SPN 789 FMI 2 — Left front wheel speed sensor erratic
Applies to: Bendix · Wabco · EC-60 / EC-80 · Wabco/ZF (common on Volvo/Freightliner)
Left front wheel speed sensor erratic.
Your SPN/FMI rides along, so nearby mechanics see the fault before they quote you.
How serious is it?
Serious. Power derate likely — plan to stop soon.
Most likely causes
- Sensor pushed away from tone wheel
- Wheel bearing slop
- Corroded sensor
- Damaged tone ring
- Sensor pushed out
- Tone ring damage
- Bearing
First moves (roadside)
- Push sensor fully to tone ring (press-fit to bearing)
- Inspect tone ring for missing teeth
- Resistance check sensor (1000-2500 ohm typical)
- Reset sensor to tone ring
- Inspect tone ring
- Resistance check
Common misdiagnoses
- Replacing sensor when it just needs pushing in
FAQ
Left front wheel speed sensor erratic.
Serious — Power derate likely — plan to stop soon.
The most common cause is: Sensor pushed away from tone wheel. Other possibilities include Wheel bearing slop; Corroded sensor; Damaged tone ring; Sensor pushed out; Tone ring damage; Bearing.
Power derate likely — plan to stop soon. When in doubt, get a qualified mobile diesel tech on the truck before continuing.
Find a mechanic or related guides
General diagnostic guidance for heavy-duty diesel engines (Cummins, Detroit, PACCAR, Volvo, Mack). Not a substitute for a scan-tool diagnosis on your specific truck.