1991 420 SEL. After replacing the dreaded Expansion Valve, the shop that did the evacuation and R134A refill told me the mechanical fan was just kinda luffing along. Also, the drivers side electric fan isn't running at all.This is with the car as hot as it ever gets (~90c) and A/C running. When I got home I put on a heavy welders glove and slowly approched the mechanical fan. Sure enough I could stop it with little effort. Soooooo.....I know thats toast.
Does anyone know how to trouble shoot the electric fans. And, which one is the low temp, high temp fan? Rather than just through a new fan on I would like to know it's not a sensor or relay etc.. OH. Both electric fans did work before.

TIA

The shop shouldn't have had anything to do around the aux fans if they were replacing the expansion valve, as I believe it is under that drivers side of the dash against the firewall. You can always just the low pressure switch with a piece of wire or something, you unplug it and go from the female ends. This should activate you aux fans and let you know if they are working.

jump the low pressure switch

I found the info provided by Mr Arthur Dalton in the following link to be very useful for testing the aux. fans(2 aux. fan circuits) :

cooling fan problem

PS from past personal experience, i found a ton of debris in each of my fans. the debris combined with rain caused the fan internals to corrode, creating an open/short circuit , resulting in 2 non-functioning auxiliary fans.

good luck

Just some information about a 91 420SEL.
The two fan motors are wired in parrallel, meaning if one is runnning then both should be running. They have 2 speeds, low and high. Low is used when the AC high side pressure goes to around 15 bars or around 215 lbs. High speed is used if the engine temp goes above 105 C or 220 F.
The engine fan engages at around the same temp as above. Below that temp, it free wheels.
So, at 90 C the engine fan will free wheel and the electric fans will not be in high speed.

Paul

__________________
84 500 SEL (307,xxx miles)

Thanks. I removed both fans which involves in my case removal of the bumper. Which by the way has three bolts per side not two as the CD shows. Jumpered 12V from the battery. The right side fan kicks on instanly. The left fan requires a little nudge but runs full out. Must be a dead spot in it like a starter motors get sometimes. I'll try the sensor by- pass test from the post above.
I see all the high temps refered to for fan cut in and so forth. Maybe my gauge is hosed up, but here in the heat of a Florida summer mine never goes above what I'm guessing is 90C. I say guess due to the highly inaccurate centigrade scale and lack of graduations on the temp gauge.
And using the open hood, heat in the face test it just doesn't run that hot. Not complaining just observing.

Thanks again. Great info from all.

Thanks T for the link. Very helpful.

Thanks P. So the techs observation of the fans was spot off. Nice to know. When I hooked up 12V to the harness plug (tandem and singly) only the right fan came on. The left one requires a nudge to get started, but runs right up to full speed. Must have a dead spot.

Thanks again for your help.

Red: I replaced the expansion valve myself. Just had it at the shop for evac. and fill. The fan thing was an observation on his part. And got me wondering. Thus the post.

Thanks for the help.