1991 XJS Convertible


My heater / AC controls are not working. Pulled fuse #11 and it looked good.

I figured out that if the ignition switch is on and AC fan is on, when you re-install the fuse the fans start to blow for just a second. Sounds like a couple of clicks either under the center of dash or in engine compartment and the controls are dead again, no fans, etc. No fuses are blowing…

All of that may be happening every time I turn the key on, but too many other noises to tell…

Is there a relay somewhere that I should be looking at?

It was all working when parked a couple years ago…

 

Have you started the engine and turned the a/c on to see if the clutch on the a/c compressor is engaging although the fans aren't blowing? It sounds like you may be having the same issue I have. I posted about it here, but unfortunately I haven't diagnosed the problem yet: Delanair MkIII issue, initial pulse of air then nothing - Jaguar Forums - Jaguar Enthusiasts Forum

There are relays inside each of the fan/blower housings, and each fan/blower has its own fuse in the left and right footwell fuse panels, separate from fuse #11. But with both fans turning on briefly and then not working after that I don't think it's a fuse or relay issue on mine. I've been working through the MkIII tests of the ECM pins. On a couple of the tests the fans come on and stay on when I connect the voltmeter pos to an ecm pin and the negative to a ground point. I plan to clean all ground and other connections I can find to see if that helps. I'll add to my post above with results.

 

Thanks for the reply Noah. I read your post and saw you haven’t got any help yet.

it does sound like a similar situation.

I’ve since figured out it gives me the brief surge of air (I THINK from both fans) whenever I turn the ignition on. I’ve checked both blower fuses and they looked good.

NOW, all of a sudden, with the AC controls set in the OFF position, when I turn on the ignition I get a brief burst of air, then it stops for a second and then a fan kicks on and continuously blows…. Not on a really high speed, and I think it’s coming from the defroster ducts…. The fan noise is coming a little left of center, is that just one of the fans???

This is a new problem…. It was working fine when last drove a couple years ago…

 

I’m looking at the Repair and Operation Manual


it says to check the Single relay NY terminals for voltage. Do you know where that is?

if 0 volts, Change switch ‘C’. Where is it?
if 12 volts, Change relay. Which relay and where is it??

thanks for any replies!!

 

The electrical guide has all the info on locations and pins related to relays.

S57_90 is the Publication number for 1990/1991 XJ-S.
The .pdf is almost 14mb file size.

 

Where can I gain access to the guide?

 

I found a hard copy online at Faxon for $119 plus shipping. Is the pdf available online?

They don’t have the supplement for 1990-91 MY

Will the 90 MY manual cover my 1991 (US Registered) XJS?

my vin is 176851 if that helps

thanks for the info!!

 

You can download S57/90 here. It says 1991 MY so I would think it would cover your car.

http://jagrepair.com/images/Electric...Electrical.pdf

 

Thanks!! That link will be very helpful!!!

now, if I just understood this chart.

is it saying to check voltage between the wires N and Y if I can find them on the ECU?

and if you find voltage, change Switch C??? I don’t see that on the wiring diagram

if no voltage change relay? Which one???

I’ve got a multi-meter, but don’t know where to go with it!!

 

The 1990 and 1991 XJ-S are the same car, there were no MY changes in the electrical system.

Every year we would get new electrical guides and if there were no changes, we simply got a new COVER PAGE instead of a reprint of the entire guide. (no changes)
The 1991 XJ-S guide was the same as the 1990MY.

 

thanks!!

I’m guessing I need to gain access to the ECU / ECM (is it the same thing?) and look at voltage between pins N and Y? is that on the main air conditioner unit up under the dash??

Then test for voltage with ignition on?
And knobs selected to 75 and OFF

 

I pretty sure the diagnostic fault chart is referring the the wire color NY. (Brown/Yellow)

 

I've looked all over the wiring diagram Fig 27.1 and can not find a wire labeled NY. I’ve now just looked over it again…

Either way, it seems like I need to change either a switch or a relay…. I’m willing to do both just to see if it works.

Any idea what or where switch ‘C’ is?

The only relay I see in Fig 27.1 is the compressor clutch relay? Easy enough to change if that’s what they’re referring to…

Thanks again for any assistance!!!

 

I might be wrong, but is it possible the Testing/fault finding chart that you are following is for the MkII HVAC system on earlier XJS models rather than the MkIII? My car is an SIII sedan rather than an XJS, but the service manual has a test section for the MkII then a separate section at the end for the MkIII system. The testing for the latter primarily involves using a voltmeter on the set of test pins on the ECM. I'll try to add links to two other publications specific to the MkIII I've been relying on, in addition to the service manual.

See Post #4 in this thread for a couple of links:

https://www.jaguarforums.com/forum/x...em-mk3-217644/

 

I bet you are right! I only have a copy of the Repair and Operation Manual for XJS I don't see where it designates the type of the HVAC system.

The Mk II system was used until 1987, MkIII systems were used after that, so i definitely have the later system.

I'd appreciate any links to the correct information!!

thanks for the help!

 

Take a look at this thread,including post #37 with photos of the MkIII ECM in place. https://www.jaguarforums.com/forum/x...-manual-83961/

It is helpful to have some of these plastic guides to slide over the ECM test pins to conduct your tests with your multimeter without shorting anything out:

https://www.onlinecomponents.com/en/...E&gclsrc=aw.ds

I may also have an excerpt from the XJS service manual that outlines testing the ecm.

 

I’m at a loss regarding the plastic sleeves needed for testing. The link is for a minimum quantity of 10 of them? Can’t you buy just one?

Do the sleeves go over the probes on the multi-meter??

 

You need insulated probes to keep from shorting pins and blowing up the control unit. I used completely insulated terminals that would just fit the pins. That's what is really required to prevent any possibility of shorts. Most of the testing is done with one lead on the ground pin, and the other on the particular pin under test, so most of the time you are only moving one lead.

 

I’ve started the 82 diagnostic tests found in the 32 page manual titled Delanair MK III Air Conditioning Service Introduction 1987 Model Year


On test #1, when checking for continuity to ground on pin 45, I got a 0.039 reading on a 2M scale. Think I’m good there?

Tests 2-7 were good

Test 8 said 3.9 max, I was at 3.98

Test 9 says 0-50 mv. I’m at .105 volts on 2 volts max scale.

Test 10 should be 0-1 volt. I’m at 10.27. I’m guessing the problem is there. Especially with it wanting to run the fans at low speed even when it’s off.

I’ve got the xjs ROM manual. The whole console has to come out to get to the mode control switch???

Does anyone see anything different??? Any point in doing any other tests since that switch has to be good for everything else to work???


Not the news I was hoping for

Look forward to hearing from anyone that has ran the diagnostic tests.

 

I also started the testing in the XJS Service Manual, Section 82

Bottom of page says 82-14

On the first set of tests with mode switch OFF,
pin 9 only has 0.21 Volts
Rest looks ok

On second set of tests with mode switch LOW
pin 13 has 4.07 volts and should be 150-350mV

I’ve looked online and there are a couple of used mode control switches on eBay with 30 day exchange warranty…

Not sure if I should buy one and start taking apart the console??

I appreciate any input and/or feedback

Thanks,
Rich