Thread: HEVAC Screen Illumination and Associated lighting

The good old HEVAC has done it again.
It started out occasionally fading out to a black screen then it would operate correctly again. Every time it did this I would tell myself to look into the problem but as life dictates, I did not do that until the damn thing went out and stayed out (with random occasional illumination)
Note that this is not a lighting problem per se but seems to be an issue with the LCD screen failing to illuminate...lack of data input I am thinking.
Interestingly the Xmas tree lights for the AES (position indicator lights) would fade and disappear at the same time and now that the HEVAC is dark, so are the Xmas tree lights and switch lighting for the top switches (except the hazard switch).
After many hours of relay checking, fuse replacement, bulb checking and even HEVAC replacement, still no luck. Even checked the red/white cable at the HEVAC and there is not 12v there.
After the HEVAC was replaced, the 'new' unit momentarily lit up (with the Xmas tree lights) then faded out and has not re-lit. This indicates to me that the problem is not the HEVAC or the lights but a supply issue somewhere.
A torch across the LCD screen does not show any sign of readout and although it is difficult to say (because there is no indication of temp settings or switches off or on) it appears that the A/C or heater is now not working either.
My testing gear shows there are no faults on the system but I only have fault diagnostic gear not a full diagnostic suite.
I am stuck for ideas guys....any suggestions would be greatly appreciated.
Cheers Bob

Ok guys...for those of you that have had similar problems, the issue was in fact the fuse box and more precisely around relay 7 which is one of the 2 relays that provide for the HEVAC and associated equipment.
There is no simple way to fix the problem other than replace the entire fuse box with a known 'working' one....BUT.
The dis-assembly and repair if using the Rob Beckett Fuse Box Repairs approach is quite easy if you have some moderate skills, particularly with a soldering iron.
All up I identified 7 dry (and cracked) joints not all of which were in the HEVAC circuitry and re-soldered all of them.
All up the repair including removal and replacement (with numerous coffee breaks) took me about 2 1/2 hours. As a final inspection I used good magnification and good lighting and thoroughly inspected every joint on the circuit board at least 3 times.
Care needs to be taken when re-soldering the pins (see Ron's info) as it would be easy to create a dry joint during this operation...a good inspection (as above) in this area is time well spent.

Lastly....daunting as this may seem to many, it is not difficult at all....good luck to anyone who embarks on this fix.

Thanks to the contributors above for their input.
Cheers Bob