How to repair the cruise control?

[Stormer]

The speedo is off by 7 at 65 & the cruise does not work.

Where do I start?

 

[commando1]

By telling us which car.
And a service manual.
There's an extremely detailed troubleshoot and repair section in there on Cruise Control.

 

[1978 NYB]

Tire size is mostly likely the problem with the speed being off. Same problem with mine.

 

[D Cluley]

1st check your odometer. If it is off the same amount as the speedometer, then your speedometer pinion gear and tire size don't match up. If the odometer is accurate, then the speedometer itself needs adjustment/recalibration.

 

[1978 NYB]

I've never seen any adjustment in the service manual for any adjustment to the speedometer itself. If you know of a adjustment procedure to the speedometer let us know. The only adjustment I've seen is the speedometer pinion gear chart for the transmission. That would be hit or miss because the chart doesn't have modern metric tire sizes. I guess you could play with adding or subtracting the pinion gear ratio. The gear ranges from 24, 25, 26, all the way to 37 teeth per gear. You could keep swapping them out until you get it close. You could use a GPS to check your actual speed to speedometer accuracy.

 

[Stormer]

Yea, thanks guys. I'm using a GPS, that's how I know it's off.

As Stan mentioned, there is a large section in the FSM for troubleshooting & I will follow that procedure.

 

[1978 NYB]

The speedometer information is in the transmission section of the FSM.

 

[commando1]

You could use a GPS to check your actual speed to speedometer accuracy.

No need to experiment.
There are on-line tire calculators that show %-age changes and allow you to pick the right gear the first time. Those damn things are not cheap.

 

[1978 NYB]

I'll have to look that up. Mine is way off too.