Long post sorry
I bought my 85 Z51 back in October. It had been sitting since 2001 and was a retirement project for an old man that just gave up. So its was sitting outside in the Texas sun for years.. I like punishment so I bought her and drove her home, well almost. She ran out of gas 1/2 way home 6 miles away. That should have told me right there I was in trouble. Shortly after that I had to have the tranny rebuilt.. A month after that the engine started knocking, New Jasper Class 1 installed. New MAF, New oil cooler ( what a pain to find) etc etc etc, New everything pretty much. The a new gas tank and sending unit due to 500 pin holes and gas all over the parking lot. New windshield installed last week with all new weatherstripping on the whole car. The mismatched factory wheels were replaced with a perfect set of factory 16x9.5" all around. The interior has been completely redone except for the carpet (next project). New paint two weeks ago back to factory Bright Red and emblems all around. I guess my point is to show Im very serious about getting this car back to pristine condition as I can and enjoy it while I can. She is a daily driver and I love the old girl. Now the issues I can not seem to solve and hope some of the Vette gods in the forum can guide me on.

1. The speedometer will not hold a steady speed reading. When driving the mpg fluctuates a few mph even if going a steady speed. I know the MPH reading on the 85 gets it's speed from the VSS signal and sent to the Dash cluster and then is sent to the ECM. The cruise gets it speed from the cluster also. I think I have that right..The I spent 2 weeks working on the cruise issues and I think its related. When the cruise is engaged at say 70mph the dash speed is fluctuating +/- 2-3mph . SO the cruise will surge and let off. If set to 70 and the cluster shows 68 it give gas to the car then the speed says 71 to 73 and it lets off the gas. This is not a steady ride! it's like riding a bucking horse as it surges up and down trying to hold a steady speed according to the jumpy speedometer.

SO my issues Im working on now is why is the cluster not holding a steady speed reading, and an advise on this and the cruise,. I think fixing one should fix both.,

Thanks

 

Below is my only experience with the system... so take it for what it's worth and what I remember from years back... I would recommend getting the FSM because I'm pretty sure it has a trouble shooting section on this... that being said, read on if you wish.

First off... my cruise isn't exactly smooth either. It works but does wander. It isn't a precise setup but isn't bucking or annoying by any means. In my situation I feel an automatic would take care of most of what I feel.

When I pulled my cluster I vaguely remember being able to supply power to it and make a harness that allowed me to test it... iirc it had a built in signal generator (on one of the cluster pins?) or I used a scope with a fixed wave output... I don't recall. What I do remember is that batee had instructions on how to do that... if you suspect it is an instrument cluster issue I would try that...

Now what I think... the VSS driver under the car on the trans... what condition is it in? Mine was failing and had road grime in it from years of use and would occasionally jump around. I suppose you could use a signal generator at the VSS plug too and see if the dash holds steady as well. Anyway I figured this out when I went to remove it and it basically fell apart.

 

Thanks for the reply. im at the point im going to send the cluster off to bantee for a complete rework.
The tranny was just rebuilt a few months ago with the engine. im assuming the VSS was checked at that time. i may be assuming to much

 

it's worth a check. if the cluster comes back bad then it is either the driver or a wiring issue imo.