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 28 faulty.
^ Break in power lead to power stage.
^ Break in lead between ECM and ECC-ECM.
^ Lead between ECM and ECC-ECM short circuited to ground or live.
^ Break in lead between power stage and compressor.
^ Lead between power stage and compressor short circuited to ground or live.
^ Break in signal lead between power stage and ECM.
^ Signal lead between power stage and ECM short circuited to ground or live.
^ Defective power stage, pressostat, high pressure switch, or compressor connector.





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:
- Check lead between connector at left A-post, contact 32 and compressor for break.
If value differs:
- Thermocontact or connector on compressor defective.
- Fix as necessary.





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 28 is OK.

If fuse OK:
- Proceed to: Check Voltage to Power Stage, section below.
If fuse not OK:
- Check lead between:
^ power stage contact 1 and fuse 28,
^ power stage contact 2 and pressostat,
^ pressostat and high pressure switch,
^ high pressure switch and compressor,
^ ECM 40 and compressor,
for short circuit to ground.





Check Voltage to Power Stage
^ Disconnect power stage 6-pin connector.
^ Ignition ON.
^ Connect a voltmeter between power stage 8-pin connector contact 1 (to battery) and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Lead From ECM, section below.
If value differs:
- Check lead between power stage contact 1 and fuse 28 for break.





Check Lead From ECM
^ Power stage 6-pin connector disconnected.
^ Ignition ON.
^ Connect a voltmeter between power stage 6-pin connector contact 1 and 5.

Voltmeter should read battery voltage.

If value OK:
- Test with a new power stage.
If value differs:
- Proceed to: Check Voltage in Lead, section below.





Check Voltage in Lead
^ Power stage 6-pin connector disconnected.
^ Ignition ON.
^ Connect a voltmeter between power stage 6-pin connector contact 5 and ground.

Voltmeter should read 0 Volts.

If value OK:
- Check lead between power stage contact 5 and ECM 23 for break.
If value differs:
- Check lead between power stage contact 5 and ECM 23 for short circuit to live.





Check Voltage From Power Stage
^ START engine.
^ Switch ON A/C.
^ Power stage 6-pin connector connected.
^ Connect a voltmeter between power stage 6-pin connector contact 2 and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Voltage at Pressostat, section below.
If value differs:
- Test with a new power stage.





Check Voltage at Pressostat
^ Disconnect pressostat connector.
^ START engine.
^ Switch ON A/C.
^ Connect a voltmeter between pressostat connector contact I (to power stage) and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Pressostat Resistance, section below.
If value differs:
- Check lead between power stage contact 2 and pressostat for break.





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

Ohmmeter should read approx. 0 ohms.

If value OK:
- Proceed to: Check Voltage to High Pressure Switch, section below.
If value differs:
- Check pressure in A/C-system.





Check Voltage to High Pressure Switch
^ Disconnect high pressure switch connector.
^ Connect pressostat connector.
^ START engine.
^ Switch ON A/C.
^ Connect a voltmeter between high pressure switch connector contact 1 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 pressostat and high pressure switch 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 ECM contact 40 for break.
If value differs:
- Test with a new high pressure switch.





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 Power Stage Signal Lead
^ Ignition ON.
^ Disconnect connector B at left A-post.
^ Read off value of parameter A/C-compressor.

Value should read OFF.

If value OK:
- Check lead between connector B at left A-post and ECM 23 for short circuit to ground.
If value differs:
- Check lead between power stage contact 5 and connector B at left A-post for short circuit to ground.





Check Power Stage
^ Ignition ON.
^ Disconnect power stage 6-pin connector.
^ Read off value of parameter A/C-compressor.

Value should read OFF.

If value OK:
- Test with a new power stage.
If value differs:
- Proceed to: Check Lead to Pressostat, section below.





Check Lead to Pressostat
^ Ignition ON.
^ Power stage 6-pin connector connected.
^ Disconnect pressostat connector.
^ Read off value of parameter A/C-compressor.

Value should read OFF.

If value OK:
- Check lead between power stage contact 2 and pressostat 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.
^ Pressostat connector connected.
^ Disconnect high pressure switch connector.
^ Read off value of parameter A/C compressor.

Value should read OFF.

If value OK:
- Check lead between pressostat and high pressure switch for short circuit to live.
If value differs:
- Check lead between high pressure switch and ECM 40 as well as between ECM 40 and compressor for short circuit to live.