08 Express 2500 4.8 Intermittent speedometer fail

The speedometer intermittently fails to display any speed

as I move away from a stop.

The failure only occurs as I move away from a stop it never falls to zero while moving

As I move away from a stop, the meter remains at zero.

As long as I am in motion, the meter will always remain at zero.

With the van in motion and the speedometer reading zero,

the transmission will not shift out or 1st gear.

To work around that issue, I stop the van for a few seconds.

When I come to a complete stop, then start moving,

the speedometer will function normally.

The meter never fails as long as the van is in motion.

As said, this problem is intermittent.

What would be the first step toward a solution for this problem??

 

Since it won't shift, or will it shift at high rpm's? I'd say it is a speed sensor.

 

I had found some information related to the VSS. There are 2 on this van.they are located at
the rear area of the transmission.
I replaced both of them.The issue still remains as an intermittent fail.
I was hoping that I could force this fail to occur on demand, but I cant make it fail.
The transmission will not shift out of first gear at any rpm, or speed until I come to a complete stop.
As I mentioned,When I come to a complete stop for only a few seconds and then proceed to move, the meter
will show proper speed.

 

Check the connections at the sensor(s).

 

What shape is the transmission mount in? If there's slop in it maybe it moves enough to wiggle the connectors in a way that causes the circuit to be interrupted.

 

That is a very good question. I hope to have time this week to inspect the mounts and the VSS connections.

 

Inspect the wiring harness under the dash. There’s a TSB stating there’s parts of the harness chafing and rubbing through in the section that runs along the parking brake pedal/bracket. Multiple issues were caused by this, including speedometer and shifting operations.

Make sure to pull the harness down far enough to inspect all sides.

The TSB# is 18-NA-102 if you want further information.

 

Thanks for the TSB information.I completely forgot about looking into tsb's,

 

Well, this van is fixed. The fix was something so simple that

it would be overlooked by most mechanics, including myself.

I am a retired mechanic,but, In my 40 years as a mechanic

I have seen this part fail maybe 6 times.

When I have seen this part fail, the vehicle would not start.

Until now, I have never seen this part cause a partial failure.

The part that failed is GM part # 88986775



I am not the one that fixed it.I give that credit to a GM dealer mechanic.

 

That part number comes up as a battery cable but it looks like a ground strap. Was the entire cable replaced or did the tech just replace the damaged section?