Finally finding the time to diagnose my AC system failure. So far it looks like a low refrigerant charge tripped the pressure switch on the back of the AC compressor. The action of closing the contacts on the switch which completes the thermal fuse heater circuit burning out the fuse. Does anyone know where this fuse is located?
Thank you.
Roger

 

Good morning from the northwest!This one is tough but i will try.On our cars[both 91my}the power comes from fuse A2,20 amp,to power up both the ac cooling fans,and the compressor clutch.Jumping the brn/blu to the grn/brn wires with the compressor relay removed should engage the compressor.Doing this should bypass the lo press cutout switch,as it is wired on the "small" side of the compressor relay,and provides ground.I tested this by doing a resistance test from the blu/yel wire with the relay removed ,to a known good ground.With my car running during the clutch test, the juices seemed to flow enough to get the sticky switch to work again.

Now on the fan relay side,{same fuse}pull relay,jump the brn/blu to ppl fan wire,and the fan should run.

There is a diode in the circuit that can be checked at the same time as we are doing our fan test by pulling out the 3 amp # 5 fuse in the center fuse block.Voltage at this fuse point indicates a bad diode.

This circuit feeds the breather heater relay on the small side through the ambiant temp. sensor.

The power side of the breather heater relay is fed by fuse B6,7.5 amp,on the left fuse box,thru the relay,and to the little heater in the breather hose.


theoretically,a bad diode would close the fan relay all the time and cause the fan to run,if it wasn,t for the thermostat in the way.

I hope this helps out,coffee and schematics go well together in the morning

 

Thank you manwich66.
I was able to do quite a bit of research on my problem over the weekend and did make progress.
My A2 20A and B6 7.5 A fuses are OK. When I jump the contacts on the fan relay the fan turns on. When I jump the contacts of the compressor clutch relay...nothing. Battery voltage is present.
Service documentation for my '90 Majestic and '95 XJS convertible both describe the action of the low pressure switch and the thermal fuse - contacts on the switch close completing the circuit to the fuse thus burning out the fuse. Based on my observations and review of wiring diagrams I will assume that the thermal fuse is located between the compressor clutch relay and the compressor and mounted very close to the compressor. I plan on crawling under my car today to look for the fuse - I do know what I'm looking for now. The service documentation for my cars doesn't provide a location for the fuse, however...? I'm going to look first in my XJS, may be easier to find in my XJS, if so it should point the way in my XJ40.


I was able to cross reference the NLA Jaguar part number to a 4 Seasons thermal use part number 35759. I bought three at Advance Auto Parts for $2.99 USD each. The service documentation provides a test procedure for the pressure switch which I will perform today as well. If the pressure switch is stuck closed it will continue to blow thermal fuses. I have been able to cross reference the Jaguar P/N to a Santech W0133-1614301.

 

Nice bit of work!Sounds like you have got a decent handle on electrical theory.Standing over my open hood,looking at the single wire leading current to the clutch on the compressor,also makes me wonder where there is a fusible link to protect the winding.On the control side of the relay on our cars, the pressure switch is a tad sticky,even with so-so pressure in the system.
I gave our clutch a quick bonk last season to get her going,and whump-there it is!
I will take a closer look see this weekend,and see if I may be able to help some,In the meantime keep on,and good luck!
keep us posted,
Cheers,Jim

 

Like this?

Four Seasons 35759 Thermal Limiter Fuse Switch | Cheap Motor Parts

I'm a million miles away from being an XJ40 authority but I'd be surprised if your car has one. Jaguar went to a different circuit/system (from Superheat switch to HSLP switch) on the XJS and Ser III XJ6/12 in late '86/early '87....and I couldn't find hide nor hair of it in the XJ40 parts catalog.

I struggle a bit to think they'd go back to the old limiter fuse after having abandoned it earlier...but I guess strange things can happen.

On the Jags that I know DID use the old limiter fuse it was clipped right onto the a/c compressor bracket

Cheers
DD

 

Thanks Doug, Jim; the most commonly used spot for these fuses is/was the compressor bracket. I'm following the service DVD for my '90 and service manual for a 94 XJS. My' '95 XJS has a much simpler wiring scheme and doesn't have the fuse. I spent Monday afternoon tracing wires and found nothing... At this point in time the fuse is either located in a difficult to access spot somewhere in the front body work (contrary to standard practice) or it doesn't exist.


I do have battery voltage at the compressor clutch relay, but the compressor clutch itself doesn't energize when I jump the relay's contacts. I want to probe the compressor clutch connector for voltage when the relay is jumped. If I have voltage the compressor is bad (at 25 years old OK); if I don't have voltage I have a burned out fuse.


My car was also converted from R12 to R134A some years ago.

 

The latest and greatest on the AC in my '90 Majestic:


There is no thermal fuse, the service DVDs for both the XJ40 and XJS are wrong. I had to rebuild my AC complete from the evaporator to the condenser - everything is new and I now have AC!