Hottest day of the year, both blowers quit

As is usual in XJ40 land, a problem only arises at the most inopportune time.

Driving home yesterday, A/C chugging along merrily in auto mode, I decided to try and increase the flow so switched up to manual, selected recirc, max, and defrost from time to time as well trying to see what would be the coldest. So system seemed to work fine for a while but after a couple of miles I couldn’t feel any air from the vents and no matter what setting I selected, the blowers didn’t come on.
I could hear the blend doors and flaps etc opening and closing but got zero flow from the vents. So I drove around with the windows open.
Today, same thing - no blower activity - however, parked at the supermarket in hot sun and when I returned to the car it was baking in there and I guess the panel was still set on med and A/C because when I started the car the damn blowers fired up and I could feel the airflow again. That only lasted a few minutes and then they quit again.


Hello Larry … and the beat goes on

As I understand the working of my 1989 system the normal thing that causes a dual fan failure in the manual mode (bypassing sensors) is a problem with the climate control panel. This is 99.9% caused by the very inexpensive generic micro switch. When both your fans failed did you check to see if the lights were still on the panel ?

Yes all the lights work normally and the blend doors move as I select one thing or another.

I had the same problem on my 88. I removed the hvac controls and shot some WD-40 in the micro switch. It cleared mine up. Verified with a continuity check while it was in my hand.

As far as I know, if the CC panel is “live” i.e. the panel lights up etc., the problem isn’t with the micro switch?

I pulled the passenger side kick panel and saw the lowest of the three ECU plugs was partially unseated so re plugged it and went for a drive.

Blowers worked for about 10 mins but I could hear them slowing down and eventually they stopped. As it happens I had a spare climate ECU on hand so I swapped the unit out with my spare.

Everything working again, it was the ECU.

Larry …

Glad it turned out to be something that didn’t require a ton of troubleshooting (always good to have spares)

Yes, my heart goes out to you with your “hot” weather, looks like low 70’s to high 60’s. We’ve had temps over 105 for several weeks straight now :fire: We’re use to hot summers here in Dallas but this is RIDICULOUS.

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It’s 33° celsius (~91f) in my apartment ATM, we’re in a bit of a Pacific northwest heat wave. So glad I bought a parts car all those years ago!

So today, same problem. No blowers at any setting, panel controls all light up and I can hear the blend doors operating as I switch to demist/defrost etc.

Pulled the knee bolster again and checked connections - according to Haynes 93-94 climate control wiring diagram, the auxiliary power relay (which I’m guessing is part of DBC10009) may have something to do with things.

I had a spare DBC10009, but swapping it out made no difference. I’ve just opened up the spare and it looks like there are a few dry/cracked joints - hoping that’s why it didn’t work … Of course my soldering tools are at the workshop about a half-hour drive from here, so tomorrow I’ll fix the spare and open up the one still fitted to the car and see if the module(s) is the problem.

Reflowed both DBC10009 modules, didn’t fix the problem. Crazy part is the blowers blow a gale if set on max when car is first started, ambient temps cool at that time but as the day warms up, they slow down and eventually quit after about 10 mins. I’m baffled!

Larry, I feel your pain (figuratively speaking). Lacking an obvious suspect I would be inclined to whip out the multimeter and start testing inputs to, and outputs from, the climate control unit. There are three 15 pin connectors (AC2, AC3, AC4) to the CCU:

Inputs come from, among other things, the climate control panel. You can see from the expanded subset of the above chart (below) that when the control panel is on, pin 9 of connector AC2 should receive 12 volts (when off pin 9 is open circuit). When the panel is in auto mode pin 4 of AC3 should have 4 volts. When manual mode is selected pin 4 switches to ground. When the panel is on and manual mode is selected pin 7 of AC3 (an output to the left blower motor) will be 8 volts if low speed is selected, 5 volts for medium, and less than 1 volt for high or defrost. Checking some of these should reveal whether the inputs to the CCU from the control panel, and the outputs from the CCU to the blower motors, are as expected.

It has been a good while since I’ve had my climate control panel out so I don’t recall how easy it would be to shoot some electrical contact cleaner into the off-on / fan speed selector. Might be worth a try in case oxidation or crud is preventing the selector from outputting the correct voltages to the CCU.

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Thanks Mike, I’ll take the panel out tomorrow and give it some spray to drink and print off your notes and charts. Equipped with my meter I’ll see what readings I come up with and report back …I really did think/hope that reflowing those cold solder joints in DBC10009 would fix the problem :frowning_face: …Oh well, onward!

Mike are those screen shots from the CD-Rom? I have one around here somewhere :roll_eyes: :thinking:

No, they are from Jaguar publication S-65/93, XJ6 Sedan Range, 1993 Electrical Guide. If you don’t have it, here is a link to it in my Dropbox account:

It is invaluable for troubleshooting our cars.

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Thanks Mike, for some reason I only had the 1990 Electrical guide. Appreciate it!

Well I had the panel out today for an inspection/contact cleaner spray also checked power to the blowers on the brown (which showed 12v) swapped the CCU again and cleaned and re-plugged every connection I came across, but blowers still don’t start at any setting.
One thing I thought odd was that I found the micro switch on the panel was a replacement -which seemed strange given that this is a 94MY which apparently doesn’t suffer from that particular issue.
However, reviewing my notes from 2012, (when I first had blower problems) it looks like I replaced the micro switch before discovering the issue was with cracked solder joints at the fuse boxes! DUH!
At this point I’m wondering if it could be the replacement micro switch failing? Up to this point I’ve been ignoring that possibility as the panel lights all work and the blend door motors all impulse normally.

The micro switch issue was seemingly corrected in the later cars as the problem was limited to cars prior to VIN 649176? All very confusing. I did read a post where an owner described a condition where the resistance in his refitted microswitch increased over time and eventually needed a second replacement. Just clutching at straws here, really out of ideas.

Larry, while that brown wire carries 12 volts to both blowers it is to the relays in the blowers, not directly to the motors. The orange wire on pin 1 of AC3 (the middle of the three connectors to the CCU) carries the triggering signal (12 volts) to the relays when the blowers are activated. Do you find 12 volts at that pin when the blowers are switched on?

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Thanks for the reply Mike - I believe that pin only showed 9v but I’ll check again this morning.
I did find the spare control panel from my parts car last night - however it isn’t exactly the same - DBC11817 in the 94 Sovereign vs. DBC11814 in the VDP.
DBC11817 doesn’t have the heated windscreen button layout so the circuitry is slightly different but the connector is identical. I’m guessing the switching functions should also be the same.
The circuit board on the original panel did show some discoloration here and there so possible that there is high resistance in there somewhere - at least that’s my best guess at this point.

I’ll try using the DBC11817 to test today.

Well not the result I hoped for, the replacement control panel worked just like the original, impulsed the blend doors and lit up the panel but nothing else. No blowers.
Refitted the original panel. Tested both blowers for power from fuseboxes, both show 12V.

Yes Mike, that pin has 12V when car is running.

After lunch I started searching the archive for SOME hint as to what may have occurred with another car in the past with the same symptoms. After a few red herrings I stumbled on these posts from 2006 by @David_Boarder:

So next chance I get, I’ll dig out some spare blower relays or if not, new diodes and see where we go from there.