I have a 1986 F150(2WD) . When I apply the brakes my turn signal slows down . When I turn on the headlights I completely loose my turn signals , brake lights and running lights . I have checked grounds , replaced headlight switch and alternator . What am I missing ?

 

Have your battery load tested, any parts store will do it no charge..

 

I would see of brakelight bulbs are installed correctly, then maybe replace the turn signal switch

 

You have checked grounds you say. Battery cable to engine block? Engine block to somewhere at fire wall? Take a booster cable, attach to Neg battery post. Other end of cable to a clean connection on the firewall sheet metal. Then try attaching to a cleaned spot on the frame. Try your problem to see if corrected by using the booster cable ground at the two locations I mentioned. Inspect all 4 bulbs the two stop and tail in the back and the two park and signal bulbs in front. Look into the bulbs and see if one of the two filaments is broken and laying against the other filament. The stop and tail lamp housings, run the mounting screws out and in 3 or 4 times to freshen, clean the mounting screws connection to the body sheet metal. Are your 2 battery cables original or do the have replacement emergency ends that go on the battery posts?? If replacement type, see about corrosion of the cable under the clamp bar on the end. Finally all the wiring at/around the starter solenoid. Is it all neat like factory or has some of it been messed with?? Look for a wire that may have many strands broken near a terminal end and no longer able to carry the load that needs to go thru it.
Cap

 

THIS is some of the best advice I have ever seen on here!!!! I 99% guarantee he does all this his problems will end!!

 

THIS is some of the best advice I have ever seen on here!!!! I 99% guarantee he does all this his problems will end!!

Thank You DonW for the comment/compliment. Gotta go easy tho, I don't need to have the space between my ear's swelling up. All kidding aside, Thank You and I hope 1eyedjac finds his problem. If NOT, I hope he comes back and we'll dig deeper. Cap