1. Vehicle speed: Diagnostics

WARNING: This page is about a different car, the 2001 Saab 9-3. However, it is still accessible from the selected car via links, so may be relevant.

The ABS has extremely good diagnostics for its wheel sensors. This means that with a high degree of certainty the wheel speed signals will be present on the control module's four outputs if no diagnostic trouble codes have been generated in the system.

Neither the main instrument unit nor any of the other bus systems have speed signal diagnostics for the LH rear wheel. If the speedometer is working, however, it is certain that speed information is present on the bus and consequently available to all systems connected to the bus.

If the speedometer is not working and the speed reading obtained on one of the bus systems is always 0, and there are no diagnostic trouble codes in the ABS, then the fault is probably in the lead between the ABS and MIU.

Trionic conducts diagnosis of the front right wheel speed signal (wire from ABS) but it may require a long journey without a speed signal before a diagnostic trouble code is generated.

With Trionic, the two speed signals used in the car can be read on the diagnostic tool

Fig 1: Identifying Bus Function Chains (Vehicle Speed - 1 Of 2))

Courtesy of SAAB-SCANIA OF AMERICA, INC.

Fig 2: Identifying Bus Function Chains (Vehicle Speed - 2 Of 2))

Courtesy of SAAB-SCANIA OF AMERICA, INC.