280CE Air Con Advice

[C337G]

All,
My air con in my 1981 280CE has decided to stop blowing cool air. Some years back I changed over to a Sanden rotary compressor, replaced the condenser, TX and dryer, and it's been fine up to last week. When I turn the fan switch on, it runs but not the compressor or condenser fan. even if I cycle the temp thumbwheel to the lowest setting. If I use a jumper wire (tee it in) from the battery to the compressor everything works, compressor, condenser fan and the AC is cool. I note that someone has added a relay down by the dryer and I suspect that was a DIY mod to get the condenser fan to run while the compressor is engaged. I have bridged out the SW on the dryer and swapped the 2 relays around under the plastic cover near the brake booster but no joy. Its clear that this has been modified by someone and I cannot find any drawings for it. Any ideas where to look? Is the thumbwheel temp regulator in the pic attached still available?

Tony

[T-Modell]

Hi Tony,
I would first check, if there's enough cooling liquid in the system and if there's no leakage. Afaik there's a pressure and temperature sensor, that switches off, if there's now enough or too much pressure. Over the years it could evaporate or there's a leakage.

Kind regards
Thomas

[C337G]

Thomas, as mentioned in the first post, the AC works fine if I use a jumper lead to the compressor, there are no gas leaks, air con is cold and there is no temp sensor/switch on the dryer. Suspect the thumbwheel control is either faulty or there is a loose connection in there.

[tsharkey]

Are the relays OK ? There is 1 for the compressor and another for the Fan (not withstanding the aftermarket relay bypassing the fan one). Have you tested that the trigger pins (85 & 86) are getting voltage ? If yes, then relay, if not than thumbwheel thermostat combo (and associated fuse ?)

[T-Modell]

Thomas, as mentioned in the first post, the AC works fine if I use a jumper lead to the compressor, there are no gas leaks, air con is cold and there is no temp sensor/switch on the dryer. Suspect the thumbwheel control is either faulty or there is a loose connection in there.

Sorry, I missed the "AC is cool" part ... then follow Tim's advice and go along the electric circuit ...
Regards
Thomas

[Chai]

In the 1974 C107, the A/C temperature dial (also) sends 12V to cause the idle speed to increase when the cabin temperature is above/hotter than the dial setting. This is in addition to sending 12V to the compressor, etc. This idle up is achieved by disabling the vacuum in the "distributor idle retard". The round contraption next to the relay (near the receiver drier) in your picture looks like that idle up control. It has a 2-pin electrical connector and two vacuum line connectors - one end to the throttle body and other to the distributor retard. I've used the 12V to this component to confirm the A/C temperature dial is operating normally - the blue/white cable in the picture connects to a LED.

AC idle changeover valve.png