740 760 Heating / Air Conditioning Faulty Sensor?

[jeremy]

Hi all, my 1989 760 Turbo's fans do not work when it is cold (nearly freezing) outside. I have read that it may be the sensor under the glove box or the solder on the circuit board behind the control knobs.
When cold, and I switch the blower on, nothing happens, except the Aircon light flashes on and off constantly.

I can sometimes get the system to work normally, when the outside temperature rises - yesterday I switched the engine off, Aircon off, recirculation off, and blower to 1, and that worked, and once the blower was going, turning the switches got everything working normally.
Sometimes there is a slight delay when the blower kicks in, almost like the sensor? is deciding if it wants to work.
The blower motor is slightly noisy.
Is there a sensor above the dash? I can't see one, only in the cabin light console.
What do people think the issue is?
Many thanks in advance for your assistance.

[Laird Scooby]

First, do you mean the cooling fans under the bonnet or the fan for the heating system?

Second, is it a manual heating control or is it the ECC - Electronic Climate Control - system?

They behave in different ways and what you're describing sounds as if it is normal behaviour for the ECC system but you probably need a regas. You also don't say what position the temperature selector is in or the vent selector.

If you switch it to screen, hottest temp and fan to auto (assuming you have ECC) you should get a blast of air at full fan speed to the screen and the A/C button will flash ( no gas probably) but if you switch it to feet, hottest temp and fan to auto, the fan won't come on until the coolant temperature is enough to blow warm air.

How long have you had the car and does it have a handb ook with it?