So my cruise control is not working, and after searching and trying all of the posted diagnostics, I cannot find the issue. 2002 F150 5.4 Lariat.

Test results:
Self test, the light flashes when engaging test, and flashes once when starting the sequence with the "on" button. Nothing happens with any of the other buttons.

10 pin connector voltage and resistance test:
All buttons corresponded perfect with their required resistance. Voltages with key off/on, brake, etc were perfect as they should be.

The only thing not correct, is the pin 5 voltage when pressing the "on" button is only around 3 volts, not 12. However, I have been unable to find anything that suggests what could cause this, or how to test/bypass to figure out the fault.

Any ideas where to look would be appreciated!

 

Have you tested for power at the master cylinder switch and be sure it's getting through the switch.
Nothing will work unless power gets to the Cruise control modual.
Testing will not even start without it.
Good luck.

 

That's a different part of the circuit.

Does the horn work normally when you press the horn pad?

If "Yes", the press the ON + Coast buttons at the same time. Does the horn blow again?

 

Well that's a little odd...the horn does work, if the ignition is off. When I had the key on, the horn does not work.

As for the on + coast buttons, with the ignition off, the key-in-ignition chime goes off, not the horn...when the ignition on, on + coast does nothing.

 

TTT...any other ideas where the problem could be coming from?

 

I looked at the schematic of the system.
Fused power comes through the Horn relay winding.
Branches off to other points.
If any of those branches are complete to ground, you will not measure 12 volts because of the voltage drop across the Horn relay winding that supplies pin 5 under those conditions.
The voltage under various conditions will change.
The only time you will see full battery voltage is if all circuits are open.
*
Any circuit that closes through will be in parallel such that the current through the horn relay will increase and voltage drop will also increase.
That is Voltage E = Current I divides by resistance R of the relay winding.
This dictates that the higher the current through the relay winding the greater voltage drop across the winding.
If you measure voltage on the 'load side' of the winding, it will be 12 volts minus the relay winding drop. That means the voltage drop across the relay winding is 12 -5 (you measured), or 7 volts drop due to the other circuits in conduction at the time of measurement..