I've got a 940 turbo wagon with some electrical quirks. The controller for directing the air or heat seems stuck on the defrost setting no matter what the dial says.

Also, the bulb light comes on haphazardly. Lately, every time I brake. I checked all the lights, they work. This is just annoying but the heater control problem is worrisome with cold weather coming.

Any ideas would be welcome. Thanks

At one time, perhaps even today, auto HVAC systems run by vacuum defaulted on defrost if vacuum was lost.

In the case of our '90 744 TI, the problem was in the plastic switch guts in the dash - I was able to repair it, but I forgot what was wrong!

Dear sandpiper32,

Hope you're well. While you could install a matched pair of brake light bulbs, I'd first spray the bulb-holders with Deoxit, an aerosol corrosion-remover. An invisibly thin layer of corrosion can disrupt a circuit and trigger the bulb failure warning. If application of Deoxit (or similar corrosion-removed) - to the bulb-holder and to the contacts on the light cluster - does not improve matters, then try installing a matched pair of bulbs.

As to the heater control, it seems unlikely that all servos would fail at the same time. I suspect a vacuum line - likely that which goes from the intake manifold, through the firewall to the climate control unit - has come loose. Look inside the engine bay for a loose vacuum line.

Without engine-supplied vacuum, none of the servos will work, that move the doors which govern air-flow direction.

Please post back with your findings.

Hope this helps.

Yours faithfully,

Spook

It is important to reiterate the importance of the vacuum line which passes through the firewall. Going over my vacuum lines recently, I noticed that the vacuum line that goes thru the firewall right angle boot and to the dampers was severely chafed. I was amazed that my dampers were still functioning. Instead of trying to fish a new line thru the right angle boot, I simply drilled a small 1/4" hole in the firewall, added a rubber grommet and ran a new vacuum line thru the firewall and spliced it into the vacuum line near the damper. Looking from the inside of the car (at least my 1992 940) you will find this vacuum line directly above the accelerator pedal.

As Spook said, the dampers that control the air under the dash are operated pneumatically (via vacuum) from the intake manifold.

As he recommends, check for a (small) vacuum hose being disconnected at the firewall or intake manifold.
There should be a one way check valve in the vacuum line near the firewall that looks like a small Oreo cookie.

If that vacuum line gets disconnected.....you will have no action on the air dampers.

Hope this helps

Agree that new bulbs may solve the bulb warning issue. Also, at the age of these cars, corrosion in electrical contacts is common. When you replace the bulbs have a look at their holders and sockets to check if clean and free of white, powdery corrosion. Insert and remove the bulbs a few times to ensure good electrical contact.

The vacuum servos which control the flaps that direct heating and cooling airflow can incur ruptured rubber diaphragms with age. Check here:

https://www.brickboard.com/FAQ/700-900/HeatingVacuumServoReplacement.htm

--
Bob: Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), 98 S90 (recently sold) and XC60. Also '77 MGB and four old motorcycles

In order to stop a leak at the heater control valve, my floor vents were disabled. I wonder if this is also the case with yours.

Regarding the bulb out light, try replacing both brake lights with two new matching bulbs. Sometimes that light can be triggered by a disparity between the two bulbs.

Thanks for the help. I'll check those vacuum lines.
In regards to the bulb out light, I noticed the other day that it's not
coming on when the light switch is on.