My fans seem to be running all the time, even with the key in the accessory position. Whether it is hot or been sitting over night. Any ideas why it might be on constant?

 

Stuck relay, stuck control switch. Tap all of the switches and the relay gently, like with a 10mm combo wrench. That might identify a stuck contact if it isn't welded too much. If that doesn't narrow it down, start pulling wires from the temp switch in the radiator, high temp/press AC switch, etc to find the problem. If none of these resolve the problem, pull the relay and see if the normally open main contacts are closed with a continuity tester.

 

I'll give it a shot. Just for quick reference where is the high temp/ac switch? Would the sensor in the intake have something to do with it?

 

Joe--

Mine did the same thing after I did a little wash on the engine. Seems like the temp switch on top of the intake didn't like the water. To test: Pull the wires off that switch and make sure they don't touch ground. With wires off, the fans should stop if that switch was shorted. You can confirm the diagnosis with a meter, testing the switch for zero ohms and knowing that's a fault condition. Jim had a new switch when I needed one.

There are no relays in the fan circuits on the S4, so save yourself the problem of pulling the carpet and the electrics panel cover.

That switch is the --only-- thing that should cause the fans to run after the key is off. If it's not the switch, you might find that the FET-based control head on the front apron has failed. IIRC, FETs don't normally fail in the shorted state but it could happen. Key diagnosis would be that one fan is runninng but the other isn't; the odds of failing both FETs at the same time are pretty small. That module is a plug-and-play replacement. If you are brave and frugal, you can follow the repair instructions at Theo's tips page, and replace the FET's themselves for a very low price. Again from memory, they are off-the-shelf N-channel devices that solder in, and need a thin fim of heat-sink grease to survive. Don't forget this! I don't know if Radio Shack sells the FET's any more. I think there's a R-S warehouse store at Knott/Golden West at Lampson in Garden Grove not far from your house. Otherwise it's a run maybe to Fry's or one of those surplus places down on Harbor.


Try a swap with a known good unit before you pry yours apart.


With the history that car has of fan overload problems, a failed front module is a very possible failure to look at.


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On your hi-temp AC switch question-- The S4 uses a pressure transducer to measure the high-side freon pressure in the system, and from that it determines the heat load on the condenser. The transducer is connected to the smart fan/flap module next to the passenger's seat. The transducer itself is located on the top of the liquid plumbing to the receiver-dryer, right side of the car, in front of the condenser and, coincidentally, just a few inches behind that fan controler module on the front apron. The transducer has the screw terminals with the rubber boots on it.



Good luck!

 

Thanks Bob. Now the fans go off when the car is off. When you turn the key to accessories the fans come on. Start the car and fans stay on. I wanted to make sure that was understood. Now when you say take of the wires off the sensor in the manifold and ground it out do you mean touching the 2 terminals together or just taking off the wires. Also when testing that sensor is it with car on or off that I should get a zero reading. Sorry But I am clueless with this cars electrical system.

 

I'm not intimately familiar with the later electrical systems as dr bob is... I was just offering some general thoughts on troubleshooting. Anyway, I think he was saying DON'T ground the wires....

 

Joe-- I think I wrote to take the wires off the switch and DON'T ground them. One of those wires is already a ground lead (the brown one...) and the other will cause the fans to start if it is grounded. (green IIRC....)

The zero reading on the switch with the wires disconnected: Don't care if key is on or off, as the wires are disconnected. Zero ohms means the switch thinks it's hot, and that's what turns the fans on. If it runs with keys off and those wires connected, then shuts off when you pull the wires off, it's the switch.

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With the switch disconnected and the fans going on and off with the key, that's normal if there's much temp in the radiator. If the car is cold and it does this, look at the temp sensor for the fan circuit. Others have mentioned a switch on the front of the radiator that may be a problem, but I've never found that part on my '89 in my casual tinkering. The FWSM shows a device called a "thermoswitch" that's really a temp sensor element/thermistor, with blue-green and brown-black conductors to it. That's the one you want. It's a reverse-coefficient element, so the resistance goes up as the temp rises; an open circuit will cause the fans to run. You can check the resistance of the sensor and the wiring together by opening the connector to the fan module next to the passengers seat and probing the wires there.

 

It took a few times but I think it has sunk in now. I will update this thread. Thank to all.