HVAC Help Needed

[TurboD]

Story time:

Hi all,

It seems that each time I am on this forum asking for help, it's generally about the HVAC system on these cars. See my previous thread caused by a bad thermistor here viewtopic.php?t=2013.

In prep for some brutal Texas summers on the not so distant horizon, I thought it would be an appropriate time to test my '87 Turbo's AC to make sure that everything worked. I turned the AC on in the car while on a short drive, and everything ran perfectly. Parked the car for less than 15 minutes, and start the car... but no AC. Not sure if it's relevant, but I generally shut the AC snowflake button off on the car before shutting it off, and turn the button on once the car is running. This time, I left the button depressed in the on position.

Diagnostic procedures:

I've watched this video more than a few times now to help me better understand this car's HVAC system and found it to be a useful guide for diagnosing potential problems. NC944er, if you're on here bro... I owe you a beer or three if we ever cross paths! https://www.youtube.com/watch?v=tY7ydikX7ng

Fuse 29:


The first thing I checked on the car was fuse 29, labeled "heating, air condition" on the fuse box. See the pic of fuse box diagram below. Checked the fuse visually, all good. And then tester it in the tested relay. Again, all good. I have glow fuses on most fuses, and nothing was illuminating as blown. I checked every non-glow fuse with a multi-meter for continuity, and every fuse had continuity.

With the ignition on, I toggled between snowflake and no snowflake while taking voltage readings at the fuse. With the snowflake on, it was around 6V. With snowflake off, it was around 4V.



Compressor relay

The next thing on the checklist was the compressor relay. With the car off, ignition on, and a friend toggling the snowflake on and off in the cabin, I didn't hear any clicking from the relay. I removed the relay and gave pins 85 and 86 power and heard the relay click as designed. I then checked for resistance between the two and it appeared to be in spec with around 55 ohms of resistance. I have continuity between pin 30 and 87a. But, when placing the relay back in the panel, it is definitely not clicking over as it's supposed to.

I then tried running the car and jumping pins 87a and 30 with a wire on the junction block. Nada. I tried jumping this while the car was running, and while I also had the low-pressure switch jumped (more on that below), and still no engagement from the compressor clutch. As a note, my low-speed radiator fan relay clicks on each time I hit the snowflake switch, and the fan follows shortly after—I'm sure its designed that way, but figured I'd mention it in the event it's not.

One thing is for sure, this relay does not seem to be clicking over when I hit the snowflake switch.






Low-pressure switch

Next, I tried to jump my low-pressure switch. Pulled the connectors off the pins, sprayed them with an electrical contact cleaner, and jumped them with a wire. Toggled snowflake on and off with the ignition on and car off, no clicking from the compressor relay or movement from the compressor clutch.

I tried a combination of jumping the low-pressure switch with the compressor relay, and that did not summon the compressor clutch to kick on.



AC Compressor

Next, testing the AC Compressor. The compressor on this car is a very new Denso rebuild, replaced in the last 2 years and with less than 3,000 miles driven since. Based on the video above, I pulled the power wire on the compressor unit with the unit still on the car, gave the power wire a 12V power source... and voila... compressor clutch engaged and disengaged as designed. So it's probably not a bad compressor.

HVAC Head Unit

I have some familiarity with the head unit, as I tested mine extensively during my last HVAC repair journey. I have a freshly rebuilt head unit on the car now, but figured it was worth swapping out with my spare one just to confirm the headunit wasn't a contributing cause. I had previously sent this spare unit out to Tom to test on his car, and he tested it and said the compressor kick on was working fine on it. Sure enough, I put my spare head unit in, hit the snowflake on with car on, and no change at all from the response with the refurbed unit in.

Snowflake light was turning on on both my main head unit, and my spare.

Fuse Block

I figured checking the harness that the Compressor relay plugs into wouldn't hurt. I tested all the below in both polarities.

With snowflake OFF;
- Resistance/continuity
+ 85 to 86, no continuity
+ 31 to 87a, 7.3 ohms
+ 30 to 31 and 30 to 87a, no continuity

- Voltage
+ 85 = 11.5V
+ 86 = 2.5V
+ 31 = 18 milliV
+ 87a = 160 milliV
+ 30 = 2.5 milliV

With snowflake ON;
- Resistance/continuity
+ 85 to 86, no continuity
+ 31 to 87a, 11.5 milli ohms
+ 30 to 31 and 30 to 87a, no continuity

- Voltage
+ 85 = 11.5V
+ 86 = 53 milliV
+ 31 = 20 milliV
+ 87a = 200 milliV
+ 30 = 2.5 milliV


What do I do next?
Not really sure what is left to do diagnosis-wise. I hope I gave enough info to point me in the right direction! Really sincerely appreciate the help.

[TurboD]

Update:

It seems that no power to pin 30 on the board is the problem. No matter what combination I do, I can't seem to get power to it. Wiring diagram below... can someone help me decipher this? Not sure where to start with diagnosing this as I'm not sure what to make of the mess that appears to be going on at the "Heater wire harness" or where to find said harness in the first place.

That, and when I cycle the snowflake on and off while measuring voltage to and from pins 85 and 86 on the AC Compressor relay with the relay plugged in, 85 reads 12V and 86 doesn't read voltage no matter if the snowflake is on or off. That probably explains why the control isn't clicking the relay over?

[Tom]

When you turn on the snowflake button, pin A8 of the cc unit should power up to 12 volts. A8 is a white wire with black stripe. First thing to check is if A8 powers up when you turn on the snowflake button (with everything hooked up and running). If not, then your cc unit is not telling the system to turn on the A/C, either because the cc is faulty or one of the sensors it relies on is bad. If you have power on A8, then the cc unit is working and trying to turn on the a/c. The signal from A8 splits in a couple of directions, but for purpose of diagnosing no power to terminal 30 of the relay, the power from A8 goes through the ice-protection switch on the side of the heater console, and then on to terminal 30 of relay G17. So, if you have power on A8, but not on terminal 30 of the relay, then there's a good chance the de-ice switch isn't working and is cutting power to the relay . The de-ice switch cuts power to the relay when the evaporator starts to ice up, but if that switch is faulty it will just prevent the relay from getting power all the time. Look for the white/back wire on pin A8 and follow it to the de-ice switch on the side of the heater core console under the dash. The power from A8 should come out of the de-ice switch on a black wire with white stripe. Find that wire and see if it has power when A8 has power. If not, then that de-ice switch is bad. If the black/white wires does have power and you still have no power on pin 30 of relay G17, then you'll need to physically trace that black/white wire from the de-ice switch to the relay board to see where the break is. Hope that make sense....

[TurboD]

That did it!

Checked 12V at A8 was properly cycling when I hit the snowflake. Then, I jumped the de-ice switch, and voila... we have compressor kick on now!! I've left the de-ice switch jumped as I cannot find a replacement part online anywhere, and can't imagine I'd freeze the evaporator while using the AC in 100+ ambient temps. Any real risks in disabling the de-ice switch in normal hot weather conditions? Pic of the very odd part number below, and the location of the switch in the driver's footwell for anyone who finds this post years down the road.

Tom, I can't thank you enough. Your incredible knowledge of these cars has saved me from 2 would-be expensive HVAC problem trips to my local shop already. Carpokes and 944 communities are lucky to have you!

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[blueline]

That did it!

Checked 12V at A8 was properly cycling when I hit the snowflake. Then, I jumped the de-ice switch, and voila... we have compressor kick on now!! I've left the de-ice switch jumped as I cannot find a replacement part online anywhere, and can't imagine I'd freeze the evaporator while using the AC in 100+ ambient temps. Any real risks in disabling the de-ice switch in normal hot weather conditions? Pic of the very odd part number below, and the location of the switch in the driver's footwell for anyone who finds this post years down the road.

Tom, I can't thank you enough. Your incredible knowledge of these cars has saved me from 2 would-be expensive HVAC problem trips to my local shop already. Carpokes and 944 communities are lucky to have you!

I couldn't agree more - he is amazing!

Glad you were able to solve the a/c issue.

[Tom]

That did it!

Checked 12V at A8 was properly cycling when I hit the snowflake. Then, I jumped the de-ice switch, and voila... we have compressor kick on now!! I've left the de-ice switch jumped as I cannot find a replacement part online anywhere, and can't imagine I'd freeze the evaporator while using the AC in 100+ ambient temps. Any real risks in disabling the de-ice switch in normal hot weather conditions? Pic of the very odd part number below, and the location of the switch in the driver's footwell for anyone who finds this post years down the road.

Tom, I can't thank you enough. Your incredible knowledge of these cars has saved me from 2 would-be expensive HVAC problem trips to my local shop already. Carpokes and 944 communities are lucky to have you!

How awesome -- so glad you we able to pinpoint the issue. You did all the legwork and pretty much had it isolated before I came along -- whether you knew if or not Very hard to say whether you'd have icing issues. I'm guessing you'd need to run the A/C for quite a while before it could become an issue. That said, when my house a/c ices up, it's always in the dead heat of summer -- I assume because the pipes drop below 32 and freeze humidity after running for hours on end, regardless of outside temps. I have a couple of spare heater core consoles collecting rust in my side yard. Let me take a look at the ice switch and see if it can be removed in one piece. I've never removed/replaced one, but believe it has a capillary tube that might make it a challenge to get out undamaged. If I can get it out, and if it works, and if it seems likely you'll be able to install it in your car, I'm happy to give it to you.

p.s., thank you for the kind words. We are trying hard to make Carpokes a valuable resource for Porsche owners, without the high noise to signal ratio you often find elsewhere (especially in the 944 groups for some reason). The very best way to say thanks is to spread the word elsewhere about us!

[TurboD]

Tom, no need to go digging through your spares! Removing that capillary tube is probably a much bigger job than I am willing to take on at the moment for the likely low probability that my evaporator freezes up. Something tells me that if the system hasn't relied on the de-ice function to-date, that I likely won't need the function during the relatively short drives I take in the car.

I've been spreading the Carpokes word, believe me! This is the only forum I frequent these days. I used to have BMWs... and the BMW forums may even be more hostile than some unnamed other Porsche forums! Carpokes is a breath of fresh air. Thanks for the efforts.

[Tom]

Tom, no need to go digging through your spares! Removing that capillary tube is probably a much bigger job than I am willing to take on at the moment for the likely low probability that my evaporator freezes up. Something tells me that if the system hasn't relied on the de-ice function to-date, that I likely won't need the function during the relatively short drives I take in the car.

I've been spreading the Carpokes word, believe me! This is the only forum I frequent these days. I used to have BMWs... and the BMW forums may even be more hostile than some unnamed other Porsche forums! Carpokes is a breath of fresh air. Thanks for the efforts.

I took a quick look earlier and the switch itself is just held onto the side of the console with 2 screws, but the capillary tube is embedded in the evaporator. It did not pull out easily, and I was afraid to really yank on it when I thought you might want it. Can't find anything online about how to remove, so I'm pretty curious now... If it's not in the factory manual, I might still tear this thing apart to figure out how that tube comes out. The world needs to know.

My guess is that you're right about being fine without it, especially if you aren't running it for hours on end...

[Tom]

I went ahead and dug into a spare heater console to figure out how that switch is installed, etc. I poked around the Internet and could not find anything on point, other than a few references to where it is located and its basic function. The manual just says not to damage the tube, and to insert the tube up to the red mark. The part number is 944.659.221.01 and it 'looks' like the part is still available for a little over $100. So, without further ado, here's a Carpokes exclusive on how to R&R that freeze-over switch.

[dr bob]

Great video!

Guessing that switch has the same internals as the one in my 928 from Behr. The fluid in the capillary tube expands with heat, and the business end inside the little box is plumbed to the end of a little beryllium-copper bellows. Warm, the expanding fluid extends the bellows and actuates a common microswitch. Cold, the bellow shrinks with the fluid, and the switch opens. There's a small amount of adjustment available at the microswitch mount to move the actuation range. That range is determined by the 'deadband' in the microswitch itself. Colder-but-not-quite-freezing offers lowest vent temp, with the switch closer to the bellows. I had mine set for 34º open, and it would close at about 39º.

But... That bellows work-hardens and the action against the switch gets soggier and soggier with use. Eventually, the shrinking fluid with cold is no longer able to draw the less-flexible bellows smaller, and the switch stays closed. The only real solution is replacement of the whole thing.