michaeldean

Guys:

My car would not start after work today - I heard the cooling fan running slowly. It turns out that its stuck on - as if the coolant temperature never declines and that flattened the battery.
So, I pulled out the big silver relay in the fuse box (fan switched off), opened it up and it smells a bit burned - though no evidence of any of the contacts stuck in the closed position. Could be the coolant temperature sensor I guess.
Is this a common failure - if so, what causes it?
That big silver relay is looking pretty spendy

Thanks!
Michael

mouse

Don't know if this is what you're experiencing but a somewhat common problem that I had was the thermoswitch going bad, so the fan would just run forever.

83 TX 944

Bingo,[mouse] On the lower driver side of the rad, there's a little switch that screws into it, 2 wires attached, don't get agressive with the switch, take your time. I forget the size, 17mm? Examine the wires and, connections. Replace as necessary.

jason952

sounds like the switch, it just happened to me. pull the relay and leave it out until you fix the sensor. you can pull the AC relay (to cut out the compressor) and just run the ac to put the fan on the slow speed to keep cool, this also tells you that the relay is running right. The sensor is like 29 mm deep socket but a 1 and 1/8" deep socket worked for me. YOu can do it from below without removing anything, but some contortion is required. At this point, some people will put in a lower temp switch to run the fans earlier. However, this works best if you switch to a lower temp thermostat as well. The stock turbo one is 92 and 102 deg C, low and high speed, respectively.

Jason

gleamingred944

I had the same thing happen. It was the thermoswitch stuck closed. But, you've got sharper ears than I do, or my fan is newer - I can't hear it running at low speed unless I lift the hood. However, now I know that if, when I turn on the ignition, the fan immediately comes on high, that's the problem. Hopefully next time I won't need a tow :-(

ERAU-944

had the same problem, it was the FAN RELAY, check it with the battery charged, and find a spare relay if there's someone near you with a good one. cost like $45 from ians for a good used one. don't break into that cooling system unless you have to. check the relay first!

jason952

IMHO, I would switch the sensor first. If the car's fan runs on low when all relays are plugged in and the AC switch is depressed while the car is on the accessory ignition setting (engine off), the relay is fine. The sensor failure is more common, it gets hot and gets "stuck," failing to switch the fans off as the fluid cools. and it's only about $25. relays are up to 150 bucks new. If you switch it quick, you won't even lose that much fluid. I only had to refill 1/2 of a qt. You should still bleed it though.

Jason

MHT

disconnect the thermoswitch, plug in the relay, if the fans run then the relay is the problem. If the fans do not run then the switch is the problem.

MHT

small posibility exists for a wiring problem between switch and relay, but that would be unusual. Good luck Mark T. P.S. if you do replace the switch you can top up the coolant then run the engine untill the fans have cycled a couple of times, turn the eng. off and let it cool down pref. overnight. Next morning top it up and you should be done.

jason952

something else I forgot, if you unplug the switch wire and jump the pins (with relay and batt on) the fans should run both low and high speed, that would also be a good test of the relay. But, you may want to pull the relay long enough to run your hand up there to unplug the wire harness.

Jason

michaeldean

Yep - it was the switch.
With the fans still running (ignition off, cold engine) I unplugged the plug to the coolant temp sensor and the fans swithed off.
As an interim workaround (till I get a new switch), I left the plug off the sensor & will use the AC to switch the fans on.
Its pretty chilly in the Northwest so no worries about overheating - I've never heard the fans on high speed anyway.
Cheers,
Michael