ABS sensors, Tranny/engine mounts 100Q

[scott.john.mockry@bangate1.TEK.COM]

Andrew,

Regarding the ABS sensor problem, if you have access to an 
oscilloscope and are familiar with its use, you can connect across 
the sensor with the sensor connector disconnected and view the AC 
signal while you spin the wheel. (You could also jack up all four wheels
 and spin the tires with the engine running  and trans in gear.) The AC 
signal produced looks similar to a sine  wave . The sensor is an 
"inductive"  type sensor which consists of  a permanent magnet 
with wire wrapped around it (coil).  When the square cogs from the 
CV joint pass by they disturb  the magnetic field which induces a 
current in the wire and produces the AC voltage.

The distance the sensor is from the cogs on the CV joint will determine
the amplitude (peak to peak voltage) so you may want to look and see
if one of the sensors has been hit by something to disturb its position.

The later 100's and 200's have a sensor that uses a retaining clip that
 allows you to just push in the sensor. I noticed when I rebuild my front 
end recently, that the sensor can be pushed in a little too far and will 
rub slightly against the  cogs on the CV joint. I assume they are probably
 designed to be pushed in all the way and then the clip withdraws them 
slightly to keep the tip from getting ground down too much.
The older 5000's used a replaceable plastic PVC tip  that set the 
distance correctly. 

I have seen other strange ABS problems that occur if the small 
square cogs on the  CV joint get smashed or damaged, if the CV joint 
was dropped during a repair etc. When these bent or damaged cogs 
rotate by the ABS sensor, the ABS signal  gets distorted enough to 
confuse the ABS controller (brain). In some cases the
damage to the CV joint cogs can be very slight or hardly noticeable. 

A friend at GM ran across a new vehicle that had the wheel hub 
incorrectly machined which allowed the wheel stud  to stick out too
far and get too close to the ABS sensor. This created 5  disturbances 
(DC voltage  shift) that repeated with every wheel rotation. The only 
way he was able to find the problem was to connect up an oscilloscope
to the sensor output  and view the distortion on the signal.

Another problem that  can show up on rare occasions is a  build up 
of metallic particles (brake pad/rotor dust) on the  sensor  which can 
distort  the output signal slightly from the sensor. In extreme conditions,
 If the hub is bent or the wheel bearing is wasted the sensor output 
can be irregular as well.

If you are not familiar with using an oscilloscope, a Digital Multimeter
 can be  used to measure the output voltage (AC voltage) at each 
sensor assuming the wheels are turning at the same speed during 
the test. The frequency and amplitude (voltage) of the
signal increases with increasing wheel speed. You could also have
 an intermittent connection from one of the sensors that is confusing 
the ABS controller. There are some resistance checks across the 
sensor that you can do to verify if the sensor itself is dead and open 
circuit. 
I believe the Bentley manual outlines the resistance values you can expect
to see.

The rear tranny mounts are easily replaced on the 100/200. There is a 
exhaust heat shield  on the passenger side that  cover up the mounting 
bolts.
There are 3 or 4 small sheet metal screws that hold this shield on, one of
them is a little tricky to get to. A 1/4 drive 5/16 or 8mm flex socket 
works well.
There are two small bolts that hold the rear  mounts to the subframe and 
one long bolt that goes through the mount  and into the rear trans mount 
bracket. (wings) Just support the rear section of the tranny with a floor 
jack before you remove the mounts. Those rear mounts can get drenched
over time  with Pentosin/engine oil  and  get mushy.

The passenger side engine mount may need to be replaced, more so on the 
Turbo
I5 engines. That small hose that directs cool air to the mount sometimes 
gets crushed
and the mount will get cooked. Recently when I replaced the subframe 
bushings 
on my 89 200TQ car I went ahead and replaced both engine mounts and found 
that the passenger side engine mount top mounting bolt/rubber  had broken 
loose
completely from the lower portion of the hydraulic mount. PDQSHP pointed 
this problem out 
some time ago to the list, (thanks Scott!) as the dead motor mount can 
place a strain on 
the exhaust manifold to cause  problems (cracks, stud breakage etc.)

HTH 
Scott M.