AC Relay? or Drier temp switch? or??????
 

Hey all.

I just had my AC compressor replaced and my aux fan isn't coming on at the low speed for AC. It comes on when engine temp is too hot though, full speed like it's supposed to.

Does this sound like the AC relay or the reciver drier AC temp switch? Or the aux fan resistor? or???????


BTW, It's an 87 300E

Thanks

Angel

Yes.

There are so many variations of Aux fan circuitry, I never test one without an exact wiring diagram. The sensor on the drier is almost never bad. Relays, fuses, and the blower resistor are the common problems.

A common test would be to test for power at the resistor. If its there the resistor is probably bad and there won't be power to the other side of the resistor.

Easy test for circuit integrity is to jumper the high pressure sw. at reciever/drier. If fan runs on low w/jumper, the circuit is fine , so I would suspect low freon charge [ not allowing high side pressure to get to fan cut-in spec .[ 20 bar on '86-87 124.030].
Gauges are best for testing the pressures , but on R12 systems [ yours], a peek at the sight glass should show no foam/bubbles if you have proper charge.
If fan does not run with sw jumper, than SB post of fuse, relay,resistor are next steps.......[ the load fuse for low fan is #7-16A on that chassis...]

Thanks for the replies fellas. I will check everything this week and post the results. BTW, the high side is 250psi and the low side is 40psi. Inside temp is 49f

The fan comes on full speed when the engine gets too hot, just not coming on when AC is on and freon gets hot.