im still looking for a chart to use the coolant temp sensor to determine the temp.

Justin

Ok I found a chart... Now...can anyone tell me the trick to measuring the resistance at the coolant temp sensor? Which wires do I use the meter on...or which wire + ground do I check. I got a number off one wire but it wasnt even close to that chart.. Any ideas?

http://www.angelfire.com/my/fan/CTS.html