Aircon power supply

XJ-S
1

My 1982 HE aircon clutch does not engage when the fan automatically switches in. I jumped 12volts to the compressor and the clutch works fine but not otherwise.
Any ideas why the power supply to the clutch is dead?
Trev

2

Hi Trev,
Your car may have a thermal fuse/superheat switch setup which will shut off the power to the clutch if your refrigerant has leaked out. Later cars have a HSLP switch which opens the ground and shuts down the clutch for the same reason.You probably should check the pressure in the system.

3

Thanks Dave. We discovered the problem while testing the system after a replacing some pipes and a regas. Air con works fine with a jumper wire.
Does this switch need a reset?. And where exactly is this it?
Trev

4

Assuming you do have the thermal fuse- once the fuse melts, shutting off the 12v to the clutch, the unit has to be replaced. No re-setting! It should be attached to, or very close to, the compressor. It will likely be a black plastic gizmo, maybe an inch and a half long, with (IIRC) three terminals. GM used these things for years.The GM Part no is 6551258.

5

Thanks Dave. I can’t get to it until early next week but I’ll look for that fuse and get back to you.
Trev

6

The thermal fuse was blown but I also noted that there is no 12 volt supply to the fuse when the air-con dash fan engages. I would expect that setting the climate control to its lowest temp (65deg F) on a warm day the AC would engage with the fan…but no. Is there a relay in the dash for the AC?
Trev
PS I forgot I started this post and started a new one… sorry about that.

7

Your expectations might be incorrect. There’s a temp sensor on the heater pipe that prevents system operation until the engine is warmed up a little. If you’re testing while it’s cold, it won’t engage.

8

Thanks for that Kirbert. We had just regassed the system and had the compressor running (jump wire to the clutch) for about 15 min at 1200 rpm…It was well warmed.
Trev

9

There’s also the sensor on the evaporator that normally cycles the compressor on and off. If your regassing kept it on too long, it may very well have iced over, especially if humidity was high. It’d take a while for the ice to melt and the thing to start working right again.

10

Trev,

There is a long list of items to check that could prevent the AC compressor from powering on.

First, you need to confirm the climate control system has power. There are 5 fuses in the system…4 of them could keep the system from powering.

We can eliminate 2 by simply confirming some basic system operations. If you switch the system on and change the temp selector from 65 to 85…do you hear the whir of the servo from the right side of transmission tunnel. If you confirm the servo operation…you know you have a good amplifier, servo and fuses.

Next you need to check the fuse to the Ranco thermostat. It is secured to the right side of the heater box…high up about level with the glove box. You generally will need to remove the under dash closing panel to access the fuse.

Hope that helps

Cheers

Gary

11

Thanks Gary.
Switching from high to low temp I can hear the servos and vane changing. Unfortunately I have created another problem in the process of trying to find the A/C fuse. The blowers work on the screen but intermittently for the other settings and the air is only to the screen when they do work.
I recalled this morning that there is an earth on a panel next to to accelerator that I removed to access the panel … maybe the problem but it will have to wait until I get home early next week. I’ll get that right before I get back to the A/C.
Trev

12

Trev,

Check that you did not dislodge a vacuum hose…windscreen (defrost) flaps as well as dash and footwell flaps are controlled by vacuum. The defrost flaps are controlled by a vacuum switch on the end of the mode control switch.

Checking your grounds is a good idea. I will also note…that I have seen a high number of intermittent power problems caused by a loose micro switch securing bolt for the mode control assembly. The bolt gets ever so slightly loose and the micro switches can shift enough to cause failure to activate. You can generally test for power on the associated wires without digging into the mode control unit ( which can be difficult to access). Check for power on the amplifier brown feed wire while moving the mode control switch.

While there could be any number of causes if your symptoms…these are the first places I would check for my own project.

Hope this helps

Cheers

Gary

13

Thanks Gary. I’ll get back tp you when I give all this a go.
Trev