scan master reads different

[mitchell.cox1]

I have a spare ecm from a 86 regal tb. will it interchange with an 87? only cause my ecm reads different mph then what is acctually happening. Like i am doing 25 and the sm reads 11...

 

[mikestertwo]

I have a spare ecm from a 86 regal tb. will it interchange with an 87? only cause my ecm reads different mph then what is acctually happening. Like i am doing 25 and the sm reads 11...

You wouldn't happen to have a 140mph overlay on your speedometer would you? If so, that is the problem. If the speedo was calibrated for the 140mph speedo with a gear reduction box the computer/sm will read low.

 

[mitchell.cox1]

so if in fact the speedo is calibrated then that would be the right speed?

 

[mikestertwo]

so if in fact the speedo is calibrated then that would be the right speed?

If the 140mph speedo is calibrated to read the correct speed using a gear reduction box then the computer/SM will be off. The problem is the computer reads the speed off of a sensor mounted in the back of the speedo head. The sensor basically reads the rpm of the rotating magnet in the speedometer. When you install a 140mph overlay the rotating magnet has to spin slower (using a gear reduction box)to make the speed read correctly on the overlay. This gives the computer the wrong info for speed.