I have a 1988 Pontiac Trans AM GTA, 5 speed, 305 5.0. getting wrong speedometer reading on digital dash. What could be the cause...malfunctioning speed sensor, or VSS?

Any insight is greatly appreciated so I can restore this car back to safe functionality.

 

Describe in what way it's wrong.

 

Greetings!

Thanks for the reply. I apologize I've been away from the forum.

Still experiencing the same issue with my '88 Trans Am GTA......

It would appear that I am getting a false speedometer reading (Digital Dash) no matter what speed I drive. For example (I live in Canada) :

Driving in a 50 km/h zone, my dash reading will be 65 km/h and if I am in a 100 km/h zone, my dash will read 145 km/h

While I haven't been driving my Car as much as I'd like, I am still experiencing this problem, and reluctantly have to almost assume my speed by driving with the flow of traffic while driving etc.

Again, not sure if such malfunction(s) are due to a bad speed sensor or even a corrupt VSS module/interference.

Whenever you have a moment, any insight you may have is greatly appreciated.

 

Have you made any changes to the car before this issue?

 

Rear end gears been changed?

 

Assuming the digital dash works on a 4000 pulse per mile system:

100 km/h is 62mph so that is 248,000 pulses an hour
but your dash is reading 145 km/h or 90mph so that is 360000 pulses an hour


50 km/h zone, my dash reading will be 65 km/h
31mph is 124000
40mph is 160000

So the percent error is different at the two speeds you listed.

You can use something like this https://spicerparts.com/calculators/...rpm-calculator and plug in your values and see if maybe a rear gear is not stock in the car (you will know your rpms etc)

or you can hook a multi-meter that has frequency in hz up in-between the speedometer and the speed buffer box (does the digital dash use the buffer box?)

at 31mph you should see 34.4hz
The math is follow order of operation do the part in () first then do the equation left to right:
(speed in mph x 4000)/60/60
or
(speed in mph x 4000)/3600
-- or if you want to stay in metric --
(speed in kmh x 2484)/3600 = pulses per second (or hz)

if the buffer box is putting out the right frequency its in the cluster, if not its either gear ratio, buffer box, or speed sender. I don't recall the pulse per mile of the sender before the buffer box but you can do the math there and test the frequency. I think the sender to the buffer is 2000 pulses per mile (1242 pulses per KM) but I could be wrong on that look up the value.

Was your car a factory digital dash? Scooter you probably know this was the digital dash 4k pulse or some other value?

 

You can also buy this and bench test the speedometer if you are decent with electronics
I use this anytime I have the needle off a cluster to re calibrate the needle position.

 

Looks like the digital cluster still works on 4000 PPM, which is out of the buffer