'97 inline 6

Electric fan is not kicking on when it should (when engine hot and when AC is on).

I put twelve volts to the connectors at the computer (where the wires connect to pin 2 and pin 22 on computer, the white on blue and pink on blue wires from the relay for the fan in fuse block) and the fan DOES come on.

Am I looking at a bad computer or a computer flashed with slightly wrong firmware? How can I test further in the situation???

THANKS

 

As long as the engine is running correctly, and seeing how hard and expensive it would be to replace that computer where you are, I'd install a temp sensor in the upper radiator hose and a stand-alone relay to turn the fan on when needed, as well as an over-ride switch in the cab in case it tends to overheat. Simple, down, and dirty!

 

I'm actually in Arizona Wish I was in Lhasa, though

Is there just the one temp sensor on the '97? The one on the thermostat housing? (The gauge in the dash is working well, which would seem to indicate the temp sensor is OK).

Is there a way to test the computer? I saw on one of CCKen's posts about putting a resistor on the temp sensor connector to simulate high temp. Also, connecting the connectors for the ac gauges together to simulate high ac pressure.

Are those legit tests on the '97? Any other diagnostics I can do?

 

OK.

1: jumpering both ac sensors, fan comes on.

2: Unplugging temp sensor, fan comes on (Guess this would simulate high ohm also?)

So, not the computer?

Could it be the temp sensor somehow, even though gauge is accurate? Is gauge a little low in reading (it runs about a mm below middle line, where it used to run just under middle line on heat gauge). I also read sometimes the temp sensor goes in to far an touches the back of the housing, causing low readings?

Thinking out loud here...

 

I do that all the time, sometimes when in the middle of a post! The too deep thing can be verified by taking the housing off and looking inside it. It's entirely possible that it can change the reading. I like the idea of a fan over-ride switch just for peace of mind in case I need it. I put one in my WJ when the pulse modulated relay s*** the bed and it's still there if I need it.

 

The fan should not come on until the temp reads 220-225 that's pretty far past the 210 mark. If your a/c compressor does not actually engage when you turn it on...the fan will not turn on...you should only have the 1 temp sender on the t-stat housing...that feeds gauge and computer...check left side of the head near the back for a sender and wire...if you have it you do not have a 97 and that's whats feeding gauge so the signal to the computer might be bad..either way best bet is to change sender in the t-stat. When you disconnect the sender in the t-stat you said the fan comes on...that's good...do you also loose your gauge reading...you should

 

Alright, got it!

I backed out the temp sensor about 1/2 MM, and everything started working as it should.

Probably my ignorance: I bolted the sensor in as far as it would go when I installed it, and it probably was pushing against the housing, causing low readings.

I'd say if you are experiencing electric fan issues, troubleshooting should look like this: (for a '97 anyway)

1) Unplug temp sensor, turn on Jeep
2) Does fan engage? - YES: Either your sensor is bad, or it is plugged in too far. Otherwise, the wiring, computer, relay and fuses were all just verified as functional
3) NO, fan doesn't engage, continue with:
4) Apply direct battery power to fan relay in box: if fan doesn't engage, the fan is problem (replace it)
5) if fan does engage, problem is: relay, computer, or wiring between them
6) Eliminate relay by swapping with known good relay
7) If that solves your problem, its just the relay. Replace.
8) If problem persists, it may be computer or wiring. Do this:
9) Unplug the connector from computer nearest the cabin. The fan relay wiring arrives at pin 2 and pin 22. Apply direct battery power there. If fan engages, you know the wiring from computer all the way to fan is good. If not, wiring is bad from computer to relay: hunt down the breakage, its not that much wiring to follow.
10) Assuming at this point the problem persists: its either your computer of the wiring from temp sensor to the computer. Does your temp gauge work? If yes: probably a bad computer (sorry). If no: probable bad wiring from temp sensor to computer.

Hope that helps someone else.