MY 87 S4 retrofitted with 134 system by Porsche via the original/PO. Problem is AC perfect EXCEPT when you turn the lites on at position 1 or 2. Clutch will disengage when you do this. Clutch will remain engaged if you pull back the steering column lite switch. Alternator output at 13.8v. Clutch wire at 11.0v after freeze switch (new) and pressure switch. With lite switch on the clutch wire is at 0.0v. I do all the work, and I can not find this Gremlin ! Any thoughts ?

 

Sounds like it is time to drag out the wiring diagrams and a DVOM.

 

I have gone thru thru WD's and have found no interactions with the lighting circuitry at the level of the CE board regarding the HVAC circuitry. Someone mentioned 'it sounds like a ground feedback problem', but I am not familiar with this term. This car is 'virgin' except for alarm and sound system by PO. the sound has no interactions with lighting. The Code Alarm does.

 

I am new to posting on Rennlist. I wanted to post to you, but posted to myself.

 

That's a weird one. Something is definitely wired strange or wrong. I'd check that AC clutch wire and circut to make sure the po didn't hack in for a 12V source somewhere.

 

In the engine compartment everything is wired as should. I am contemplating pulling the CE board for that is where the alarm is tied into.

 

I just can't wrap around why everything works as should with the lights off. If tapped into 12v somewhere, it would steal and not function regardless of the light switch status.

 

The very first thing I would do if it came into my shop would be repair the alarm, and by that I mean put it in the trash can and put every thing back to factory specs.

Most times that will fix a alarm related issue.

 

The alarm works very well. I'm just wondering how the alarms tie in to the light switch affect the the circuit to the AC clutch

 

no idea, but I know if it is in the thrash can, that is not the problem.

 

There is a link between the lights and the HVAC, and that is the lamp inside the HVAC control unit.
Check that the earth wire to the HVAC control unit is good (brown wire to ground point V which is above the fuse board, via connection point 1 and the cigar lighter) if it isn't, the HVAC control unit could be getting its ground via the internal lamp and the light circuit. If this is the case, the lamp in the HVAC control unit will not be working (check that).

 

I have never seen an aftermarket stereo or aftermarket alarm system that was PROPERLY installed. And by that I mean using correct wiring points for signals and voltages, correct connectors at respective points, properly routing of wires and cables, tying off extra wire lengths. Most often it is completely cobbled, wires crammed and stuffed, things like B+ wires off illumination wires, plastic tape wrapped ***** nilly about badly connected wires...wires never soldered, just twisted together, original connectors cut off, no conventional wiring codes followed, etc. i am sure there are exceptions, I haven't seen them....and not to really pick on the stereo and alarm installers...they are usually untrained people (usually entry level folks) with little experience, who are expected to wire up the most complex of electronics in 30 minutes or less, using only a hammer and a cresent wrench.

My personal worst experience was in an old Corvette I bought, a car in a remarkable condition body and mechanical wise....no start one morning, found there were 4 alarm systems in it, one on top another, a rats nest of wires that was completely unbelievable, and how it ever worked is beyond me. Amazing what stripping it all out, and correctly repairing all the kludged wiring back to original as best possible, did for the car.

So, have to agree, the trash can is the right place for the aftermarket alarm system....

 

Thanks for the replies gentlemen. The light in the HVAC controller function. The sliders are a little dim, and it was on the agenda for today to remove-disassemble and clean/replace the optic cables. Regarding the alarm, I feel this has to be the culprit. I was viewing the WD online for Code Alarm. Wire/Harness 3/B from alarm goes to parking lights. I will start there, and see what kind of 'spaghetti' connections I may find.

 

I agree with blown 87. In looking at the WD there are lots of places for "interaction" on the 12v supply to the compressor clutch. My guess is the current at the clutch is marginal, and turning on the lights draws down the flow enough so that there isn't enough to keep the clutch energized. Break out the DVOM and start checking where the voltage drops.

 

With light switch off potential in clutch wire is 11.0v. With light switch on the potential in the wire is 0.0v. Not just a draw down to below the threshold the clutch activation. Actually while driving and turning lites on (while AC running) you can hear recirculation flap shift. While driving having the AC on barely causes voltimeter needle to drop - clicking the blower fan speed up does so more relative to pushing the AC button on.