Greetings,

I've been unable to find the problem with the circuit that activates the fan with the refrigerant high pressure switch.

Here is what I've tried:

1) When I jump the two leads that plug into the switch on the AC receiver dryer the fan is not energised.

2) When Grounding the Blue/Brown wire, I hear what is termed the "Low Speed Relay" contacts close.

3) I have replaced the fan "ice cube" relays with no results.

4) Jumping the coolant switch leads will activate the fan at high speed.

How do I access the wiring under the ice cube relays to follow the circuits?

The wiring diagram shows a preresistor. Could that be at fault? Where is it?

What else should I be checking?

Lastly what does the single post 50 degree temperature switch located on the block just behind the water pump do?

As always thanks for any insight, help or suggestions.

David Jonas
'86 300SDL 138k mi.
'84 300TD 240k mi.

It would be great if anyone could offer any help on even one of the questions.

Thanks

Dave Jonas

1) When you jumper the leads at the red or green switch on the drier, the low-speed fan relay should engage, and turn the fan on low speed. If not, then the relay, fan, or low-speed resistor is bad.

2) Good - the relay is working then

3) Good - proves relay is not the problem

4) Good - proves the fan works!

5) You shouldn't need to access the wires under the relays. The pre-resistor is near the receiver-drier. I bet it's faulted open, or has bad connections.

6) The 3-prong switch on top of the engine, that triggers the high speed at 105C, is often bad after 15+ years. A new one is ~$25 from aftermarket sources. I highly recommend replacing it as a preventive maintenance item.

7) The 50C temp switch near the t-stat is for the tranny. It triggers a solenoid that re-routes the vacuum signal to make shifting softer until the engine warms up past 50C, then allows normal firm shifting. My cars slip a little with this circuit functioning. I disconnected it and my cold shifts are fine IMO.



HTH,

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Dave
Boise, ID
Check out my website photos, documents, and movies!

gsxr, thanks for lending me some of your experience and knowledge. I was able to verify that what I think is the pre-resistor (it looks like an old ballast resistor from a '40s or'50s farm tractor) is at fault.

On the 300 SDL, the pre-resistor is located behind the false firewall in the well directly beneath the brake fluid resevoir. Has any one replaced one of these? I am wondering how to get at it without having to remove parts of the hydraulic brake assembly?

Thanks...

Oooops! The resistor is near the drier on the W124 chassis. Sorry to steer you in the wrong direction. I didn't know it was so hard to access on a W126. Hopefully someone else can provide some tips to access it, as I have never worked on a W126. At least you have nailed down the problem. If you use a length of wire to short it out, this will allow the fan to operate (on high speed) until you can fix it properly - if so desired...



Good luck,

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Dave
Boise, ID
Check out my website photos, documents, and movies!