Volvo 850 ABS- / TRACS system
The 850 model has a history of failing MOT due to the ABS- / TRACS- system. A lit warning light for ABS will in some countries give you a welcome back ticket, even if the ABS-system isn't tested or even required to work ( dep. on manuf. year and country ).
A orange light attracts inspectors as light attracts flies a warm summer night..
The ABS and TRACS system rely basically on the same components in the vehicle and therefore both lights ( if the vehicle is equipped with TRACS ) will come up if there is a failure in one or several of these components.
On several accounts it has been the ABS-unit that fails, but there are some other issues that should be checked out before the ABS-unit is suspected. A faulty Sensor, a broken connecting wire and a faulty Speed-Ring all gives you the same symptoms as a faulty ABS-Unit. Therefore, the components supplying the ABS-Unit with information need to be ruled out.
Sensor-, sensor connecting wire - or Speed Ring- faults are the most common reasons for a lit ABS-Light, NOT a failing ABS-Unit.
If you have access to a OBDII-reader the stored DTC codes can specify a faulty component, or at least tell you which wheel you should focus on. Even though the code readers not always give you the exact or correct info, they are rather accurate when it comes to problems in this area.
The DTC codes can also provide information that point to a broken or missing Speed-Ring.
Regardless what measuring method you use, you only test the sensor and/or the wiring. The ABS-/Speed- Rings on the CV-Joint isn't measured.The condition of the Speed Rings is a vital part of the ABS- TRACS- system. If a Speed Ring is missing, displaced, cracked or rusted the sensor cannot accurately read the speed.
Faulty Speed Rings can be replaced, but require that the CV-Joint is removed.
If the rings after inspection seems to be in good physical condition, clean the gaps to remove all dirt that could cause a faulty reading.
ABS Socket (A3)
| 1-1-1 | No DTC set |
| 1-2-1 | Left front wheel sensor, faulty signal below 40 km/h |
| 1-2-2 | Right front wheel sensor, faulty signal below 40 km/h |
| 1-2-3 | Left rear wheel sensor, faulty signal below 40 km/h |
| 1-2-4 | Right rear wheel sensor, faulty signal below 40 km/h |
| 1-2-5 | Signal faulty from at least one wheel sensor for a long period |
| 1-3-5 | Control Module faulty |
| 1-4-1 | Faulty pedal sensor, shorted to ground or supply |
| 1-4-2 | Faulty brake pedal switch, open or short circuit |
| 1-4-3 | Control Module faulty |
| 1-4-4 | Brake discs overheated ( TRACS equipped vehicles only ) |
| 2-1-1 | Left front wheel sensor, no signal when moving from standstill |
| 2-1-2 | Right front wheel sensor, no signal when moving from standstill |
| 2-1-3 | Left rear wheel sensor, no signal when moving from standstill |
| 2-1-4 | Right rear wheel sensor, no signal when moving from standstill |
| 2-2-1 | Left front wheel sensor, ABS operation signal missing |
| 2-2-2 | Right front wheel sensor, ABS operation signal missing |
| 2-2-3 | Left rear wheel sensor, ABS operation signal missing |
| 2-2-4 | Right rear wheel sensor, ABS operation signal missing |
| 3-1-1 | Left front wheel sensor, open/short circuit |
| 3-1-2 | Right front wheel sensor. open/short circuit |
| 3-1-3 | Left rear wheel sensor, open/short circuit |
| 3-1-4 | Right rear wheel sensor, open/short circuit |
| 3-2-1 | Left front wheel sensor, irregular over 40 km/h |
| 3-2-2 | Right front wheel sensor, irregular over 40 km/h |
| 3-2-3 | Left rear wheel sensor, irregular over 40 km/h |
| 3-2-4 | Right rear wheel sensor, irregular over 40 km/h |
| 4-1-1 | Left front wheel inlet valve, open/short circuit |
| 4-1-2 | Left front wheel return valve, open/short circuit |
| 4-1-3 | Right front wheel inlet valve, open/short circuit |
| 4-1-4 | Right front wheel return valve, open/short circuit |
| 4-2-1 | Rear wheel inlet valve circuit, open/short |
| 4-2-2 | Rear wheel return valve circuit, open/short |
| 4-2-3 | Traction control system valve circuit, open/short ( TRACS equipped vehicles only ) |
| 4-2-4 | Pressure switch for TRACS, faulty or short circuit ( TRACS equipped vehicles only ) |
| 4-4-1 | Control module faulty |
| 4-4-2 | Pump pressure low |
| 4-4-3 | Pump motor, electrical or mechanical failure |
| 4-4-4 | No power supply to valves in hydraulic unit |
The reading from each sensor should be in the limits of 900Ω - 1380Ω.
Left Front Sensor
|
pins 11 & 4
|
Right Front Sensor
|
pins 18 & 3
|
Left Rear Sensor
|
pins 10 & 2
|
Right Rear Sensor
|
pins 17 & 1
|
DISCLAIMER
If you are inexperienced working on projects like those mentioned on this blog, especially but not limited to
electrical or mechanical projects,please remember to read all safety precautions and use safety equipment.
Also remember that during the work, it is very possible to cause more harm to the vehicle than it initially had.
You might also void your vehicle insurance, create a hazardous condition, harm or even kill yourself or others.
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