ABS Pressure Modulator Valve Codes SPN 795-800: Which Axle, What to Check
Updated: 2026-11-13
What the pressure modulator valve does
SPN 795, 796, 797, 798, 799 and 800 are the ABS pressure modulator valves, one per wheel on a three-axle layout. The valve sits on the frame near the wheel, in the air line between the brake valve and the brake chamber. Inside are two solenoids: the inlet one holds the pressure, the outlet one releases it. On command from the ABS unit the modulator holds, releases or builds up pressure so the wheel does not lock. The ABS unit energises the coil and checks the circuit, so nearly all of these codes are electrical, not pneumatic.
Which SPN is which valve
| SPN | Valve | What is special about this position | FMIs listed |
|---|---|---|---|
| 795 | Axle 1, left (front) | Front axle, steered. ABS here lets you keep steering while braking hard. | 3, 4, 5, 6, 13 |
| 796 | Axle 1, right (front) | Same axle as 795. The harness flexes with every steering movement and suspension stroke, and the connector on the kerb side takes spray from the tyre. | 5, 6, 3, 4, 7 |
| 797 | Axle 2, left (usually the driven axle) | Second axle, usually driven. | 3, 4, 5, 6, 13 |
| 798 | Axle 2, right (usually the driven axle) | Same axle as 797. It belongs to the tractor's own axle: a trailer cannot cause it, because trailer faults are reported through the trailer ABS unit. | 5 (the most searched), 6, 3, 4, 7 |
| 799 | Axle 3, left | Third axle of a multi-axle tractor or trailer, often a lift or steering axle. | 5, 6, 3, 4, 7 |
| 800 | Axle 3, right | Same axle as 799. The cause is often shared between both sides: the same harness, ground or supply. | 5, 6, 3, 4, 7 |
FMI and what they mean
| FMI | Meaning | Factory wording and what it points to |
|---|---|---|
| 3 | Voltage above normal / shorted high | Solenoid shorted to voltage (Bendix, Meritor "ShortUb", WABCO "Shorted to UBATT"). Renault Trucks: inlet and outlet valves, short circuit. |
| 4 | Voltage below normal / shorted to ground | Solenoid shorted to ground (Bendix, Meritor "ShortGnd"). Renault Trucks: short to ground. |
| 5 | Current below normal / open circuit | The commonest entry. Open common terminal or open solenoid (Bendix), "Interruption" (Meritor), "Open Circuit" (WABCO). Renault Trucks words it as a short on one of the supply wires. Usual finding: a broken wire near the axle, corroded connector pins or an open coil. |
| 6 | Current above normal / grounded circuit | Chafed wire rubbed through against the chassis or suspension, water in the connector, shorted turns. WABCO words it as "Shorted to Ground". |
| 7 | Mechanical, valve not responding | The valve sticks although the circuit is healthy. |
| 12 | Faulty device | Renault Trucks: circuit pneumatic fault or module internal fault. Check the module inlet and outlet for air leaks, the air lines for contamination and kinks, the exhaust at the valve and at the module with a PC, and the minimum pressure switches. |
| 13 | Calibration / configuration | Bendix "Configuration Error"; Meritor "OverEquipped". |
Renault Trucks also lists FMI 2 for the front and axle 2 valves: the ABS valve does not meet requirements, so check that the correct valve is fitted.
What it should read
- One coil, 24 V valve: 10.7 to 25 ohm, usually about 15 ohm.
- Both coils in series (between the two coil terminals): 21.4 to 50 ohm.
- Same for a 12 V valve: 4.0 to 9.2 ohm per coil and 8.0 to 18.4 ohm for both.
- Coil to body: infinity.
Checks that apply to every valve
- Read all ABS codes. If several axles are listed at once, test the supply voltage and ground of the ABS unit before the valves.
- Switch the ignition on. During its check the ABS unit clicks the valves one after another, audible at each wheel. A valve that stays silent points to its circuit.
- FMI 3, 4, 5, 6: unplug the valve connector and measure each coil against the common terminal. Zero means a short, megohms means an open circuit.
- If the coils are in spec, check the wire from the valve to the ABS unit for continuity while flexing the harness. The valve connector on the frame sits in dirt and salt. Clean and dry the contacts.
- If the valve bleeds air constantly, suspect dirt or water under the diaphragm and check the air dryer.
- Run the valve test in diagnostics: the chamber pressure should fall and rise on command. Check the air supply to the valve.
- Replace the modulator last.
Common causes, most to least frequent
- Valve connector on the frame: oxidation, water. After tyre or brake work, look for a connector that was not refitted or a wire trapped under a bracket.
- The wire to the valve, especially where it flexes on the front axle.
- The valve coil. An open wire or connector is far more common than a failed coil, so test resistance first.
- Dirt and water in the valve from a failed air dryer.
Can I keep driving?
All six codes give the same basic verdict: drive with caution, and you have enough time to reach the yard. The service brakes work and normal braking pressure still reaches the chamber, but ABS on that wheel is off. 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. Let the air system build to working pressure and listen for leaks. A faulty ABS can fail a roadworthiness test. The detail differs by axle:
- SPN 795, 796 (front): a front wheel can lock on a wet or icy road and you can no longer steer around an obstacle. Avoid motorway speeds and long trips until it is repaired, and repair it sooner than a rear axle code.
- SPN 797, 798 (axle 2): a locked drive wheel on a loaded truck can make the rear swing out. Keep speed down, drive smoothly and repair soon.
- SPN 799, 800 (axle 3): ABS on that wheel is disabled and it may lock on a slippery road. Brake early and gently, keep a larger gap, and get the plug and wiring fixed.
FAQ
Is a front axle ABS code more serious than a rear axle code?
In terms of handling, yes. A locked front wheel means loss of steering, so repair SPN 795 or 796 sooner.
What is the difference between SPN 797 and SPN 798?
The same modulator on the two sides of axle 2: 797 is left, 798 is right. Check both if both codes appear.
Does SPN 799 FMI 5 mean the modulator is dead?
Not necessarily. An open wire or connector is far more common than a failed coil. Test the resistance first.
Could the other side of axle 3 fail as well?
Yes, because the cause is often shared: the same harness, ground or supply. Read both SPN 799 and 800 before you replace anything.
Is the pressure in the brake chamber affected?
During an ABS event the modulator can no longer hold or release pressure, but normal braking pressure still reaches the chamber.
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