1998 BMW M3 chirping behind dash when A/C on and I apply br

Tiny
98M3
  • MEMBER
  • 1998 BMW M3
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 89,500 MILES
I am getting a chirping sound behind the dash when I apply the brakes. The chirping sound inside the car only occurs when I have the ac on.

If I brake with the ac on and I turn off the ac while it is chirping and I am still applying the brakes to stop, the chirping goes away as soon as I turn ac off.

thanks for your input
Do you
have the same problem?
Yes
No
Saturday, February 13th, 2010 AT 11:14 AM

2 Replies

Tiny
DR. HAGERTY
  • MEMBER
Noises travel at a different rate through cyberspace, and they can have a tendency to become a perception issue. I can't think of a reason for the A/C to be related to the brakes, but there are several reasons A/C systems make noise that might be heard as chirping. The refrigerant level in the A/C is low and the expansion valve is the noise maker, normal operation is restored when charge level is nominal. The fan is noisy. There is a sympathetic resonance that is somehow coincidental and a personal reconnoiter is needed.
Was this
answer
helpful?
Yes
No
Saturday, February 20th, 2010 AT 10:20 AM
Tiny
JEPH
  • MEMBER
Hi, 98M3.I'm not a certified BMW mechanic (I'm a physician who owns a 99 M3), but I CAN give you a few reasons that the chirping in your (and my) car is related to the brakes: 1. The chirping is not related to a perception issue or anything in cyberspace, the fan shaft is either under lubricated or has an old worn plastic bearing and squeaks at just the right (wrong!) Speed. When the brakes are applied, so are the brake LIGHTS and they will steal a small amount--but enough--current from the fan motor to slow it down enough to resonance frequency (the frequency human ears can hear the defect."Chirp"). 2. The brake system vacuum line could be involved. Any number of complex feedback systems in these over-engineered cars may not be opening an air dam all the way or stealing vacuum from an air inlet and changing the afterload on the fan--thereby causing a chirp. Any bad vacuum lines on your M3? This last one is really doubtful--I understand that the baffles that open and close various air vents onthe car are electric-servo controlled, but you never know--THEY might be malfunctioning. I had a German car years ago that had a brakes-caused-weird-problems. And it was the bad vac lines.

Well, i've got the same chirp as do many others. From the answer you got from the above BMW "pro" I'd say we both need to keep looking for the fix. (I'm done with BMW dealer service, BTW.)
Was this
answer
helpful?
Yes
No
Tuesday, August 3rd, 2010 AT 5:42 PM

Please login or register to post a reply.

Recommended Guides