Freelander 1 1.8 Air Con Pressure Switch Test - solved

Air con has packed up on the G4. 1.8k.

Have switched the air con compressor clutch relay on and off with diagnostics but clutch is not engaging.

Centre part of compressor pulley is free to turn but guess clutch coil is broken.
I don't really want to remove the alternator and stuff, but can't find the control wire to the compressor to test for supply and the continuity of the coil.

Can anyone show me a picture with the connector and wire on the compressor please?
Rave says remove half the engine before starting the job !!!

The control wire is visible from the bottom, once the sup guard is removed. The actual connection is on the top of the compressor, between the high pressure outlet and the drive pulley. The clutch assembly can theoretically be removed and replaced, with the unit still fitted to the car, but space it tight.

You can see the compressor best from below, as the alternator obscures it from above.

I don't have a picture if the 1.8 I don't think, I do have a picture of the V6 one though.

Thanks. Hope I can unplug from below!!

Further non invasive testing reveals:
Diagnostics (Delphi) operates the air con relay on a working vehicle, but whilst it says it operates the air con compressor relay on the vehicle with the fault, it actually doesn't. I guess it just instructs the CANBUS as the switch would from the ipack.
I have tested the relay and the compressor clutch (from the relay socket) and they both work fine so no need to pull the compressor off - thankfully!

So the question is, what does the ECU have as inputs to prevent air con compressor relay operation?
Looking at circuits I think it is either low pressure in the system and the pressure valve preventing operation, or the evaporator temp sensor.

Before getting the system recharged, does anyone know of any other interfaces relevant?

Can I bypass the evap temp sensor to test it then and if so, where is the plug coupler? It seems to be to prevent icing - not a problem with a short test.

Thanks

If the tri-mode switch is reporting low or high pressure in the AC, then I suspect that the system won't activate with the diagnostic, as running the compressor without AC fluid, would cause damage to the compressor.

The engine ECU is what actually controls the AC, based on information from the tri-mode switch, and the centre console buttons.

Learning about air con as I go!

The evaporator stat goes to a 2 pin plug behind the centre console by the drivers left foot. Easy to unplug and test, measures around 5k ohms. Compared with another car and seems OK.

The Delphi can engage the air con clutch on a working system with the engine off so no risk of damage, but think the low pressure switch is the only thing inhibiting mine from working now.

Would like to unplug pressure switch and insert resistors to simulate normal pressure and see if clutch works then, with engine off.

Any idea what resistance to provide between the pins?? Assume high and low as 3 pins.

Thanks.

andyfreelandy said:

Any idea what resistance to provide between the pins?? Assume high and low as 3 pins.

Click to expand...

The tri-mode switch is just a switch, so can normally be linked with a wire. However I'm sure it's a bit like a changeover relay, so when one switch opens, the other closes. I tend not mess with AC myself, as it's generally pretty reliable, requiring an occasionally re-gas.

Made myself an air con tri-mode switch tester out of an old TD4!!

Used the plug coupler and found the following on working and ungassed systems.
Pin 1 purple / black to pin 2 green / blue gives 46k ohm.
Pin 2 green / blue to pin 3 blue / orange gives 15k ohm.
Pin 1 purple / black to pin 3 blue / orange gives 60k ohm.

So looks like it could provide a signal rather than being a switch.
All vehicles, ungassed, working air con and suspect air con gave same readings.

Going to get the suspect regased now as looks like the pressure sensor is same as the others, whatever it does, measures and sends as a signal to the ecu!!!

Bit of time this afternoon confirms, air con pressure switch is actually an active pressure sensitive transducer. Pin 1 and 3 have a reference 5v from the ecu and pin 2 wrt to pin 1( - ve) gives a voltage proportional to the air con refrigerant pressure.
My working car gave 1.1v back to the ecu on the signal line, pin 2.
My G4 (duff air con) have 0.1v back to ecu which disables the compressor clutch relay.

By disconnecting pin2 from the pressure sensor and providing a 1v feed back to the ecu, via a 470k resistor from the 5v reference, I was then able to operate the air con compressor clutch correctly from my Delphi test program.
So, about 1 v represents good pressure and 0.1v is naff. Worth checking before you pay for a re gas as it may be another part of the system that has failed. This was my objective, to prove all will work well when recharged. There may be a leak, but will get it pressure tested first and always ask for a leak dye to be put in with the regas as this will find the leak if it starts at a later point.
I didn't run the compressor with the low pressure, just operated the air con clutch with the engine off and ignition on.

Interesting!!!

Very interesting Andy, and good to know my AC information is about 20 years out of date.

This could help a bit: