A/C system, the latest challenge

Now that the engine is running fine, I decided to charge the A/C. For the 1st time ever, the clutch is not engaging. I do have an impedance at the clutch and one terminal is grounded.

I believe the in-line, 10A fuse is just under the starter relay. I found this fuse to be blown and replaced it with no luck having the clutch now working. My 1st question is, what are the two relays next to the starter relay? The electrical guide shows only one relay. One is standardly situated vertically (the middle one) and the other is situated horizontally and nearest the front of the engine bay.

If I have some time this weekend, I will also give a run-down on the changes I’m experiencing with the Norman’s water rail restrictions that are only installed on the A Bank water rail. The change is very noticeable.

The relays may be compressor clutch relay and I have a listing for an “inhibit relay” but not sure what it’s function is. I think maybe EFI / emissions system.

One of those relays acts with the inertia switch to shut down the EFI system, I think.

The A/C compressor clutch is really turning out to be a puzzle. Anybody know how to check the “high side low pressure switch”? Sounds kinda funny but that’s what it’s called on the electrical guide.

Also, anyone know if it’s normally open or closed and should it be closed with the AC on?

Several things on the A/C. The clutch relay is the vertical relay and it has a yellow plug base and is the only relay out of the two with a G wire on pin 30. There is no stripping just solid green (G) Not GW (green with a white stripe).

I also ohmed all common points out from pin 30 on the clutch relay. Everything is connected properly. Then I put +12V at the top terminal of the compressor and there was no engagement of the clutch. To make sure, I took the connector off of the compressor grounded the bottom terminal of the compressor and then applied +12V to the top other terminal. The clutch clutched.

So this tells me that the bottom connector does not see gnd. And sure enough there is 40ohms at the bottom terminal to gnd. Apparently this is at all times. Looks like the high side low pressure switch is open at all times.

Can anyone explain what the high side low pressure switch is and does? According to the diagram below that switch should ground the bottom terminal (closed switch) and the clutch engages or float the magnetic clutch coil in which case there is no magnetic field and no engagement and the compressor does not spin. It looks like that low pressure switch is not switching at all. I would presume that it is normally closed so that when the clutch relay provides +12V, the clutch will engage and the compressor will spin. That is not happening.

Here is the attached picture that does not have to be downloaded


You should be able to get the clutch to engage if you short out the low pressure switch.

It is there to protect the system from low pressure … so either there is low pressure, or the switch is stuck/stuffed. I am no expert on Air Cond systems, but I have a vehicle with the same symptoms.

Only other thought is I am concerned about the blown fuse. Fuses don’t just blow for fun. Something must have drawn too much current.

Putting it all another way, Is there any current flowing through the circuit? If not, then find where the disconnection is. You have a resistance on your clutch coil. You should be able to jumper 12V to that, and it should pull in.

Good luck.

Hi Mark,

I have arrived at the point where everything is operating properly. I believe I now have to go to an A/C servicer who can take the steps necessary to charge the system and in so doing will be able to get the High side Low Pressure Switch to operate properly. Right now the switch being open is the correct state for ‘low’ pressure in the high side of the system and is in this state to protect the system. So nothing is broken and in need of repair or replacement.

Thanks for the help. I will let you know what the servicer recommends.


Shuts off the compressor in the event of loss of refrigerant.

I went to an A/C servicer today and he said the clutch is disconnect thru a low pressure switch on the high side. He also said he would get it fully charged and operational for $160 max. What a deal !