Mechanics question: cruise controls stops when headlights on?

[soupcxan]

A friend recently bought an RV. While driving it around this weekend, we noticed that the cruise control works fine, except when you turn the headlights on. If the CC is active, it will disengage when you turn the headlights on. If the headlights are on and you try to set the cruise speed, it won't engage.

I've tried googling this but so far my only ideas are to check the rear brake lights. As far as I know they're working. Any other ideas?

[bruce1]

This may be a long shot.

The cruise control will shut off when the brakes are applied. In my car this is accomplished with the same switch as the brake lights. If there were a short somewhere between the headlights and the brake lights the cruise would shut down whenever the headlights were activated.

Power from the headlights, brakelights and parking lights all go through the signal/hilo beam switch so that may be a possible point of trouble.

A defective signal switch managed to shut down my brake lights while leaving my signals working perfectly once.

Bruce

[nwsteve]

Quote:

Originally Posted by soupcxan My dad recently bought a used Coachman RV, built on a 1999 Ford E350 chassis. While driving it around this weekend, we noticed that the cruise control works fine, except when you turn the headlights on. If the CC is active, it will disengage when you turn the headlights on. If the headlights are on and you try to set the cruise speed, it won't engage.

soup
Try this link and see if it give you any ideas http://www.rv.net/forums/index.cfm/f...4.cfm#16244074
When I have "issues" with my RV--rv.net and the rv-owners site for the manufacturer seem to help the most.
good luck
nwsteve

[yakers]

I like Bruce1's suggestion. It could be connected to the brake light as a disconnect. There are two kinds of CC, the older type used vacuum and the newer ones are electronic. Its probably electronic and it could be how it is hooked up, maybe just changeing a wire. Also could be an issue with grounding. More electrical resistance when the lights are on. Anyway, an auto electrican should be able to sort it out.

[cute fuzzy bunny]

Yep, many ford cruise controls use the same switch that controls the brake lights and cruise control; problem with that switch could do it. A broken brake light bulb also may "tell" the switch that the brakes are always on.

Another oddball is the switch is sometimes on the brake pedal, and bumps/potholes can jiggle the pedal, causing the switch to think the pedals been pushed; having the lights on might be increasing/reducing some electrical load in the switch that makes it more sensitive to this. Even a slight 'sag' in the pedal thats ok with the headlights off but not ok with them on might do it.

Note that these switches usually dont look for voltage to tell them the pedals been pressed or the brake lights have come on, they usually look for the appearance of a ground signal. So anything thats cut, broken or fused in the headlight circuit connected to the rear lights might be creating a "false ground".

Ugly, hopefully replacing that switch blindly does the trick, providing all the rear bulbs are working.