1996 F-250 XL, 7.3 Power Stroke, 181,000 miles; My cruise control will turn on, and it will engage. It will hold for about 100 yards and then shuts off. If I hit "resume" it surges back towards the set speed and shuts off again. I can do this all day with the same result. It will not stay engaged. I've replaced the magnetic pick-up and harness plug on the rear differential, but it didn't change a thing. I'm suspect of the cut out switch located on the master cylinder, but need to ohm it to see if it's closed, and will open when the brake is applied (for $15 I may just replace it). My speedometer does begin to "jump" around 65 mph, and I wonder if that has any bearing on the cruise control, however the problem presents itself at 40 mph when the speedometer isn't jumping.......any input would be greatly appreciated!.....Thank You!

 

The brake pedal switch is on the pedal arm / box, not on the master cylinder. If it's that, you'll also see (well, someone behind you will see) your brake lights come on. If you are having any speedometer issues, then yes, that will affect cruise control. You might want to inspect the tone ring in the diff; if the diff lube is ready or near-ready for replacement, now would be the time to drain it and remove the cover, to get a good look at the tone ring.

Automatic or manual transmission?

 

Edit - deleted.

 

I hadn't considered the brake pedal switch. I was referring to the cruise control cutout (release) switch located on the master cylinder (motorcraft PN 1L1Z9F924AA). This is that dasterdley switch that has caused all the fires! I even removed the factory recall supplied "fused" harness section that was put on to avoid potential fire thinking that might be a problem, but it made no difference.

 

Oh yeah, I'd forgotten about that secondary switch on the M/C. Did you check that fuse?

 

Haven't checked any fuses yet. I did go out and check to see that the brake lights are working, which would lead one to assume that the brake light switch and circuit are good.

 

Isn't there another cut out switch on the ebrake assembly? There is on my manual just not sure about the autos. A weak ebrake spring could make for a sloppy pedal causing intermittent issues.

 

My truck doesn't have any switches on eh ebrake that I'm aware of

 

Well, I say that. I'm referring to any type of switch involved in control of the braking or cruise control circuit. It does have a switch that lights an indicater light on the dash when you depress the ebrake.

 

The PCM can see the status of the dash brake failure warning light, which comes on when you apply the parking brake. I haven't tested it, but yes, it's possible that the PCM will disable the cruise when it sees that light on. But if that were the cause, I would think you'd see the warning light blink briefly as the cruise disengages.

The fact that it disengages at a rather predictable period after engagement suggests that it's more related to the VSS, tone ring, etc. Esp. since you're having other speedometer issues. It may just be that at lower speeds, the speedo itself isn't sensitive enough to pick up whatever the issue is, but the PCM is.

 

Did you check the 3rd brake light too?

 

It would appear I have embarked on a search for the holy grail!...I checked all the pertinent fuses and all were good. All the brake lights work. So I have embarked on a plan. I ordered a new VSS and Cruise Control Release Switch (goes on the master cylinder), and I should have them by this weekend. I'm going to open the rear differential and check/clean the tone ring, I'll then install the new VSS, inspect and replace if necessary the VSS harness connector, and check the VSS/Tone ring clearance. If all checks out I'll reassemble the differential, and install the new Cruise Control Release Switch. If none of that corrects the issue, I'll probably remove the dash cluster and ship the PSOM assembly off for repair. I found a company online (www.clusterfix.com) that has warranty, reasonable pricing, and good turnaround. I spoke with them today and I'd have to ship my PSOM assembly to them, they would diagnose and repair ($175), and return the unit to me. The lady said they get them in and out in a 2-3 day time frame, so I'd still be looking at ship time to and from their location. I figure 10-12 days depending. If anyone knows why I shouldn't use this bunch, or knows of a better place to repair PSOM assemblies, please speak up. Hopefully I won't have to go this route.

 

I'd just get a cluster from a junkyard truck. Then you have the option of getting your PSOM rebuilt/repaired, and still have a working cluster.

 

......Any idea if the PSOM units are "fuel type" specific?......could you temporarily run a PSOM from an F150 gas burner in your F250 Diesel truck?

 

I would guess there all the same.