940T 1998 ABS lamp goes on and off

V850B5254T · Post by **V850B5254T** » 13 Feb 2012, 06:39

Car specification

Volvo 945
B230FT, High pressure turbo (165HK)
Manual gear
No aircon
European car (Denmark)
1998
278.000 km

NOTE
Though the car is build for the European marked I believe that the ABS system is the same as is used in Northern America.
These European market 940 do not have an OBD system that allow ANY reader to read out errors except the original Volvo reader.

Description of problem;
In my wife's 1998 945T the ABS lamp began to turn on and off for some time ago.
Since we have snow on some roads by now the effect of the lamp has been tested: ON = no ABS, OFF = ABS works.
During the changing outside temperature it became evident that the lamp was more prone to be on in 'hot' weather than in 'cold'.
The reason for the ' is that the problem has accelerated, - that is, it has to be colder and colder for the problem (the turning on of the ABS lamp) to stay off.

I have measured the resistance of the sensors in the engine compartment and read a low value (~ 1 Ohm).
The battery has been disconnected for 30 minuts to reset all systems, but the ABS lamp still comes on as soon as it is reconnected and the ignition is turned on.
This leads me to believe that the sensors and the sensor rings in the front wheels and the differential are not the problem (as the car did not have to move to provoke the error).

I have read that the ABS electronics in the 850 series have a problem with the soldering of the connector, but I can't find any information about similar problems with the 940 series.
If the ABS control box have to come apart and be re-soldered I can do it myself, but before splitting it apart I would like to know if it is necessary.

Any suggestions are welcome.

/oakey

Post by **jimmy57** » 13 Feb 2012, 07:24

The initial test done when system powers up checks the wheel speed sensors for continuity and also the power supply relay.
The pump relay and sensor signal production (part of test that would detect sensor ring issues) occurs at 6-8 kmh.
I am assuming the ABS system used on 98 MY was the same Bosch supplied system used on s90/v90 that year in USA. I can't imagine it would not be.
The power supply relay is the smaller of the two under the cover on pump assembly.
With the key on you can back probe the wheel sensor circuits using a voltmeter. Good circuits will have approximately 2 volts DC on one wire and 0 on the other. If there is an open circuit due to a faulty wire to a sensor or a failed sensor you will see 5 V on the signal wire and 0 on the ground (earth) wire.
The wiring diagrams on this site under "repair database" tab will not have a 98 wiring diagram listed but the 1995 940 or 1998 s90/v90 would work. The wiring diagram may not give you info as to which wire is earth and which is signal for each sensor but probing both for each will give you the results you need. Backprobe with a slender probe fashioned from a straightened paper clip if your probes with your meter will not reach the terminals from back side.
Generally the rear sensor being the problem will also make the speedometer have problems.
The leads to the front wheels can break inside in the insulation and temperature does affect their failure.

V850B5254T · Post by **V850B5254T** » 14 Feb 2012, 03:19

Thanks Jimmy,
Its snowing right now here in Denmark and I haven't got a closed garage.
Thus it might take a couple of days before your suggestions can be put into actual work.
As soon as I have news I'll post it here.

I love this forum
/oakey

V850B5254T · Post by **V850B5254T** » 15 Feb 2012, 04:50

I promised to come back with news.
The car is in for inspection (MOT or whatever it is called in the North Americas).
This called for the local car shop to be involved, - new brake disks, pads, - etc.
I told my mechanic to look for obvious trouble i connection with the ABS system.
He has called back to tell me that though the impedance measurement of the front sensors didn't indicate an error there, the sensors where compressed by rust from the surrounding metal and after removing this rust, the ABS error disappeared.
The sensors will be exchanged with new ones.
Pictures would have been nice, but by the time my mechanic called me he had already removed the rust.

When the car is ready and has been driven for some time, I will return with the my experience.

/oakey

V850B5254T · Post by **V850B5254T** » 20 Feb 2012, 02:30

Latest about ABS problems.
Yesterday the ABS sensors for the front wheels were exchanged with new ones.
The relays in the pump assembly was pulled out , disassembled, inspected and tested, - no problem to be found.
No corrosion was found on the relay pins or in the pump assembly.
I looked for the ABS control module, - according to the '1995_940_wiring_diagram.pdf ', - it should be placed in front of the driver door, but it was no where to be found... a change that must have been introduces between 1995 (the document) and 1998 (this car).
Does anybody know where to find this module in a 1998 945T?
I must admit that the voltage test that jimmi57 proposed hasn't been done yet, but it will.

The road worthiness test was due this morning and on during the drive to the test station the ABS lamp turned on after 10 minutes of driving

.
However, - during the test and on the way home, the lamp was off

.
I will continue to look for the cause of the problem, - it is after all my beloved wife's car.
To be continued....

/oakey