My car is a 1990 XJ40 4.0 Vanden Plas. When I turn the dash switch to turn on the A/C the engine fan and the A/C compressor aren't turning on. The in-dash fans work but the A/C compressor and fans aren't. The fuse is good and both relays also appear to be good (unlikely they're both crapped out?). Where should I begin looking to troubleshoot?


Thanks,
Roger

 

Hi - There's a low pressure switch that prevents the system from coming on if you are low on refrigerant. This low pressure switch prevents the a/c relay from energizing and starting the compressor.

Check the a/c pressure first. Low refrigerant is the primary cause of no a/c operation.

If want to check the compressor operation you can by-pass the low pressure circuit by jumpering the a/c relay socket, but you should only do this momentarily and use a fused jumper. If the compressor starts, at least you know the spendiest component is ok and the problem lies elsewhere.

There are lots of posts in the archive regarding the a/c system, check the posts by Nick (NTL1991) as it was his advice that got my system up and running when I faced a similar situation last summer.

good luck

 

Thank you Lawrence! The low pressure switch is the switch on the back of the compressor - two wires coming off of it?

 

Not on my '94 Roger, it's on the a/c line near the bulkhead ..mine is a late model factory R134A system so quite different to yours -

You should check if you are low on gas before you do anything else!

 

Lawrence,you have much experience on the ac stuff,on my 91,I jumped the relay near the hood hinge on the right,by pulling the relay.

Just wanted to see if the fan clutch would spin.It did,but not cooling..
Roger95,take a look the relay might be close by...may not be a fix
But you can try...hope this helps...

ps..good to see ya Larry,thx for that cool ac tool.....

Jim..

 

Hey Jim, great day up island, learned a lot! You're one talented guy! I'll be on here asking you about bushings pretty soon for sure - not just yet though, they're calling for -18c (0f) this week w/windchill ...Not carport weather

As far as experience with the a/c - I just learned a/c in order to fix my 40, brought on by taking the car to a local garage to get it recharged when I was baking in traffic last summer!

My mate who knew a bit about it showed me there was SOME gas in the system when he poked his car key into the low side valve- there was quite a bit of pressure in there, but that didn't mean much to me at the time - total a/c greenhorn.
I did figure I needed to know how much was in there though - I'd read on the forums that low gas could stop the system from working.

I picked up a pressure gauge at walmart, told me I had 20 psi in the system. btw, I took the reading in the parking lot then returned the gauge!

As the a/c hadn't worked when I got the car and it was a year later, figured it could be a slow leak if it still had 20psi - I think it's around 26 that the pressure switch engages. Hoping a top-up was all that I needed, I took it in to get charged and the guy asked me to pop the hood and fire it up with the a/c on high - put his hand on the hoses, told me this one should be cold etc, scared my wallet a bit with the "your compressor isn't even running" and the "if we've got to replace .." comments..

The ballpark estimate was the clincher though

Anyway, eventually I did the job myself. My first real a/c diagnostic tool was a ten-dollar 21 LED UV flashlight, ebay bucks well-spent. You have to have one of these - it REALLY lights up the engine compartment and leaks just pop right out with the dye trails.

The rest of the basic kit, a set of manifold gauges and a vacuum pump set me back about another $150 or so, which isn't too bad. Best part of the story is the a/c works now!! I leave it on all the time with the auto setting. It's great going from no heater and no a/c to having a working climate control, sure makes a difference tooling around, the car didn't even mist up at all this winter - when it's working, it sure works great!

all the best

Larry

 

Thanks a lot fellas! I'm going to try to get into it this weekend.

 

I don't throw out a persiflage air but I ain't surprised at all to hear that . I had 2 XJ40s and none of them worked as supposed to do

Our forum members seem to hit the nail or spot-on on their diagnosis .I had to replace a coupe of items from a feckin' a few bucks valve to evaporator , condenser , countless times recharging A/C blah blah blah
I trust you can get this sorted out

 

I got into this job a bit this afternoon and so far I have jumped the fan relay and the fan works fine. The AC compressor relay has battery voltage (12.43 volts) between pins 30 and 87. When jumped the compressor clutch doesn't engage. I really doubt the compressor has crapped out; the car has 79,000 miles only and the AC wasn't used much by the PO. Next step; unplug the pressure switch on the back of the compressor and hook up my AC gauges.
Any other possibilities?


Thanks,
Roger

 

Hey Roger, not sure what's up with your system - like I said, when I jumped the pins on the relay socket on mine, the clutch engaged and the compressor energized ..AFAIK that's the test for the compressor - maybe somebody with experience with your 1990MY can chime in?

p.s. I know it might be obvious but make sure the A/C is set to "on" when testing!

 

Hey Roger,chase down the clutch wire from the front of the compressor,and energize the wire with a fused jumper.

With the car idling,briefly give er a zap,and you should hear the click of the clutch,as well as see the front of the compressor engage.

No squeezy-no cooly.I think there is a diode in the circuit as well,only to protect the upstream circuitry from the voltage spike caused by the breaking of the circuit when shutting the system off.

Hope this helps
Jim.

 

Thanks fellas, that's the next part of my "journey"; energizing the compressor clutch
directly from the battery using a fused jumper. Don't think I'll be able to get to it until early next week.


Roger

 

Speaking of electrical fan.. Is the one in the front for the AC only? Or does it help in cooling of the car or transmission.