1989 740 ABS light

jeff c · Post by **jeff c** » 19 Jun 2010, 11:56

Question on a project car.

The ABS light in my 740 turbo comes on after reaching @ 5 miles per hour. The rear brakes and calipers are new. The front brakes appear to be in good condition except the drivers side drags a little, but the wheel can still be turned by hand when jacked up. (It is not totally frozen)

I pulled the front sensors and cleaned them off, but the light still goes on. It shuts off when the car is turned off and does not come on until I am driving forward and step on the brake.

What are the next steps to solve the problem?

This is a car I am rehabbing for the fun of it.

Thank you very much.

Post by **billofdurham** » 20 Jun 2010, 12:25

Welcome to Matthew's Volvo Site.

You cleaned the sensors but did you clean the tone wheel? This is on the hub and is what the sensor monitors to feed info to the ABS control unit.

You should try to get the dragging brake sorted out as it could be confusing the ABS by not rotating at the same speed as the other wheels during braking.

Bill.

jeff c · Post by **jeff c** » 20 Jun 2010, 16:05

A little more information -

I shot brake cleaner onto the gear looking item between the rotor and the dust shield and then rotated the hub to make sure I got all the way around it. Is that what you mean?

Also - I have not done anything to the sending unit for the rear wheels.

I backed the car out of the garage today. The light did not come on. It came on as I approached 5 miles per hour driving forward back into the garage.

Thanks for the help.

lummert · Post by **lummert** » 21 Jun 2010, 08:49

The tone rings get loaded up with rust from the brakes. When my ABS on my 88 760 Turbo Wagon acted up I cleaned the notches of the tone rings with a small screwdriver and a file. I have had no more ABS malfunctions for about a year.

Post by **billofdurham** » 21 Jun 2010, 15:21

And that is the only sure way to do it. Any build up can play havoc with the system. The rear tone wheel is mounted in the diff so doesn't usually need attention.

However, ABS does not turn on until the vehicle is travelling at 12mph or more and it will turn off at 12mph if you have been travelling at a higher speed. Your light coming on at 5 mph would appear to be caused by a malfunction of one of the sensors or the ABS control unit itself.

If you require it I have the test procedures for ABS on the 740.

Bill.

lummert · Post by **lummert** » 21 Jun 2010, 16:45

I forgot to mention that the ABS can have symptoms that don't make sense. When my ABS malfunctioned, the left front brake would pulsate. If the ABS light was on the brakes would work fine, but, if the ABS light was off the brakes would act up. I disabled the ABS for almost 4 years before getting around to checking for a problem.

Post by **billofdurham** » 22 Jun 2010, 12:33

Most drivers when confronted by an ABS light immediately think the brakes are going to fail. Not so, it is only the ABS that has a problem. The brakes are fine. You are reverting to what all cars had in my youth, brakes that stopped the car by pressing the brake pedal with no fancy electronic add-ons to go wrong. If the roads were icy or slippery in any way we used cadence braking, that is pumping the brake pedal, giving a human form of ABS.

Bill.

jeff c · Post by **jeff c** » 24 Jun 2010, 17:06

Well - I am still working on it. I ran the test proceedures and found out that the wire to the brake switch was off and I had two bad bulbs. Unfortunately the light still comes on.

Funny that when I back out of the garage, the light does not come on, but when I drive back in, it does. Do the sensors not work in reverse?

I guess my next step is to pull the wheels and clean the notches. I cannot find where the sensor wires come back into the car and connect. I should be able to see them, but I cannot.

Thanks for your help.

Post by **billofdurham** » 25 Jun 2010, 12:14

ABS doesn't work in reverse unless you can exceed 12mph going backwards.

The wiring will go to the ABS ECU which is shown on the attached. There is also a wiring diagram for the ABS. They may help.

740 ABS components and wiring.pdf: (498.32 KiB) Downloaded 732 times

Bill.

jeff c · Post by **jeff c** » 26 Jun 2010, 06:41

Problem solved!

After looking for the fifth or sixth time, I found the connections for the sensors under the hood. When I checked them with my ohm meter, the passenger's side did not register while the driver side did. I picked up a sensor from a donor car and bingo - the light did not come on.

This was the first time I had used an ohm meter to check wiring. It was not as hard as I thought it would be when I first read in my repair manual the proceedure to check the ABS.

Thanks to everyone for their help.