Code Details
| Display code | WABCO RSSplus SID 4 FMI 7 |
|---|---|
| SPN | Not applicable or not verified |
| FMI | 7 |
| OEM code | WABCO / ZF SID 4 FMI 7 |
| Manufacturer | WABCO / ZF |
| System | Trailer ABS / RSSplus |
| Component | Wheel Sensor d |
| Source address | Unknown or not applicable |
| Severity | high |
| Review status | ai source checked |
| Source confidence | high |
| Last reviewed | 2026-04-09 |
Plain-English Meaning
The signal coming from sensor D has a characteristic that is not consistent with a clean, evenly-spaced tone ring. This is not a wiring fault — the circuit is intact and the sensor is powered — but the pulse pattern suggests a mechanical issue at the wheel end. Damaged, bent, or unevenly spaced tone ring teeth are the most common cause.
WABCO MM-0888 FMI 7 differentiates between wiring faults (FMI 3/4/5/6) and mechanical signal quality issues. The ECU evaluates pulse spacing and amplitude consistency across a rotation. An irregular spacing pattern that exceeds the ECU's tolerance triggers FMI 7. This can also occur if the wrong tone ring tooth count is installed for the ECU's calibration.
Common Symptoms
- Trailer ABS lamp on or blinking — may be intermittent and related to road conditions
- Irregular pulse pattern visible in WABCO PC diagnostics for wheel D at consistent wheel speeds
- Code may be less likely to set at very low speed where one irregular pulse is less distinguishable
Possible Causes
Possible causes may include the items below. The list is not a parts diagnosis.
- Tone ring with missing, bent, or chipped teeth — most common cause of FMI 7
- Tone ring installed with the wrong tooth count for the ECU configuration
- Tone ring with heavy rust buildup that has effectively reduced or bridged tooth spacing
- Incorrect tone ring installed during a hub or bearing replacement
First Checks
- Inspect the tone ring at wheel position D by rotating the wheel slowly and examining each tooth — missing, bent, or corroded teeth are visually identifiable.
- Check the tone ring tooth count against the WABCO RSSplus configuration for this trailer — a mismatch after a wheel-end repair is worth confirming.
- Inspect the sensor tip for debris or buildup that could create an irregular air gap as the ring rotates.
- If the tone ring and sensor are undamaged and the tooth count is correct, use WABCO PC diagnostics to monitor the sensor output at slow wheel rotation to confirm irregular pulse spacing.
Can I Keep Driving?
Trailer ABS and brake-related codes should be handled conservatively. Stop safely for brake warnings, red stop lamps, abnormal braking, wheel-end concerns, or any severe warning condition, and follow fleet or OEM guidance.
Related Lookup Pages
Sources
- WABCO RSSplus Trailer ABS Maintenance Manual MM-0888 WABCO / ZF Commercial Vehicle Solutions · oem · accessed 2026-05-05 · confidence high
Source: WABCO / ZF Commercial Vehicle Solutions, WABCO RSSplus Trailer ABS Maintenance Manual MM-0888. This page paraphrases factual fields only and is not a substitute for the original document.
Open source - TOOLBOX PLUS Diagnostic Software ZF Commercial Vehicle Solutions · oem · accessed 2026-05-05 · confidence medium
Source: ZF Commercial Vehicle Solutions, TOOLBOX PLUS Diagnostic Software. This page paraphrases factual fields only and is not a substitute for the original document.
Open source
FAQ
Can rock or road debris damage a tone ring enough to cause WABCO RSSplus SID 4 FMI 7?
Yes. Tone rings on trailer axles are exposed to road debris, and a direct impact can chip or bend a tooth. The damage does not need to affect multiple teeth — a single damaged tooth creates one irregular pulse per revolution, which the ECU detects as a periodic irregularity. This is typically a straightforward inspection item.
Does WABCO RSSplus SID 4 FMI 7 mean the tone ring needs to be replaced?
If a damaged tooth is confirmed, the tone ring should be replaced. Minor surface rust is usually not sufficient to cause FMI 7 unless the rust is heavy enough to substantially reduce the effective tooth height. A clean tone ring with no visible tooth damage that still sets FMI 7 may indicate a configuration mismatch rather than physical damage.
Can WABCO RSSplus SID 4 FMI 7 appear after a wheel bearing replacement?
Yes, if the replacement used a hub or bearing assembly with a different tone ring tooth count. If FMI 7 appeared immediately after wheel-end service, verify that the replacement parts match the tooth count in the trailer's WABCO system configuration.