Vss

[azblazer]

Just a note for those familiar with the 89-91 drac set ups....
Decided to ditch the cable driven speedo in the 79 in favor
of an 89-91 electric speedo....

Before i got to this point...i was driving around my yard watching
the live data on my snap-on scanner as i had a no code chuggle
just off idle...which ended up being an egr valve....but i noted at
low speeds the scanner (snap-on) would show mph....but erratic.
Anything above 10 - 15 mph it timed out at 255 which with ecms
is usually atributed to said ecm either being times out (ie: said fault
has occured 255 times and the fault count is full) or the data recieved
either via data bus or sensor is out of range and it has no idea what to
do with said data and then goes into or uses a default value.

So i jacked up the 79 and used my fluke 88 and test leads to watch the
vss's a/c output...which looked just fine. So i jacked up the chalet and repeated said test as the chalet runs the howell 2 pulse unit...readings between the two units was similar.

So i swapped in the drac...feed both vss wires to the drac...ign-gnd...brown wire to ecm...rewired ic plug and road test....works great...speedo off as to
be expected until i have time to locate some dip switches to solder in to cure said issue....however i noted will driving with conversion done and the scanner...i now see mph clean and steady from the scanners point of view.

So i will assume until the advent of the drac unit in 89/90 the ecm never really saw a clean enough (analog to digital( french to chinese)) signal to do anything with it and or gm changed the parameters in which manner a vss set a code....ie...i have signal....can't do shiz with it...but i have signal so we are just fine.

Just one of those odd things you notice i guess...

DW

 

[dave w]

Good info. Before I got laid off ... I had access to a few different O-Scopes to look at the AC waves of a VSS.

I am wondering? What if someone applied ground to neutral / park switch, would the ECM still go into error?

dave w

 

[dyeager535]

Grounding P/N was what "Painless" had (or still has) people do instead of making VSS work.

My own experience with VSS (documented here in a post I documented, which got heated response from the TBI guys) was that it was absolutely critical to proper operation.

I *think* the TPI setups (at least my vintage, '89) were using a buffer of sorts by then, so that makes sense on the signal quality. '87-89 R/V TBI has trouble codes built in for VSS failure, but I can't recall off the top of my head what is supposed to trigger them.

But...wasn't GM running ABS in the R/V's by say, '89? '90 was the first year for DRAC in the R/V's FWIW. From the literature I've seen, early ABS uses 128,000PPM, which isn't even possible with the '81-89 VSS setups. Perhaps that was only on other vehicles (new body C/K, etc), and the rear wheel ABS in the R/V's at that point was extremely crude.