Aux Temp Switch Bad? Aux fan running when cold.

The other day the Aux fan came on when the temp read about 80 C. It is supposed to come on at 100 C. It went off but now when I turn the key, the Aux fan comes on when the motor is stone cold. I pulled the lead from the switch and it stops. Reconnected and the Aux fan runs all the time. I haven’t driven it since this problem came up.

Does it sound like the Aux Temp Sensor? Could it be anything else?

Thanks in advance for those who help.

Steve
’85 500SL Euro

[taaboo]

Our 300TE did that last year once on a really hot day. I found the relay coil would not pull out. I removed the relay and re-inserted it and it's been fine since. Probably should replace it though.

When the car is cold check the resistance across the switch to confirm that it is OK.

HTH

Brian

I assume it should be 0 or infinite. Is this so? I assume it should be 0 when cold. Is this right?

To replace it, do I need to drain the coolant? Are there any special tips?
What is the torque spec?

Thanks!
Steve
’85 500SL Euro

[taaboo]

I guess my line of thinking was that it's the relay sticking, based on the experience with our car. Your description does indeed sound more like the switch. I can't remember from the ETM whether the relay is powered directly from the switch or not ..... Anyway, the resistance should be a high resistance, as in infinite (open circuit when switch is not 'made').

When I've replaced these on my cars (don't have an SL) I've usually been able to do the "quick change" - unscrew the old switch just to the point it comes out. Have the new switch handy. Unscrew the old switch and *quickly* put in the new one OR plug the hole with your finger/thumb and then replace finger/thumb with new switch. Wipe up the coolant that came out and you're done.

YMMV, HTH, etc.

Brian

Thanks for your responses Brian.

I bought thte new Aux temp switch and put the wire on it. The fan didn't run. On the old one it runs full speed.

I changed it out in about 5 minutes. The coolant only dripped out and there was no mess. THe only test left is to take it up to 100 C and see if the fan comes on. I'm sure it will.

Steve
'85 500SL Euro

Typically the aux fan is engaged by high side refrigerant pressure, but sometimes is also turned on by high coolant temp. It was probably coming on because of excessive high side refrigerant pressure.

Good luck,
Larry

Hi Larry

What if the A/C doesn't work? I am not sure if it's low pressure or something else, but it hasn't worked since I got the car. I have other priorities so I haven't started it yet. The parts are a lot of $ too.

The Aux fan was coming on from the coolant temp sensor because that is what triggered the fan running and I replaced the temp sensor an it seems to be working fine.

I guess they can go bad.

Steve
'85 500SL Euro

[Ron Johnstone]

They pass quite a bit of electricity and the interior contacts finally get enough arcing on them to stick. I had one on a 240D that stayed on and the only way I could get it to release is to let the engine cool down and then swat it with a wrench. As they are sealed, all you can do is replace them or unplug them.