Aux electric fan troubleshooting - PeachParts Mercedes-Benz Forum

renaissanceman · #1 04-29-2015, 12:07 AM

I finally evacuated and recharged my A/C a few weeks ago (with envirosafe/UV dye -- didn't want to risk charging my R12 reserve into a possibly leaky system...), and it blows nice and cold at speed, but the electric fan does not come on. The first thing I checked was the resistor, and I found it was open, so I ordered a new one from Pelican. Tonight I put it in, and I still get no fan with the engine idling and the A/C on max.

Where to look/what to test next?

BillGrissom · #2 04-29-2015, 02:43 PM

What is "the resistor"? My 84 & 85 have a "temperature switch" that screws onto a blind hole in the "filter/drier". It switches on at what seems too high a temperature. In my 1985, I have seen the fan come on when idling, and perhaps "about right". In my 1984, I haven't seen it come on while idling (fooling w/ car) even when the filter/drier tubes get almost too hot to touch and the AC blows too warm. When I manually turn on the fan (remove relay & jump 30 to 87), the "liquid tube" cools down and the AC starts working fine. I have several temp sensors I pulled at the junkyard I will test in a pot of water when I get a chance.

It could be a design problem, degradation over time, or a batch of bad after-market switches, since many people complain. RollGuy has a post showing how to wire a relay so the fan comes on whenever the AC clutch is engaged. Downside is the fan will then run even on the highway when not needed.

renaissanceman · #3 04-29-2015, 04:00 PM

The resistor is located behind the driver's side front headlight, and below the ABS pump on my 1987 300D. It supposedly is used to run the fan on at a reduced speed whenever the A/C is on as indicated by the high pressure switch...the location of which I am not sure of. Full speed fan is initiated by the temperature switch near the upper

I realized afterwards that the ambient temperature was in the 50F range, so it may not have been building sufficient pressure to engage the switch...

I need to jumper that or the engine temperature switch to see if the fan even runs, I suppose.

jay_bob · #4 04-29-2015, 07:52 PM

On the 87 124s the auxiliary fan is controlled by the two sensors directly. (88 and up they changed the control of the high speed to the PBU)

Low speed power path: fuse D -> relay K10 -> resistor -> fan motor
High speed power path: fuse D -> relay K9 -> fan motor

Low speed control path: fuse 7 -> relay K10 coil -> S32 (refrigerant pressure sw at dryer, directly behind the left headlight) -> ground

High speed control path: fuse 7 -> relay K9 coil -> S25/5 X1 pin 2 (this is the 3 pin sensor switch on the water neck on top of the head just to the left of the fuel filter - pin 2 of the 2 pin connection, ground is pin 1 of the 2 pin connection) -> ground

If you short the leads at the pressure switch, K10 should pick up and you should get low speed.

If you short the leads on the 2 pin connector at the temperature switch K9 should pick up and you should get high speed.

renaissanceman · #5 04-30-2015, 02:07 AM

That's exactly the info I was looking for but could not find.

I'll give those a shot in the coming days.

renaissanceman · #6 04-30-2015, 03:10 PM

I jumpered the connector on the dryer, and the fan reluctantly started...made some grinding and clicking for the first 5 seconds, and now runs great and strong. The connectors were quite corroded, so I cleaned them with a razor blade, so hopefully that will correct the issue. Something tells me it will not, though.

We shall see. I've got a new temperature switch with the lower temperature version (Red top -- 100C fan on, 110C cut compressor version)

I may test on full speed when I tackle that, most likely over the weekend.

renaissanceman · #7 05-05-2015, 04:08 PM

Jumpering the connector at the coolant temperature switch resulted in a healthy flow of air. While swapping in the 100/110C switch, I cracked up the connector pretty well

It is still very functional, but now on my junkyard list (since I'm OCD about having broken things on my stuff...

Still no aux fan operation with A/C on. Maybe the switch is not getting up to a high enough pressure to cause it to close, as I'm running envirosafe, which apparently operates at lower pressures. Maybe when the weather gets hotter it will start kicking on. (doubtful, but hey)

ROLLGUY · #8 05-05-2015, 05:36 PM

Quote:

Originally Posted by BillGrissom

RollGuy has a post showing how to wire a relay so the fan comes on whenever the AC clutch is engaged. Downside is the fan will then run even on the highway when not needed.

It is in my signature. This is one of the easiest and best A/C performance modifications one can do. Enough said

renaissanceman · #9 05-05-2015, 11:51 PM

Does your kit work in the W124 300D? Also, do you happen to know which compressor these cars have stock? I see mention of an R4 and a Sanden...or another?

ah-kay · #10 05-06-2015, 12:27 AM

The kit is for the W123 chassis. W124 already has a good compressor.

ROLLGUY · #11 05-06-2015, 12:46 AM

Quote:

Originally Posted by renaissanceman

Does your kit work in the W124 300D? Also, do you happen to know which compressor these cars have stock? I see mention of an R4 and a Sanden...or another?

As far as I know all the 60X engines use the Denso compressor. As ah-kay says, they are good compressors. However, they all need the manifold seals changed if original (black). This is the common leak area. Not that hard to do with the compressor removed. Replace with the green HBNR seals for use with 134a......Rich