300SDL- would a non-funtioning aux fan trip a/c compressor? - PeachParts Mercedes-Benz Forum

a2aa · #1 08-19-2010, 01:32 PM

Would a non functioning auxillary fan trip the a/c compressor >1min after the a/c is turned on?

At first, turning the engine off and on again would reset the system and re engage the a/c compressor. Today, after some inspecting, I found out that the aux fan is faulty,thus it doesn't come on during start ups - (when head pressures are high), would this trigger the relay to dissengage the compressor clutch? Other wise, the system works beautifully. BTW , ambient temp in Texas now a days is 102 degrees.

Thanks for the help

compu_85 · #2 08-19-2010, 02:29 PM

If the system pressure is too high it will shut the compressor off. By using the system with a dead aux fan you risk damaging it

-J

compress ignite · #3 08-19-2010, 09:00 PM

The Auxiliary Fan Temperature switch on the Receiver/Dryer will shut Off the
Compressor Clutch Electrical Flow,If Refrigerant Temperature rises Too High.

The Auxiliary Fans (When Activated)lower Refrigerant Operating Temperatures...

So it's possible for lack of Auxiliary Fans to cause Compressor Clutch Dis-Engagement.
The Question is: @ Ambient Hades Like Temps will the Refrigerant Temps
skyrocket quickly enough to trip the Temp Switch on the R/D.

AND I enthusiastically agree with Compu 85 that running the A/C with out
functional Auxiliary Fan(s) is risking even more trouble.

boblo_home · #4 08-19-2010, 09:30 PM

AKAIK 300SDL controls the aux fan in 2 modes.

A pressure switch on the high side of the recv/dryer to control the fan at slow speed. Earlier MBZ used a temperature sensor but NOT the SDL. I think it is switched in at n bar as per the schematic. May be 20, cannot remember off hand.

A 3 prongs sensor on top of the engine block to control the fan at high speed. It cuts in at 105C to cool the radiator.

Another pin of the same sensor switches in at 120C to switch off the A/C. The signal is fed into the Kilma relay. Unless your engine runs very hot and trips the sensor at 120C, it is likely to the Kilma relay.

sixto · #5 08-19-2010, 09:31 PM

Doesn't sound related. AC compressor quitting then coming back after a restart sounds like a Klima relay or compressor speed sensor issue. The Klima relay doesn't know about the aux fan.

This thread might be useful in your investigation -

http://www.peachparts.com/shopforum/showthread.php?t=280522

IIRC, there are two triggers for the aux fan:

1) Low speed = bridge the AC pressure switch with pigtails to see if the aux fan engages.

2) High speed = bridge the paired contacts in the 3-pin temp switch by the upper radiator hose.

If only low speed is unavailable, check the inline resistor. It might be somewhere behind the driver side headlight, but 126s are a long time ago for me.

Sixto
87 300D

a2aa · #6 08-20-2010, 12:14 AM

Friends..... after close inspection of the circuit and the pattern when it disengages, I found: 1st a faulty relay. 2nd, busted aux fan fuse.

I pulled the relay beside it (same numbering on top of relay cover) and switched it around to see if the clutch would stay engaged using a DIFFERENT relay.

I sprayed some brake cleaner between the clutch space to clean oily debris if any. Then the obvious, I replaced the busted fuse of the aux fan.

Everything seems to work well.The clutch stayed engaged and the fan is running the show. NOw whether any of the three things I did fixed the problem would be the mystery now.

I'm ordering 3 new relays and replacing all of the same silver covered relays right above the fuse section of the box.

My next guess was the klima relay but for now, it is spared. It sure was a great feeling to fix a problem w/o spending a DIME. Thank you forum friends.