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2005 F150 - Possessed?

When my turn signals are on, they blink normally until I press the brake pedal at which time BOTH flash quickly UNLESS the headlights are on in which case they BOTH flash slowly. Stranger still, when I have the headlights on, the tail lights go out when I press the brake pedal. Also, the cruise control refuses to work when the headlights are on and while it does work when the lights are off, it turns off when I use the turn signal.

I’ve checked all fuses, swapped out bulbs, and even checked the ground connections. All fine. I checked the connections on the multi-function switch and they all appear to be fine as well. I didn’t try swapping out the flasher relay because a) the parts store was out of stock and b) it was $35 and it seemed a bit steep for a part I’m not even sure is bad.

Thought about taking it to the dealership, but thought a priest might be a better alternative.

Any ideas what might be wrong? What have I missed?

It really sounds like a grounding problem.

Where would I begin to check that? I checked two grounding points above the spare tire. Both were solidly screwed to the frame and ran into the harness that went to the taillights.

Try replacing both rear brake lights (on some cars that might be four rear brake lights.

Ok. I’ll give it a shot tomorrow. I only have one spare here. I did swap with each and had no change, but I suppose there could be a problem with both.

Replaced both lights this evening and tried it. Still the same problem. I don’t remember if I checked for continuity between the socket ground and the frame. Will try that later this week.

In the meantime, any other ideas?

Checked continuity between the socket and the bumper. Meter read 1.5ohms.

Lets try a different approach. Based on the fact that you ohmed the ground for the rear bulbs, that means you have a volt ohm meter. Ohm testing for all intensive purposes for automotive repair is unreliable. (many will argu with that statement, the reality is that in the field an ohm meter is a waste of time especially in cases where significant amperage is involved) Go ahead and remove one rear lens assembly and turn the lights on…

Assuming the concern is still present, take your volt meter and connect the negative lead to a known good ground, (Best location is the negative battery post, you’ll need a long jumber wire for this. If not try to find a clean piece of sheet metal or a clear spot on the vehicle frame). Next, get yourself a paper clip and straighten part of it (T-pins from staple or office max work great for back probing plus there really inexpensive and have a nice pointed end which aids in getting pins pass electric seal grommits). Inset the paper clip into the back side of the rear bulb electrical connector where the black wire enters the connector. (The bulb should still be plugged in, you will be inserting the clip into the connector from the wiring side)

Put your meter into the volt position and physically check for voltage on the black wire (lights should be on) Because you are checking a ground wire, there should be no voltage on that wire… Ground should always be ZERO Volts. There is an acceptable voltage drop accross wiring so it won’t be perfect Zero, but better be below 0.5 Volts. If you see voltage in excess of .5 v on the ground, the ground path is experiencing excessive resistance.

Your truck has three connectors located on the ground path as the current attempts to get back to the body ground. Two are located behind the bumper, to the left of center. (fairly close together. The third connector is sitting on the frame rail, almost immeditely below the drives door area. The ground eyelet for the rear tail lamps is actually on the left A-pillar inside the passenger compartment, behind the left front kick panel.If you do dtect voltage on the ground I would inspect these connections for corrosion. If no corrosion is found you can also perform the back probing method on these connections. By back probing these you can isolate the poor ground to a particular harness or connector. Also keep in mind that when a connector is plugged together, you can back probe from either end of the connector Voltage measure on one side but not the other indicates that the connection is poor within the connector.

Most PU’s now come with hitches and either pre-wired for trailer lights or actually have the plug next to the hitch. Check out the wiring in the area of the hitch, and/or plug. Perhaps it has gotten damaged, pinched, or munched on by a critter.

That’s exactly what I needed – a step-by-step approach to the diagnosis with explanations. Using the back-probe technique, I was able to trace it to the connection below the driver-side door. Was getting 0.05V on the front-side and 4.4V on the back. Cleaned the connection with appropriate cleaner and works like a charm. Appreciate the help!

Great Job,

Now that you found the point of resistance you may want to think about replacing the corroded/damaged/spread terminal. (Think of it as a little CYA. Cover Your *&^) Corrosion (if this is what it was) started likely from water intrusion, you will want to ensure no additional water gets in.

If you know a good Ford dealer with a good parts man, he would be able to tell you which pin you need and order it for you. You can also go to, they have wiring pigtails and terminal illustrations. You may need to hunt a little to find them but in my opinion it would be worth the effort.

At the very least get some electrical contact grease and apply a small amount to the terminals. You don’t need to pack the assembly like a wheel bearing, just a small amount will do you. You should also apply just a little on the wire side as well, just to ensure the grommit is sealed as well.