different overheating question

Well I finally overheated. Yesterday at the Marquette, MI run I got into the red zone. I shut it down and noticed my aux fan wasn't working. With some quick thinking I cut my fog light power wire and wired it to the fan and presto I had my fan again.

The problem is that the thing still overheated with the fan on. Then when I was driving home last night it would get well over 210 and then drop within 5 seconds down to 180 then a little later go back up. It repeated this several times. I am going to flush the radiator and put in a new thermostat this week.

My question is does anybody have any idea which temp sensor turns the fan on? I don't know why it wasn't turning on but it should have. So I was wondering if the temp sensor for the fan is different than the temp sensor for the gauge inside? My thinking was that the radiator was staying cool enough to not turn the fan on but the lack of coolant flow was keeping the engine much warmer. So if the fan temp sensor was in the rad and the gauge sensor was in the block then this would make sense. Any idea's, thought's, etc?

the thermostat that turns on my fan is mounted in the radiator side resevoir.
HTH

Yeah, '87 the aux fan sensor is in the radiator tank. If you got into the red zone your t-stat is probably toast. I believe the aux fan sensor feeds a relay, so you should probably check to be sure the relay is working. Not sure where you spliced into the circuit -- you might have bypassed both the sensor and the relay, in which case you don't know which might have been the problem.

The aux fan sensor is located on the radiator tank near where the transmission cooling lines attach. If you've never replaced it, then my guess is that the sensor rather than a relay has failed, although as Eagle suggested you should check both. The sensor costs about $30 and it isn't difficult to change as long as you can maneuver around the transmission lines and air intake duct work, but for the money I would just wire a manual dash switch via a relay so that you can energize the fan whenever you want. The problem with the sensor is that it has no failure indicator--the only way to know it isn't working is when the fan fails to switch on causing you to overheat. And if your engine is prone to overheating anyway (which the closed systems tend to do), then even an functioning sensor usually energizes the fan too late to make much difference. Jeep apparently figured this out and moved the sensor on later models from the radiator--which by definition runs cooler than the engine--to the engine thermostat housing.

Erik
88 Comanche

If your radiator is plugged up the sensor will not work properly.I have been told this is a common problem in the older system.
Wayne

Well good I am almost 100% sure that my t-stat isn't working and I know that my fan clutch is so I think the fan just didn't turn on because it wasn't supposed to according to the temp sensor.

Thanks guys you told me exactly what I wanted to hear.

Stupid T-stat I guess I will have to find my spare wherever that may be...


Oh yeah I wired the fan so it works with my fog light switch so it is easily replaced back to stock and the only downfall was that I didn't have fog lights!

This may not apply to your situation, but I was having the same problem, my aux fan would't come on and it would boil over. I had a bad overflow/pressure tank. I replaced the bad tank. Now the truck doesn't get hot AND the aux fan comes on.

I can't prove it, one of my theories is that there wasn't enough pressure in the cooling system to get the water up to the temp sensor in the radiator.

The other one being that there was air traped in the system not allowing propper flow to the sensor.-----------Dave