I have a 1997 Cherokee SE 4.0 with a 'mystery' problem. Randomly, my coolant temp "dummy light" will flash on and the cooling fan with turn on. The vehicle "thinks" it is overheating, however it is not. The vehicle is running at very normal temperatures.

I have replaced the coolant temp. sensor and the connector. I have paid several mechanics to fix this problem, and a few hundred dollars later I still have the same problem. I have also replaced the forward O2 sensor and the continuity from the cluster to the sensor has been checked. Just installed a new Cat and Muffler.

Does anyone know what else could cause the temperature "dummy light" to come on randomly???

 

My guess would be that you are overheating and the dummy light is doing its job. Could be your cooling system is failing someplace. I would check it carefully.

Good luck.

 

How do you know that the vehicle is running at very normal temperatures?


See if you can get a scanner that displays live data and check the coolant temp when the light comes on and the fan starts.


The fan is set to kick on around 218*F.

 

Can you read live data on a RENIX?

 

^ I don't know.


The OP has an OBD II '97 so I would think he could.

 

temp sending unit ?

 

iirc there's 2, one on the rear driver side of the block and one on the thermostat housing did you replace both ? Not sure which runs the light or fan.

 

The '97 only has one ECT, and he replaced it.

 

OP has a 97 so there is only a single sensor (in the thermostat housing), which feeds the ECM and the dashboard both

 

I thought it said 87. I didn't have my reading glasses on...getting old sucks.

 

OP, are you checking the temp with a temp gun or anything? To me it sounded like it was happening quickly? Not sure if I read it right though. could be an issue in the cluster as well, sure not common But I have seen it.

 

Yes...

 

Thanks for helping me out with this problem, and I do have some more information....

Live reading of the temperature after running on the highway for 45 minutes was 198 degrees... Except for the brief period of time the light flashes. Then it jumped up to 308 for the second the light flashed. The vehicle is not running sluggish or showing signs of overheating. In fact, the top radiator hose can be touched without burning yourself.

Both O2 sensors, cat and muffler, coolant temp sensor and wire connector has been replaced.

The continuity from the cluster to the sensor was checked as well.

Also, I drove it up to Georgia this weekend and it ran great the whole way, at 75 mph for over 3 hours.

Is it possible for it to be a problem with my ECM??

 

What brand temp sensor did you use? That may be the problem.
How are you verifying "real" temps? IR, mech test gage, thermometer in rad?

 

How did you check continuity from the cluster to the sensor? The ECT sensor signal goes to the PCM, not the cluster.


Why did you replace the ECT connector?


Does your radiator electric fan kick on when the temp jumps to 308 degrees?


It sounds like you are getting an open in either in the live ground network wire to the ECT (Brown/Yellow tracer wire) or an open in the signal wire to the PCM (Tan/Black tracer wire).


Inspect the ECT wires where they enter the fuel injector wiring manifold for defects, and inspect the wire loom where it exits the manifold for chafing through to the wires within. There have been a few reports of the loom chafing on the end of the fuel rail.


The ECT Brown/Yellow tracer wire splices in to the ground network inside of the fuel injector manifold. If you can't find any obvious defects in the wiring/looms outside of the manifold, open the manifold and look for the splice. Check it.


If all else fails, wiggle the wiring harnesses while observing the coolant temp on your scanner to see if it jumps.


I could show you how/where to check voltages at the ECT connector but do the above checks first.