Weird ABS issue on '95 850 turbo sedan

My ABS light has been coming on and going off at random time while driving.
Once it comes on it stays on till the next time the car is started and the
system does it's test at 12 to 15 mph. Then it goes off after the test...
and it might not come on for several days.
Anyway the Volvo place tested the code and it was a 3/4, they said that was
the right rear wheel sensor.
Fine, that's easy to replace. I haven't got the part yet, am waiting...

The WEIRD thing is this... lately (sometimes) when stopping the car the
brake pedal will feel solid with the car slowing down nicely... but at
about 15mph during the slowdown the brake pedal will all of the sudden give
an extra 1/2 inch or so down. Not exactly 'mushy' but real weird. The ABS
light will come on at the exact second this happens. (as opposed to the
random times indicated above.) After this 'slip' in the pedal the car stops
fine.
And one time, and only one time, the brake pedal gave it's little 1/2 inch
extra at a stop sign and there was a growling sound from near the brake
pedal and I could feel the grinding/growling sound in the pedal as a
vibration. That seems to not be related to a RR wheel sensor!!

Any ideas, appreciated!

Just so you'll know... the battery/alternator system is 100% and my tires
are at the right inflation.

 

USA
Can you sort of flesh out why you feel certain it's the ECU? I know where I
can get a used one for $75, so I'm very curious.
By the way, the Volvo tech that read the code (I'm not doubting what you
say. but,) is very expereinced, etc and typically good even though I don't
use the dealer for repairs unless forced to. He said the code was 3/4, rr
sensor... now I'm curious what to make of it all.

 

ABS ECU is shagged - nothing to do with the right rear sensor. What
country are you in?

Sean

 

The rr sensor error is typicl of the abs ecu error being faulty...the
abs ecu is a known weak point....perhaps the extra travel happens as
the ABS system turns off and activates a valve somewhere?

 

Its a common fault on the 850s caused almost certainly by overheating
of the ECU. Its always the right rear sensor.

Have a look at this URL

http://home.earthlink.net/~vicrocha/Volvo_Parts/Volvo_ABS_feedback.htm

Sean

 

Looks like the repair is probably pretty straightforward for anyone with
electronics troubleshooting skills, likely cracked solder joints.

 

I would have thought so as well. However the guy with the URL I posted
says he also uses larger wattage components and extra heatsinks for
the repairs. So there may be a bit more to it than that...

Sean

 

The ABS computer monitors the resistance in the circuit to the sensors
among other things. If the circuit shows abnormal resistance, then the
code for that sensor is set. If the break in the monitoring circuit
actually occurs inside the computer, then the sensor code is still set,
but changing the sensors won't help. And the rear sensors come as a
pair, with a long harness that routes over the fuel tank, so changing
then is not a simple job.

 

The ABS computer monitors the resistance in the circuit to the sensors
among other things. If the circuit shows abnormal resistance, then the
code for that sensor is set. If the break in the monitoring circuit
actually occurs inside the computer, then the sensor code is still set,
but changing the sensors won't help. And the rear sensors come as a
pair, with a long harness that routes over the fuel tank, so changing
them is not a simple job.

 

Well as I said, some troubleshooting skills are required, perhaps there's
some power resistors that overheat, or underrated transistors that can
benefit from added heatsinking. Hot spots will often show as badly darkened
circuit boards in the areas that get hot, if the components have standard
markings you can go to www.nteinc.com and look at their crossreference to
get datasheets for compatible parts. If anyone has a dead unit lying around
I'd be glad to take a look at it.

 

Maybe a brake fluid flush could help. If you haven't done that ever, it is
time. Fluid absorbs water over time and can rust components from the inside
out. Not to worry you too much, but Volvo does recommend a flush every 3
years I think. Make sure whoever does it that they can handle bleeding ABS
cars.

 

I would bet big $$$ that the ABS control unit has gone bad. This is a well
known weak point of that vintage Volvo. I just replaced the one on my '96
950.

The error code readout is almost always wrong.

I've read that what happens is that the metal conductor lines on the
internal circuit board develop small hairline cracks which open and close
internal connections.

There is a guy on the 'net offering repair and rebuild services. Search
google for Volvo 850 ABS light and you should find it.

I replaced my car's controller with a new one from Volvo ... almost $500 for
the part. Ouch.

Personally I think that Volvo shipped a substandard product in this regard
and should be replacing them for free, but that has not happened.

John

 

Do you still have the bad one? I'd be happy to pop it open and investigate,
if I can fix it I'll do the first one for whatever return shipping costs.
Cracked solder joints are a very common problem with electronics in general,
particularly those that have hot running components.

 

Do you still have the bad one? I'd be happy to pop it open and
investigate,

Unfortunately the $480 part price at the dealer for the replacement part was
an "exchange" price and would have been much higher if I wanted to keep the
old one. I suspect that Volvo may be doing that to keep cores out of the
repair/rebuild market in order to keep their monopoly on the part. The
replacement was clearly of new manufacture and not a rebuilt unit.

John