IroczInOz

Cruise has never worked in the car until I started to do some fault finding. I found the cruise switch was bad so I installed a 2nd hand stalk and that checked out fine.

Took the car for a drive and the cruise does engage. But it will not hold speed and the speed drops down my about 10-15mph and then the cruise kicks in again and accelerates the car up to the set speed. Then it is a constant repeat cycle as described above. slow down - accelerate - slow down - accelerate etc...

I tried changing over the vac canister that had no change.

Any cruise experts out there know what the problem is? Could it be a bad throttle cable that sticks?

Fullsizewagon

I assume youre talking about an Iroc, don't know what year. I don't know the Iroc specifically or what could cause such behaviour but I guess most GM CC's are identical so here are some thoughts.:
You'll want to check the one-way vacuum valve in the line between the manifold and the canister.
Cruise control "motors" (the bellows) have a built-in position sensor so the control module knows where the "gas" is at, independent of spring strenght, but check linkage for friction. The sensor should have a resistance of 20 Ohms. Measure between pins B & D (tan & lt blu/blk). Pin D should be grounded, via the control module, to pin C (blk). Maybe a faulty position sensor circuit could lead to the symtoms you describe.