Thread: Air Conditioner problems

I had a no go on my A/C in my 1999 Disco 1.

Fault finding proved to be interesting.

After having a the A/;C liquid pressure and volume checked with the correct gadget using the correct refrigerant . ( hi low pressure switch cuts out and will not allow clutch to engage if refrigerant is over or under pressure. )

That eliminated and still no go .

Started out by mud mapping the circuit from the discombobulation of info on the Electrical repair manual.

The manual has different colour wires to that on the vehicle and mis-numbers some pins on the relays. That aside I managed to get a diagram and a slip manual that gave me the Information I needed.

First I checked the two fuses in the system. The one in the engine bay that supplies power to the clutch via clutch relay. Check the clutch in the compressor is engaging by disconnecting the plug and jumpering to the battery. Lett terminal ( black wire) to earth , right terminal ( green black wire) to positive.

If the clutch is ok and compressor compressing the engine will load up a bit and cold air will spew forth from the vents . OK ?? reconnect plug.

Next with an alligator clip and probe wire ( bit of wire that will slip down behind the terminals on the plugs. ) You can earth out the pins on the Hi Low pressure switch. If you can get the plug off , and thats a tricky thing to do the plug has a little tang on the side that has to slide down ...I got mine off and jumpered across both pins. If that doesn't get the compressor going the Hi Lo switch is OK .

Next I pulled of the drivers side (rh) kick panel and found the A/C logic Relay and the A/C Clutch relay.

Both relays can be pulled out for a look while still connected by easing the base upwards and it will come of the metal tang at the rear.

If you have a multimeter and are ok measuring voltages you can check you are getting 12 V at pin 85 of the relay Next check for any voltage on pin 86 ( both tests done with fan on and A/C button selected. If you have 12 V on both pins then the relay is not cutting in .

If pin 86 is 0V then your relay contacts should have 12 V on pin 87 ( the contacts are closed and power is going from engine bay fuse through the relay to the clutch. The clutch should be working) if not the wire between the relay and the clutch is at fault. ( connector pins etc) If no Volts to pin 87 relay is stuffed. ( while 12 V on pin 85 and earth on pin 86 . )

Don't wanna do all that ...replace the relay for 20 bucks or so.

Now for the dirty rotten mongrel low side of the whole shooting match.

The switches in the cabin do not directly operate the clutch relay. The switches send and earth to 29 of the Engine Control Module requesting Air Conditioning power be sent to the Clutch relay.

The Circuit diagram is less than helpful in telling one where all the wires connect too.

First thing to check is that the A/C logic relay is working. The A/C logic relay is next to the Clutch relay ( see photos)
The logic relay gets its power from the Ignition load relay. If that wasn't working a lot more than your A/C would have problems. But check for 12 V on pin 85 anyway. ( ignition on fan on a/c selected).

If thats Ok then see if there is 12 V on pin 86 . If there is then the problem is further down the line. Short pin 86 to earth and the A/.C clutch should kick in. If it doesn't then change the relay . ( if you dont want to do these checks then change it as a short cut. )

If the logic relay is working then shorting 86 to earth should put 12 V on pin 87 of the relay. ( there must be 12V on pin 30 if not a problem with the wire from the ignition load relay to the logic relay. )

Everything check out OK ??

Now to check the last things in the chain .

To check the heater fan switch and the A/C selector switch .

with ignition off key out connect a beeper, multimeter ohm meter to pin 1 on the duel pressure switch plug turning the a/c switch on with the blower switch to any speed selection ( not off) should give you a circuit from pin 1 of the duel pressure switch to earth.

No circuit means removing the centre panels to get to the A/C selector switch and the Blower switch and remove them to clean/replace. Repair to the club for a few snifters.


if it does there is only one thing left in the chain and its a beut.

The last switch is the front A/C evaporator temperature switch. The little bugger is supposed to turn the A/C off in the event of Ice forming on the core in the cabin .
Mine is a Siginmilya 443550-0030 heat activated switch. It is located on the left hand side of the heater box. The glove box and lower trim panel have to come of to get to it.

It has a long "Burdon" tube that disappears into the heater box. The Bourdon tube is filled with a fluid that expands and contracts a bellows inside the switch operating against a spring that opens and closes contacts.

This was the fault in my system . The contacts were dirty. I dismantled the switch cleaned the contracts and reassembled it being careful to count the number of turns I had to loosen of the adjusting screw to get it together and assuming I had got that right when I managed to get it back together.

I would not recommend dismantling the switch to anyone but an expert. It is a very difficult job.

The alternative is to replace the switch. I cant see how the Bourdon tube can be extracted from the heater box without dismantling half the interior of the car and pulling the heater box to pieces. It appears the end of the tube is attached to the cooling radiator inside the heater box.

It might be possible just to short the switch out and not have an ice detector ...I dont know what the consequences of that might be. There might be a simple replacement switch on the market ..I didn't find one.

Thats my adventure with the crummy electrical fault finding manual and the very very difficult wiring problems .

Pictures are Clutch relay with logic relay removed just to the right of it. showing mounting tang . Mounting bracket for Temp control switch then temp control switch with bourdon tube disappearing into heater box.

Originally Posted by northiam

Yes but the one in the pic is just a electro-mech one, isnt it?

It can be changed to a thermistor type very easily.