Communication Problems With Scan Tool

FAULT TRACING INFORMATION

Fault Sources

^ If engine STARTS:
- Low battery voltage.
- Break in lead to DLC.
- Lead to DLC short circuited to ground.
- Defective wiring in Scan Tool (ST).

^ If engine DOES NOT START:
- Break in 15-feed to ECM.
- Break in 30-feed to ECM.
- Break in feed from main relay to ECM.
- Break in control signal from ECM to main relay.

NOTE: If communication problem occurs when engine started, connect battery charger or extra battery.

TESTING PROCEDURE





Start Engine
^ Test START engine.

If engine starts:
- Proceed to: Check Diagnostic Lead Motronic 1.8, section below
If engine does not start:
- Proceed to: Check ECM Power Leads, section below.





Check Diagnostic Lead Motronic 1.8
^ Ignition OFF.
^ Connect an ohmmeter between DLC contact 2 and ground.

Ohmmeter should read approx. 10K ohms.

If value OK:
- Check diagnostic wiring to ST for damage.
If ohmmeter reads approx. 0 ohms:
- Check lead between ECM 55 and DLC contact 2 for short circuit to ground.
If ohmmeter reads Infinite resistance:
- Check lead between ECM 55 and DLC contact 2 for break.





Check ECM Power Leads
^ Ignition OFF.
^ Connect breakout box and check control points.
^ Connect ECM.
^ Ignition ON.
^ Connect a voltmeter between:
- #18 on breakout box and ground.
- #27 on breakout box and ground.
- #37 on breakout box and ground.

In all three cases, voltmeter should read battery voltage.

If values OK:
- Check all signals to and from ECM.
If value differs on #18:
- Check lead between ECM 18 and fuse 1 (11B/1) in central relay unit for break.
If value differs on #27 and #37:
- Proceed to: Check Lead, section below.
If value differs on #37:
- Proceed to: Check Voltage to Main Relay Contact 1, section below.
If all values differ:
- Check main fuse.





Check Main Fuse
^ Check if main fuse 4 (11A/4) in main fuse box behind battery is OK.

If fuse OK:
- Proceed to: Check Fuse, section below.
If fuse not OK:
- Check circuit for short circuit to ground.





Check Fuse
^ Check if fuse 1 (11B/1) in central relay unit is OK.

If fuse OK:
- Proceed to: Check Feed Before Fuse, section below.
If fuse not OK:
- Check circuit for short circuit to ground.





Check Feed Before Fuse
^ Ignition ON.
^ Connect a voltmeter between upper connector on fuse holder for fuse 1 (11B/1) in central relay unit and ground.

Voltmeter should read battery voltage.

If value OK:
- Check lead between fuse 1 and ECM 18 for break.
If value differs:
- Check lead between fuse 1 and main fuse 4 for break.





Check 15-Feed
^ Ignition OFF.
^ Disconnect Fuel Pump relay in fuse box at left side of instrument panel position 2.
^ Ignition ON.
^ Connect a voltmeter between relay base contact 2 and ground.

Voltmeter should read battery voltage.

If value OK:
- Check lead between Fuel Pump relay contact 2 and ECM 27 for break.
If value differs:
- Check ignition lock.





Check Ignition Lock
^ Ignition Off.
^ Disconnect ignition lock connector.
^ Turn ignition lock to position II.
^ Connect an ohmmeter first between contact 15 and contact 30 on Ignition lock and then between contact 15 and the second contact 30 (on Ignition lock).

In both cases, ohmmeter should read 0 ohms.

If values OK:
- Proceed to: Check Voltage to Ignition Lock, section below.
If any value differs:
- Test with a new Ignition lock.





Check Voltage to Ignition Lock
^ Ignition OFF.
^ Ignition lock connector dIsconnected.
^ Connect a voltmeter between Ignition lock connector contact 30 (to battery) and ground.

Voltmeter should read battery voltage.

If value OK:
- Check lead between Ignition lock contact 15 and Fuel Pump relay contact 2 for break.
If value differs:
- Check lead between Ignition lock contact 30 and fuse 1 for break.





Check Voltage to Main Relay Contact 1
^ Ignition OFF.
^ Connect ignition lock connector.
^ Disconnect main relay in central relay unit position C.
^ Ignition ON.
^ Connect a voltmeter between main relay base contact 1 and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Voltage to Main Relay Contact 2, section below.
If value differs:
- Check lead between main relay contact 1 and main fuse 4 for break.





Check Voltage to Main Relay Contact 2
^ Ignition ON.
^ Main relay disconnected.
^ Connect a voltmeter between main relay connector contact 2 and ground.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Control Signal to Main Relay, section below.
If value differs:
- Check lead between main relay contact 2 and main fuse 4 for break.





Check Control Signal to Main Relay
^ Ignition ON.
^ Main relay disconnected.
^ Connect a voltmeter between relay base contact 4 and battery positive pole.

Voltmeter should read battery voltage.

If value OK:
- Proceed to: Check Fuse, section below.
If value differs:
- Check lead between main relay connector contact 4 and ECM 36 for break.





Check Fuse
^ Check if fuse 5 (11B/5) in central relay unit is OK.

If fuse OK:
- Proceed to: Check Voltage From Main Relay, section below.
If fuse not OK:
- Check lead between ECM 37 and fuse 5 for short circuit to ground.





Check Voltage From Main Relay
^ Ignition OFF.
^ Disconnect fuse 5 (11B/5) in central relay unit.
^ Main relay in central relay unit position C connected.
^ Ignition ON.
^ Connect a voltmeter between upper connector on fuse holder and ground.

Voltmeter should read battery voltage.

If value OK:
- Check lead between fuse 5 and ECM 37 for break.
If value differs:
- Check voltage from main relay.





Check Voltage From Main Relay
^ Ignition OFF.
^ Disconnect main relay in central relay unit position C.
^ Ignition ON.
^ Connect a voltmeter between relay base contact 3 and ground.

Voltmeter should read battery voltage.

If value OK:
- Check lead between main relay contact 3 and fuse 5 for break.
If value differs:
- Test with a new main relay.