Cooling fans crazy run
 

1991 350SDL the pair of cooling fans out front of the radiator come on, shut off for a few seconds, come back on for a few minutes, shut off for a few seconds, come back on,...etc, etc. the come on when the key is turned to the position before start. Also, the car will sometimes have trouble turning over like the battery is weak, then start suddenly and be ok (unrelated?)

thanks:confused:

unfamiliar with vehicle, although sounds like a bad sensor and/ or relay

Try to get the engine operating temp to 80-85C. Turn the air conditioning on. If the electric fans turn slowly or come on by lightly tapping the hub of the fan and then they stop, buy 2 new fans before it kills the cylinder head. I had this happen to me and that is how I ended up with a #22 head. Hope this helps.

Can you correlate fans turning on and off with AC compressor engagement? If they're correlated, you probably have an AC issue rather than a fan issue.

Fans are driven by 2 circuits - one is the AC high pressure switch, the other is the engine coolant temp switch. Disconnect the AC high pressure switch and see if the problem persists. This is the switch near the AC dryer with pigtails. The switch with only contacts is the low pressure switch. If the problem goes away, consider a new high pressure switch considering you will need to evacuate the system. Don't be tempted to convert to R134a if it's cooling well on R12. The law says you can't manufacture R12. Nothing against certified techs using existing supplies. But I digress, if that's not it, disconnect the 2-pin connector at the temp sensor on the upper radiator hose fitting. If the problem goes away, consider a new temp sensor. These sensors are color coded to correspond to different fan turn-on and compressor turn-off setpoints. There is information in the archives on which setpoints correspond to which part numbers and which is original equipment on which cars... in case you want to take the opportunity to use a sensor with lower temperature setpoints. If neither of these is the problem, hunt for the aux fan relays in the fusebox. I don't know which they would be in your car but they'd be among the silver boxes. These are pretty generic MB relays. If that's still not it, there's probably a frayed ground wire between the fan and the ground point.

Sixto
87 300D

This sounds aftermarket to me.

I never knew of any w126 that had dual electric fans. Was this a 1991 thing that I'm unfamiliar with, or could this be the work of hack wiring somewhere?

Around 88 or 89 MB switched all 126s to dual fans. 124s also got dual fans at some point.

Sixto
87 300D

My mistake, sorry :o

... he says as the runs to the pick-n-pull :)

Sixto
87 300D