Got a strange cruise control question.

98 GMC Sierra 4x4
had 5.7, but replaced it with a HT383 stroker from GMPP
Prior to replacement, cruise worked fine.
Post engine swap, cruise takes 2-5 minutes to engage, and does not hold
speed on grades. Very erratic operation.
No other known issues, engine runs fine. Cruise cabling seems fine.

Ideas where to start?

Outside of testing all the electrical inputs/outputs per the factory service
manual to be sure everything is working as it should (like speed sensor,
etc)...

And adjusting the mechanical linkage per the service manual
specifications...

It is possible that there could be some sort of internal "error detection"
software in the cruise control controller/computer. This would allow the
cruise control to work within a certain set of values only.

You could program into the software that if it needs to increase the speed
say 3 miles per hour faster, then it should expect to pull the throttle
cable no more than 1/8 inch to get that increased speed. And if it needs to
pull the cable more than that to get the speed increase, then it should
shutdown because something is not right. (For safety).

Also you could include the opposite. That it needs to pull the cable a
minimum amount to get a certain speed increase. And if it is pulling less
than that, then something is wrong, so shutdown.

Or the cable should be in certain positions for certain speed ranges. Say
pulled back 1/8 inch to 1/4 inch for 35 mph. 1/8 to 1/2 inch for 45 mph,
etc. And say it is pulled back 1/2 inch for 35 mph, this would not be the
way it should work, so something is wrong, so best to shutdown for safety.

I have no idea if any "safety checking" like this is built into the cruise
control module or not. But I know if I was a large vehicle manufacturing
company and saw all those lawyers out there, I would sure do my best to make
everything as safe as possible. And include everything I could to prevent
anything like this from "going haywire" and causing the vehicle to "lurch
into something" due to a malfunction.




"BikeRider50" wrote in message

"BikeRider50" <BikeRider50 (AT) takethisoutGmail (DOT) com> wrote

The key phrase is "Prior to replacement, cruise worked fine."
I would look for a vacuum leak.
Regards,
JR

BikeRider50 wrote:
Sounds like the tach signal or speed sensor is screwed up. Which
induction and ignition are you using? What other changes were made?

--
Steve W.