Need help diagnosing a run-on aux fan. - PeachParts Mercedes-Benz Forum

pwogaman · #1 09-21-2006, 10:53 AM

My 85' 300SD auxiliary fan has taken on a mind of its own lately. This morning after parking at work the auxilary fan was running full steam. I started the car again to check the engine tempurature and it was still on the low side of normal - the car did not have the opportunity to fully heat up (short drive from home). Even with the a/c climate control functions off, the fan persisted.

There have also been at least two occasions where, when starting the car cold, the fan kicked in at full speed, but this time the fan was going full steam with the car off.

Fearing a dead battery upon returning to the car I disconnected the auxilary fan until I return.

Any suggestions trouble-shooting / diagnosing this? I suspect an engine tempurature sensor may be malfunctioning in the closed position. Could it be the coolant or the refrigerant tempurature sensors? Any suggestions on how to quickly elemenate possibilities?

Many Thanks

Zerohour3k · #2 09-21-2006, 11:05 AM

Same thing happens in my 84 300D Euro. Aux fan will run when the car is off, if the engine gets hot enough. I ended up just unplugging it, since my A/C is broken, anyway.

SD Blue · #3 09-21-2006, 11:28 AM

Another possibility is the aux. fan relay is sticking. I believe it is in the fuse box.

pwogaman · #4 09-21-2006, 11:59 AM

Quote:

Originally Posted by SD Blue

Another possibility is the aux. fan relay is sticking. I believe it is in the fuse box.

Damn good thought. I'll check that one right away. A little ultra-fine sand paper between the contacts should fix that if it is the problem.

RAYMOND485 · #5 09-21-2006, 01:31 PM

1984 300d Turbo 140,000
They Are Two Relays The Same On The Left Front Fender Engine Under Black Box Cover, One For A/c Compressor, One For Aux Fan, Switch The Relays

MS Fowler · #6 09-21-2006, 04:39 PM

On a similar topic...
Where do I begin looking if the aux fan starts when the key is first turned "on"? --while the GPs are heating, the fan is roaring.

winmutt · #7 09-21-2006, 05:16 PM

Is the ac on? Does it only happen when GP is on?

pwogaman · #8 09-21-2006, 08:15 PM

Quote:

Originally Posted by SD Blue

Another possibility is the aux. fan relay is sticking. I believe it is in the fuse box.

That was it. I got in the car after work (engine cold) and started it up. The ac was off and the aux fan was roaring. I popped open the hood and pulled the aux fan relay. The fan stopped. I gave the relay a little bap and replaced it. The fan did not turn back on. I next switched the a/c clutch relay with the fan relay pursuant to the above suggestion. This way if the a/c clutch relay sticks open if will not matter too much when the engine is off.

I will order a replacement relay with my next batch of parts.

Thanks to all who helped.