No Communication With Engine Management System

Fault symptom
Car fails to start and no Intelligent Saab Tester (ISAT) scan tool communication with VSS.

Condition
Communication between VSS and the engine management system absent or faulty.

Diagnostic help
- If diagnostic trouble codes P1640 or P1641 are stored in the engine management system they cannot be read because there may be a short circuit to ground/B+ or an open circuit in the data link (K lead).
- On starting, when VSS and the engine management system exchange alarm codes, communication takes place between pin 21 of the VSS electronic control module and pin 33 of the Trionic electronic control module or pin 55 of the Motronic electronic control module.







Diagnostic procedure
1 Check ISAT scan tool communication with VSS
- Connect an ISAT scan tool.
- Select 'THEFT ALARM".
Does the ISAT scan tool display a THEFT ALARM submenu?
[YES] Continue with point 2.
[NO] Check via the LED that the alarm is not immobilized. If the LED flashes twice, disarm and repeat "Check VSS communication". Otherwise, continue as described in the "fault diagnosis, data link connector" section under "Anti-theft alarm, fault diagnosis without diagnostic trouble codes".







2 Check ISAT scan tool communication with the engine management system
- Unplug the VSS electronic control module.
- Connect a jumper lead between pins 19 and 21 of the VSS electronic control module connector.
- Select 'TRIONIC" or "MOTRONIC", depending on the engine management system in question.
Does the ISAT scan tool show the submenu for the selected engine management system?
[YES] The K lead is OK. Continue with point 3.
[NO] Proceed to point 4.

3 Check for diagnostic trouble codes in the engine management system
- Select "READ FAULT CODES".
Is diagnostic trouble code P1640 or P1641 stored?
[YES] Continue fault diagnosis as described for the stored diagnostic trouble code
[NO] Proceed to point 5.








4 Check the K lead
Check the wiring harness for continuity between pin 21 of the VSS electronic control module and pin 33 of the Trionic electronic control module or pin 55 of the Motronic electronic control module.
Is the wiring OK?
[YES] Continue fault diagnosis on the engine management system as described in Engine Management System.
[NO] Rectify or replace the wiring (including connectors).

5 Final test
Start the car and check to see if the fault symptom persists.
Does the trouble persist?
[YES] Continue as described in "Before Changing The Electronic Control Module".
[NO] The remedial measures taken were correct or the fault is of intermittent nature.