cruise control surge

Cole · Jul 20, 2014

Hey guys, I have a 1989 ranger with factory cruise control. Whenever I use it the RPM's surge from 1000 to 3000 no matter what speed. I have checked for a binding actuator cable, kinked vacuum lines, and I even tried adjusting the cable. Anyone have any ideas as to what it is...

RonD · Jul 20, 2014

There is a check/release vacuum valve on the brake pedal in the cab, this is a mechanical valve to release vacuum pressure in the cruise control module in case the electric shut off for the cruise control fails.

Diagram here: http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Cruisecontrol_1of2.JPG

It is called the Vacuum Dump Valve in above diagram, if this is disconnected or leaking then when cruise was engaged you would get a vacuum leak and RPM changes

Cole · Jul 21, 2014

I checked the dump valve and the dump valve's vacuum line and they both held a vacuum. Any other ideas?

RonD · Jul 21, 2014

In '89 a speedometer cable was still being used.
Is your speedometer needle steady or does it fluctuate?

You can pull off the cable at the transmission end and inspect the plastic gear
Pictures here of what it should look like: http://www.therangerstation.com/tech_library/speedometer.shtml

Not sure what year they added the VSS(vehicle speed sensor) to the mechanical cable, but both were used at the same time until '94.
The VSS signal was used by the computer and cruise control.

In '95 the cable was eliminated and only a VSS was used

Cole · Jul 21, 2014

It does fluctuate below 25 mph. The cable has been replaced and the gear looks fine. I also checked vss resistance and it was 210. The manual said it is supposed to be between 180-250

RonD · Jul 21, 2014

Just to see if it is the computer(PCM) or cruise control that is causing the RPM issue, open throttle all the way and remove the cruise control cable.
Then drive and set cruise control, if RPMs change then the computer is doing it, perhaps via IAC.
If RPMs don't change then I would hit the wrecking yards for a new cruise modules

Cole · Jul 21, 2014

I will try that tomorrow. It is raining like cats and dogs here now and the truck doesn't go outside if there is a cloud in the sky. So if the rpms are normal then I need a servo or an amplifier?

RonD · Jul 21, 2014

Yes, but I wouldn't think the servo could "think" on it's own, unless it is a failed vacuum internally that unseals and reseals because of cable movement.

So Amplifier would be best guess, misreading VSS signal maybe???

Here is the page 2 of the Cruise diagram: http://www.therangerstation.com/tech_library/EDiagrams/files/Diagram_Cruisecontrol_1of2.JPG

Both pages do include some ohm readings

Cole · Jul 21, 2014

Ok after i take off the actuator cable tomorrow I will go to upullit and pickup a servo and an amplifier and see what happens

Cole · Jul 23, 2014

It was the amplifier, I went to upullit yesterday and picked up a servo and amplifier. I replaced the amp only and now it works perfect. Thank you!

RonD · Jul 23, 2014

Good work

cruise control surge

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