I just purchased a 1986 F-250 Lariat Diesel this weekend, and was pleasantly surprised at the condition. However, driving home at night, I noticed that the headlights were pulsing. My son began to investigate, and the entire lighting system in the truck was pulsing, dashlights, radio lights, cablights, everything. I am almost totally ignorant about electrical things, my gut tells me alternator (?) going out, or a poor connection. The wire going into the alternator seems to be tight, and I have yet to run to auto parts store for system test. Anyone out there with more experience got an idea? Thanks

 

Several times on here it's been a bad ground from the regulator case to the sheetmetal. If it looks good, see if there are any ground straps from the battery/engine block to the sheetmetal that may be missing.

This is assuming you have the alt with the seperate regulator.

 

IT sounds like either the voltage regulator is bad which is inside of the alternator (I think) or a bad ground on a headlight.
before you change the anternator pop the hood and check the grounds on both headlights....look for a single wire coming off each headlight pigtail that is grounded directly to the fender....make sure that the screw that grounds it to the fender is tight...I had this issue on my 80 and chased everything but a bad ground for 6 months. also check to make sure that your battery cables are tight....

 

Thanks for the hints, will check connections next free afternoon. Will keep all posted.

 

Sir, do yourself a favor and check ALL the grounds on the vehicle! Yes, it takes some time and now is a bad time to be clambering about underneath vehicles doing so, but it may help. Make sure all the ground cables/wires from headlights, body grounds, battery grounds etc. are clean and secure. Recently I spent $700 in repairs and went thru 3 batteries in less than 30 days until the culprit was finally found. Corrosion on all the ground connections. The vehicle (NOT a Ford) spent some time on the East Coast near saltwater. In my case turning on the headlights would kill the engine..so would going from high-beam to low-beam!! My next suspect would be the Alternater/Voltage Regulator. Keep us posted! Audie..the oldfart!