1989 cruise control problem

[70Stub]

89 k1500 with the 5.7, auto trans. Truck runs fine down the road. Set the cruise and it starts sputtering, misses. Take it off and truck runs fine again. Has new plugs, wires, rotor, cap. Nothing else has been worked on. Cruise did work fine but now I can't even use it. I ran a separate ground to it thinking maybe that was the issue. Any ideas what is causing this? Thanks

[roboticjunky]

Symptoms of a Bad or Failing Cruise Control Vacuum Reservoir

1. Speed surges while cruise control is engaged

One of first symptoms most commonly associated with a problematic cruise control vacuum reservoir is the speed surging while the cruise control is engaged. If the cruise control vacuum reservoir fails, the cruise control system will be left without the vacuum it requires to hold the vehicle at a steady speed or rate of acceleration. The cruise control may still try to hold the vehicle at a certain speed, however, it will eventually run low on vacuum which will cause the vehicle to slow. The speed of the vehicle may continuously fluctuate as the cruise control system uses up and then regains vacuum pressure.
2. Cruise control will not maintain speed

Another symptom of a problem with the cruise control reservoir is a cruise control system that simply cannot maintain speed. If the cruise control vacuum reservoir fails, the cruise control system will be left without its necessary vacuum reserve, and as a result, will not be able to hold the vehicle at the set speed or rate of acceleration.
3. Engine performance issues

Another symptom of a potential problem with the cruise control vacuum reservoir are engine performance issues. If the vacuum reservoir breaks or leaks, it may cause a vacuum leak that can affect the performance of the vehicle. Depending on the severity of the leak, the vehicle may experience a decrease in acceleration, power output, and fuel efficiency.

The cruise control vacuum reservoir is a very important component to vehicles that use them as part of their cruise control system. It supplies the cruise control system with the vacuum necessary for it to operate, and without it, the system cannot function.

[70Stub]

Hmm, I don't see any vacuum lines going to the cruise control, only wires. Where is this located?

[roboticjunky]

You're right yours doesn't have a vacuum reservoir.

I thought it did since I had 2 vehicles that had TBI that had vacuum reservoirs.

Another website suggested to unhook the harness going to the cruise control under the hood and clean the contacts/plug.

[hatzie]

1989 usually has the electromotor cruise control box.

The electromotor cruise control setup is literally an electronic gas pedal with no vacuum connections. Just a throttle cable from the cruise box to the throttle and inputs from the brake & clutch pedals, VSS, control switches, and ignition switch.

The tech training manual for this system is in the 1973-1991 manuals page in my signature.

The only reason I can think of that the electromotor cruise system might interfere with the engine controls is if the cruise control box is somehow loading the VSS signal. That's a big reach tho.