Cruise control

[ThatOneGuy]

Not sure where this fits so I put it here, yesterday on my way into work. To pick up the tow truck for the week. I tried the cruise control on the 89. Seams to work great however it wouldn't let me set the speed above 55. Is this a normal thing?

 

[RonD]

No, not normal

Any Cruise Control just goes by "pulse" count when its SET, it doesn't know what "speed" or MPH is
If you set it when pulse count is 100/min then it will try to maintain pulse count of 100/min using the throttle cable
Using the Decrease or Increase just lowers or raises the "set" pulse count in memory

Pulse Count can come from speedo cable or VSS
1989 should use VSS, but just look at the actuator in engine bay, if you see 2 speedo cables hooked up then it uses those for pulses, if no speedo cable then it used a VSS

Actuator uses Vacuum to control its throttle cable position, so you could have a vacuum leak on one of its hoses so it simply can't pull open throttle and hold it open above 55MPH, because of the leak/low vacuum

I wouldn't think its a pulse issue because if it was it probably wouldn't work at all

1989 should have an emergency "Vacuum Dump Valve" inside the cab on the brake pedal rod where it goes in to the booster, vacuum hose runs from actuator thru firewall to this Valve

The brake lights should deactivate the Cruise but just in case the Vacuum Dump Valve releases all vacuum in the actuator when brake pedal is pressed down, which of course releases the actuator's throttle cable pulling power
If this valve or hose leaks then low vacuum available to actuator

 

[ThatOneGuy]

Perhaps it has to do with it being a manual and I noticed this on a hill in 5th gear, I'll have to do more testing