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Two requirements for the relay - Has the contact capacity for the clutch, and it fits where you want to install it. The original industrial relay I used cam from a local electronics store near 928 International. The formerly-available Radio Shack pinouts have been documented well. But you can use virtually any DPDT relay, connecting the pins on the new relay to the PC board connections corresponding to those same contacts on the old relay. There's a picture of the contact arrangement and the pinout on the old realay. Match it up to the same diagram on the new one, and note the pin numbers so you know what to connect.
There are several discussions about fitting a secondary relay in the engine bay, using the existing relay to trigger that. Power would come through a fused feeder attached to the jump-start terminal. I'm not a fan of jury-rigged electrical stuff in the car, especially connected to primary feeds and/or exposed in the engine bay. Just a personal thing, think mods like that look a little too trailer-trash than the car deserves.
Greg Brown offers a full refurb service on the control heads, including the relay upgrade. If you fry the old controller board by accident, by ham-fisted soldering, etc, it's a LOT more work to fix, and may be beyond repair if teh damage is extensive. If you have a doubt about your abilities, contact Greg and get it fixed right.
Interesting subject matter. My control unit has been a bit dodgy recently in that occasionally the a/c would not latch. I found that switching it off and back on again [sometimes several times] brought it back to life so my presumption is that the relay is probably on it way out.
I looked at Dwayne's excellent write up and considered it was probably beyond my [not very good] soldering capability so may need to do something shortly. Good to learn there is an option for repair with Greg- just wondered if anyone can advise me whether my spare 90 S4 head unit can be transplanted into my 92 GTS chassis. It is physically slightly different to the later unit that has a bit more functionality but whether plugging the earlier unit in will fry anything no idea- any thoughts?
I traced the Compressor Clutch not engaging in my 1991 S4 back to the Head Unit, and have replaced the Relay as described in this thread - very helpful, thank you. The Clutch now engages, and now also with a full charge of gas the system gives lots of good cold air. However the Compressor only runs when the Heater Slider is all the way to the left - Full Cold. This does not seem how it should be (Would demist not be assisted by a bit of heat and aircon?), but I do not have a basis of comparison. Or errors could include my dodgy soldering skills and mapping (Old Relay was Yellow). Thoughts?
Thank you. Cheers, JimR
AC is always on in the coldest position of the slider, it is also on during DEF mode (as you suggest) and during recirc mode (where equipped), it is of course also always on whenever the AC button is pressed.
This suggests to me that the AC switch doesn't work properly - it has a separate plug - is it plugged in (e.g. does it illuminate?).
I followed Dr Bob’s recommended location for externally mounting the new relay as well as the area for the wire leads to emerge from the inside of the head unit. This area is at the unused port at the back of the unit depicted by the green arrow below.
The AC Switch was plugged in, and the light illuminated when the switch was pushed in and ignition on, or when off (in out position) but lights on. With the latter settings the light is not as bright. All as expected.
I was a wee bit anxious about touching anything after being so long without cold air (Have had the car almost ten years, and not sure the AC ever worked properly), and did not want to lose what I now had. However this morning I pulled the plug, and switch cover, cleaned the plug contacts (Male & female), sprayed contact cleaner on the plug contacts and in the switch. Operated the switch a number of times, and then left it to sit and muse on its shortcomings for a bit.
Now seems to be working more as expected, although compressor clutch disengages when the heat slider is about half way across. Perhaps this is normal operation with the system saying it is not yet warm enough in the car, and shutting down the air conditioning at that outside temperature? I had previously cleaned the outside sensor a while back, and then the inside sensor when replacing vacuum diaphragms a few weeks ago.
Sorry for the long explanation - although took even longer backwards and forward working through the various settings, but felt it best not to involve my wife.
BTW: Phoenix is a great spot. We have visited several times (Our youngest daughter lives there - even shouted me a ride in Sentimental Journey last month), and we ran (rather waddled) the Scottsdale runway earlier this year.
Unfortunately not - when AC should be on the compressor runs continuously regardless of temperature setting (unless the freeze switch is activated). All temperature regulation is still controlled (as with the non-AC case) by directing hot water through the heater core and varying the mixing flaps. So yes with AC on you cool all the air by the maximum amount possible and then warm it back up again to the temperature you really want... Really. This is also how most cars work (certainly of this vintage) - remember all the heating is for free.
Phoenix is great until June/July when the temperature hits 110F - time to head out of town!
Well after studying the wiring diagrams, and the relay , I realized my PCB is different to that used earlier in this thread. Being a 1991 it is of course same as described in the following thread:
So although my special "Feature" has been deleted, my HVAC is now working as per Factory Spec, I now know how to test a Diode, and I better understand the PCB - Although have not figured out where the Red wire to Connector 12 goes to, as no wire is shown on the Circuit Diagram?
Thanks for the extensive information and pictures.
I have read this topic with great interest, also had some issues with the temp control unit and resoldering the circuits solved a variety of non-functioning controls. The relay works fine. Never forget to check the vacuum controls, they are not suited for eternity.
Even here in The Netherlands it is warm now and I have repaired the AC system on my 1989 model. Converted to R134a and it worked fine (too cold actually) and have a leak in the compressor Denso 6E171. The new o-ring of the rear cover is to blame.
Also I had a leaking sensor, on top of the AC condensor to dryer tube pipe. Luckily I had a replacement, an old one and not confident it will last. This is also the question I have: I know there is a low pressure switch on the lower side. But what is the function of the one on top? Is this a temperature control or pressure control? Settings and possible replacement that can be ordered?
Shown on the edited photo as taken from the excellent write-up that started this topic.
Thanks in advance.
Dr. Bob
I don't think Greg Brown still provides the control head rebuild service. I sent my HVAC control head to Roger who then sent it to GB this time last year. I still have not gotten it back.
I think you should do some follow-up with Roger and Greg to find the controller.
That sensor on top of the drier manifold reads pressure. The controller under the cover next to the passenger seat reads that, plus the ignition on, plus the signal passed through the second set of control-head relay contacts, and decides how to operate the fans and flaps. Refrigerant pressure there is very predictably related to refrigerant temperature, so the fans and flaps are functionally responding to heat load on the high side of the system.
Darn good thing it doesn't get too warm there in Natchez!
I spoke with both Roger and Greg's wife to no avail. I just bought another used one from a list mate. I am in the process of putting in a beefier relay. It may warm up later this summer.
Actually Natchez is a little too far north. I had a frozen pipe several years back.
You may not remember but when I first got my 928 you advised me on getting it cranked some five years ago. I really enjoy this list. My middle child lives in Portland. Maybe our paths will cross one day.
Take care
I spoke with both Roger and Greg's wife to no avail. I just bought another used one from a list mate. I am in the process of putting in a beefier relay. It may warm up later this summer.
Actually Natchez is a little too far north. I had a frozen pipe several years back.
You may not remember but when I first got my 928 you advised me on getting it cranked some five years ago. I really enjoy this list. My middle child lives in Portland. Maybe our paths will cross one day.
Take care
That ain't good because I sent him 3 and only got one back.
07-30-2015, 03:10 PM
