StratfordShark

My MY 91 S4 had the yellow relay shown here (wide shot for orientation).

New grey relay was direct replacement and had 5A rating, above the clutch draw.

To open the HVAC head just remove the Philips screws on each side and then pop the plastic tabs around edge (this is the fiddliest part as you get one tab free and it will pop back when you loosen its neighbour - in the end what worked well was wedging ends of wooden cocktail sticks into tab slots). The two haves of the case just fall away.

Desolder pump very useful for getting old relay out of board. You could use desolder braid instead.

If your voltage is dropping to near zero when you try to engage clutch then it does sound like the relay is suspect.

I also experienced corroded connections in the freeze switch which showed same symptoms, even though the head relay was fine. The freeze switch should have been just a virtual short but had higher resistance which meant once it passed the clutch current most of the voltage was dropped across it. I replaced the freeze switch even though I wonder if it ever operates (last week was driving for hours through France in 30+ degrees with aircon on, and I wasn't aware of freeze switch ever cutting in).

You just unbolt the freeze switch (seem to recall it's an odd nut size - 12mm?) and pull out that yellow wire from the hole where it goes through to contact fins of evaporator. The yellow covered wire in fact contains liquid which contracts and trips freeze switch when evaporator freezes. You are correct that it's soldered to freeze switch - it's integral to it and when/if you buy a new switch it comes with this attached.

Bill Ball

I've seen what you describe with a bad headunit relay, as SQLGuy suggests.

If you haven't done so already, verify that the compressor clutch responds properly to 12V provided directly from the jump post to the clutch wire, as I suggested before. [EDIT after post: Ooops, I see you did this...and I see now others have already covered most of what I said below...oh, well.]

The first step in removing the headunit is to pry out the plastic trim piece. It takes skill to do this without snapping it in half. This will expose 6 small screws. You need to remove all of them. Keep a magnet handy or use a magnetic screwdriver. Even with that, I think the small washers on these screws are aluminum. Then pull out the small panel below the headunit and the headunit. The wire harness usually has a short tether so it can be a chore to get to the two plugs. After you get it out, you split the case by removing phillips screws on both sides and gently prying the case apart. You'll see a small relay, usually blue . There are no direct replacements AFAIK. You'll need to desolder and remove the relay, get a matching by higher rated replacement, solder in jumper wires on the circuit board to connect to the new relay. There are a number of threads on this.

You got a little confused by whether the freeze and low pressure switches needed to be plugged in to test things. You can do this either way and learn different things. I usually just put jumper wires in the plugs, bypassing the sensors. If things work then, I know I have a bad sensor and then go on to test them as others described. If things don't work with teh jumpers, I verify that the clutch will respond to 12V from the jump post, and I check for 12V at one of the freeze plug wires. If no 12V there, it's overwhelmingly likely to be the headunit relay.

Bill Ball

I see Stratfordshark shows a direct replacement for the relay in the 91 headunit. I don't believe this will work in 89 and earlier. The relay and circuit board are different.

Here's pictures of relays I have found in headunits. The blue Gruner is from my 89, and the grey Matsu****a is from a 90. Totally different pinout.

AO

The grey relay failed on my car (don't recall which one - '91 or '87). I replaced it with the larger external one from Radio Shack. Works fine now. If you determine your relay is dead, I'd replace it with the external one from Radio Shack. DrBob has an awesome writeup on how to do it. Takes about 30 min.

Dan87951

Can't seem to find the relay you are referring to by Dr. Bob can you give me a hand?

Thanks

dr bob

Dan--

I did my 1989 car very soon after purchase, using a surplus relay I found at a local supplier. Since then others have substituted a Radio Shack part that's more commonly available. It has the same function as the one I used but the pinout for the relay is different. You'll need to adjust the wire connections slightly to match whatever you buy, but that's really a matter of looking at the circuit pictures on the relay to get them right.

The original maintenance documentation I did for my car is attached. Whenever I make a change to something on the car, I do a detailed description so that a future owner/mechanic will be able to see why something is different. I don't want to ever be the sucky PO for a future collector or something. Been a victim enough that I don't want to be the perp.

dr bob

PM your e-mail and I'll send it as a much more readable PDF. It's just barely too big to attach here with the limits set so low.

Dan87951

Well I got the parts from Radio Shack but can't get the stupid relay to solder in. I got the old one removed but the new one is not soldering in.

I think this is the final straw that broke the camels back. I have sunk thousands in restoring this car (torque tube, AC, t-belt, trans lines, power coating, etc) and spent half the summer to get the AC working than the control unit burns out. Oh yea the AC was nice and cold but go figure the unit gives out. Unbelievable how my luck is. I'm taking a break from this car, its no longer fun to work on anymore. Thanks for the help.

dr bob

The new relay is attached using jumper wires that extend out to the side of the controller case, as shown in the diagram. I used a small drill bit in a pin vise to open the PC board holes up a smidge so they could be soldered.

PM your e-mail addy so I can send you the PDF version of the original CAD that's actually readable. There are folks who have done this already in your area that may be able to help, and I know that SeanR and Docmirror in DFW area do it for others too once in a while.

Mrmerlin

Your wires are backwards. No wonder it's so hot in that car!

Dr BOB would you please explain this picture in more detail how do you know where the correct wire position is?
I didnt see any numbers on my unit other than a 6 on the forward pin

dr bob

Not much to explain. The freeze switch is a thermally-activated microswitch inside that case. There's a capillary tube that (typically) contains a silicone fluid that expands a lot when it's heated. The tube connects to a cavity under a little copper accordian bellows, so the warming silicone expands the bellows. On that bellows is a little lever that actuates the switch. The switch is wired externally to the Normally Open (NO) contact, and is actuated when the capillary tube is warm to close the switch. The lever arm has a spring on it, and the spring is adjustable within a narrow range so you can do a little adjusting of the on-off temp range. Problem: The copper accordial bellows work-hardens as it flexes. Over the course of it's use, it eventually loses flexibility and doesn't collapse any more when the silicone fluid shrinks from the cold. So the switch stays closed, and the compressor is allowed to run even when there's ice building in the evaporator.


Oh, yeah... The order of electrical connection on the switch makes no difference. Ultimately it's a simple snap-action switch, single-pole, so there's nothing inside sensitive to polarity.

-- You can easily verify the integrity of th switch with an ohm meter with the switch isolated from the wires.

-- You can check the voltage going to the compressor clutch circuit (actually back to the CE panel, then to the pressure switch on the drier, then the 14-pin fenderwall connector, to the front-of-engine wiring harness, through the connector on that harness to the clutch wire by the engine oil dipstick.

shadowknight

question if I am getting .46 volts from my meter off the freeze switch does that indicate my relay is bad within the HVAC controller.

Schocki

Pulling the unit is not hard, just be carefull with the plastic part around the radio and center vent.

the symptoms are very typicall for a bad relay. Voltage at the tester and clutch does not engage due to the previously described load factor of the coil. Use the search funtion, get a new relay from Radioshack and solder it in and you will stay cool again

SQLGuy

Probably, though not definitely. If you're getting 12V into the A/C head and only .46V out of it, then it's more likely the A/C head relay, or the A/C switch, or the A/C system relay, or wiring...

Do you still only see .46V when setting the system to defrost? If you get 12V when set to defrost then you have a problem with the A/C switch itself.

dr bob

That's a pretty good indication that the relay is failing. Unplug the compressor clutch and check again. If you now get battery voltage at the freeze switch, the relay needs to be replaced. 928Int'l sells a rebuilt head unit with a new relay if you aren't up to doing the dr bob 'fix'.