@NTL1991 if you have the 91 diagram I'm interested to look at it please

I haven't been able to find it in my electronic files, but I do have an Electrical Guide binder somewhere, I'll give it a look when I dig it up.

It works

So, thanks to @NTL1991 I checked the diagram.
When the engine coolant is above 40°C, 5V is applied to AC4-13. I never had this 5V ! So I joined AC4-13 with AC3-6 to force 5V.
And I have the fans spinning, whatever temperature I ask

Thanks a lot @NTL1991 if found the solution because of you
(but don't misundestand me, I'm still interested with your Electrical binder )

The shunt to test :




Now, I have to make the shunt nicer, and put all the stuff back on.

That’s great news!

I’m curious if you happen to see a part number on the Climate ECU.

Do you know if it’s ever been replaced?



The 1990MY and 1991MY Diagrams show the 5V signal through the inhibit switch. The 1991/1992MY diagram shows the ground signal through the inhibit switch.

No, I can't see part number on Climate ECU.
I don't think it has been replaced.
I "found" All Data, and the diagram you gave me is the same in 1990, 1991, 1992, so it is the good one : when the coolant is above 40°C, the ECU wants AC4-13.

Once again, thanks @NTL1991

So you have the “older” ECU which looks for 5V when the heater core is hot to enable the fans. But, you have the “closed when cold” inhibit switch and grounded wire at AC15 installed in the car. There seems to be a mismatch here. If you replace the inhibit switch with the “older” style, that closes when hot, and tie AC15 to the correct spot (5V source) your system should work as designed, inhibiting the fans when cold, and enabling them when warm.

I’m just not sure why you have a mismatch

Yes indeed, it seems to have a mismatch.
Is it possible to change the inhibit switch without removing the coolant ?

Just wondering if all these tests are performed on a hot engine?
On my car, the blowers will not start on warm/hot setting either if the engine hasn't reached operating temperature. The delay varies of course, but usually it takes about 2km of driving before they start. The blowers do work on cold or a/c setting immediately with no delay. This is how the system is designed, correct?

Larry

@NTL1991 can you take a picture of the 1991/92 diagram please ?

@Lawrence Yes exactly, the system is designed to not send cold air when the engine is cold and the driver asked for hot air. But in my car this does not work.
By the way, as @NTL1991 stated, it seems I have a mismatch : I don't have either the good A/C ECU either the inhibit switch.

That is correct Larry, the inhibit switch is looking for a sufficient temp at the heater core pipe. I don’t have the manual in front of me but I believe it’s 40*C before the switch closes (or opens, depending on your model hear) and provides the signal to the Climate ECU to allow fan operation.

The ECU calculates the heat/cooling load based on the sensors and will look for the inhibit switch signal before allowing fan operation if there is a heating load. That signal is ignored by software when there is a cooling load.

Okay, reviewing your pictures I can say with some confidence that your Climate ECU was replaced with an older (likely used) unit.

I can tell this from the fact that your inhibit switch is normally open, that your ECU is commanding fans on with 5V at AC3-6 instead of a ground signal doing it. And the fact that your inhibit switch wires are green and black, the green wire being grounded as it should according to the later diagram.

How long have you had the car? How long has this been an issue?

You could swap in a correct late 1990-1991/1992 ECU as per the TSB I posted early on, which shows the correct matching ECU and inhibit switch part numbers. Jumping out will work fine (you will lose the inhibit feature) but to be perfectly correct you can swap the newer ECU in.

Thanks Nick for the Diagram
( AllData is not good for this diagram !)

Hello again

I just ordered a DBC 5722 calculator to test if it's work as it should do
Just because I like my car to be perfect

Good evening everyone. have a recently purchased 1989 Jaguar xj40. When I turn on the air conditioner, the fans start at max but then stop. They only operate slowly in defrost mode. This happens at any temperature. What can it be?

Hi Giuseppe,

Welcome to the Jaguar Forums! It's great to have you with us, and congratulations on acquiring a Jaguar XJ40.

There is a lot of good information about the blower motors at this link:

Jaguar XJ40 Air Con Blowers

Please visit the New Member Area - Intro a MUST and post a required introduction so we can learn something about you and your Jaguar and give you a proper welcome.

Also, please read the content at this link:

New Members - a MUST Read if You Have Just Signed UP

Cheers,

Don

I disassembled, cleaned and checked the fans and they are ok. Checking the voltages I realized that the control unit sends a voltage of about 1.25v to the fans but then it goes back to 0v and they stop. I can't find any information about it. Reading a guide on the Mark 3 system suggests a problem with the control unit.
Can anyone help me?