Cruise Control Issues

So let me see if I can describe this right since there are actually two things. Lately at slow speeds when the cruise control is engaged, it seems like the speed fluctuates + or - 1 mph. Well yesterday, I took the truck to Midas to get an alignment done and the cruise worked fine at 55mph. I got the truck back and now it doesn't work. I don't blame them because they only did an alignment and didn't mess with anything. I was there so I know the work they did. I checked the fuse inside the truck and it was fine. I'm not very well versed on cruise control problems since I've never had to deal with it. Thoughts??



Second, the crankshaft position sensor is bad so I'm not sure its affecting this or not. When I'm driving at 55 mph down the highway, I'll let off the gas to coast to a stoplight. The truck gradually will slow down and downshift, but my RPM's stay around 1800. They will lower when I hit the brakes, but I thought it was odd. Could it be the TPS or something else? Any help is appreciated.

Change the sensor I am not an expert on this one but I am pretty sure that would affect the cruise control.

The brake light switch can also be a culprit with cruise issues. Worth a look anyway.

Take a look at the wire harness where it comes out of the turnsignal stalk.

The single most-common cause of cruise non-function is damaged wires at the turnsignal stalk.

Of course, there are many other things that can cause problems, too.

All the above for your cruise control issue, especially the column switch and it's wiring. But check your stop lamp switch cruise input also, but speed input from sensor needs checked. Does the speedometer fluctuate at all? Also, when coasting to stop, the transmission should free-wheel, but when you apply the brakes, the torque convertor clutch may still be engaged. This is one area to eliminate first. Do you have a scan-tool like a Tech1 or Tech2?
As for the 1 - 2 MPH variation, that can be in the cruise module itself. I may have service info that goes back that far, with schematics if needed. Is this the 98 Silverado 4X4?. Which engine? I can only look for the part a RockAuto to see shat type you have. If it is the electronic cruise, in the rectangular box, this would be the most common type. I have 8-9 new ones in box, all ACDelco. They would use the 4x4 VSS sensor for high resolution input w/out buffer. It goes into the VCM and shared with the Cruise Module for input. They like to use the VSS signal for rear ABS and single sensors for front wheels. You indicated crank sensor (CKP), do you have a Check Engine Light with code?

I'm thinking it could be the VSS itself. When my cruise quit working, shortly thereafter so did the speedometer. I replaced the VSS and all is right again. And on level ground, the cruise doesn't vary from the set speed at all.