VSS affecting ABS ...?

[m***@gmail.com]

hello all,

vehicle -- 1998 S10 4x4 automatic trans.

I was driving along at about 50 mph when the speedo went to zero.
About 3 seconds after that, the ABS light came on. Another 3 seconds
after that, the trans. kicks down to first gear...

I made an about-face, and headed home. About 20 miles later, the SES
light came on.

when I got home, the code came back as the Vehicle Speed Sensor.
supposedly, the cascade of events is that : once the computer saw
"zero" as the speed, but the ABS wheel speed sensor saw 50 mph, it
decided that the discrepancy meant the ABS was out. *then*, the
computer put the truck into "limp home" mode, restricting the speed
(since it thought I had no brakes) by only giving me first gear.

I changed out the VSS, and the speedo works fine, the truck has all 4
gears back, and the brakes work fine. The SES light is out, but the
ABS light comes on as soon as I start rolling.

does the ABS have it's own, separate/independent computer ? and if
so, am I experiencing a leftover code ? can I clear it by
disconnecting the battery overnight ?

any comments appreciated...

[Steve Mackie]

Post by m***@gmail.com
any comments appreciated...

None of that sounds right to me. If the ABS was in error, it would disable
it. The ABS controller is typically separate from the PCM and relies on it's
own wheel speed sensors and not the VSS. Not sure about the trucks though.
Do they use the VSS for rear wheel speed reference?

I haven't heard of a "limp" mode where the trans is stuck in 1st. This would
likely be a result of the PCM detecting a VSS fault. Limps modes are usually
related to engine performance.

With all those things happening at once, first thing I would do is have the
battery tested. Check the battery connections while you're there.

Steve

[m***@gmail.com]

Post by Steve Mackie
None of that sounds right to me. If the ABS was in error, it would disable
it. The ABS controller is typically separate from the PCM and relies on it's
own wheel speed sensors and not the VSS. Not sure about the trucks though.
Do they use the VSS for rear wheel speed reference?

I think so (the factory service manual is severely lacking in
explaining how to diagnose this), the VSS was on the tail of the
tranny, so I reckon the driveshaft revolutions equate to rear wheel
speed...

Post by Steve Mackie
With all those things happening at once, first thing I would do is have the
battery tested. Check the battery connections while you're there.

will do -- I'll report back.

[Shep]

Get it scanned on an ABS function scanner and look for a bad wheel speed
sensor. Hello Steve, Shep.

Post by m***@gmail.com
hello all,
vehicle -- 1998 S10 4x4 automatic trans.
I was driving along at about 50 mph when the speedo went to zero.
About 3 seconds after that, the ABS light came on. Another 3 seconds
after that, the trans. kicks down to first gear...
I made an about-face, and headed home. About 20 miles later, the SES
light came on.
when I got home, the code came back as the Vehicle Speed Sensor.
supposedly, the cascade of events is that : once the computer saw
"zero" as the speed, but the ABS wheel speed sensor saw 50 mph, it
decided that the discrepancy meant the ABS was out. *then*, the
computer put the truck into "limp home" mode, restricting the speed
(since it thought I had no brakes) by only giving me first gear.
I changed out the VSS, and the speedo works fine, the truck has all 4
gears back, and the brakes work fine. The SES light is out, but the
ABS light comes on as soon as I start rolling.
does the ABS have it's own, separate/independent computer ? and if
so, am I experiencing a leftover code ? can I clear it by
disconnecting the battery overnight ?
any comments appreciated...

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[Steve Mackie]

Post by Shep
Get it scanned on an ABS function scanner and look for a bad wheel speed
sensor. Hello Steve, Shep.

Long time no see! He should still have his battery checked. ;)

[Pouta]

I got everything checked... the only thing that comes back is that the
ABS unit itself is bad.

I find this hard to believe, since it would be a monumental
coincidence that the ABS unit went bad at the same time my VSS went
bad.

screw it, I don't need ABS anyhow, the truck runs/stops fine... just
wish I could knock out the ABS light in the cluster !