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BMW 740il Auxiliary Fan Not Coming On

ASKED BY:

Emil Nikolov

FSC MEMBER:

Yes

CATEGORY:

Body

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Question

I gave a 1997 BMW 740il and they auxiliary fan will not come on when the a/c is on. The fuses look good and test the fan is ok. Don’t know where else to look.

Answer

Hello Mr. Nikolov, sorry for the delay responding, but we have been really busy at the shop. I have a couple of questions, when you say the fuses look good, did you check them with a voltmeter? Also, did you apply power and ground to the fan to make sure that it comes on? If not I would do that first.

If the above-mentioned tests are ok you will then have to move to test the relays. At the fan there should be 4 wires, the brown wire should be ground all of the time. The other sires are power wires that are not going to all have power at once, the only way to know for sure which one should have power at a specific time will be to look at scan tool data. If you do not have a scan tool it will be somewhat of a guess. The black and green wire should have battery voltage from the high-speed relay, the black wire should be battery voltage from fuse #78 (50 AMP) and the black and brown wire should have battery voltage from the normal speed relay.

I don’t know how familiar you are with electrical testing, but you can also check the function of all 2 relays, if the relays do not supply voltage it does not mean that they are bad as they may not be getting the signal to turn on or may not have voltage bring supplied to them. Both relays should have battery voltage at the green and yellow wire coming from fuse #20 (5 AMP) and also on the red and green wire coming from fuse #9 (20 AMP). The high-speed relay is triggered by the ground at the black and grey wire, that ground comes from the A/C high-pressure switch, and the normal speed relay by ground at the black and yellow wire, that ground should come from the A/C control module.

If the relays, fuses, and fan motor test ok, then the diagnostic process will have to move to the A/C control module and the A/C high-pressure switch. Please let me know if you have any further questions. Thank you.

Gil Neves

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