The climate control panel on my 1994 XJ6 has a power supply issue for a few weeks now.
The symptoms are that the fan switch does nothing and the various buttons on the climate control panel are also inactive.
The only thing that does happen is that when the fan switch is turned from High to Defrost, a click can be heard under the passenger's side of the dash.
The fault seems to VERY rarely suddenly disappear and the system will work normally for a short period of time but this happens very infrequently.
This evening I noticed that other interior subsystems were also inactive.
Specifically the radio and the power seats.
As I was riding around I think I might have hit a very minor pothole or road irregularity and suddenly the radio display lit up and the power seats worked.
So I tried the fan switch and that was working too.
I continued driving down the road and shortly there after all of these functions just as suddenly went dead again all at the same time.
Can anyone think of a reason why all three subsystems, climate control, radio and power seats would all loose power at the same time?

Theories or speculation always welcomed.
--
Steve

 

Bryan has suggested I look at the auxiliary power relay H1 with the yellow base under the passenger's side dash.
When I looked the wiring diagram this morning I discovered that a whole bunch of stuff is fed off that same relay including the horns and cigarette lighter in the front console.
Since these have always worked I was puzzled so this afternoon I went out and checked the car again to see what is or isn't working.
The power seat controls are working at this point (was dead last night). The climate control panel is completely dead and the radio is dead.
The horns and the front cigarette lighter have always worked.
So I guess this means the auxiliary power relay module H with the yellow base is probably not the root cause of the fault because then the horns and the cigarette lighter would be failing at the same time since the power for those is via light green with orange wires off that same terminal on the aux power relay.

I will continue looking closely at the wiring diagram to see where this fault might reside.

 

I'm with Lawrence on this. Fusebox first. You could splice in direct 12V from battery to confirm, just check Haynes where exactly, but my bet is on fusebox anyway, especially since they are original , unopened and therefore highly susceptible to cracked solder. Do these first and then if it doesn't help we could look elsewhere. There is plenty posts on howto do this, if you're not comfortable with soldering iron, any mobile phone repair place should do it cheaply. My bet is on the big bolt in the fusebox, you'll have to cut it open to get inside but its easy work anyway.

 

OK, this is very strange. After spending quite a bit of time going over the wiring diagrams this afternoon trying to figure out my radio and climate control panel power issue, I went back out to the car to see if sudden inspiration would hit me if I looked at the various bits on the passenger’s side of the dash. I made sure the wiring harness plugs in the climate control ECU were firmly seated and then when that had no positive effect I decided to look at the auxiliary power relay. A few months ago I was having an issue with my side marker lamps, tail lights and front parking lamps not functioning. At that time a whack on the DBC10009 relay did the trick and I was able to rejoice at my good luck at having working lights again. However just for insurance I also purchased a spare DBC10009 relay off of an ebay seller.

So as I am sitting in the passengers seat looking at the various components I decided to try swapping the spare DBC10009 relay in to see if it might have any effect on my current problem. I did not have much hope since as I said other things that were powered off of the SAME TERMINAL on the DBC10009 relay were working fine. Anyway I removed the original DBC10009 relay and installed the spare, started up the car and WOW! The climate control and radio were now working! Since I didn’t really believe this relay was the problem I swapped the two relays in and out of the car a few times just to confirm my findings. I even went for a ride to run a few errands with the spare WORKING relay installed and the climate control and radio seem to be functioning fine now.

Long story short, never doubt that a buggy DBC10009 problem could cause strange issues.

Thanks Bryan for pointing me towards looking at this relay in the first place.
--
Steve

 

DBC10009 suffers from exactly the same issue as fuse boxes and it's just a matter of time before both will need attention. Plenty of repair howtos on xj40.com forums but generally as it seems to be the most expensive relay out there, it's worth fixing it by opening it up, resoldering and if the individual coil still having problems operating contacts, the whole line(4 of them in each relay) can be replaced with either new parts(resistor/diodes) and/or replacement coil from another quad relay, that isn't DBC10009.