I have an '89 Skylark custom 4 door, L4 2.5l, with about 190K on it. When I got the car about 12 years ago, the speedometer seemed fine (maybe showing I was going a little faster than I actually was). Over the years the speedometer gradually started moving more and more to the right, showing I was going much faster than I actually was. Is the vehicle speed sensor, the ECM, or the instrument cluster actually causing this? I've guessed the speed sensor and have really had a hard time finding one identical to the one on my transaxle. What I have is a little gear driven shaft which comes out of the top of the tranny into a housing and the gear on the other end drives a little gear on a shaft which plugs into that housing at a right angle, with the wire connector on the end of it (I guess this is the transmitter shaft). During all of my online and local part store searches all of the ones listed look nothing like this. They just contain a driven shaft coming out of the tranny with vanes on the end which send a magnetic impulse to the wire connector on the flat side of it. Anyone think the speed sensor is the problem with my speedometer and if so, where can I find my "unique" speed sensor? Thanks

you look on rock auto? the ones on there are just different between auto and manual. put your car in and go to electrical/speed sensor. if its different maybe they made a design change that will still work

Yep, I just received the speed sensor I ordered for Rock Auto and installed it. Got the same result, although better than my original-when I was doing 30, my speedometer was showing 65 (15 mph less). I'm wondering if it'* in the instrument cluster.

You need a schematic on this and a scan tool......to see if the signal goes directly to the cluster or to the ECM.....if it goes to the cluster, bad cluster....if it goes to the ECM, then you need a scan tool to read the sensor.....if the sensor data reads the correct speed, but the cluster doesn't, bad cluster.....if it reads the same as the as the cluster, sure you have the correct sensor(have to have the correct amount of teeth on sensor gear).....if the sensor is ok, then you need an ECM.....

Can I read the sensor on an OBDI system though? The other day I couldn't access it through my Innova scan tool when was asked to enter the 8th digit of my VIN which on my title is U (can't find the plate on the car). Yesterday, I looked up info. for engines in Skylarks of that year and found out R is the digit used for Tech IV or 4 tech 2.5l engines. I guess it'* possible someone replaced the engine years ago. Haven't tried R yet on the scan tool which leads me to my next question-if it is the ECM, will one out of '91 be compatable with my '89? Thanks

You have to determine if the speed sensor goes directly to the cluster or if it goes to the ECM first.....need a schematic....

According to the schematic in the Haynes manual I have, the sensor is routed through the ECM. Now what?

As I said in post #4, since the signal goes through the ECM first, you compare the speed of the data from the speed sensor to the speed on the speedometer.....if the speeds read the same, bad sensor.....if the speed data from the scan tool is correct, but the cluster reads wrong, it'* the cluster....

The other day I tried to access my car'* 'puter with my Innova 3120d (OBDI&II) scan tool and it said unable to access after I entered the year and 8th digit of the VIN. Took it to O'Reilly'* and they had a tool by the same company and he managed to access it with no problem and find an EGR code. When I emailed Innova'* tech. support, they said to try using the tool at normal operating temp. which I had done by driving to O'Reilly'*. The second thing this lady said to do was to try hooking the scan tool up with the engine running. I don't think that'* a good idea...and why should I when O'Reillys Innova tool had no problem connecting?

Is this the speed sensor you went with?
Note: The sensor sold here is the same one RockAuto has listed for a lot less, just linking this to show what the part looks like, since RockAuto'* part image was so small.
Cruise Control Sensor Vehicle Speed Sensor ACDelco GM Original Equipment 213 293 | eBay

Or did it look like this one?
ACDelco 213 293 Cruise Control Vehicle Speed Sensor for A T GM 10457264 | eBay

It is odd that both have the same part number, but look different, maybe the image for the first one is wrong, I do not know for sure.

Also, it does not hurt to hook up a scan tool to an engine that is running.

Get a scan tool on there that works, and verify if the sensors info is the same as the dash gauge, like Gus said to do, only then will you know how to proceed.