ABS Wheel Speed Sensor Codes SPN 789-794: Which Wheel, What It Means
Updated: 2026-11-14
What these sensors are
SPN 789, 790, 791, 792, 793 and 794 are the ABS wheel speed sensors, one per wheel on a three-axle layout. Each is an inductive, two-wire sensor. It has no supply: it generates an AC voltage itself when the toothed ABS (tone) ring turns past it, and the signal grows with wheel speed. If the signal is missing, weak or erratic, or the wiring is open or shorted, the ABS/EBS unit sets the code for that wheel. The SPN number tells you which wheel to look at.
Which SPN is which wheel
| SPN | Wheel | Where the sensor sits and what to watch | Wheel number in Knorr-Bremse EBS / MAN wording |
|---|---|---|---|
| 789 | Axle 1, left (front) | In the steering knuckle. The cable runs to the frame in a loop that flexes at every steering turn, so it breaks at the bend by the knuckle. | Wheel 1 |
| 790 | Axle 1, right (front) | Same as 789, on the right side. | Wheel 2 |
| 791 | Axle 2, left (usually the driven axle) | In a holder on the axle beam behind the brake mechanism. The cable runs along the beam to the frame and is clamped by straps, most often by the spring. | Wheel 3 |
| 792 | Axle 2, right (usually the driven axle) | Same as 791, on the right side. | Wheel 4 |
| 793 | Axle 3, left | At the hub of the third axle. On many vehicles this is a lift or tag axle, and the cable between the beam and the frame tightens at every lift. | Wheel 5 |
| 794 | Axle 3, right | Same as 793, on the right side. | Wheel 6 |
Common FMI and what they mean here
| FMI | Meaning | What the factory wording points to |
|---|---|---|
| 1 | Data below normal | Sensor air gap too large (Bendix "Excessive Air Gap", Meritor "AirGap Failure", WABCO "Air Gap"). |
| 2 | Erratic or intermittent | Unstable signal: air gap, dirty or damaged tone ring, wheel bearing play. Renault Trucks words it as open or short circuit and gives the sensor resistance as 950-1930 ohm. |
| 3, 4, 5, 6 | Voltage or current out of range | Short to supply (3), short to ground (4), open circuit (5), current above normal or grounded circuit (6). Usually a broken cable or connector. |
| 7 | Mechanical, not responding | Large air gap, damaged tone wheel or ring periodicity error (WABCO "Incorrect Tone Ring"). Knorr-Bremse and MAN word FMI 7 as "short" and FMI 4 as "no signal". |
| 8, 10 | Abnormal frequency; abnormal rate of change | Erratic signal, or signal lost while driving. Renault Trucks adds: check that there is no residual pressure in the brake chambers and no mechanical binding in the brakes. |
| 11 | Root cause not known | Meritor "Plausibility Check"; WABCO "Abnormal Speed (Chatter)". |
| 13, 14 | Calibration; manufacturer-specific | Bendix: tire size calibration (13) and output low at drive-off (14). Renault Trucks FMI 13: wheel size mismatch above 30 percent; check the tyre size, drive a few kilometres, and change the tyre size setting if the fault persists. Meritor FMI 14: run-out failure. |
For SPN 789 to 792, Renault Trucks also lists FMI 12, an ABS unit internal fault, with the repair "replace the ABS unit". It is the only code in this family that points at the unit, not the wheel end.
What it should read
- Sensor resistance, WABCO: 0.9 to 1.3 kOhm. Knorr-Bremse: about 1.7 kOhm. A sensor of another make has different resistance, so do not mix them.
- Sensor-to-ring gap: not more than 0.7 mm.
- AC voltage with the wheel turning at 3 km/h: not less than 100 mV.
Checks that apply to every wheel
- Disconnect the sensor connector at the ABS unit and measure the resistance together with the extension cable. Compare it with the sensor on the other side of the same axle, the best reference.
- Open or short (FMI 3, 4, 5, 6): if the reading is out of spec, remove the extension and measure directly at the sensor. If the sensor is in spec, the fault is in the cable.
- Weak or erratic signal (FMI 1, 2, 7, 8, 10): push the sensor home against the ring. It sets the right gap on the first turn of the wheel. Check the hub play.
- Turn the wheel and look at the ring for broken teeth, dirt, rust and run-out. Look at the hub seal too: oil from the hub settles on the sensor and the ring and weakens the signal.
- When you replace the sensor, replace its clamping bush and grease with it.
- Measure the AC output while spinning the wheel. It should be steady and rise with speed.
Common causes, most to least frequent
- The sensor has moved away from the ring: old bush, hub play, a repair done without pushing the sensor home.
- The cable at its weak point. On the front axle (SPN 789, 790) that is the bend by the steering knuckle: turn the steering to the stop both ways and see whether the cable pulls tight or rubs on the knuckle or track rod, or watch the resistance while turning lock to lock. On axle 2 (SPN 791, 792) it is the axle beam and the spring, where the straps clamp the cable. On axle 3 (SPN 793, 794) it is the section between the beam and the frame: raise and lower the lift axle and watch whether the cable goes taut.
- Dirty or damaged toothed ring.
- The sensor itself.
- A sensor of another make.
Can I keep driving?
For all six codes, SPN 789 to 794: drive with caution; you have enough time to reach the yard. ABS on that wheel or axle may be off, but the service brakes work. On a slippery road brake gently and keep a longer following distance. If the red brake lamp comes on or the pedal feels soft, stop. The manufacturer flags this code as critical, so if in doubt, stop and check on the spot.
- SPN 789 and 790 (front axle): a locked front wheel on a slippery surface means no steering. Drive slowly and repair soon.
In all cases, let the air system build to working pressure, listen for leaks, and do not drive with faulty brakes.
FAQ
Is SPN 790 the left or the right wheel?
Right. SPN 790 is the front axle, right-hand wheel sensor. The left front wheel of the same axle is SPN 789.
Why does the code come back after cleaning the sensor?
The cable may be broken inside the insulation, most often at the steering knuckle on the front axle, or the gap may be wrong. Test the resistance while turning the wheel or the steering.
Can a different make of sensor cause the fault?
Yes. WABCO and Knorr-Bremse sensors have different resistance, 0.9 to 1.3 kOhm against about 1.7 kOhm, so a sensor of the wrong make can set the code.
Which wheels are SPN 793 and SPN 794?
The third axle, left (793) and right (794). On many vehicles this axle is a lift or tag axle, and the cable between the beam and the frame tightens at every lift.
Should I check the other side of the axle?
Yes. Compare the resistance with the sensor on the other side of the same axle. It is the best reference.
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