No power to AC pressure switch and compressor wont turn on

[cgrphotos]

So the compressor on my 1998 M3 only turns on when I jump the relay. Pins 85 and 30 on the relay both show 12v, but no pins on the plug for the pressure switch show any voltage.

Anyone have any ideas?

[heifetz17]

If the system is similar at all to my e23, check you're evap temp switch. My power goes from the relay to the temp switch to the pressure switch to the compressor.

[cgrphotos]

If the system is similar at all to my e23, check you're evap temp switch. My power goes from the relay to the temp switch to the pressure switch to the compressor.
Must be this part

64-51-1-468-411

However, where does that go?

[freddybeemer100]

Get a wiring diagram and find the break in voltage going to the switch. Have you tried jumping the switch? Does the switch when closed complete a ground?

[CROWN]

Low pressure switch

[medic1dl]

Is the system charged? If it isn't then the low pressure switch is doing it's job by not allowing the compressor to start.

[cgrphotos]

Yes it's charged. If I jump it, the aux fan turns on but no clutch. Once again, jumping the relay makes it blow perfect.

[PJ325i]

I'm going through the same thing. I'm beginning to think it's the actual pressure switch that is bad . . .

For mine, the aux fan comes on when I switch the A/C on, but no compressor.

[TwinturboZ32]

a relay has two parts... the contact side (relays main power to the device)

the other side is the "coil" side. You have 12v going to a coil of wire that pulls the contact side open or closed.. there of course needs to be a ground to the other side for it to operate.


Since it DOES work when you jump it then you must have power to the contact side and a good connection from the relay to the device.

Which leaves the coil side in question.

I'm not sure how bmw does it but you can either have constant power (12v) and have a switch that connects/completes the ground circuit to energize the coil.

Or you can have a constant ground signal and a switch that applys/connects power to the coil and energizing it.

Focus on the smaller wires (if there are any....)

they will either have...

A: constant ground, and power ONLY when it should be activated.

B: constant power, and ground ONLY when it should be activated...


Figure out how its wired, see what you have, either constant ground or constant power, then trace/follow what your missing...

here is a basic diagram I drew of general relay operation, it will be either config A, or config B (as shown above)

[PJ325i]

Take a look at this . . . http://forums.bimmerforums.com/forum/showthread.php?t=1474309

Might be helpful. I'm going to try changing out the the evaporator temp sensor in the morning and see what we get.

[cgrphotos]

a relay has two parts... the contact side (relays main power to the device)

the other side is the "coil" side. You have 12v going to a coil of wire that pulls the contact side open or closed.. there of course needs to be a ground to the other side for it to operate.


Since it DOES work when you jump it then you must have power to the contact side and a good connection from the relay to the device.

Which leaves the coil side in question.

I'm not sure how bmw does it but you can either have constant power (12v) and have a switch that connects/completes the ground circuit to energize the coil.

Or you can have a constant ground signal and a switch that applys/connects power to the coil and energizing it.

Focus on the smaller wires (if there are any....)

they will either have...

A: constant ground, and power ONLY when it should be activated.

B: constant power, and ground ONLY when it should be activated...


Figure out how its wired, see what you have, either constant ground or constant power, then trace/follow what your missing...

here is a basic diagram I drew of general relay operation, it will be either config A, or config B (as shown above)

Thanks a lot. I had to read this a couple times, but I think it makes sense. I'm going to do some searching tomorrow.

[CROWN]

still say low pressure switch ensp since system is charged. i've had a few cases same problem. could put power to clutch or jump relay and it works. low pressure switch in bam everything works.

[TwinturboZ32]

still say low pressure switch ensp since system is charged. i've had a few cases same problem. could put power to clutch or jump relay and it works. low pressure switch in bam everything works.

yes very possible, I was just explaining he needs to find out what controls the coil circuit.

I've only had my bmw for a few days so I'm not sure what controls the relay coil. But just general electrical troubleshooting.

[PJ325i]

My issue turned out to be relays . . .

The PO had the wrong relays plugged in. However, if you go by the BMW Electrical diagrams, he did us the correct relays . . .

I used this (http://forums.bimmerforums.com/forum/showthread.php?t=1693088) to straighten out everything. . . .