Porsche 928 Climate Control Troubleshooting

I. Basic (no tools required)

a. Move temp slider to minimum temperature - with A/C button "on" compressor should come "on".
Servo unit (above the accel pedal) arm should move to the maximum position (water valve closed
with vacuum - water valve is normally open without vacuum resulting in maximum heat).
Notes:
1. high/low pressure switches must function properly - freon O.K.
2. compressor clutch current from climate control unit (CCU) must be less than 3-4 amps
or the CCU will be damaged.

b. Move temp slider to maximum temperature - water valve should open ( no vacuum) and heat
should be developed. Servo unit arm should move to the opposite maximum position of "a".

c. If either test "a" or the test "b" fails, the problem is not the CCU.
Possible causes for failure; power to CCU, bad servo unit, compressor clutch, water valve,
freon level/switches, wiring, loss of vacuum.

d. Move temp slider to middle setting - servo unit arm should be @ approximate middle of
positions of "a" and "b". Possible causes; open temp sensors (inside/outside), bad servo unit.

Note: The min/max temperature change (no A/C) with the temp slider is a function of the inside
and outside temperatures; i.e. At very cold ambient temps, the min to max temp change may
be 40 - 50 deg. F. At very hot ambient temps, the min to max temp change may only be
15 - 20 deg. F. Also, an "open" outside temp sensor will cause full heat (no vacuum on water valve).

e. With the blower on max., move the diverter lever (lower) to various positions
and the air should be diverted accordingly. Possible causes for failure; bad servo,
no vacuum, vacuum solenoids bad, and/or vacuum actuators bad.

II. Technical (test light and/or voltmeter)

Measure the various CCU pins and check for proper voltages with the CCU connected.
Pin numbers are only applicable to late 928 CCUs (07/09/11 types). Pin numbers on early
CCUs are different as is the wiring. The CCU pins are numbered from left to right as viewed
from the CCU's side with the CCU upright and horizontal (as mounted in the vehicle) with
the front to your right.

Pin Number Measured Value (volts) (Note: Notch between pins 3 & 4)

3 interior lights "on" power ( 12 )
4 ground ( 0 )
5 compressor clutch power "in" (A/C "on") ( 12 )
6 compressor clutch power "out" ( 12 ), to freon switches then to clutch
7 A/C switch power "in" (A/C "on") ( 12 )
10 CCU power "in", ( 12 ), ignition key "on"
11 power "out", ( 12 ), lower slider @ defrost mode
12 power "out", ( 12 ), lower slider @ "OFF", vacuum to water valve from solenoid (12)
13 power "out", ( 12 ), lower slider @ max heat
14 temp sensors input, must indicate a resistance to ground, not an "open"

 

Excellent! Many thanks for posting!

Brad

 

Just what I needed. Thanks.

 

Reading this old thread gives me an option to ask a, perhaps, silly question: what happens with the AC on, does the climate control still rely on the temperature control by the slider or not? My impression is that with AC on the slider control does not work at all: it does not change the speed of the vent fan and it does not control the AC by any other means. Am I right that the sliders and the "automatic" control only work with AC off and only affects the heater? Am I right that the only way to adjust the AC blow is by operating manually on the fan speed **** in its positions from mi to max? My car is a 1988 S4...

 

 

You mean the AC switch should be engaged (clicked) and depending on the position of the slider it should automatically switch on-off depending on the temp/need? I think that being 30 yrs old the clima is actually an air cond and not an "inverter"/climatizer as in modern cars. Sorry for being so "basic"...

 

There are several pages in the WSM that detail what should happen for various positions of the sliders. It's been a while since I went through them in detail.
As far as I know/recall:
The AC switch is more-or-less independent of the temperature control. On '91+ the A/C is automatically switch on if you select recirculation mode or defrost. The latter is true for pre-91 as far as I know. I don't know if the former is true for your '88.
The AC is not automatically switched on or off as a result of sliding the temperature control. That slider controls mixing of hot air from the heater core and cold air from the evaporator along with the operation of the hot water valve from the engine.

 

After looking at this post and searching all over, I have a question.

What is typically the problem if the A/C works brilliantly with the lever in the recirculation location (whether the A/C button us pushed or not), but if you have the lever in the middle (where you control which vents you want) and push the A/C button in the Compressor won't engage and button is lighted...? Is this vacuum related and should dig in there or this a sensor issue mote likely in the tray below he hood where some of the sensors are? 94' GTS

Thanks,
Bryan

 

 

I agree. When it's on the vent choice position and I push in the A/C button it lights up but compressor doesn't engage. when I add to recirculate it turns on an works well.

 

To ensure that observations are correct: How are you verifying that the compressor clutch is engaging?

 

Isn't there only one true way? By looking at it once it engages on the compressor. I realize some people may feel a draw on the idle or car but we are using direct visualization to know for sure.

In addition, when not engaged its not cold air after a minute like it would with it engaged and working properly. You still think it's a Head Unit or could it be one of the sensors in the cowl under the hood? Thanks

 

 

I have a slightly different problem with my car. I'm on my third day of ownership and I wanted to ease into the multiple items that need addressing so I thought I'd start on the A/C system.

With the A/C on, the car blows ambient temp air through the center console only, none of the diverter positions change the air flow. OK, that's going to be one or more of the actuators gone bad I think. Maybe a bad water shutoff valve as well But I do get hotter air when I slide the slider into the red zone. I do see a constant stream of bubbles in the sightglass on the receiver/dryer and about 45 psi on the low side so I know the compressor is doing something. The expansion valve is also ambient temp, never gets cool, never caused condensation to collect on the valve. So I added 1 pound of R134a (PO had it converted many years ago) to see if stopped the bubbles and cooled any better. No improvement. I still get a constant stream of bubbles and the expansion valve is still air temp. Now the low side is up to 50 psi but still no cooling. Strangely the air in the rear A/C unit is slightly cooler than ambient, but not cold.

It seems to me that if the expansion valve never sweats, and I have constant bubbles, there is no meaningful cooling going on in the system so that's where I need to start before I run down the actuator problems. Am I right and what is the next step, replace the expansion valve or receiver/dryer or both? Any suggestions will be appreciated.

 

Since this thread's topic is specific to the HVAC head, I would suggest starting a new thread for your non-head A/C troubleshooting. This will get more eyes/ears/fingers on your problem(s.)

I think I've seen ONE maybe TWO 928s correctly converted to r134. You likely have a bunch of black (r12) OE o-rings still in the system. You may not have r134-specific expansion valves. Your compressor hoses might not be r134 barrier hoses. And, likely - based upon your increasing low-side pressure with more r134 - the refrigerant charge is wet and contaminated with something(s) that isn't (aren't) r134a.

Based upon your description, I would probably nuke it from orbit with new o-rings everywhere, new R/D, new valves, rebuilt/new compressor, flushed components, etc. Or, at least, get the system discharged, flushed, and filled with the proper weight of r134 and oil. That will at least get you a starting point with a known baseline. This of course depends upon which resource you have in excess: your time or your money. If time is scarce start from scratch.

Last, workshop manual specs for pressure assume a warm engine at 2500 rpm. You didn't specify your test conditions. Idle RPM doesn't turn the compressor fast-enough to do much cooling or get the low-side down very much. A good compressor with a clean dry charge should pull the low-side down to low 20s PSI at 2500 rpm.