I have just purchased a 1979 240D. After driving it for a bit yesterday, the auxiliary fan would not turn off - I ended up having to disconnect the power cord.

The temp guage never registered above 175, so, from what I understand, the fan should not have turned on in the first place.

So...

1. Is it possible the Aux fan switch is the problem - if so where is it located ?

2. Is there a temperature detection switch that could be stuck on - again where is it located ?

3. Is there a relay that could be the cause - again where is it ?

Any and all advice is welcomed.

Jeff

There should be a set of two relays on the driver's side fender under the hood - maybe under a black cover. One is the fan relay, can't remember what the other one is. Try swapping, they're the same. If no difference, you've ruled out relay. I can't remember if that model has a sensor for temp or just for the AC pressure. If there is one for temp, it is probably on the thermostat housing.

Someone may have monkeyed with the relay or wiring in order to keep the fan on. If you have AC, look at the top of the reciever/dryer and see if there are any wires disconnected from it.

__________________
1984 300Sd 210k

Former cars:
1984 300D 445k (!!) (Strider) Original (and not rebuilt) engine and transmission. Currently running on V80 ( 80% vegetable oil, 20% petroleum products). Actually not, taking a WVO break.
1993 300d 2.5 275k. Current 120/day commuter
1981 300SD 188k (Hans) Killed by a deer

I wish I had that problem. My auxillary fan won't turn on. Insufficient voltage. Still haven't tracked it down. The engine seems to run at acceptable temps even with the A/C. Runs much cooler then my newer gassers though I know they are designed to run hot for fuel efficiency and pollution reasons.

__________________
My Daily : 96 E-300 Diesel with 195,000 miles
Retired: 92 300D 2.5 T 345K miles and for sale
Retired: 95 E320 157K miles and currently parked with blown engine

Both retired cars are for sale as is my w124 shop inventory

My Saab 99 used to do that with the fan and it was the (Bosch) relay. The contacts welded shut.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Someone mentioned putting a manual switch for the aux fan on the dash. I always thought that was a cool idea.
Maybe one day...

danny

__________________
1984 300SD Turbo Diesel 150,000 miles

OBK member #23

(\__/)
(='.'=) This is Bunny. Copy and paste bunny into your
(")_(") signature to help him gain world domination

Well this is an old thread but not much help. My 300SD aux fan has started to turn on as soon as the key is turned on. Does anyone know what may be going the cause? It does this when it is cold and this is a new problem/situation/thing for this car. Thanks.

__________________
If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon

Find the dryer.

There are two electrical switches on the dryer.

Disconnect the wires from the upper switch.

See if the fan shuts down.

Report back.

Given that the fan starts when the key is placed in the run position, it pretty much has to be the temp switch on the r/d. The aux fan relay is hot at all times, so the fan would run with the key off is the relay was not opening.

Brian, OK finally had a chance to go check this out, YES the fan stops when the wires are unplugged from the top switch. Now what? What does this tell me? Is this the same switch tangofox007 is referring to? Thanks.

__________________
If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon

Yes. It sounds like that switch is not opening as it should. Just to be sure, check for continuity through the switch with the car cold. If it has continuity, replace the switch.

If the switch looks like this, you can change it with no refrigerant loss:
http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23H0N2A6324F16YR7A&year=1979&make=MB&model=240-D-002&category=R&part=A%2FC+Temp.+Switch

Thank you. I'll check for continuity. I also followed that link for the SD, looks like I will be recharging the system. As always Thanks for the help here guys.

__________________
If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon