Speedo shows 10 to 20 after startup

[AndyL]

Any particular reason my speedometer jumps from 10 to 20 for about 5 seconds after my truck starts in the morning? I have never noticed this other than after a cold startup. No other issues, the mileage increments as well.

[gmctd]

Might check the connections\grounds in and around the instrument cluster.

Pcm gets engine rpm frequency and vehicle speed sensor frequency.
Tach gets alternator frequency.
Speedo gets vehicle speed sensor frequency.
ABS gets vehicle speed sensor frequency.
Radio gets vehicle speed sensor frequency.

[hump]

Mine does that as well, I've just written it off as typical crappy GM engineering to match the other gauges in the dash. Still, we all love these trucks. (most of the time)

[catmandoo]

do you have a manual or auto ,my 92 will jump like that and it's a manual.if say i'm setting somewhere in heutral it will just set there and bounce up and down,but push in the clutch and it stops.if ya let it set and idle too long you can rack up some miles.

[markrinker]

Its just because your truck is so damn fast...

[AndyL]

It is an auto. I have already checked all the grounds on the engine, maybe I will reseat the PCM and anything else I can find.

[gmctd]

I believe it's tach signal feeding into instrument cluster power bussing - power and ground - via bad connection(s)

The gages are on regulated cluster supply line - supposed to keep signals from passing back and forth, confusing instruments and PCM.

Oxidized connections result in signals "modulating" power and ground lines.