Speedometer Quits, Brake Light Flashes

[Oriley]

Just read through 6 pages in search and I'm getting tense.

89 Chev 350 4x4
Rear wheel anti-lock brakes
Deluxe Cluster-Gas

When driving in rain or even a light mist when the road isn't even wet, suddenly the speedometer quits, cruise doesn't work and the brake light flashes. Didn't notice if the odometer still works. Truck drives just fine.

To correct this I give the top of the dash a whack a few times and the speedometer/cruise come back.

To stop the brake light flashing I have to remove the two wiring harnesses from that box under the master cylinder, plug back in and the light goes out. Brakes work fine at all times.

Any ideas what this could be and how I could fix it? I sure don't want to take the dash off, scares me to death.

Thanks,

Oriley

[crabtruck]

89 had ABS on 4x4"s, thought it was couple years later for them.
Anyways, all those symptoms you have share the speed signal info from tranny,
but it is most likely either a bad (loose) ground connection or hot to the speed buffer.
Which I think, is located about where you're hitting.
Turn yourself upside down under dash with flashlight and
do a visual and push/pull inspection for loose connections.

[Oriley]

Whilst looking for a picture of what I was looking for I ran across this page:

http://www.customconversion.com/V8-c...ed-Sensors.pdf

"Earlier (1987-1989) Full-size trucks use a programmable

DRAC that is incorporated into the instrument panel. All Chevrolet trucks with rear wheel anti-lock brakes

(1989 and newer) use electric speedometers.

......goes to the DRAC which divides and conditions the signal for the electric speedometer,
ECM, cruise control and anti-lock brakes."
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

So, I guess I'll look for this DRAC thingie and see what I can find.


Couple thoughts.....

I've had the speedometer quit a few times in the past but never the brake light till recently when driving in the rain.

I've washed the truck maybe 3 times in this 'touchless' car wash. It sprays water on the undercarriage as you drive in. This got me thinking, why did I never get a brake light doing that? Doesn't make sense to me.

All this electronic stuff is way over my head anyway.

Thanks,

O'Riley

[crabtruck]

It was interesting reading the info on your link.
Not too sure I agree with all they say the standards were.
DRACs for controlling radios is nice but they didn't have them by 94 (my year)
Car wash is not likely problem.

The VSS, taking the place of the old style speedo gear in the tranny,
is an electrical "generator" basically, that sends info (square/DC/AC etc)
to the vehicle speed buffer, which then sends it along to the speedo,
the ABS (RWAL), and the ECM/PCM. Buffer may divide the "counts"
depending on where it's sending the info. Just a "2 to 1" deal, no biggie.
Some rides the bufffer is part of the ECM but I think yours is located
in the dash under the right side. I suggest you start with a visual inspection.
Lots of times, even if you have no idea what you're looking at,
you'll recognize that "that plug shouldn't be half off" or "geez, that's corroded"

[Oriley]

Couple observations since the last post:

Month or so ago I drove 7 miles in light rain, no problems. On the way home I turned on the cruise.....maybe 30 seconds and the speedometer and cruise quit.

Today I drove the same in a good rain and didn't use the cruise and no problems.

So, it 'seems' to me it is the cruise control that is doing this....not sure.

Also, no ABS light has come on since the very first time.


Oriley

[chadwickthomas]

since the problem exists in wet conditions I would try unplugging the connector on transmission and installing it back with a circular wiggling motion. Ihad this same problem on two cadillac's and found a bad connection at the ecm from the speed sensor try un plugging ecm connector and reinstalling it.