Aircon and Heater Controller Unit

[craigcaf]

I have a problem with the A/C on my T350. The heater controls, flap stepper motor, fan etc are all working fine, but the A/C is not clicking in when the heater control button is pressed and the blue LED illuminates. It was all working fine a couple of weeks ago when I last used the car. So far I have (i)checked fuse 28 which is the main feed for the HVAC system, (ii) have checked the aircon fluid pressure (all fine - was checked and regassed last year after the trinary switch leaked), (iii) checked that the A/C clutch engages when energised by supplying an independent 12V to the compressor feed wire, (iv) disconnected the trinary switch to see if the engine fan comes on. (i) - (iii) are all fine, and in the case of the trinary switch when disconnecting the fan does not come on, and also when connected the fan is not on (which it would be for example if the refrigerant was over or under full).

The heater control/amplifier unit I presume is the next place to look - so far I have checked the connector to the control/amplifier unit (not melted or visibly corroded or broken). I have checked at the compressor end that when the A/C switch is pressed, the feed wire is definitely not getting 12V, so the compressor clutch is not engaging.

So all in all nothing obvious and I'm starting to wonder if it is something deeper in the heater amplifier unit.

Anything obvious I have missed?

C

Edited by craigcaf on Wednesday 20th August 21:47

[LeeT350c]

Hi

There is a ribbon to wiring loom adaptor behind the POD. These often come unstuck from their crap velco / glue mountings and something might be loose? It makes the connection between the ribbon cable on the back of the fan switch into the wiring loom that goes down to the ecu. Just a thought.

Cheers

Lee

[EvoOlli]

Hi Craig,

the Heater Control Unit is not a solo ECU, it's the Dash ECU which controls it.
But in the Dash Pod there's a small diagnostic possibility of the signals the A/C uses. When I'm at home I will have a look at mine and then I can tell you which status the signals have to have.


BTW, in the passenger compartment is a small part which looks like it's coming from a refrigerator: It is there to prevent the A/C to ice up and switches off the A/C. This switch could be faulty too.

[craigcaf]

Thanks Lee / Ollie.

I will check the dingly dangly thermostat thing (I know the bit you mean Ollie) and see if that is the issue - I think you can just bypass it if faulty as it's set to max according to GV. I will also check if it is receiving the signal for A/C on or off and also check the POD menu. If it's not receiving the signal think it could be worth checking the POD ribbon cable as suggested.

If it's none of those, will need to think again.

C

[EvoOlli]

Here are the Pictures from the Menu 5, the first without A/C enabled, the second with the Button pressed:

PR is the Trinary Switch
IC is the Iced Control switch
Both have to be ON
CL is A/C Clutch engaged

[craigcaf]

I checked the pod 5 menu. IC was ON but PR was OFF and so CL was OFF. Cycling the capillary thermostat dial from max to min to max cycled IC from ON to OFF to ON, so that looks fine. Taking the trinary switch multiplug off there are 4 pins one which looks like a ground, two at +12V and one which is the output of the switch. Bridging one of the 12V feeds to the switch output turned PR ON as expected, and then the Aircon worked, CL was ON and you could cycle this ON OFF ON by pressing the aircon dial button. Then air was nice and cold.

According to my refrigerant system pressure gauge there is plenty of refrigerant in the system. So I think it is the trinary switch even though its only a year old. The only puzzling thing is that I expected the right hand rad fan to come on with a failed trinary. When I reconnected the trinary switch and put ignition on, the fan did come on, with PR showing OFF, but then on pressing the aircon dial on and off and on again it went off and stayed off which I didn't expect.

I think next step is to degas the system and replace the trinary switch, unless there are any better ideas....

C

[Englishman]

I had trinary switch failures a couple of times in quick succession back in 2005/6. Was simply rain getting into the units. A bit of sealant around the wires into the unit stopped it happening again.

[EvoOlli]

As far as I know:

The trinary switch has two outputs (hence the name ?)
One is for pressure too low and the other is for pressure too high