Electrical 89 fox AC help please

[Fast89GWILL]

Hi everyone, please help if you can as I'm STUMPED!

89 fox 5.0 and no AC

Brought in to test charge and it was full of 134a and no leaks confirmed.

They stated no power getting to the compressor but they claim to have jumped it and it worked.

I replaced the low pressure switch and WOT relay and still no power. Fuse is good and all other controls work or so it appears. Ground behind engine is tight and no other issues with car.

Any thoughts? Help is greatly appreciated as I've been chasing for almost a year.

Thanks!

 

[Mustang5L5]

The AC compressor circuit on these cars is relatively simple.

You didn't say what they jumped out to get it to work so we can't assume anything. What i'd do is turn the AC on and test for power at wire 348 at the low pressure cutoff switch. You should have 12V on the light green wire. If you do, try installing a jumper wire here and see if the AC turns on.

The WOT relay is a common problem area. The wire insulation cracks and falls apart and the plug needs to be replaced. The relay should be normally closed and only opens at WOT, so you should have continuity across the terminals for wire 883 and 347

This wiring diagram is for 1993 but should be the same wire colors and numbers for 1989. This one is just a lot easier to look at vs the '89 diagram.

 

[Fast89GWILL]

Thanks much!

When I jump it, it comes on and now when I connect the LPS back, the compressor kicks on and off and goes from 45psi when not engaged and drops to 20psi when engaged. Doesn't seem to want any more freon if we it drops and I try to out more in.

Thoughts in intermittently kicking in and off?

 

[Mustang5L5]

What’s the ambient temp and what’s the high side pressure?

If it’s cooler out, the system will cycle. The ambient temps keep the system pressures down so you will hit the 20 psi limit a lot sooner.

Generally the high side pressure should be 2.25-2.5x the ambient temp.

hold 1500rpm, turn ac to max with doors open, let stabilize for 30 seconds and then record high and low side pressures and make not of outside temp. That will allow us to make a better assessment of system performance.

 

[Fast89GWILL]

Thanks much. Am traveling this week but will take a shot on Saturday and let you know.

Appreciate you taking a minute to assist. Thanks!

 

[Mindseye007]

Hi everyone, please help if you can as I'm STUMPED!

89 fox 5.0 and no AC

Brought in to test charge and it was full of 134a and no leaks confirmed.

They stated no power getting to the compressor but they claim to have jumped it and it worked.

I replaced the low pressure switch and WOT relay and still no power. Fuse is good and all other controls work or so it appears. Ground behind engine is tight and no other issues with car.

Any thoughts? Help is greatly appreciated as I've been chasing for almost a year.

Thanks!

You might have a faulty wiring harness that connects between your main harness and A/C compressor and need to buy one of these to restore power to the compressor

https://lmr.com/item/LRS-8793SCH/Mustang-A-C-Compressor-Harness-87-93