Compressor shuts down at idle, fan does not turn on
 

Just had my compressor and receiver drier replaced. While driving, the AC is great. When stopped at a light, the compressor shuts down at 400 psi and the fan never kicks in, and it blows warm air. The mechanic jumped the AC switch and the same thing happened. Is it a relay? What would you look at next? Appreciate the thoughts and ideas.

Other then that the AC works great when the car is moving.

What model car?

That would be good information to have, huh? It's a 1988 190e 2.3, 4 cylinder. Thanks for asking

Exact thing happened to my '93 190E 2.3 a few years ago and it turned out to be a blown fuse for the aux. fan. Replaced the fuse and no problems since.

If that is not your problem and the aux fan is working for temperature control but not the high side pressure for cooling the condenser, then I would verify that the high pressure switch with the pigtail on the drier/receiver is working correctly.

If the fuse is good and the aux. fan doesn't operate, also check the aux. fan relay in the rear of the main fuse box. I always check mine when I change the oil by shorting the connector at the aux. fan temp. switch. I can hear the relay click before the fan operates.

Thanks slk230red! The aux fan does work for temp control and not the high side pressure. Just as you described. Is the high pressure switch easily replaceable? My understanding is you have to drain the system before replacing the switch. What do you know about that? I have read where someone tried to replace it and they sent all the freon into the atmosphere!

Are you still using R12?

With the engine running with the ac on, will the aux. fan operate if you short the 2 wires at the switch on the drier? This will verify that the aux. fan wiring circuit is good.

Yes, you have to evacuate the refrigerant, replace the switch, vacuum the system, and recharge. While you're at it, you might want to replace the drier/receiver and replace both switches.


Option 2: Some guys have used the aux. fan switch connector at the drier/receiver to install a manual switch inside the car to operate the aux. fan when needed. Something you could do until your a/c need other repairs, then you could replace the pressure switch at that time. Just a thought.

Actually using R12A. It's something I found on Amazon, didn't require a retrofit and seems to work better than r134 which, according to my mechanic, doesn't cool as well and makes the system work harder. AC works great when I'm not idling. I did read that R12a is pretty flammable. More than r134 or R12.

As I just had the compressor and receiver/drier and r12a replaced, can the mechanic put back the freon after its purged or do I need to get more?

Thanks!!! Appreciate your time!

400psi?

Yup, that's when the compressor shuts down. Actually between 350 and 400

I don't know how the mechanic can evacuate and recycle the R12a unless he has a separate system designated ONLY for R12a. He can't use a standard recycle jug as this would contaminate the jug/bottle making it unfit for any other refrigerant.

95% of all automotive A/C shops will refuse to work on a R12a conversion. Just something to keep in mind.

Sounds about right, as I recall the switch remains closed between 2 and 26 bar to protect the compressor. Any thing above or below the compressor shuts off.

Before doing anything else, I would verify that the other switch (the one with the pigtail) is defective by checking it with an ohm meter. If it remains open when the compressor shuts off due to high side pressure, then it is defective.

Good luck

Either there's moisture in the system or it's way overcharged. 400psi on the liquid side is too high even in 100f+ ambient temp here in Texas.

Did you pull a vacuum for at least an hour prior to charging it?

That's not the problem. If the aux. fan fails to operate at idle to cool the condenser, the high side pressure will continue to rise until the high pressure switch opens causing the compressor to shut off. The other pressure switch on the drier/receiver with the pigtail is what cycles the aux. fan for a/c operation.

Well, it's been over 100 the last few days so I didn't get a chance to try anything. But I went out today and started with the basics. I looked at the fuse for the aux fan, first it was the wrong fuse, a white 8 amp. I took it out, and the metal literally fell out of the fuse. I replaced it with a red 16 amp. Turned AC on while idling and the AC seems to do fine at idle. I have to do a test drive to really prove this out.

My question is, when I press the AC switch, should the aux fan turn on immediately? And stay on as long as the AC switch is pressed?

Thanks

No, not always depending on the high side pressure. there are 2 pressure switches on the air dryer. I know this has been mentioned above
One is the high pressure/low pressure cutoff. This switch only controls the compressor and will turn the compressor off if the pressure is too high, as well as if the Freon charge is low; it will not allow the compressor to turn on.

the 2nd switch is the low speed fan switch. (on my w126 for example), it turns on at 20bar(290psi); and enables the low speed fan. And it turns off at 15 bar (218psi). So your system high side pressure will need to be atleast 290psi for it to even enable. Which is a typical operating pressure after about 15-30sec of running. Then it will turn off once pressures drop below 218psi... (usualy when you are driving down the road on a avg temp day).

Not knowing the exact wiring setup on your w201, but it triggers a relay which passes thru a "preresistor" to give the electric fan slower speed.