MainePorsche

The outside alarm has been cleanly vacated.
The 14 pin connector at the left post is good. It was part of the fabricating work of the power cables/harness that I did. I am at the Board now. If needed, and I can't work out activating the original alarm I'll shout out. Thanks

I forgot to mention now that the alarm is out.... the f*$#%&g AC still doesn't work with the lite switch on !

Alan

OK so I think we need some better diagnostics in a methodical way to narrow the focus...

Disconnect the compressor clutch connection and see if it still drops from 12v with AC on and lights off to ~0 with lights on? if no and it still does when connected this suggests a resistance issue somehow under these consditions (not sure how).

Do this test from the powered AC Clutch line to a solid chassis ground - then test between the 2 clutch wires (this may not the same thing...).

If it still drops to ~0v either way then lets back up...

1. Check connector G21 on the bottom of the CE panel - this is the HVAC feed from the blower relay -
powers everything HVAC - should be a solid 12+v at all the time running?

2. Check the freeze switch under the cowl (near the evap) - both terminals should read the same -
powered ~12+v when the AC is on ? should change just with AC switch.

3. Check also at H15 - this should be the same as the freeze switch terminals.

4. Check at M12 - - this should be the same as the freeze switch terminals.

5. Check at the low pressure switch on the AC dryer - both terminals should be the same as the freeze switch terminals.

Report back on where (if anywhere) things change...

If the root of the drop off is at the initial 1. point - then its an AC head unit power drop out issue

If the root cause is at 2. then it seems likely its a head unit relay issue

If its after that point is seem its a connectivity issue (though quite how that is related to lighting is unclear).

Alan

MainePorsche

Alan,
Thanks for the attention. Really most gracious of you to take the care to delineate a diagnostic algorithm. I will get to it the week of 8/22. Am going to PEI (Prince Edward Island) with my 12 year old to UPEI for Hockey for a week. He's a AAA player and it's nice bonding time with old dad. We'll shack in a dorm for the week. At least he's not so prissy that not having AC a night (remember lights on !) is an issue. Actually we both like to top and windows open. Can't say as much for my wife and daughter. Thanks again.

Alan

Well at least you have that option - agree nights are not usually so bad - except here in Phoenix where its still 100F @ midnight... makes AC rather important.

Enjoy the time with your son - you have your priorities straight - the car can wait...

Alan

MainePorsche

For anyone who has read this thread, the situation is now resolved with satisfaction.

Synopsis: My AC ice cold, works fine EXCEPT when I turn the lights on - the compressor would turn off.

I have tested all components of the HVAC system and they were fine. I've removed the rheostat for illumination - checked all the black/blues - fine. Checked grounds especially MP IV (if my memory serves me correctly) the HVAC controller ground - all other grounds to this point were OK. I've dissected the 14 pin connector - fine. I've removed the original power harness to alt/starter/points - rebuilt it. I've checked all wiring behind pod and center console - no 'foreign' connections to grounds. I've diligently removed a Code Alarm that was installed for keyless entry by PO - did tap into the light switch for signal light. I've gone tooth and comb thru the CE panel -found nothing, but serviced it anyway. Serviced EVERY ground in the vehicle. Became proficient in deciphering Wiring Diagrams.
I've probably done some more small things that I thought might be connected to this dilemma as a ground feedback but I can't remember what they are and I don't want to take any more time by going thru my very detailed Maintenance and Preservation Log, for I want to get to the conclusion.

I bought a new light switch from Roger, for the switch was not sitting 'tightly' in the pod. I also noted that the illumination in the switch was at best dull, and sometimes out. I attributed this to the 'looseness' - manipulating the bulb would get it back. Well today put in the new switch - BEHOLD the AC compressor works with light on and off.

I can't say I'm frustrated by the circuitous route I took to get here. I've actually got to know 928 electrics - the 928 en toto - a lot better. I, and the car, have benefitted from the journey.

Addendum: I would like to thank Alan for his diagnostic algorithm, and it was he who got me looking at ground feedback.

smiffypr

A do the lamps in the HVAC control light up too?

MainePorsche

Everything in the car worked fine except the compressor clutch wire would go to 0.0v when the light switch was turned on. I actually changed out the illumination of the controller to LED's with new fiberoptic cables prior to changing the light switch today.

MainePorsche

Been a long time with my relentless Gremlin. Now he is vanquished.
New hvac controller was the final solution. Several threads on this and the fault was a diode on Track 4 of the controller. Went for a drive with the AC and the lights on for the first time since I've had the car. Conquest is a good feeling...

Next... ?


Here are some of the threads related to this hunt.

https://rennlist.com/forums/928-foru...-relay-xi.html
https://rennlist.com/forums/928-foru...ne-update.html
https://rennlist.com/forums/928-foru...-question.html
https://rennlist.com/forums/928-foru...-specific.html

fraggle

Whew great follow up. Some of these problems can be hard to figure out.

GregBBRD

Wow. Great diagnostics.

Tracing the problem to that one diode is genius stuff.