95 e320 a/c Low Aux Fan
 

My question is the opposite of most postings concerning A/C aux fans. I have low speed aux fans that cycle on immediately and remain on even though high side pressure never reaches the cut in spec of 325 PSI.

My question is this. Can the sw/32 fail in a closed position (constant closed circuit)? If not, can this be caused by a failed low speed relay?

Jack

Either will cause your complaint

The diagnosis requires a simple test:

With low fan ON , disconnect the pigtail wires at the sw. If fan still runs , the relay contacts are stuck closed .
If fan stops with disconnect, then sw is suspect [ as long as PSI is below spec cut-on]
BUT ...be aware that cut-in is lower than gauge reading by approx 20PSI b/c gauge reading and sw reading are different b/c of gauge port and sw locations in the system.

Thanks Arthur. Disconnect at pigtail stopped the fan so failure points to the blue relay. Is this a solder joint issue that possibly can be fixed or do I just buy a new one?

BTW I understand your caution about the 20 PSI variance betw. reading on gauge versus the pressure at switch. I am undercharged and high side was at 240 PSI @ 82*F and 70% humidity so I was not close to triggering the aux fans.

Jack

<<Thanks Arthur. Disconnect at pigtail stopped the fan so failure points to the blue relay.>>


Incorrect ..re-read my previous posting.

If the fan STOPS when you disconnect the pigtail wires to the switch, then the relay is working as designed and the low fan circuit has been verified as Ok EXCEPT for the switch.
The suspect is the S/32 switch....or someone has wired the low fan to come On w/compressor...

Simply do a continuity test at sw w/OHM meter.

Low speed aux fan should come on at approx 16 Bar (about 235-240psi)

...and OFF at 12 Bar....

Thanks to all. Yes I did misread Arthur's post. Based on above, it doesn't appear I have an issue with sw/32. I checked pressures this morning at 82* F ambient with 75% humidity and reading was 30/280 @ 1500 rpm.

Since high pressure was over 16 bar and never under 12 bar, yes, fan should run continuously. I checked center vent this afternoon at 101* F ambient and it read 46*. I'm quitting while ahead. Will check aux fan in October when temp gets back to the 70's and pressures drop.

Jack

As a quick test, You can monitor the high side on that chassis with the eyesight glass.
If it is clear and there is a thermal load , you should have low fan.
..And we use the S/32 jumper trick as a quick test for Low fan circuit integrity..saves lots of diagnostic time..............

Arthur - what started this whole concern about the aux fan goes back about 30 days ago when I noticed they did not seem be coming on at all as the temp started to climb. I searched the posts here and came across yours from a couple of years back where you mention the #6 fuse, the possibility of blue and green relays being reversed in their socket position and the update for the 15A fuse on the low fan relay.

I did not have a blown #6 but rather the D fuse which also is on the aux fan circuit. I went ahead to check the relays and found they were reversed. I guess this was done many years ago when MB replaced the wiring harness. I repositioned them and changed out the 15A with 20A.

I guess I never had properly operating aux fans. Seeing them run constantly in this hot weather was something I was not use to and made me think I had a problem.

Very Good..
Glad you found that posting in the Archieves and glad you used it...there are so many answers in there now that this Forum has 10 matured years of service .
Wish we could get more people to use it....

All I see lately is " Where is XYZ part ??? ..and could we post a Picture , with our Finger in the picture right on the part " .. when all this info is right there at their finger tips...
I will go to the extremes in diagnosis, but I ain't taking pictures !!!