battery drain from Heater/AC circuit - BMW Forum

Grandpa2011 · 01-05-2011, 05:20 AM

95 BMW 528I Series 5

My wife said the heater would sometimes go off by itself then come back on. Until yesterday it did not affect the battery voltage.

Now a faint buzzing sound behind radio with key off drains battery down to 7 volts. Recharge to 12.5 volts and in 10 minutes back to 7 volts Battery is only 3 months old,

Taking out #9 fuse in the glove box stops the buzzing. I see #9 supplies the Heat/AC control Module circuit. Before I do any further dismantling I thought I would ask is there a specific motor or other electrical device more prone to failure.

Any ideas are appreciated.

Ron

brettski · 01-05-2011, 06:20 AM

Welcome to the Werkz.

Does the buzzing noise occur when the ignition is on, but all of the heater/fan settings are turned off? Are you able to tell if the noise is electronic or mechnical in nature?

Grandpa2011 · 01-05-2011, 02:01 PM

The faint buzzing sound occurs ignition on or off. It sounds like its coming from directly behind the center LCD display.

With ignition on I ran the air volume switch all the way both directions.
Although the red bars showed increases and decreases the buzz sound seemed to stay the same. I don't think the blower is working. The schematic
shows output from the control module going to " blower relay, blower , blower motor output stage" I'll try to check that direction.

I seldom drive the car so I am unfamiliar with the proper sound of a BMW blower motor going through it's speed range.

Maybe tomorrow I'll pull the radio to better access the IHKA control panel module since the service manual says it's there and best I can tell so is the buzz. Sound seems more electronic than mechanical.

Grandpa2011 · 01-08-2011, 12:25 PM

Well I pulled the radio and CD player. Then slid the climate control
unit out. The buzz was in that unit.

Next day. In reading up on blower motors, relays and FSU on the web I see many people with problems resolved by replacing the FSU. I ordered one but while waiting thought if I put the control unit back in then take out the FSU I might find the FSU was causing the control buzz.

The buzz was gone when I put the control back in prior to taking out the FSU.
Typical of electrical problems.

Took out the FSU and will install the new one as soon as it arrives.
Hope that does it.

Grandpa2011 · 01-11-2011, 06:22 PM

Installed the new FSU.

Blower works good in all ranges.

No abnormal sounds, everything
appears to function as intended.

brettski · 01-13-2011, 05:50 AM

Well done, what did the FSU end up costing you?

Grandpa2011 · 01-13-2011, 09:33 AM

Quote:

Originally Posted by brettski

Well done, what did the FSU end up costing you?

About $78 including shipping. I googled the part number
and just picked one of the suppliers around $60. that said OEM.

Having read up on the web of many others that had the same problem
and stating they fixed it in 20 minutes my 3 hours in a cold driveway is the learning curve.

It's fixed, I learned and next time I can be one of the 20 minute guys.