Speedo slows down at speeds above 100km

[racer55]

96 C3500 with NV4500 trans.
Question for the cluster/DRAC guru's,when traveling at speed on the highway my speedo works prefectly fine up to 100KPH-after that speed if I increase my speed to 110KPH the speedo reads 90KPH and continues to back off in an almost linear rate as speed increases.

Where do I start looking for this problem?
Speed sensor in the trans,grounds for the DRAC or wiring to the cluster.

The cluster was replaced for other reasons but the brand new cluster behaves the same way as the old one did.

I have yet to check the speed reported by the ECM to compare what it sees to the cluster since it is tough to drive that fast and monitor the laptop by yourself.

Minor problem but an annoying one.

 

[Burning oil]

Does the 4500 have input and output sensors like the auto? Try swaping them if yes. Or just buy a new, they are cheap.

 

[racer55]

Not sure,will have to look.
I have not spent any time on this yet so all info is welcome.

 

[Burning oil]

Tanner and GM guy know alot about them, maybe PM them incase they dont see this post.

 

[THEFERMANATOR]

Pull the speed sensor in the tailhousing of the trans and check to see if it has some metal filings on it. It will only have 1 speed sensor in it, but if it gets a few metal shavings on it it will do this.

 

[racer55]

Ok will do tomorrow time permitting.

 

[Raleigh]

So this is a question I have as well, mine is auto.
Mine usually reads lower than what I am actually driving, every once in while it will read correctly.

I assume it is a sensor that needs to be replaced, I did not know there were 2 though!

 

[Burning oil]

Auto has two so the TCM/PCM can monitor slipage.

 

[GM Guy]

havent had that issue yet.

I am having issues of the speedo not reading enough on dad's 92, I havent done anything to it yet, since it has a cracked bellhousing and is sitting.