2001 F-150 XLT with 5.4 Triton Engine, automatic, with 41800 actual miles

The cruise control is intermittent. The switch test passes with no problems, including pulling in the throttle at the end. The brake-cylinder recall is fitted but even with that switch "jumped" the symptoms are the same. The speedometer is working fine, and is stable and accurate at any speed.


It appears that temperature has somethingto do with whether it will work. If it works, it works fine. I can turn it off and on, set speeds, increment the speeds, etc. with no problem. If it doesn’t work, no use trying again for the rest of the day. It seems to be ambient temperature, not engine temperature, that makes the difference. The cooler it is outside, the more likely that the system will work. But we live in the coastal south! Makes for bad odds.

I wonder if it might be a speed control issue with a defective part not sensing that the car is at a suitable speed to set the cruise (for instance, maybe sensing that is it under 25 mph).

Does that make sense to anyone? Can anyone think of a component which might be affected by the outside air temperature? Is there a speed sensor someplace that might be the problem even with the speedometer working?

Thanks.

(Sorry if this is a repeat post. Search gave over 500 hits.)

 

When the fault is present the test from the steering wheel has to fail!
Are you testing when it fails? not when it's cool and good.
What you describe is usually a faulty master cylinder pressure switch.
If the switch or an aftermarket replacement becomes sensitive to rises in the brake fluid pressure in the system from engine bay heat, power to the CC will be lost and fail to operate.
After market pressure switches have been known to fail by this sensitivity.
Be sure your testing when the fault is present.
The other possibility is the brake pedal switch.
If it is faulty or out of adjustment it applies an inhibit voltage to the CC.
What this means is when the CC is active and you need to brake down quickly, the pressure switch opens power from fluid pressure and the brake pedal applies inhibit voltage as a means of fail safe operation so CC does not drive you into an accident.
Take a new look at your testing based on the above info.
Good luck.

 

From my original post:
The cruise control is intermittent. The switch test passes with no problems, including pulling in the throttle at the end. The brake-cylinder recall is fitted but even with that switch "jumped" the symptoms are the same. The speedometer is working fine, and is stable and accurate at any speed.

I'll add that everything else on the truck works perfectly, so it doesn't appear that the ECU or any other sensors or computer components are failing. But we all know how that goes......

The switch tests always work, whether the cruise control works or not. And the brake-cylinder switch cannot be the cause since nothing changes if I jumper it completely out of the system. Same thing for the brake light switch.

That is why I suspect a speed sensor thing. Since that would normally be at 0 when testing the switches but if it is always reading 0 (like if it fails when it is hot outside) the cruise control will not set because of bad speed input. That's only a guess, but I have been chasing this problem for well over a year, seeing it work in cool weather, but not in hot weather.

(Ford dealer has been no help. They spent a lot of time on it, couldn't get it to work on a hot day, but couldn't find out why!)