1991 350SD - Electric fan running all the time
 

As soon as you turn the key to the run position, the left (driver side) fan starts running at ful blast and wont turn off when the car is running.

Any ideas?

Stuck relay.

Hmmm, possible, possible.

I was wondering if the PO wired it direct, but I didn't have this problem before the engine swap. But I touched nothing in that area. I guess it could be a coincidence.

Any idea where the offending relay is located? Fuse box?

Yes.

Sixto
87 300D^2

The relay was identified, and seems to be good (pulled the cover, and circuit is open) called my father to check with him. He said to study the harness differences in the 300 vs the 350. He said a sensor is pushing the current to the relay, I need to track that down. Never ran into this on any of the other 300 to 350 swaps. But, this IS the first 91 I've swapped into. All others have been 90's and SDL's, not SD's for whatever that's worth.

The fans run in parallel so whatever triggers the left fan triggers the right fan simultaneously. Either the left fan is stuck on in isolation, or a sensor is stuck and the right fan is dead.

If there's a 3-pole temp switch on the turret fitting for the upper radiator hose, remove the connectors and see if the fan turns off. If so, the temp switch and the right fan are bad. Or your engine is overheating ;)

If there's a temp switch with connectors on pigtails on the AC receiver/dryer, remove the connectors and see if the fan turns off. This temp switch taps into the refrigerant circuit so don't remove it without first discharging the AC system.

Those are the fan triggers on an 86-87 SDL. The 90-91 SD/SDL might be different.

Sixto
87 300D^2

I was mistaken. Both fans are running full bore. I couldn't see that well the other night when I observed it.

Disconnected high pressure switch on dryer, cycle switch, 3 pole upper hose wiring, cruise control (it was handy) all at once, both fans still blaring. I dont recall this happening. There is a sensor under the injector lines screwing into the head, but I think that's just the sender for the gauge.

I have this exact problem in my E300D W124 - the fans switch on full when the engine is cold and the ambient is warm. They do shut off when the engine comes to normal temperature. But the relay clicks about 10 times in rapid sequence when it shuts off

The other bit I have noticed is that if the ambient is about 75 degrees - they dont switch on.

The blue temp sensor that goes to the HVAC PBU which actually controls the high speed function of the fans - shows correct resistance and when the fans shut off engine running - when I disconnect the plug to the sensor the fans work high speed just as they would normally.

I would have to dig into the dashboard to test the wiring and see whats wrong. Some people say the sensor goes bad and causes this while a dodgy PBU also causes this - but my HVAC works perfectly.

Correct. If there's a temp sensor beside it it's for the afterglow relay.

Sixto
87 300D^2