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Old 11-20-2015, 06:33 AM   #1
headlock
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Default AC Compressor clutch not engaging

Hi Guys

Using this post as a reference

http://blog.bavauto.com/6932/bmw-e36...ot-turning-on/

I'm trying to diagnose my AC problem. The AC compressor clutch does not engage when you turn on the AC using the control switch. The light on the control switch illuminates, but no power is received through the harness to the compressor.

In the relay socket, i tested terminals 30 and 87. Neither is showing any voltage. 12v is NOT being received by any of the terminals at all.
Fuse 39 and 16 appear ok.
If i jumper relay terminal 30 and 87, the compressor clutch engages - so at least the compressor is ok.

what should i check next?
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Old 11-20-2015, 01:58 PM   #2
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Maybe the refrigerant pressure switch is not made because the charge is very low or gone due to a leak in the ac system.
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Old 11-21-2015, 08:40 AM   #3
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where do i need to look for the pressure switch, and what is the best method of testing it without a pressure gauge? AC refill kit's are illegal in australia, so i cant just go get a can and gauge and refill it unfortunately
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Old 11-21-2015, 01:24 PM   #4
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What year model?

1. You need to test the system if it still has refrigerant charge. With engine off, remove the charging valve cap and depress slightly the schrader valve and see if some R134A will escape. If it is low, then it may not have enough pressure to close the low pressure switch. It protects the compressor from getting damage in case the charge is low.
2. If you cannot diy, that's too bad, because you have to take it to an ac shop.
3. Can you try buying R134A and gauge manifold set online?
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Old 11-21-2015, 09:16 PM   #5
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Is the low pressure switch before or after the relay in the chain of events?

My understanding from that link is terminal 30 in the relay socket should be hot at all times. In my case it has no voltage.

I just want to make sure I've checked everything I can before I go pay someone a bunch of money. Ac re gasses are not as cheap as they used to be.
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Old 11-22-2015, 12:29 AM   #6
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The ECU supplies power to the dual pressure switch power then the pressure switch output goes to the integrated climate control unit. To test, put a jumper across the dual pressure switch and turn on the ac compressor and see if it engages. if it engages, system is low on refrigerant or the pressure switch is bad. Test the compressor for a few seconds only as running it with no refrigerant will damage it.

Fuse 39 only supplies power to the terminal 30 on the compressor relay. Then 87 goes to the compressor clutch coil.
What controls the relay are terminals 85 and 86. The compressor relay 12V control voltage is supplied at terminal 86 of the relay at F16 then the integrated climate control unit provides the ground path at terminal 85 of the compressor relay.

See diagram.
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Last edited by bazar01; 09-18-2016 at 03:57 PM.
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Old 11-22-2015, 04:17 AM   #7
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that's great info, thankyou!

--

so i tested the low pressure switch and it's receiving power. Met up with a friend of a friend who has a pressure tester and sure enough it's totally empty of refrigerant.

So it just needs to be leak tested and then filled up. thanks for your help.

Last edited by headlock; 11-22-2015 at 06:47 AM.
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Old 12-01-2015, 12:53 AM   #8
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FYI...The system has to be pumped down into a vacuum before adding Freon.
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Old 12-01-2015, 02:42 PM   #9
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You probably meant that the system needs to be evacuated or a vacuum pulled to remove air and moisture. This is a standard procedure after the leak has been identified and fixed whenever a system has completely leaked the refrigerant or the system has been opened.
Pumping down is used to collect refrigerant inside the condensing unit by closing the high side service valve and using the compressor to pull all the refrigerant and store it inside the condensing unit on most residential split ac units.
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Old 12-02-2015, 05:00 AM   #10
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That's very helpful to the layman...
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