Speed control amplifier

[Docktergreg]

My cruise control stopped working a few months ago. I downloaded a diagnostic paper for it, and everything checks out ok, within specs. Went through it twice. I even hot tested the servo to see if it would raise and hold engine rpm. worked as it should. So, I suspected the computer, also called the amplifier, and purchased a used one, said to be good, from EBAY. Still no worky. So now, not knowing if the amplifier is working or not, Is there a way to bench test it? Amp number is E9AF-9D844-AA. Thanks for any help.

 

[XOLATEM]

and everything checks out ok, within specs.

What is 'everything' that you checked...and what is the year of the truck...?

As far as bench testing the amplifier....do you have a wiring diagram for it...?...if so...let's see it....

If you can get your hands on a break-out box...that would go a long way to helping you see if the inputs and outputs are in order...or not...

 

[Docktergreg]

I have attached the diagnostic testing procedures, and wiring diagram. I'm not sure what a breakout box is.
Truck is 1992 f350 with 7.3, and 5 speed manual.

 

[franklin2]

Many common places for failures. Your checks should have picked them up though. The electrical switches on the clutch pedal and the brake pedal commonly fail. And the slip rings behind the steering wheel are a common failure point.

Easiest way to make sure the brake and clutch interlocks are good is to just jump them out and see if it starts working. That would be jumping power to the lightgreen wire. Here are some more diagrams if you need them.

The buttons can be difficult to diagnose, since it uses resistors to tell the box what to do. I assume your checks went through this also. I can't open your diagnoses file but those checks are usually in there.

 

[Docktergreg]

I saved the diagnostics as a pdf and attached it. Hopefully you can open it. It does include checking all of the resistance and voltages from the slip ring. They are all within specs. So unless the diagnostics are missing one or more checks, I am suspecting the amplifier module. So if there is a way to bench test it, that would be great. Otherwise I will check around to see if someone has a working module that I can try.

 

[Docktergreg]

Actually, the only thing on the diagnostics that I was unable to check was the speed sensor input. Apparently the diagnostics are for a different sensor input mounted from the transmission to the firewall. My sensor is located on the rear differential, and routs to the speedometer, then to the amplifier. I assumed that the signal is good since my speedo works fine. But, diagnostics don't show a test for the sensor input.

 

[franklin2]

I think your assumption would be correct on the speed sensor. If the speedometer is working, then you would think the speed sensor input would be good.
On the earlier trucks, they used a sensor in the speedometer cable. Those trucks actually had two speedometer cables, a short one from the trans or transfer case that ran up below the brake booster, where that plugged into a little mechanical/electrical sensor, and then another short speedometer cable went from the sensor into the firewall and into the speedometer.

Can you get the control box apart? If you could, you have nothing to lose to take a small soldering iron and re-heat all the solder connections on the circuit board. And look it over for anything that might be burnt.