Hottest day of the year, both blowers quit

Sorry to keep pecking away on this topic. I had recalled that many years ago (10, as it turns out) Don B posted a good writeup and photos in the old site photo album describing and depicting the cleaning of ground wires / studs. I had been unable to access the old site for the past few days due to a site certificate error. I was just now able to access the site and here is a link to photos showing those black/pink wires and the grounding stud:

Great help Mike, just back from camping trip - will check trunk grounds tomorrow …fingers and toes crossed!

Cleaned all the grounds (which were pristine/still coated with factory dielectric grease) and was preparing to test 12v on the orange wire at Pin 1 when the car developed an idling/starting issue. After checking the usual things I noticed I couldn’t hear the fuel pump priming …there is a chance that I might have disturbed the fuel pump relay as it lives in the general area of the grounds, but possible fuel pump has also failed.

I’ll check again tomorrow, for now the car won’t start.

These things are sent to try us Larry. Hope you sort it out soon :hot_face:

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Finally got back to the blower situation this morning. I put 12v on pin 1 orange wire and the blowers didn’t run. All I heard was a faint but distinct relay click each time I touched the contact. Ignition was on BTW.

OK, needing some electrical help here. IF has been speculated the relays cannot be serviced without pulling the blowers, can the diodes be tested with the blowers and relays in situ? Based on this snip from the electrical diagram:

Could you remove the wiring connectors at both left and right blowers and then, for each blower, using the connector on the blower (NOT the wiring harness connector) check for continuity between the pin (#3) that the black/pink wire would connect to and: (a) for the isolation relay, the pin to which the orange/black wire connects (#8 for the right blower, and #5 for the left), and (b) for the high speed relay, the pin to which the orange wire connects (#1 for both right and left blowers)? The expectation being there would be continuity with the negative lead on the pin #3 and no continuity with the leads switched (positive lead on pin #3)? Sorry for the convoluted description. Does this make sense or is my thinking totally buggered?

No Mike, I’m grateful for your input. I just spoke to a local pal who also has one of these 40’s and discussed the diode/relays in the blowers - referring to that old post from 2006 back in the thread where the poster had an issue with a diode on one of the relays. He agreed that the conclusions the OP came to were correct.
My thoughts were that if a 12v trigger voltage from pin 1 orange were supplied from an external battery in tandem with known good fuse box power to the blower(s) and the blower(s) failed to start, then that would tend to rule out a control issue and point to the blower(s) themselves, right?
He suggested back-probing the brown main power connector (the 12v supply from the fuse box) while applying the control signal and measuring the voltage with a meter looking for a voltage drop, his thinking leading toward a supply (fuse box) issue.
While that could be a possibility, (I re-flowed both fuse boxes years ago) yesterday I fitted my spare passenger fuse box from my low mileage parts car -with no effect. I also grabbed both spare blowers from the parts bin and will scavenge the relays from them. They both worked in the past but had darlington issues IIRC.
I’ve already tested the passenger side replacement in the car footwell and it also failed to start -so- if my reasoning is correct and there is a problem inside a blower, i.e. the aforementioned blower relay/diode problem, then it would be the driver’s side blower that may be where the problem lies.
All of my tests so far have been on the passenger side but I think my next step may be to pull the driver’s side blower and go from there. I know people don’t relish pulling the blowers but it really isn’t all that bad with a long extension - I find re-fitting the flexible connector from the blower to the CCU to be the biggest PIA, but nothing insurmountable.

Appreciate all your advice always. Thanks again.

Looking more like a power quality issue and not control circuit or logic.

Odds of both blowers failing at same time is very unlikely. Getting closer I think.

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Is it possible your evaps are icing due to low refrigerant and mimicking a no fan?

No. Heat blower settings don’t work either - although as with the A/C air flow the heat will work intermittently and very weakly on auto/medium setting.


before you start pulling out the blowers, please double check all voltages. You may want to revisit the coolant temp switch. If that is stuck closed, the blowers will not run. So, you might want to measure Pin AC 3-6 again, with the engine off (cold) and on (warm). Please report results.

Good luck

Not sure what switch you’re referring to?

The coolant temp switch is part of the ac system. It is attached to the heater pipes on the left of the AC assembly. It governs the activation of the blower motors. It is connected to the AC control module. Just look at the electrical manual pages again, it is mentioned on page/FIG 29 data. Further up in this thread, you did confirm that all voltages at the computer were present and correct - as laid out on that manual page.

I suggest you measure the control voltages again with special attention to the coolant temp switch. If that switch is broken, this would explain your ac blowers not running.

Guido, I respectfully submit this could be true if the system demand is for heat, but not true if the system demand is for cooling or is in defrost mode. See posts 33-35. This is from the Jaguar training document Delanair Mk. IV Air Conditioning System:

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Thanks Mike, I wasn’t aware the posts above. With regard to the logic mentioned in the MkIV manual, you are absolutely right.

My thinking was a bit different, still. I am suspecting a bad ground or power supply issue here. The coolant switch is the single source that I can think of that does govern both fans simultaneously. Thus, I think it is important to measure all of the control voltages again and start from there.

I’ll be yanking the blowers tomorrow and doing some swapping and testing, hopefully leading to some conclusions about what’s going on.

I’ll be checking the relay/diode theory noted in post #20 …apparently this failure can also affect both blowers …we will see!

Larry, I found the reference to to replacing the relays without removing the blowers. It is in the XJ40 ebook on the old site:

Yes Mike, I’m familiar with the method but I’m going to pull the blowers out anyway and possibly do a blower swap to get a DEFINITE assessment on each one. My “spares” definitely ran on defrost when I took them out, they are my original units that I removed when the darlingtons failed originally, replacing them with the rebuilt parts car spares.
Instead of removing the blowers and opening them up to fix the darlington etc, I did the work on the parts car blowers so I could simply replace the units and get back on the road right away. The “spares” I have ready to use for the test are my original “only on high” blowers.
I’m expecting to know a lot more after tomorrow. I might even test and replace the driver’s side fuse box - my plan is to identify the working/non-working components once and for all.
I don’t mind the removal/refit, it’s much easier than doing the fuel pump.
I already ditched the passenger air bag so that side isn’t too bad, no glove box or air bag there. IIRC, the driver’s side is pretty straighforward too…

Well, assumptions make an ass out me once again LOL

So turns out it’s the blowers after all. I pulled the drivers side blower, took it to the bench, opened up the case removed and inspected the motor brushes, blew out the dust etc with an airline and gave things a shot of contact cleaner.
Changed out the relays and powered the motor directly by connecting to the fan terminals inside the case. Blower ran, full tilt.

Hooked it up in the footwell, turned the controls to max/cold and turned on the ignition. Nothing. No go.

Hooked up the spare and to my surprise this one blew properly. Of course the slow speeds don’t work, that’s probably why I changed it out in the first place. So it looks like I probably only had the drivers side working when things finally crapped out altogether. Installed the spare so know I have at least one running, even if it’s on noisy only. Was nice and toasty on the way home!

I’ll test the passenger side spare in the next couple of days as the passenger side isn’t running ATM and have a ponder about how to get all four blowers running to spec again - not sure why the blower I replaced ran fine with a direct connection but not through the harness? Have to have another dig inside I guess. Oh well, nobody ever said owning one of these was for the faint-hearted! Onward!