I installed a brake controller in my '86 F-350 Crew Cab long bed XL trim diesel with cruise control this past week. For the brake siwtch wire to the brake controller I was going to tap off the brake light switch but the cruise module was right where I mounted it so I figured which wire was off the brake switch (engine off tested 0v at rest, .2v less than at brake switch when pedal pressed). Start the truck and it thinks trailer brakes should be applied constantly.

I checked voltage at the cold side of brake light switch and it's 0v engine off, approx. battery or alternator voltage when pressed, and when running and pedal not pressed it's 8v. Unplug the wide plug on cruise control module (closest to column) and behavior is normal, 0v at rest and battery/alternator voltage when pressed. Cruise control was non-functional when I got the truck and I've never done anything with it.

I tapped the light green wire at the far passenger side of the cruise control module plug.


PO had a brake controller at some point, there are various hackjobs in the wiring which I've cleaned up most over the years. There is still some old wiring from the previous brake controller. a constant hot running to a two conductor back to midpoint on the truck where it goes into a ball of electricla tape and wires, then a larger diameter multiple conductor wire back to another ball of tape and loose wires at the bumper near the old 6 round plug I removed. I don't think what's left of this would affect this issue. I will remove all that and am replacing it with two new 10ga conductors to the rear for power and brakes and tap the other 5 at the bumper/quarter panel to run a 7 spade RV plug+5 flat combo.

Truck has a 130 amp 3g alternator, RTRM mini fuse/relay box used for headlights, blower motor,stereo, brake controller. Fairly new 2/0 grounds off batteries and between batteries to 4ga to the solenoids/distribution to the cab and 6ga to the RTRM panel. Additional grounds have been added, including 14ga to pass fender and pass inner fender electrical mounting plate, 14ga to cab off pass battery, 10ga to inside cab off drivers battery (distribution for gauges, brake controller, misc.).

Also possibly relevant is I changes to LED backup lights and taillights. These lights will sometimes be dimly lit (less than if parking lights are on) when the truck is running (or sometimes off) and I've seen a very dim glow rarely when it's shut off.

I like to think I've de-hacked most of it but maybe I'm missing something, or maybe the cruise control module is faulty. I have zero knowledge or experience with cruise control on 80-86 trucks so no idea if this could be by design (doubtful, this wire should be to interrupt cruise and dump vacuum, activated by brake or clutch push), or where to start looking. CC is not important to me, so I could remove it, but since the truck originally came with it I've had the thought of maybe getting it working eventually. I did a C6 to ZF5 swap last year and am rebuilding a manual column to swap in, so not even sure how the wiring compares yet as it may not be possible to swap the column components over.

 

Maybe relevant diagram. The pinout on the cruise controller is different than mine, but I assume same concept should apply?

 

Gary has the 1986 EVTM on his garysgaragemahal website.

​​​​​​​Proper diagrams, and broken out circuit by circuit.

​​​​​​​ http://www.garysgaragemahal.com/speed-control1.html

 

Here's the turn & stop lamps.... Turn, Stop, & Hazard Lamps - ???Gary's Garagemahal


And trailer option...
​​​​​​​Trailer Option - ???Gary's Garagemahal

 

I bet you put your regular taillights back in and that ghost voltage will go away. It is from the cruise control brain, on the later trucks it sometimes comes from the RABS board.

The stray currents do no harm when you have a big old timey bulb back there that drains it away. But put LED's in there and everything changes.

 

I could try incandescent again and see, but I doubt it will drop 8 volts. I prefer brighter light from the LEDs (mainly for reverse lights) but the big reason is lights that don't burn out.

I'd imagine I could put a diode in this wire off the cruise control module, don't know why I didn't think of that once I figured out where ti was coming from.

 

Diode might work. Just do a test run and make sure the cruise drops out when you touch the brake. What also may work is putting load resistors across the LED bulbs in the rear. Just across the brake/turn filaments.