A fault finding process to check electrical system of your A/C.

Go onto 928 Specialists web site and download their fuse and relay chart for your car (1987 S4)
 

1. Disconnect negative terminal from battery
2. Remove relay IV (Ignition Relay X)
   Check continuity of terminal 30 and 87 by supplying 12 volts to terminal 85 and connect 86 to earth and whilst having an ohm meter across terminal 30 and 87 and see whether you have a circuit.
   Remove 12 volt supply and you should have an open circuit as the relay solenoid coil is de-energized.
   If no circuit replace relay
   If OK proceed to next step.
3. Check fuse 6 A/C fuse (5 amp) (visual inspection, ohm meter or via fuse tester if fitted to Central Electrical Panel - this will require the battery to be connected). If fuse is OK then proceed to next step.
4. Reconnect battery
5. Turn on ignition and this should connect 30 bus to the A/C Control Switch.
6. Turn on A/C switch.
7. Disconnect twin spade connector from Thermo Switch Evaporator Protection and connect a volt meter between spade connector on the twin wire (violet wire with green strip) and earth. You should get around 12 volt depending on the state of charge of the battery.
   
   If you don't get 12 volt then you have a continuity problem between fuse and A/C control switch, the A/C on/off switch and Thermo Switch.
8. Check plug T29 which is a 4 terminal plug (unfortunately my 1987 wiring diagrams do not have the location field, however my 1990 S4 it is indicated location as 8cN) which should be behind the passenger side panel of the central consol). Remove side panel by removing two Phillips headed screws and you should locate the 4 terminal plug at the front end near the A/C foot well air outlet.
   
   Disconnect plug and check that all terminals are clean and bright and all wires are connected to the terminals. The positive wire is via terminal 2 of plug T29 which should also be a violet with a yellow strip wire on the A/C control switch side of the plug joining to the violet with white stripe wire to the thermo switch side.
   Disconnect plug and check whether you have 12 volts from the control switch side of the plug to earth via a volt meter. If you have then the A/C control switch circuit, including the A/C on/off switch is OK and the fault is in the circuit from the plug to the thermo switch.
   
   To check the continuity of this wire, connect the ohm meter to the spade connector of the wire at the Thermo Switch and earth the other end to earth. If you don't get an infinity reading then you have a short in this wire (set ohm meter to maximum setting and with an open circuit you should get a reading of 1). When you connect the ohm meter to the spade and the other end to earth the reading should remain 1, if you get a lesser reading you have a short circuit. Connect the ohm meter to terminal 2 of plug T29 and the other end to the spade connector and you should get a lower ohm reading which means that the continuity of the wiring is OK
   
   You will need to track this wire between the terminal plug T29 and the thermo switch connection to find the earth or fracture. You may find the fault where it passes through the fire wall or any other area where the wire may have been rubbing against a hard circuit or if it has overheated and burnt through the insulation and is shorting or the wire could be broken.
   
   If the fault has not been located here with a 12 volt supply reading then the next step is:
    
9. Remove the drivers side panel from the Central Control Console and remove the plug connector from the A/C on /off switch and by using a ohm meter check the operation of the switch by connecting the ohm meter between terminal 8 & 9 ( the red and white stripped wire terminal and the black wire with the blue stripe terminal). With the ohm meter connected push the switch on and off and confirm that you have a circuit when the switch is on.
   
   If you still do not have 12 volts present then proceed to next step.
    
10. Locate terminal 7 on A/C control switch and check that you have 12 volt supply. Check this with the volt meter connected between terminal 7 and earth. If 12 volt is not present then check for shorting or continuity of this wire via the ohm meter as previously described.
    
    If there is still no 12 volt supply then move to the next step.
     
11. Locate terminal 10 on A/C control switch and determine whether you have a 12 volt supply. If you have 12 volt supply then there is most probably a fault within the A/C control switch circuitry or in the wiring between fuse 6 and terminal 10 via plug T30.
     
12. Check wiring between fuse, plug T30, terminal 1 and the from plug terminal 1 to terminal 10 on the control switch. T30 plug is a 6 terminal plug and should be located adjacent to T29 plug on the passenger side of the central console. Disconnect T30 plug and check all terminal plugs are clean and bright and all wires are connected.
    
    Using the volt/ohm meter between terminal 1 of T30 plug (Black wire with white stripe) on the fuse side of the plug check for 12 volts, continuity test and short circuit test. If loss of continuity or short circuit indication then check wiring for burning, insulation wear, cracking etc. Repair fault/s.
13. If the compressor clutch is still not engaging then the circuits from the inlet to the thermo switch to the compressor clutch are faulty.
    
    If 12 volts is now available to the spade connector (the spade with two wires) reconnect to thermo switch, remove the other spade connector and check whether there is 12 volts from this connector to earth if 12 volts present then switch is OK, if not thermo switch is faulty.
     
14. If switch is OK, then move to the High Pressure, Low Pressure Switch located on the passenger side of the car just forward of the A/C condenser and liquid receiver. Disconnect the plug from the bottom of the switch and check for 12 volt supply via volt meter to both terminals connectors within the plug. If no 12 volts is located then there could be a fault within the wiring between the thermo switch and the pressure switch. Fault finding via visual, continuity or short circuit testing to be undertaken. If no fault is located then run a temporary wire from single spade connector to pressure switch and then determine whether compressor clutch operates with ignition on and A/C switch on.. This will indicate that there is a fault in the wiring.
     
15. If no fault located and clutch is still not engaging then with temporary wiring still connected check outlet terminal connector for 12 volts. If 12 volts present then pressure switch is OK and would indicate fault maybe between pressure switch and compressor clutch. This wire is a black wire via terminal 2 of plug T18 and then via a connector just before the compressor clutch. Disconnect T18 and check plugs etc as described above and check that you have 12 volts etc.
     
16. If no faults found and 12 volts present you have the wiring and the last plug before the compressor clutch to check. Undertake checks as described above and if 12 volts is available at last plug then the compressor clutch maybe at fault.
     
17. To check clutch operation, use the temporary wire as mentioned above and connect to terminal plug to clutch. Ensure that you do not connect to the earth wire of the clutch and the clutch should operate. If it does not then the clutch is at fault and may need replacing.
    
    Initially this was to be a short process. Hope it will help.
    
    Tails 1990 928 S4 auto