Cruise control causes surging when engaged

I’m looking for help on why my cruise control causes surging of speed when engaged. The acceleration and deceleration is quick and feels as if I were accelerating and decelerating rapidly to maintain a permanent speed. Although it does maintain the set speed this surging is actually profound enough that it causes my body to shift forward and back. Pretty sure this is not good for engine or transmission function, and certainly mot my comfort when using.
Any insight as to possible cause is appreciated!

 

When I asked (in 2002), the dealer said it's common, and there is no way for him to adjust it. To keep it from shifting up & down, they said to turn off overdrive, or select a lower gear. That way it won't keep hunting gears up & down. It does depend upon how steep the hill is. Hope I got the correct symptoms you have.

 

Out of curiosity have you checked for any vacuum leaks?

 

Check the battery clamps. Loss of continuity, will mess with the computer causing the CC to lose communication.

 

if the engine can not develop enough torque/HP to maintain the desired speed then the trans will down shift, this is normal. Check for excessive slack in the cruise control cable at the throttle bell crank.

 

Thanks for the responses, but just to clarify, the transmission is not shifting down and the action is the same with overdrive on or off. What’s happening is the same effect as if I were accelerating then decelerating constantly with a resulting jerk in speed in both instances. Think of trying to stay within a few inches of a car in front of you going 60 mph and you’re trying to stay within a few inches of its back bumper only using the accelerator and not the brake.
Also, I failed to mention that this is an after market add on which was done by the previous owner. I’ve had the truck for over 6 years.
If anyone has a schematic of a typical aftermarket system that might help. I can’t seem to find one on the web. thanks again for any help or ideas.

 

Also, it’s a ‘99 model PreRunner….

 

First thing I'd do is remove the aftermarket CC.

It shouldn't surge. Changing engine speeds and trans shifting for hills is normal. Surging on level roads is not normal.

Then if you must have CC, suggest to purchase another aftermarket to install or have the dealer install an OEM system.

 

Maybe check the slack in the cruise control cable that goes to the throttle body. There should be just a teeny tiny bit of slack.

 

Outside of checking the tension of the cables, I don't think many here are going to be able to help troubleshoot an aftermarket CC.

Your best bet might be to locate the unit itself and find a manufacturer or model and see if you can get in touch with that company.