Trying to figure out A/C

[jht3]

Originally posted by mjcalabrese
Matt

There is a low pressure switch, a high pressure switch and in some cars an a/c temp switch that are in between the evaporator temp regulator and the compressor. If one of these are open the compressor will quit. Look for where the 12v stops when the compressor quits. Compressor operation has nothing to do with coolant temp.
Mark

actually, compressor operation DOES have something to do with the coolant temp. the temp sensor i was referring to is on the back of the thermostat housing. it opens when the coolant gets too hot. mine failed open.

i do agree on how to test the system. when the compressor quits, pull over and check voltage at the low pressure switch, high pressure switch, and the coolant temp switch. if any of those three are open, the compressor clutch will not engage. this assumes you have a good compressor ground.

the evap temp sensor will also disengage the compressor if the temps get low enough for the evaporator to freeze

[m3ubbs]

Thanks guys. Yes James, except the temp sensor you are talking about is located in my radiator. I have no sensors, and never have had any, connected to the thermo housing. I replaced the sensors--they were cheap and it gave me an excuse to flush my system and replace a couple worn coolant hoses. Mark, thanks for insight on testing the system. I should have thought to hook up a volt meter and go step by step. I guess I didn't get too much out of circuits in physics 2. I have a feeling its either a pressure switch or that a/c switch, but I'm hoping the aux fan resistor at least gets my fan running on low when I turn on the a/c like it's supposed to.

Again, thanks guys.
Matt

[mjcalabrese]

Thanks, glad to be here & really glad you like Roundel!

[mjcalabrese]

Matt

I stand corrected, James is correct, the second switch on the theromostat is the a/c temp switch. Are you telling us that M3 doesn't have the large thermostat with both sensors? Where is your dual temp switch? Is the wrong thermostat installed? I've never seen a M3 without both sensors in the threrostat, why is yours diffenent?

[m3ubbs]

I've never had any sensors in the thermostat. I have always had the large thermostat with black housing since I got the car. Instead of sensors I have two bolts (plugs). I never had any issues with any of the coolant system or a/c until recently. My sensors are in the radiator. I have two sensors on the lower left of the radiator. I think most cars have plugs there. The curious thing is that I can't find a 3-pronged sensor anywhere (dual temp switch?). I have 2 2-pronged sensors in the radiator...one a low temp and one a high temp rating. My a/c runs through the low temp, while the aux fan runs through the higher temp one. Records show a/c was retrofitted with a new compressor, dryer, ect. a little more than 4 years ago. I don't know how they wired it when they did it but I never had any problems. My car is also an early '87 production (March '87). Not sure if that makes a difference on this sensor issue but when I got my new radiator a couple months back I know there was a thru-march-'87 description. When I drove it today the a/c cut in and out. But not really randomly... i'd get up to about half on temp gauge with a/c full blast (was very cold), then a/c wouldn't work. Car would cool back down to right between half and the next tick down and the a/c would come back on. After a while it quit all together. I don't know maybe there is a problem with the wiring somewhere and I just can't see it. The connection to the high pressure sensor wasn't fully seated, but fixing that did nothing. Hopefully taking a voltmeter to the system will shed some light. We'll see..

[mjcalabrese]

Matt
Well, you learn something new everyday. I've never seen a system like yours. I guess BMW changed it very early in production. It should be easy to troubleshoot. Take a large jumper wire and jump the wires of the switches one at a time to see what they do. Have the ignition on, a/c switched on, jump switch, does compressor clutch pull in or does fan run?. Then try the other switch. Do you only have two? I'M going to look for an early ETM. You've got me wondering now.

[m3ubbs]

Yeah from what I've seen there's only those two sensors. There doesn't seem to be the 3-pronged dual temp sensor anywhere. I'm beginning to think it's not electrical anyway. I drove back to school last night, which is about a 2.5 hour drive, and the a/c worked for a while then cut out. I drove for about an hour without it, then tried it again and the damn thing worked. Does it sound like it could be the compressor clutch itself? Because both switches are working and voltmeter showed power to the whole system. The only question I have electrically now is the aux fan resistor, and that part should arrive either tomorrow or Tuesday and get my fan running when I turn on the a/c..but I can't imagine that alleviating the problem with the compressor cutting in and out.

[mjcalabrese]

Matt

So you're saying when the compressor quit, you had power at the compressor? Was the clutch engaged? If you had power at the compressor and clutch wasn't engaged try adding a jumper from ground to the compressor. How did you check the switches?

[John Arnold]

the a/c started acting up after i replaced my radiator a couple months ago, so that's what is drawing my attention to those sensors.

Are you sure the wiring is correct? Are you sure you have the correct sensors?
The low speed switch (closes at 91*C) should have a green/black wire and a solid black wire. The high speed switch (closes at 99*C) should have a green/black wire and a black/brown wire.

anyone know what temperature shuts down the a/c?

The coolant temperature switch for the A/C (called the "A/C temperature switch" in the ETM) opens at 108 degrees C. The wire colors for this switch are black/red and gray/red.

The reason I thought it was the temp sensor was the fact that the a/c seems to shut down at the same temperature each time, and when I unplugged the wiring to that low temp sensor the a/c would not work at all. Apparently this was coincidental because replacing the sensor did not work.

That should not happen. Unplugging the low temp fan switch should have no effect on the AC compressor operation. It is a parallel circuit.

I have 2 2-pronged sensors in the radiator...one a low temp and one a high temp rating. My a/c runs through the low temp, while the aux fan runs through the higher temp one.

Not correct. The A/C doesn't run through either fan sensor. The A/C powers the aux fan on low speed, but it is independent of the fan sensors.
FYI:
The earliest M3s did have the sensors in the radiator. In fact, the only Behr radiator I have seen in the aftermarket has the two threaded holes for these sensors. Fortunately, new thermostats have threaded plugs which can be used in the radiator for the later setup. It is advisable to move these sensors to the thermostat housing, as BMW soon discovered that the risk of overheating is reduced. The bottom left corner of the radiator sees very little flow, and it's a mystery to me why the sensors were put there in the first place.