Re: 164 ABS Fixed!, Head Removal Next?

[Talfieri61@domain.elided]

Hello All,

At John Wiltshire's encouragement, I am posting some additional details 
related to the ABS fix I shared a couple weeks ago.  

When I first began looking into my ABS problem, I went through the entire 
troubleshooting procedure without finding a single fault.  The checks in the 
procedure simply cannot diagnose weak or mismatched magnetic coupling of the 
wheel sensors; resitance measurements alone do not indicate how the sensor 
will perform in the ABS system.  Based on my experience with these types of 
circuits, although the electronics are quite reliable, adequate tolerances 
and signal conditioning are not designed into the system as a whole to make 
it reliable over time.  The design is too dependent on the mechanical 
features of the system which are subject to significant change resulting from 
wear and weathering.  The good news, of course is that it is repairable.

John asked about using a regular analog oscilloscope for the tests.  Yes, you 
can use an analog scope to see the signal; it will just be a bit more 
difficult because of the fact that an analog scope does not capture and hold 
a waveform like a digital one.  First, it is important to set up the test and 
connect your scope properly.  You will have to adjust the timebase down quite 
a bit, to about 20 ms per division; the volts per division may be set to 
about 50 mV per division.

Remove the connector from the ABS ECU (N27) and measure the signals right at 
the connector.  Connect your scope's ground to the car's chassis ground 
somewhere.  I clipped a ground strap to the latch post on the door pillar.  
Then, connect the ground lead of your scope probe to one end of the sensor 
you are testing, and the probe tip to the other end of the sensor.  No doubt 
that if you've gone this far you have the pinout information, but I'll 
summarize it here:

LF Sensor - N27 pins 4 and 5
RF Sensor - N27 pins 21 and 23
LR Sensor - N27 pins 7 and 9
RR Sensor - N27 pins 24 and 26

So, for example, to test the left front sensor, connect the ground lead to 
pin 4, and the probe tip to pin 5.  Again, make sure that the scope ground is 
common to the car chassis, otherwise extraneous noise can make the small 
signal difficult to  distinguish.

If your car is like mine, the stronger signals are from the front sensors.   
To establish a baseline signal, raise only one of your front wheels off the 
ground, start the car, put the transmission in first gear (or low, if 
automatic), and let it idle.  After you have connected your scope to the 
corresponding connector pins, observe the scope, and make a note of both the 
peak-to-peak signal amplitude and the signal period at engine idle.  Do the 
same with the other front wheel and corresponding sensor.  The important 
thing to understand is that the signal period corresponds directly to the 
speed of the wheel.  The signal amplitude of the two front wheels should be 
well within 10% of each other at the same period.  If I remember correctly, 
mine were about 1V peak-to-peak at idle (I have an automatic, by the way), 
but I don't remember the period.

To get a signal from the rear wheels, raise the wheel and spin it fast enough 
to obtain the same signal period as you observed with each of the front 
wheels at idle.  Hopefully, you can spin it fast enough with your hand to 
obtain this period.  Preferably, have some else do the spinning while you are 
observing your scope.  With the signal at the baseline period, note the 
amplitude.  Do the same for the other wheel.  I suspect you will find one or 
both of the rear wheel sensors significantly lower than the front sensors.  
Mine were about one third that of the front.

Once you have made these measurements, you can determine which sensor(s) need 
to be modified to obtain matched signal amplitudes.  The goal is to obtain 
close to the same signal amplitude on all four wheels at the baseline period. 
 The basic principle to keep in mind is that the closer the sensor is to its 
phonic wheel, the stronger the signal will be.  You can use either of the 
techniques I detailed in my original e-mail to adjust this gap.  By far, the 
easier method is to add material to the tip of the weakest sensor(s). 

As I discussed in my message about the ABS fix, I am likelly to soon begin 
the task of removing the heads and have then redone, mainly because of the 
probable crashed valves of the right head.  I read Brad's project 164 article 
for head removal but would still like to hear from others who may have 
tackled this job before.  Things to look out for, what parts to buy, about 
how much I can expect to pay, and what enhancements can be economically done 
... that sort of thing.

Any suggestions welcome!

Tom Alfieri
Electronics Technician
1991 164L
Albuquerque, NM

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