ABS System Troubleshooting

[Jimvr4]

My ABS light has been intermittant for a while now so I'm going through the shop manual and trying to diagnose the problem with the system.

Symptoms: Nothing other than the ABS light comes on after putting the car in gear and driving.

Troubleshooting using the service manual section 35-27. Note the 1995 VR4 ABS system fault codes cannot be read without a MUT III tester. Therefore the troubleshooting will focus on verifying the components and running component tests:

1) Verified the ABS warning light does function. It flashes a few times after the key is turned ON which is normal.
http://jns.jimnshar.com/217d9d00.png

2) Check the wheel speed sensor resistance values. For this test I pulled the passenger side rear panel and pulled the ABS ECU so I could tap into the connector and run the tests.:

http://jns.jimnshar.com/21480a90.jpg

Using the figure on 35-40

http://jns.jimnshar.com/2157c3e0.png

Front Left (pins 14/15) reads 1.05K
Front Right (pins 33/31) reads 1.06K
Rear Left (pins 11/10) reads 1.06K
Rear Right (pins 30/28) reads 1.06K

Standard value is 0.8 - 1.2K so all of these are normal and all wheel speed sensors are good

3) Valve Relay test using the figure on page 35-46

http://jns.jimnshar.com/216a8ba0.png

The resistance across terminals 2 and 27 reads 105 ohms. This is in the standard range so the Valve Relay is good.

[Jimvr4]

4) Motor Relay test using the figure on 35-47

http://jns.jimnshar.com/2189a780.png

The resistance across terminals 2 and 26 reads 83 ohms. This is in the standard range so the Motor Relay is good.

5) Wheel speed sensor output voltage measurement using page 35-75 of the manual. This one was kind of fun. First you have to put the car up on four jackstands and then measure the voltages at the connector while the car is in idle.

http://jns.jimnshar.com/21cf0480.png

I used a DVM set for AC voltage. All readings were taken with the car idling at about 800-900 rpm. I got readings of 1.0Vrms on the Front Left and Front Right while the Rear Left read 0.45Vrms and the Rear Right read 0.6Vrms. All readings were healthy.

6) G-sensor test using page 35-117

http://jns.jimnshar.com/219d41c0.png

My sensor measures open circuit in either position so my G sensor is faulty. Unfortunately these are no longer available new. I managed to get one from Ground Zero Performance.

http://jns.jimnshar.com/21be0180.jpg

The replacement G-sensor tested fine so I installed it and plugged in the ABS ECU. Took the car for a 50 mile round trip and the failure did not appear so it seems my ABS system is now functional once again. Looking at the manual it appears possible to bypass the G-sensor function by unplugging the G-sensor harness and connecting a jumper between pins 1 and 2 on the harness. On level ground this will function the same as having an operational G-sensor installed however at a 30 degree uphill incline it will not function properly since continuity will be maintained.

ABS System Disable

The ABS system is easily disabled by pulling the ABS fuse in the engine bay but the ABS light will continue to be lit even if the ABS ECU is removed. The light can be extinguished by making two connections at the ABS ECU connector. First connect pins 2 and 18, this provides power to the valve relay. Next connect pins 9 and 27, this connects the valve relay to ground. These connections cause the valve relay coil to become energized which extinguishes the ABS warning light. Of course this method requires a functioning valve relay. Alternatively you could pull the cluster and remove the light bulb. Either method will prevent the ABS warning light from displaying the normal sequence when turning the key to ON.

The wire connections can easily be made by cutting the terminals from a small fuse and soldering them to the ends of a short wire:

http://jns.jimnshar.com/21e80a90.jpg