Hey this is my first post of any kind and I hope I am doing this right...I have a 93 K5 FS Blazer.
1.Speedometer is very jumpy - as soon as I give it gas it pegs out and then goes back and forth.

2. The Tach seems to be reading low than the actual tachometer.

3. Odometer is always running when truck is turned on. It even moves when in park.

I have replaced VSS.

I also took down to local Dealership and they found nothing wrong except they saw the symptoms.
The truck runs fine and shifts smoothly. Just trying to fix this nagging issue.

 

i have seen a similar issue in a monte carlo. this was like a year ago so i dont really remember the year, id guess 99ish. anyways it was the actual cluster itself. im surprised the dealership couldnt give you a straight answer. you can try throwing a junkyard cluster in it if youre all out of ideas if they aint to costly

 

I should of mentioned that I had already replaced the cluster and both clusters did the exact same thing.
I have also tried to reset the system by holding the battery leads together for 15 seconds.
At a loss I think it is something electrical but I can't find the issue.

 

and what does the odometer do when it pegs???? Okay, that's cryptic. You have an aftermarket tach, too? WHICH read lower? Is idle good? The one that shows 650 rpm at idle in park is right for v6, 500-550 for v8Turned on meaning running.. or ign on? So it either stops when enigine kills, or it runs all the time. Are you sure the speedo needle doesnt bounce a little too?Good you mentioned two different cluster did this. Since your ECM is OBD I and runs fine the next question is how does cruise work. If it worked before this and now doesnt work, a different thing.

What ELSE is flaky in your truck?

What I suspect is a bad ground which you might find by gently wiggling all the wire harnesses under the dash.
I have no access to those wiring diagrams but if you do, find the wire out of the ECM that goes to tach in the dash. Will bet if you disconnect it, the odometer stops running. I could be wrong. But if i had to bet, a bad ground will cause tach to reflect over into odometer circuit.

By the way.. all you guys who have old GM's work on them yourselves and dont have wiring diags for them are nuts! Fords have about 1/4 the electrical problems and I wouldnt have one without the diagrams. Even if you cant read them you should have them.

 

I have replaced DRAC Module, ECM, Had Cluster Tested on table nothing seems to be helping at all. The only clue that I have discovered is when Brakes are pressed it seems to have a affect on the needle. But for the most part the speedometer is pegging out and the odometer is going the same speed as the speedometer. I have no idea what could be wrong. When DRAC is disconnected nothing moves except ABS light comes on.

 

If you have cruise control, does it work?

I didnt even know what a DRAC was...
This is a 94 which I saw might be slightly different
Changed following due to brain fqaart
Put a DVM {AC Volts} across the VSS inputs and note the reading. Should be a couple hundred millivolts and output relatively stedy with given speed.
No voltage standing still. Disconnect that connecotr and check for continuity to grd of the two vss wires. Should be high resistance.

Then check ac mv on line to cluster..

Check that ground to engine

 

Thanks for the diagram. I am hard at it trying to find the problem. Yes cruise control works .
When odometer pegs. I start engine Speedometer pegs (sometimes it bounces or pulses) I can get the needle to 0 by pressing brake then as soon as I touch gas pedal past 85 again. When I drive the speedometer is everywhere. No pattern at all. The odometer does what ever the speedometer is doing.
Everything is factory.
Turned on means started. When turned off needle will sometimes stay pegged out or return back to 0. No pattern.
There does seem to be some relationship to the brake pedal being depressed . Not sure what that is.

 

Well I got it fixed. I had to go to an old school automobile electrician and he discovered a faulty ground on the DRAC module that was back feeding into the cluster. Thanks for all the help.