W124/300D Red AC (Aux Fan) pressure switch failed - PeachParts Mercedes-Benz Forum

Chris_87_300D · #1 10-17-2009, 03:48 PM

Anyone have any thoughts on this...

I did significant work on my AC about 3-4 years ago on my 87 300D W124 (evaporator failed) Also replaced expansion valve, all rubber hoses, compressor seals and all other seals. Did not replace 100% metal tubes, condenser, or compressor.

Recently I noticed that it wasn't cooling adequately and there were lots of bubbles - I added dye and recharged w/ 0.8lbs. Also identified at that time that the red pressure switch had failed, it was no longer turning on the fan (fan works when switch bypassed, switch does not come on at 230+ psi.) I've jumped the red switch so the aux fan comes on 100% of the time that the key is turned on to avoid stressing the AC until I get everything fixed.

I have been unable to identify any leaks with either dye or an electronic leak tester. I've run it for 30 min in the garage, and driven for 75 miles and then looked several times. Although there are a few bubbles in the sight glass (I'm reluctant to go "all the way" to 0 bubbles since I don't want to overcharge), they don't seem to be getting worse (i.e., apparently there are no more bubbles than when I charged it).

Questions:

Is it possible or likely that the failure of the switch stressed the system and it leaks only under the very high pressure of when the aux fan wasn't working and the system was stressed?

Is there any likely permanent damage that may occur with the high pressures associated with the aux fan not coming on? Which components would be likely to fail (please, tell me that it's not the evaporator...)

Is there any reasonable workaround for the red switch short of replacing it? If I can't identify a leak that would require evacuating the system, I really don't want to do so to replace that stupid part (I'd have to go to a shop to have it reclaimed, etc.) Thoughts might be to use the old style/earlier model temp switch (vs the current pressure switch) affixed somehow to the condenser, etc.

Is it a bad idea to just jump the aux fan so it runs all the time? Will that shorten it's life significantly? I'm not sure how much they run in "normal" use - i.e., 20%, 50%, 80%... Winter is approaching (our temps are already in the 60s and will average 50 for the next 5 mos or so...) but we leave the AC on to reduce condensation as well as to keep all the seals lubricated. Maybe there's a way to switch the aux fan off the kilma so it runs 100% the time that the AC is on but not otherwise unless called for by the coolent temp switch...

Thanks in advance!

babymog · #2 10-17-2009, 05:17 PM

Jumpered it on low, probably okay. On high it will overwhelm the stock alternator on an '87.

babymog · #3 10-18-2009, 12:08 AM

Some new information here:

someone has changed the orig sw. to the newer replacement switch , [ red] which is 16/12 bar , cut-in, cut -out, so it is still short refrig b/c the high side gauge port reads 1-2 bar higher than the actual pressure at the drier switch port.
You should be just slightly short...plus you should not see any bubbles.

Chris_87_300D · #4 10-19-2009, 12:08 PM

Thanks Babymog. I'll get it up to temp and double check pressures ASAP and post results along with ambient temps...hopefully tonight.

Anyone on the other questions?

#5 10-20-2009, 07:24 AM

Is this system still on R12? If it is 134, then the fan not coming on would be more apt to result in a high side runaway.

If you still have dye in the system and have it cooling, can you look at the condensation drain water with your black light? That's a good way to test the evaporator. It's hard to get a definitive test on an evaporator with a sniffer.

The next step in the leak check process would be to pressurize with Nitrogen and a few ounces of R22 then use a sniffer.

Chris_87_300D · #6 10-22-2009, 03:39 PM

Larry - Yes, still on R12. I think that I got lucky today. I saw a tiny bit of dye showing on the compressor outlet (see pic). With a mirror, it was clear that there was a small leak at the top right corner. All in all, that's not too bad of a place if you have a leak, it's just an O ring (but the process is the problem!)

On the red switch - I tried the old mechanics trick and tapped it a few (ok, a bunch) of times with a small wood handle. Now it sort-of works, comes on at 188 psi and goes off at 184 psi so it cycles too fast but at least it comes on. I'll replace it when I do the O ring since my fix probably won't last long.

Any suggestions on brand for the red switch that has worked well for you or others?