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).
    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"

by Lorenfb
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.