I need help as my headlights started flashing off and on when we were coming home from camping. I got this truck - 96 F250 - last fall but this was the first time we had been on an extended trip after dark. I had replaced the headlight switch earlier as the instrument lights did not work. When I run the lights on dim the flashing stopped and we were able to get home OK. I also have replaced the dimmer switch assembly as the 4 way flasher did not work. It has a new battery, I had the alternator rebuilt by a local shop. What is next?

 

Cables and ground connections need to be shiny bright and tight, grind down to bare metal, block connectors and SAE connectors, through firewall harness etc, each connection that has resistance adds up bigtime, that's the classic problem as trucks get older.

New ground and starting cables are best, they are prone to corrosion on the inside crimp and under the insulation, where it can't be seen. Even an invisible layer of skunge between a connection is enough to cause starting and charging problems. 0.06 ohms resistance (hardly anything) in the charging system for example reduces alternator output by 30%! Not enough to notice at idle, but under heavy electrical load it will choke.

Here's an easy way to measure this problem so you can identify it anywhere on the truck, no special equipment other than a digital voltmeter, no disconnecting anything.

High current carrying conductors are always measured while under load. The voltmeter is connected across the connection. If there is high resistance, the juice will take the easy way, through the voltmeter. So, the actual voltage drop will be displayed as a + positive number on the voltmeter. Slick, and quick. This works on headlight cables, alternator and cables, starters, starter cables, etc. Usually anything more than a couple tenths of a volt is considered excessive.

Keep your charging and starting system happy with good grounds, and everything else - ignition and lights - will be happy too. It pays to do this. In your example, find the negative terminal on the headlight bulb. Place the voltmeter probe there. Place the other end of the voltmeter on the negative post on the battery. Turn the headlights on, try it at idle too. All of the voltage lost in the negative side of the headlight circuit will be displayed. Can narrow it down as fine as you want, from a single cable, or even between the battery post and the battery terminal. Then check the positive side of the circuit. Between the two sides, no more than say half a volt drop should be observed. Headlights need all the juice they are supposed to get, or the brightness will drop off a cliff. All of the resistance confuses the alternator and regulator too.

http://www.aa1car.com/library/voltage_drop_testing.htm

 

[Deleted. ^^^What he said ^^^. His post finally popped in after I said check the ground wire and is heckuvalot more detailed.]