Climate Control

Buzzing Noise from HVAC Control Unit

When *my* HVAC control was buzzing I learned there were at least two ground connections entering the control head. Only one was completely connected to one side of the coil of a relay inside the unit. It’s the
relay which is buzzing because it does not have enough voltage across its coil to completely do what it needs to do. The act of closing the contacts alters its concept of ground enough to open the contacts which alters its concept of ground again and closes the contacts. Buzzz.

The cigarette ligher wires are close to the clock which has a backlight. One thing that pointed me in that area anyhow was when I found 1) headlights on, 2) doors open, then 3) LCD clock died. Needless to say, P.O. had everything hooked up wrong in that area. The brown wire ground from the HVAC control head connected to the ground on the cigarette lighter, and from there another wire actually made the chassis connection. Clock wires were involved too.

If the relay is buzzing then the gound side isn't grounded. On the circuit traces in the HVAC control there is something else connected to the same ground. This something else is about 5 volts above ground when not properly grounded. Putting about 7 volts across the relay coil. Which is exactly marginal to make it work.

Memory is about 7 years old, but am fairly sure it is a brown wire coming out of the HVAC, but a black wire from the cigarette lighter to ground. As for myself, I had a test lead in hand when I caught the thing buzzing and was able to prove the cure. Haven't heard it buzz since.

'83 928S

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David Kelly N4HHE