We have had ongoing problems with the ABS system in our 95 940T wagon with 110K miles. The codes are for the Rear Speed Sensor, but that was replaced with no imrovement. The mechanic checked out the wiring today but could find no problems, so he says it must be the ABS control module. He is not trying to make extra money because his recommendation was to live with it.

The problem we have is that the system activates often at slow stops, and after doing this a few times it shuts itself off. Generally you can't drive more than about 15 minutes around the city before the ABS light comes on. I would like to get this fixed before the winter.

I emailed Victor Rocha who was highly recommened on this board, but he said problems with the ABS Module in a 95 940 are unlikely.

I don't have another mechanic I could use, but I could take it to the Volvo dealer. Maybe they would be more familiar with the specific problems in this vintage car?

If we assume it is not the control module, and the rear sensor is new, would it make sense to replace the whole wire between the two? What would be involved in replacing this?

I'm by no means an ABS expert, but I've got some TSB notes on ABS diagnostics that I thought might help identify potential problem areas from the symptoms you've given.

The closest basic fault types to what you describe are:
Type 5 -Fault occurs sporadically while driving, stays lit until ignition is switched off
Type 6 -Fault occurs during braking, stays lit until ignition is switched off
Type 7 -Fault occurs during braking, stays lit, but goes out before ignition is switched off. This would seem to be the one that matches your symptoms.

For fault type 7 where the ABS system is still functional, then the problem is listed as poor alternator charge. The ABS dash lamp lights when the D+ wire is shorted to ground. As it happens to you when braking, I don't think this applies.

For fault type 7 where the ABS system stops functioning, the problem is listed as an intermittent/poor power supply related fault. It says this can involve: a bad battery, poor alternator output, bad grounds (ABS controller, sensors), faulty wiring (ABS controller), bad fuse in transient protection relay or bad relay itself, and finally a faulty ABS controller. From what I can make of the info and your situation I would be very interested in the transient surge protector relay (I presume that's in the dash relay tray).

If the above checks out okay then here are some general notes. Most ABS problems are at the sensors or in the sensor wiring, especially the front sensors, and not the controller. That the diagnostics pointed to the rear sensor may be a red herring. Obviously the sensors need to be clean and undamaged (check the sensor tips and back mounting). Same for the pulse wheels which should also be checked for warpage. The sensor wiring must not be damaged as the integrity of the shield protects the tiny AC current from RF/EMI interference -that also means keeping all ABS wiring away from ignition wiring and modules such as the coil. Check the sensor wiring in the wheel wells for damage, especially where it goes through the fender. The sender connectors in the wheel wells should be opened, inspected, cleaned (spray contact cleaner) and re-sealed (dielectric grease). Check and clean the ground at the ABS hydraulic unit. Check and clean the ground for the ABS controller (ground rail at left A-pillar, behind kick panel). Your rear sensor wiring has been checked, but you could check it again for damage at the critical points. I don't have the routing handy, but I presume it lives in the left rear harness so I would want to remove the rear seat and check the area where that harness comes out over the wheel well and starts to head up next to the rocker.

Other thoughts:
-Is the speedometer working properly with a steady needle? If so then the basic wiring connections to the rear sensor should be okay.
-When the new rear speed sensor was put in, was the clearance set properly? For a 48-tooth pulse wheel it should be set to .6 mm (min .3 mm, max .8 mm). That appears to be for all later (1989-on) ABS systems, but you'll need to verify that. Some sensors were mounted in aluminum differential cover plates and are not adjustable.
-Are the front tires stock size and are they properly inflated? If they're radically different from stock then the rolling radius could create problems. The ABS systems are known for being more sensitive at slower speeds and this might explain your intermittent problem. There are a number of tire calculators on the internet such as at tirerack to verify that.
-Imcomplete brake bleeding can also cause ABS problems.

Hope something in all that helps you and/or your mechanic.
--
Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day

Thanks for all the details. I may not have explained it well, but our problem is actually like #6 - once the light comes on it stays on until the engine is shut off. I don't think it comes on without braking, which would be #5, but I will have to pay more attention to see. It definately never goes off once it is on.

For fault type 6 where the ABS system sporadically fails and stops functioning during braking and remains lit until the ignition is switched off, the problem areas are listed as:
a) Intermittent sensor related fault
-fault in wiring between control unit and sensor: open circuit, short circuit (weak signal) in sensor wiring
-faulty sensor
b) Intermittent pump related fault
-faulty pump motor relay
-fault in wiring between control unit and hydraulic pump
c) Intermittent valve related fault
-fault in wiring between control unit, valves and valve relay
-faulty valve relay
-faulty valve coil

I would concentrate on a sensor related fault, specifically damaged sensor wiring or a damaged sensor, especially for the front sensors as they are the most exposed. In my recollection that is by far the most common problem area reported here with ABS systems. Refer to my earlier notes for likely fault areas related to sensors and wiring. Although the other problem areas are certainly possible, I don't recall ever seeing those reported here on the brickboard. If you haven't already done so then do some heavy duty searching of the brickboard for all articles related to common ABS faults.
--
Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day

Thanks for the info. Every time I reset the codes the new code that comes on is the same (1-5-5) which points to the rear sensor, which was replaced. The front sensors have been cleaned in the past but that has not helped. From your post it sure sounds like it is in the wiring for the rear sensor but the other strange thing is that I have had no speedometer problems with either the old or new rear sensor which makes me think that both the sensor and wiring are working fine.

If you keep tripping ABS code 1-5-5 which is described in the FAQ as "Rear axle sensor: open circuit or short-circuit to battery voltage" AND if that fault code description is entirely accurate AND if the speedometer is working AND if you've replaced the rear sensor then you may indeed be looking at a wiring fault to the rear sensor as you originally surmised or a faulty controller (I still say that's unlikely).

Please refer to the proper wiring diagram for your vehicle to verify the ABS controller pin numbers described below. Unfortunately my 1995 wiring diagram is elsewhere and I am working off an 1989 diagram.

The basic diagnostic for the rear sensor wiring goes as follows:
-Open the rear sensor wiring connector. The resistance between the wires going to the sensor should be .6-1.6 KOhms.
-At the ABS controller, remove the connector. Remove the connector shell. Only probe the sides of the connector (to avoid damaging the main contacts). The resistance between pins 7 and 9 should be within the same range.

The basic diagnostic for the front sensor wiring is as follows:
-Open a front sensor connector in the wheel well. The resistance between the wires going to the sender should be .9-2.2 KOhms.
-At the ABS controller, remove the connector and connector shell and probe the sides of the connector. The resistance between pins 4 and 6 for the left front sensor and between pins 11 and 21 for the right front sensor should be within the same range.

FYI The wiring from the rear sensor to the ABS controller branches off with wiring to the speedometer at one of the large connector blocks at the left A-pillar (typically buried up by the brake release handle). You'd be looking for a connector that has green-white wires on pin 12 and brown wires on pin 11. One pair of these wires goes to pins 1 and 2 on a connector at the back of the speedometer. The other pair goes to pins 5 and 6 of a six pin connector (also at the left A-pillar) which then carries on to pins 7 and 9 of the main connector on the ABS controller itself.

To carry on with my previous symptomatic diagnosis. With the connector on the ABS controller (but shell removed to permit testing) and with the ignition on (KPII):

To test the pump relay in the hydraulic modulator (aha, that's where they live):
-Connect a wire between pin 28 and ground. The modulator should run (buzz), otherwise the relay is suspect. ***DO NOT hold the modulator on for more than a couple of seconds***

To test the valve relay in the hydraulic modulator:
-Connect a jumper wire between pin 27 and ground.
-You should then read 12 volts between pin 32 and ground, otherwise the relay is suspect.
(actually, a simpler way to do that may be to connect a jumper wire from your meters black probe to chassis ground then use the red probe at pin 32 and the black probe at pin 27)
--
Dave -own 940's, prev 740/240/140/120's & quasi-expert only on a good day