A friend has a 2001 F150 which has developed a troublesome problem. When you turn on the parking/headlamps, the brake lights (including the CHMSL) come on and remain on bright. If you hit the turn signal or 4 ways, the indicator(s) in the cluster light steadily and the front corner lamps go to full brightness with no blinking...

We checked all fuses, they are all fine. Any ideas what could cause this and/or possible remedies?

 

Lets start with some specs on the truck.

1. Trim level & body style ( the wiring diagrams are a bit different for specific trim levels and body styles ), and if he has the factory trailer tow package, with all the relays installed.

2. If the main headlamp switch is in the off position :
2.1. Do the turn signals work correctly front and rear ?
2.2. Do the 3 brake lights ( without the turn signals on ) work correctly ?
2.3. Do the 3 brake lights ( with the turn signals on ) work correctly ?

3. With the problem noted, do the trailer tow wires exhibit the same symptoms as the truck ( both L and R S/T/x and the parking lamps.

4. What happend when you turn the main headlamp switch to on, and press on the brake lamps ( no change, or something different, note both front and rear ).

Those are the start to the questions that I can see.

 

I'd suspect that your MFS has developed a short circuit, either internal or at the wiring harnes connector.

Steve

 

EDIT: I got a reply, the answers are added below in bold.

1. XL Regular cab longbed, 4x2, 4.2 L auto. The truck had the flat 4 plug from the factory, but no receiver, so I'm guessing it doesn't have the tow package.

2.1. They work until the brake pedal is pressed, at which
time they just stay stuck on.

2.2. Not really, since they are on the whole time.

2.3. They are just on all the time if the light switch is on.

3. Have not checked, but will.

4. Brake lights are constantly lit when park/headlamp switch is on, although, if you press the pedal, there is a very slight increase in brightness.

Does "MFS" stand for "multi-function switch?"

 

Yes, that is correct. Usually the Hazzard switch cleans the contacts / acts normal when used, this is why I was thinking along the lines of something could be shorted, but Steve has a point with that post.

If you want to short cut to testing the MFS, it can't hurt, and would be in the list of items, depending on the answers to the questions above.

If you want to go right to testing, remove the MFS from the steering wheel column. here is the procedure :

 

I received a reply and added the answers above to keep them all in one place.

I did think of something else, there was an issue with the trailer wiring under the bed a while back necessitating a repair, could something screwed up back there cause symptoms such as this? Is there a way to isolate that harness?

 

Well that is some good detail.

Not withstanding Steve's recomendation to test the MFS, I woudl take a slightly different approach.

Remove both rear bulbs, and see if the CHMSL still has the same problem, and check if with the parking lamps on, that the fronts are on the low element, and if you use the turn signals, or 4 Way, that th efront flash ( albeit quickly due to limited load ).

With the slightly brighter, I am guessing at a bad ground on the rear of the truck, and the stop lamp is where it is deriving ground from ( via the CHMSL or BOO switch ).

Removing both the rear bulbs will stop any ground finding.

Question on the flat 4 under the rear bumper, is this part of the factory system, or an add on kit, or DIY add on ?
Trying to figure out if it is using the trailer tow circuits in the truck, or the actual truck lights ( should not be ).

 

It is the factory wiring, but a new plug was spliced on during a repair...

I'll pass on the results of the bulb removal and see where that takes us.

Thanks!

 

If it is the factory harness, unless he cut the ground side, it should not be a problem. The ground splice is to the frame where the factory tow harness connector is at. You can see it just below the connector for the factory 7&4 pin harness, it is a screw into the frame with a 10 AWG white wire I do believe.

 

Well, that very well could be the issue, there was a ground problem with the 4 pin (which is all the truck has, no 7 pin connector) and a new plug was spliced in with a new ground. Is the lead to the 4 pin a seperate harness, ie, is there a point where that can be unplugged to isolate it? (I don't recall and the truck isn't here to look at)

 

I think it is the same connector up under the truck.

On my '01 and my '06 it was easier to see by dropping the spare tire.

Take a bulb out of one of the S/T/T and check the ground to the frame ( clean bolt ) as a starting point. Should be 0 ohms or the continuity tone.

 

SSCULLY: Thanks for the help, we traced the problem to the plug. For now it has been clipped off and will be replaced at a later date. All lights now work properly.

Thanks again!

 

Don't thank me, I had you going upstream. You guys got that one on your own !
Thank yourselves for problem diagnoses 101, completed sucessfully !