Volvo 780 ABS problems

[BertoneP]

Hi,
I am having some problems with the ABS system on my car.
It is a 780 1989 model, unfortunately without any diagnostic system ( that if fitted is at the front left strut).

First the ABS light refused to go off after cranking the car in my garage.

The car had not been driven for 10 years...... I managed to track down a blown 10 Amp fuse for the overvoltage protection that is located under the steering wheel (the car is a left hand drive). So after fixing the fuse and cranking the car, the light went off.

However when today I took her for a spin, the ABS light went on just after a little while driving. Previously i removed both front wheel sensors, cleaned them and measured them around 1kohm both...so those sensors should be ok. Do not know how to test the rear sensor attached to the diff....

I also located the two pump motor relays near the abs pump in the engine bay, and they seem to work ok as well.....

Now I am without ideas to see what could be the next step to troubleshoot....thankfull for ideas and help

[Laird Scooby]

Does your speedo work? If so, the rear sensor in the diff is working. There is a relay of some sort that links the speedo sensor output to the ABS, can't think what it's called just now but that can cause problems.

However i'd lay bets it's one of the front ones. Might be worth cleaning the tips of them again but also inspect the tone ring/reluctor ring for missing teeth, cracks/breaks etc and general dirt/corrosion.
Any of these can cause a false signal to the sensor as can breaks in the leads to the sensors.

A good way of isolating which sensor is giving the fault (whether it's a false error or real) is to use a 1200 Ohm resistor. Disconnect the sensor plug in the engine bay and fit the resistor with one leg in each side of the plug and refit the other half of the plug so the resistor is making contact. As the fault occurs once you are moving, it suggests there is either no output or a false output from one or both sensors. When the resistor is across the plug of the good sensor, it will hide the output signal so the ABS light will stay off when you get above 10km/h. When across the faulty one, the light will still come up.

Hopefully it's simply a matter of them being dirty again and that sorts it!

[BertoneP]

Hi Dave,

Yes, my speedo is working ok.

Not sure I understand how to do the test with the 1200 ohm resistor....
Will rotate the wheels to see if I have any AC output signal from the sensors.

Do you know if there is a way to get a fault diagnosis result from the system (although my car has no diagnosis port in the engine bay)?

Thanks for your help!!

[Laird Scooby]

Originally Posted by BertoneP

Hi Dave,

Yes, my speedo is working ok.

Not sure I understand how to do the test with the 1200 ohm resistor....
Will rotate the wheels to see if I have any AC output signal from the sensors.

Do you know if there is a way to get a fault diagnosis result from the system (although my car has no diagnosis port in the engine bay)?

Thanks for your help!!

There should be a self-diagnosis function on it, my Rover 827 uses the same Bosch ABS2-E system and that has self-diagnostics but finding how Volvo have wired/configured it is the problem that has eluded me so far.

I'll try to find some pics online of how to use the resistor, it's difficult to describe but has worked successfully in the past to fool the ECU into thinking the sensors are intact, just not giving an output.

It's not a true AC output from the sensors but more of a pulsed DC output, either way your multimeter should show something when you spin the wheels with the meter across the plug terminals of the sensor.

[BertoneP]

I think I have discovered one fault...not sure if there could be more...

I measured the two front sensors.

Left sensor is around 1200 ohms stationary and 0,3-0,5 volts AC when i rotate the wheel by hand....move output voltage reading when spinning faster....

However the right sensor is around 1000 ohms stationary BUT does not give any AC voltage reading when rotating the wheel.

So I guess I have one faulty sensor

It might also be something wrong with the ABS toothed steel ring in the wheel hub, but I guess buying a new sensor is the first step, right?

[Laird Scooby]

Originally Posted by BertoneP

I think I have discovered one fault...not sure if there could be more...

I measured the two front sensors.

Left sensor is around 1200 ohms stationary and 0,3-0,5 volts AC when i rotate the wheel by hand....move output voltage reading when spinning faster....

However the right sensor is around 1000 ohms stationary BUT does not give any AC voltage reading when rotating the wheel.

So I guess I have one faulty sensor

It might also be something wrong with the ABS toothed steel ring in the wheel hub, but I guess buying a new sensor is the first step, right?

Certainly suggests one faulty sensor, ideally they should be in the 1200-1600 Ohms range. Check the ring but i think the sensor will be the only problem.

I had to drill, hammer and even burn mine out on my old 740 because they were well and truly rusted in!
nce out, i used a die grinder to firstly enlarge the hole very slightly, secondly sand it then finally polish it to a mirror finish. Then when i fitted the replacement, i smeared a thick coat of either Vaseline or silicone grease round it to protect it (but not on the tip) and waterproof it.

[BertoneP]

Spot on...the fault was due to a bad sensor!! Installed it with some grease around the sensor housing...the old one was really stuck...but a bit of twisting and hammering did the trick.

Took the car for a ride and the ABS light went off...and stayed off during the entire trip.

I was so happy that I "armed myself with courage" (not sure if this is the correct English saying....) and drove the car to pass the Spanish MOT. And I passed it, so today seems to be my lucky day

BTW a good sensor should give multimeter readings of around 1000-1200 ohms resistance and 0,03-0,05 volts AC when spinning the wheel by hand. My new one was 1020 ohms. The old (but ok) sensor on the opposite front wheel, gives 1150 ohm....

Thanks all for your help.

[tofufi]

Congratulations on getting your 780 on the road - great news!

[Laird Scooby]

Pleased you got it all sorted and my advice helped! Also that it now has the Spanish equivalent of an MoT - very good news!

I know the phrase you mean, can't think of it now but what you've put is close enough to know what you mean so all good!

[BertoneP]

Muchas gracias