Re: [No EFI] ABS Help Needed

[Shannen Durphey <shannen@grolen.com>]

Mike Turner wrote:
> 
> I will check the tach filter somehow. I'll have to think about how would
> be a good method short of buying another one. I do have several scopes
> so I might just scope the line but since I've never looked at it before
> it might look a little strange. 

Look for a signal similar to an ignition primary side signal.  You'll
want to connect the scope to the wheelspeed sensor lines to look for
spikes.  I found the problem with the sensor disconnected, and the
scope connected to the vehicle harness.

>From your suggestion I take it that the
> tach signal also goes to the abs unit. 
Yes.

>Or I could infer that the noise
> was somehow being coupled into one wheel sensor's lines on the unit you
> were working with. 
Well, not in the same way that plug wires will do.  It was definitely
coming through the ABS controller.


> The trouble codes you mentioned are from the abs unit?
I mentioned trouble codes?  The only way to get brake system trouble
codes in your car is to jumper 2 terminals in the ALDL connector. 
These cause the ABS controller to flash the ABS light, similar to the
check engine light.

> I thought the only thing on the aldl connector was the ecm. 
Fuel pump, TCC, ABS, SIR, BCM are typically connected to the ALDL.

> I'm going to
> spend some quality time with my vette schematic tomorrow hunting
> connections to the abs. 
Look for the ALDL connection, too, : )

I believe the symptom appears more than once per
> ignition cycle. I also do not believe it's the self test cycle but I'd
> like more info on it since I don't know if it is designed to occur
> slowing down, speeding up, or what. 
Self test is designed to occur when the vehicle first reaches 7mph (or
maybe 8, can't remember now).  I've never tried to reach the initial 8
on a decel, couldn't tell you what would happen.  The self test will
delay starting if the brake pedal is depressed when the car reaches
test speed.

My knowledge of this abs unit is
> limited to thinking I know what the target wheel slip and threshold
> wheel
> slip numbers are (I paid attention during a talk given to NCCC by the
> head
> of GM Corvette engineering one night). 

You know more about that than I do.  I don't believe I've ever seen
those numbers in GM literature.

My wife has driven this car on
> the
> street and competitively since it was new from the dealer and has never
> seen this happen before. I noticed it after a hot autocross run as I was
> lightly applying the brakes on the return road back to the pits. She
> experienced it driving home from Asheville to Greenville after she
> reached
> Greenville (>60miles) in a light rain shower. I'd appreciate any info
> about these or any abs units.

This is not a light request. : )  Maybe someone can post the address
or phone number for GM training materials.  That would provide a good
place to start.
Shannen
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