Hello guys !

I have an issue with my heater blower in my XJ40 1990 4.0.

- when I ask cold air, in speed 1, 2 3 or defrost : it is ok, cold air, and fan speeds are good
- when I turn the temperature button to ask warmer air : the fans slow down until they stop in about 3 seconds.

I have tested the thermal switch on the heat pipe : even when the pipe is very hot the switch is opened. I shunted it : same problem, no fan when I ask warm air.

I have found the thermal sensor, but I don't know how to test it :

 

I made a video showing the problem :

 

If the fans are stopping completely, then that certainly looks like an issue with the blower motor inhibit switch circuit. The in-car temperature sensor (with aspirator motor) does not have enough authority to stop the fans dead like the inhibit switch does.

VIN 594576 and newer 1990MY cars have an inhibit switch that is closed when the heater core is cold, providing a ground signal to the Climate Control ECU. If the switch is operating properly, you could have a short to ground on that signal wire, making the ECU think the switch is closed, and thereby inhibiting the blower motors even though the heater core is hot.

Pre-VIN 594576 cars have a different inhibit switch (open when cold) and a different Climate Control ECU, so be aware of this.

Nick

 

Hello Nick,

thanks for your answer.
My car VIN is 605xxx so the inhibit switch has to be opened to allow fans.

I thought it was the opposite : when the coolant is hot enough the switch is closing to allow fans… so to test it I shunted it thinking allowing the fans.

I will remove the shunt and leave its wires not connected : the ÉCU will think the tubes are hot and allow fan, ram I right ?

 

I've just tested it : in the garage the temperature is about 16°C, and the inhibit switch is closed. I have disconnected its 2 wires, but I can only have fans when I ask cold air. When I adjust the temperature button, I hear the flap changing their positions, but the fans slowly shut off (like in the video)

 

@NTL1991 another question please
From where do you have the information that the inhibit has changed for VIN 594576 and newers XJ40 please ?

 

This is per Jaguar TSB 86-71, I have attached it for reference. There is also another TSB showing the part numbers of the matching ECU and Inhibit Switch.

So the switch is closed when cold, and open when hot, which means it's functioning correctly. There could still be a short to ground on the signal wire back to the ECU.

The new wiring is designed to allow the fans if an open circuit occurs. I'm guessing they wanted it to be a bit more fail-safe.

 

Thanks @NTL1991 I really appreciate

I'll search for a short to ground on signal wire. is the wiring of the 3 plugs on the ECU known ? So I can check the gound wire ...

 

Attached is the circuit diagram, along with the Inputs and Outputs in the system. Hope this helps.

 

Thanks a lot @NTL1991 ! I will check my wires.

 

Today I received the thermal sensor ordered before @NTL1991 told me "The in-car temperature sensor (with aspirator motor) does not have enough authority to stop the fans dead like the inhibit switch does" .

And, as expected, no changes.
Tomorrow I will carefully disconnect AC3-6 and AC4-13 (as seen on the diagram provided by Nick) to eliminate short to ground.

Stay tuned

 

I could not wait until tomorrow

I have disconnected from the ECU connectors AC3-6 and AC4-13 : now the fans are blowing, hot or cold selected !
But, the flaps are no longer moving when this 2 wires are disconnected ...

 

I have disconnected the temperatures and solar sensors :

- solar sensor
- in-car temp sensor
- ambient temp sensor
- a/c aircon temp sensor

Still no warm air

 

So it appears the diagram I posted is for the earlier cars! These had the normally open inhibit switch, one side of which is connected to the 5V common sensor feed (the AC4-13 you disconnected). When the switch closes, 5V appears on AC3-6 and the blowers start.

The newer cars use a normally closed switch which disables the fan when a ground appears at the ECU which is why a different ECU part number is used.

I’m going to look up a 91MY diagram and see what I find for you.

You will definitely need AC4-13 connected as all the sensors get their power from this circuit. Without it, I’m not surprised the flaps aren’t moving.

 

Okay AC3-6 receives a simple ground through the inhibit switch on later cars.

Have you checked AC3-6 and the inhibit switch itself for a ground condition?

You will need to reconnect the temperature sensors and 5V power feed AC4-13, it is not tied into the inhibit switch on later cars, and will be required for the flaps to position correctly.

See if you have fans with AC3-6 disconnected and AC4-13 and the temperature sensors reconnected.

Sorry for the confusion with the diagram! I will post the 91MY as soon as I can.

 

Helo Nick,

Following your last messages, I did some tests :

- AC3-6 disconnected and AC4-13 and the sensors connected : no fan
- on the inhibit switch (which on my car is closed when water is cold), 2 wires : Green, and Black. I have disconnected both cables :
- green : shorted to ground
- black : not shorted to ground



On the back of the AC ECU, there is a green cable, which seems to be the ground for the ECU. No climate control at all when this green cable is disconnected. But the green color seems to be the same as the green on the inhibit switch ones. Picture :



And, last test (I don't know why I did it, but I did it anyway and the results are ... weird) : I ask warm air, no fan. I hold the temperature sensor in my fingers (so it get warmer) and ... the flaps are moving et the fans are spinning... I don't understanding anything anymore



I also found this plug, which is not connected :

 

So your wiring to the inhibit switch is correct. Black is the logic ground signal to the ECU to inhibit the fans, green is ground, same as the ECU ground you found. And switch is working properly, so that rules all that out.

That bare connector I have no idea. It does not look like any connector I’ve seen in XJ40s. Perhaps a previous aftermarket alarm, cellular telephone, etc? The wires look factory though. Do you have a better picture of the face of the connector? I can dig in the electrical guides and see if I find anything. Where does it tie into the harness? And where exactly was it sitting in the car?

I’m curious about the fan starting when you warm the temperature sensor… which one are you warming? The in-car temperature sensor with aspirator motor?

 

Wait a second…

I have seen that style connector before, and it’s the second wire to control the blower motor speeds… The motors have a heavy gauge connector for 12v/ground and a second connector for the low voltage signals from the ECU to feed the variable speed power transistor.

I don’t have it in front of me but it looks like a possibility!




Yep!!! That sure looks to be it! No wonder you were having fan issues. But… that would only explain one fan not working. The other should still be running unless you have the very common only one fan working syndrome

Also, the ambient temperature sensor is located in the RH blower motor, and uses that connector for signaling.

 

Hmm, I have the 2 fans working ! Left and right, both are working.
The problem is that no fan, nor left, nor right, are working when I ask hot air.

Here is the disconnected plug. It seems to come frome the aircon harness :





And to answer your question: yes, it is the temp sensor with the little fan I have kept a few seconds in my hand to warm it ... and the fans have started (hot air asked on the panel)

 

Good catch Nick ! The corresponding plug was hidden behind :




So, I plugged it and ... no behaviour change

Cold air asked : both fans are blowing
Hot air asked : none