PeachParts Mercedes-Benz Forum - View Single Post

emerydc8 · #9 06-03-2007, 10:12 AM

You were absolutely right about the bad wire to the preresistor. When I looked at the old wire, it was somewhat corroded and probably not making good contact. Also, on the back of the preresistor, there is a screw that tightens the square-nut terminal base on the front. That was loose too. Once I replaced the terminal and tightened things up, I reconnected the wires coming out of the red high temperature switch (the ones that were originally disconnected when I started this investigation into why my low speed fans were not running).

I turned the key on (engine off) and jumpered the pressure switch (the black spaded connections). I left the wires connected and just pulled them back enough to get a paperclip to touch both leads. I did not hear the aux fan relay click.

I started the engine, turned on the a/c and jumpered the switch again, bringing it up to 2000 RPM. Still no low aux fans. Mysteriously, while the engine was idling and I was trying to figure out why the low speed fans were not coming on, they suddenly turned on all by themselves.

Then, out of curiosity, I pulled both wires off the pressure switch and they kept running (in low speed). Obviously the pressure switch on this particular car--as modified by someone without my knowledge--does not control the low speed fans. BTW, I checked the coolant temperature when the low speed fans came on and it was right above the 85 degree C mark (I know the coolant temp should have no bearing on the low speed fans). I'm sure they were the low speed fans--not the high speed; the high speed are much louder and come on at 92 C.

So, I took the car for a drive and checked that the a/c was working. It was 68 degrees F outside and the outlet temperature from the center vent went down to 40 degrees F. When I pulled back into the garage, I opened the hood and the low speed fans were not running. The low speed fans finally did come back on after about 5 minutes at idle.

(This next section might more appropriately be the subject of a separate post, because it's more a/c related; but, I don't know if it might tie into the fan conundrum.)

As the fans turned on, I noticed the sight glass for the R-134. About 20 seconds after the fans came on, the R-134 in the sight glass started bubbling. Then I could see the R-134 level go way down--out of sight. After about a minute it came back up and the bubbles eventually disappeared.

With the car at idle (68 degrees F in the garage), the a/c outlet temp eventually climed up to 70 degrees F. I don't know if this is because of no air flow (or at least not enough from the low speed fans) or whether it is because the rpm was not high enough to get optimum performance from the compressor. I did not rev the engine for ten minutes to see if the a/c would eventually bring the outlet temp back down again, but that may be a next step.

I only have a low pressure gauge--and a cheap one at that--so I am only able to see half of the picture; but, when I checked the low pressure side and revved the engine to 2,000 RPM for about a minute, the pressure fluctuated between 10 psi about 22 psi--back and forth--every few few seconds. Not sure what that means or whether anyone could even diagnose without a high side reading.

In any case, the low speed fans now work--just not off the pressure switch--and the a/c works when the car is not sitting at idle. So at least there is some forward progress. Thanks again for your time.