Checking the A/C System

FAULT TRACING INFORMATION

General
This fault tracing procedure applies only to that part of the A/C system controlled by the Motronic 1.8. For other fault tracing procedures, see HEATING AND AIR CONDITIONING.

Fault Symptoms:
^ A/C compressor does not start.
^ A/C compressor runs constantly.

Preconditions
^ DTCs absent.
^ Fault permanent.

Fault Sources
^ Fuse 16 (11B/16) in central relay unit faulty.
^ Break in power lead to A/C relay.
^ Break in signal lead between ECM and ECC-ECM (ECC) or heater control (MCC).
^ Signal lead between ECM and ECC-ECM (ECC) or heater control (MCC) short circuited to ground or live.
^ Break in lead between ECM and compressor.
^ Lead between ECM and compressor short circuited to ground or live.
^ Break in signal lead between A/C relay and ECM.

TESTING PROCEDURE





Preliminary Check
^ START engine.
^ A/C OFF.





^ Read off values of parameters A/C CONTROL, A/C RELAY and A/C COMPRESSOR.
^ Switch on A/C.
^ Read off value of parameters A/C CONTROL, A/C RELAY and A/C COMPRESSOR.





^ Compare values with following table to continue fault tracing.





Check Compressor Resistance
^ Ignition OFF.
^ Disconnect compressor single-pin connector. Connect an ohmmeter between engine block and connector (to compressor).

Resistance should read 2 - 5 ohms.

If value OK:
- Proceed to: Check Voltage to High Pressure Switch, section below.
If value differs:
- Thermocontact or connector on compressor defective.
- Fix as necessary.





Check Voltage to High Pressure Switch
^ Disconnect high pressure switch connector.
^ START engine.
^ Switch ON A/C.
^ Connect a voltmeter between contact 1 on high pressure switch connector contact 1 (to A/C relay) and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check High Pressure Switch Resistance, section below.
If value differs:
- Check lead between high pressure switch and A/C relay contact 3 for break.





Check High Pressure Switch Resistance
^ Ignition OFF.
^ High pressure switch connector disconnected.
^ Connect an ohmmeter between both high pressure switch contacts.

Ohmmeter should read approx. 0 ohms.

If value OK:
- Check lead between high pressure switch and pressostat for break.
If value differs:
- Test with a new high pressure switch.





Check Signal From ECC-ECM
^ Disconnect ECC-ECM 30-pin connector.
^ Ignition ON.
^ Connect a voltmeter between ECC-ECM connector contact 9 (to ECM) and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Lead From ECC-ECM, section below.
If value differs:
- Check lead between ECM 41 and ECC-ECM contact 9 for break.





Check Lead From ECC-ECM
^ Disconnect ECM.
^ ECC-ECM disconnected.
^ Ignition ON.
^ Connect a voltmeter between ECC-ECM connector contact 9 (to ECM) and ground.

Voltmeter should read approx. 0 Volts.

If value OK:
- Fault is not in that part of A/C system controlled by Motronic 1.8.
If value differs:
- Check lead between ECM 41 and ECC-ECM contact 9 for short circuit to live.





Check Signal From Heater Control
^ Disconnect heater control 22-pin connector.
^ Ignition ON.
^ Connect a voltmeter between heater control 22-pin connector contact 7 (to ECM) and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Lead From Heater Control, section below.
If value differs:
- Check lead between ECM 41 and heater control contact 7 for break.





Check Lead From Heater Control
^ Disconnect ECM.
^ Heater control 22-pin connector disconnected.
^ Ignition ON.
^ Connect a voltmeter between heater controls 22-pin connector contact 7 (to ECM) and ground.

Voltmeter should read approx. 0 Volts.

If value OK:
- Fault is not in that part of A/C system controlled by Motronic 1.8.
If value differs:
- Check lead between ECM 41 and heater control contact 7 for short circuit to live.





Check Fuse
^ Check if fuse 16 (11B/16) in central relay unit is OK.

If fuse OK:
- Prodeed to: Check Voltage to A/C-Relay, section below.
If fuse not OK:
- Check lead between:
^ fuse 16 and A/C-relay in central relay unit position F contact 1,
^ A/C-relay contact 5 and ECM 40,
^ A/C-relay contact 3 and high pressure switch,
^ high pressure switch and compressor,
for short circuit to ground.





Check Voltage to A/C-Relay
^ Disconnect A/C-relay in central relay unit position F.
^ Ignition ON.
^ Connect a voltmeter between relay base contact 1 and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check A/C-Relay Contact 2, section below.
If value differs:
- Check lead between fuse 16 (11B/16) in central relay unit and A/C-relay contact 1 for break.





Check A/C-Relay Contact 2
^ A/C-relay disconnected.
^ Ignition ON.
^ Connect a voltmeter between relay base contact 2 and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Lead From Pressostat, section below.
If value differs:
- Check lead between relay base contact 1 and 2 for break.





Check Lead From Pressostat
^ A/C-relay disconnected.
^ Ignition ON.
^ Connect a voltmeter between relay base contact 1 and 4.

Voltmeter should read battery voltage.

If value OK:
- Test with a new A/C-relay.
If value differs:
- Proceed to: Check Voltage in Lead, section below.





Check Voltage in Lead
^ A/C-relay disconnected.
^ Ignition ON.
^ Connect a voltmeter between relay base contact 4 and ground.

Voltmeter should read approx. 0 Volts.

If value OK:
- Proceed to: Check Break, section below.
If value differs:
- Proceed to: Check Short Circuit to Live, section below.





Check Short Circuit to Live
^ A/C-relay disconnected.
^ Disconnect pressostat.
^ Connect a voltmeter between relay base contact 4 and ground.

Voltmeter should read approx. 0 Volts.

If value OK:
- Check lead between pressostat and ECM 23 for short circuit to live.
If value differs:
- Check lead between A/C-relay contact 4 and pressostat for short circuit to live.





Check Break
^ Disconnect pressostat connector.
^ A/C-relay disconnected.
^ Ignition ON.
^ Connect a voltmeter between relay base contact 1 and pressostat connector contact 2.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Pressostat, section below.
If value differs:
- Check lead between pressostat and ECM 23 for break.





Check Pressostat
^ Ignition OFF.
^ Pressostat connector disconnected.
^ Connect an ohmmeter between both pressostat contacts.

Ohmmeter should read approx. 0 ohms.

If value OK:
- Check lead between A/C relay contact 4 and pressostat for break.
If value differs:
- Check pressure in A/C system.





Check A/C Relay
^ START engine.
^ Switch ON A/C.
^ Check if compressor STARTS.

If compressor starts:
- Check lead between A/C relay in central relay unit position L contact 5 and ECM 40 for break.
If compressor does not start:
- Test with a new A/C relay.





Check Signal From ECC-ECM / Heater Control
^ Disconnect ECC-ECM 30-pin connector (ECC) or heater control 22-pin connector (MCC).
^ START engine.
^ Read off values of parameters A/C CONTROL, A/C RELAY and A/C COMPRESSOR.

In all three cases value of parameter should read OFF.

If value OK:
- Fault not in that part of A/C-system controlled by Motronic 1.8.
If value differs:
- Check lead between ECC-ECM contact 9 (ECC) or heater control contact 7 (MCC) and ECM 41 for short circuit to ground.





Check A/C-Relay Signal Lead
^ Ignition ON.
^ Disconnect pressostat connector.
^ Read off value of parameter A/C compressor.

Value should read OFF.

If value OK:
- Check lead between pressostat and ECM 23 for short circuit to ground.
If value differs:
- Check lead between A/C-relay contact 4 and pressostat for short circuit to ground.





Check A/C Relay
^ Ignition ON.
^ Disconnect A/C relay in central relay unit position F.
^ Read off value of parameter A/C compressor.

Value should read OFF.

If value OK:
- Proceed to: Check Lead From High Pressure Switch, section below.
If value differs:
- Check lead between A/C relay contact 5 and ECM 40 for short circuit to live.





Check Lead From High Pressure Switch
^ Ignition ON.
^ A/C-relay connected.
^ Disconnect high pressure switch connector.
^ Read off value of parameter A/C-compressor.

Value should read OFF.

If value OK:
- Check lead between high pressure switch and compressor for short circuit to live.
If value differs:
- Proceed to: Check Lead to High Pressure Switch, section below.





Check Lead to High Pressure Switch
^ Ignition ON.
^ High pressure switch connector disconnected.
^ Disconnect A/C-relay in central relay unit position F.
^ Connect a voltmeter between relay base contact 3 and ground.

Voltmeter should read approx. 0 Volts.

If value OK:
- Test with a new A/C-relay.
If value differs:
- Check lead between A/C-relay contact 3 and high pressure switch for short circuit to live.