captcashew

Everyone, I know there are lots of AC threads out there, but I wanted to describe what is happening on our 1987 928 S4 automatic and see what people thought. I think I have a strategy for figuring it out, but just wanted to see if anyone had any other suggestions. I'm pretty much a newb on auto mechanic type of stuff.

OK, here is the history of the car's AC. I got the car last fall with a non-fuctional AC. Really, I never even tried it since a) I didn't need to use it at the time of year and b) the PO told me it didn't work when purchased, so I didn't need to test. In his 4 years with the car he:

Upgraded from R12 to 134a. It would take a recharge but only blow cold for a month or two. Thinking that sounds like a leak (which I assume it would be too), he had someone do some work on it. Eventually they replaced the compressor but when they did the guy said he didn't think it was the compressor since the old one looked good. I have the old compressor, fyi. He said that if he had to guess, he'd say it was the condenser.

With all this, I figured I'd have to do a recharge since it had been over a year since he had recharged it to get dye in the system so they could see if there was a leak. And if it held enough to last the summer, maybe that would just be what I'd do every year if the fix cost was too much.

So that brings me to what I did. I have a local mechanic that I trust but isn't a porsche guy. I knew this before I got the car, so I never intended on using him. I was trying to decide if I should take it to the porsche dealer an hour away or a europeon auto specialist about 30 miles way. Since I knew all I needed was a recharge, I decided to let my guy work on it since he said that was something he could do and he didn't mind working on something like that. But if it was something that was more engine or 928 specific where he needed different equipment, he'd let me know and help find someone who could help.

So here is were things go wrong. They recharge it. But it still blows hot. Doesn't seem like the compressor is engaging when the AC button is pushed. But does engage when you switch to the defroster. The defroster seems to blow hot for me too for the 2 minutes I had to play with it yesterday. Not sure if that is supposed to be that way or not. The HVAC controls are set to cold (blue), so I don't think it is a newb mistake like that.

I found a write up for the 1987_AC_Compressor_Relay_Replacement_Procedure.pdf from Dwayne Heinsma that sounds like it could be the same issue I am having. I also have a full list of service manuals on CD the previous owner gave me.

Does anyone have any thoughts on where to go from here? I think I'll still have my regular mechanic work on it, but I want to try and help point him in the right direction so that I don't have to pay for research time as much as possible. Any other thoughts or links I could use?

Thanks in advance

-Bill

jeff spahn

Yes I do. Went through something very similar. Sounds like the relay in your head unit has gone bad. It is an easy fix to do the relay replacement with the Radio Shack one. You will eventually need to do it in the future so just do it now and see if it fixes your issue.
You can jumper across the freeze switch in the cowl and see if that fixes it but I would bet it is your little relay on the head unit.
You can see my trials and tribulations on the same issue (from being way off base to fixing it in the end)
here: https://rennlist.com/forums/928-foru...earched-3.html

mickster

And here is Wally Plumley's everything you ever wanted to know about 928 HVAC but were afraid to ask (thanks to 928 Specialists):

http://www.928gt.com/T-WallyHVAC.aspx

griffiths

Here are somethings you can check, generic ac tests any tech can do:

1) Does the ac clutch engage? When the system is on (engine running, windows up, thermostat max cold, etc.) look at the outer hub on the compressor clutch and determine if it is spinning with the pulley or standing still?

2) (engine running, windows up, thermostat max cold, etc.) If the clutch does engage:
A) Locate the expansion valve near the windshield tray. Feel the smaller inlet pipe. Is it neutral, warm or hot?
B) Locate the expansion valve near the windshield tray. Feel the larger outlet pipe. Is is neutral, cool or cold?
C) With ac service gauge set attached to ac service ports, engine at idle, what is the:
i) low side pressure
ii) high side pressure
iii) outside air temperature
iv) cockpit vent temperature

3) If the clutch does not engage:
A) Engine off: locate the compressor clutch wire. Unplug clutch wire. At the
'feed side' end of the power supply wire to the compressor, measure voltage coming to wire with ignition in accessory, AC turned on, what is voltage?
i) If you have voltage from the power wire to the clutch, measure the resistance (ohms) of the clutch wire to the compressor body, how many ohms or do you have short or open?
ii) If you do not have voltage from power wire to the clutch, use a 12vdc power source such as a battery charger (you will need to have charger ground to vehicle body) or vehicle's 12vdc source (such as battery jumper junction on firewall) and apply power to compressor's clutch wire. Does the clutch engage?

NC928S4

Start simple.

1) Does the compressor clutch engage when you press the AC button with the sliders not in defrost? Mine didn't and I traced things down to the failed relay in the head unit. Dwayne's procedure solved my problem. Also, make sure both rad fans kick on.

2) Then check for leaks by pumping the system down to see if the vacuum holds.

3) Recharge.

If this fails then consult the Most Exalted WallyP-lama.

griffiths

Follow the logic

captcashew

Thanks everybody. I think that gives me enough to start from. Sounds like it is the relay, but I'll have my mechanic go through all the other options too. I take it back in on Wednesday. After I get a verdict, I'll post back on here the results.

GeorgeM

I wouldn't call an R12 to R134 conversion an "upgrade".

captcashew

Yeah, well, it's cheaper at least.

captcashew

Well, here is the update. Turns out just the AC switch got unplugged somehow. I had picked up the replacement relay from Radio Shack and all the components just to be ready and that was it. Oh, well. Guess that is good.

Thanks for all the feedback everyone. This is still probably just step one. Now we need to find the leak. But I have a lot more resources now and know what to look for with the relay switch in the future.

Tampa 928s

With the dye you used very easy to find the leak. Check with a dye pen the top of your compressor, connections at receiver/dryer and condenser. I would also look at your expansion valve for any traces of the dye. You can get a pen at a good automotive outlet. Easier to see when its darker. If you don't see any dye it could be your evaporator, rare to see that.

SMTCapeCod

S pipe and/or RMB in lieu of A/C repair sounds like a bargain.

captcashew

I picked it up from the mechanic. It blows cool, not quite cold. He said give it a little while, it might be the expansion valve sticking. So give it some time. I have no problem waiting a bit. I also found another write up that said that replacing the heater bypass valve can help. So if more needs to be done, maybe I'll tackle both of those. The previous owner said when he did recharge it, it blew COLD.

What ticks me off more was that he said the AC switch wasn't just disconnected, but jammed way back there. Took him a while to straighten it all out and get it all hooked up again. I had someone replace the stereo head unit a month or so back (just replace 2-3 year old one, so it wasn't like they were replacing the original). So they must have done it. It probably cost me at least another $100-150 in labor to get it all sorted. Ugh. And the new stereo doesn't even sound as good as the old one (I wanted it since it was the same as one in my other car and had a handsfree integration with my phone).

blown 87

I thought the same thing when I read that George.

Alan

Defog (DEF) is full heat and full cooling at the same time BTW - so what you saw was correct behaviour.

I know it seems odd but it does make sense: AC just for dehumidification & heating for better remaining vapor capacity & screen warming. It is not the best defroster setting by far (use defrost, console/door vents closed - max heating - much faster process).

If AC works in DEF then the relay is good and the switch is suspect as you found.

If its still not fully cold you probably have an open or leaking water valve and possibly a bypass comb flap that is not operational. May also not have a full refrigerant charge - that won't help

However - R134a working correctly can be plenty cold... conversions usually less so than purpose designed systems.

Alan