My daughters boyfriends truck hasn't charged properly for over 7 months. He asked me to take a look at it. He has literally been charging this thing with a charger every night!

The history, 2 new batteries 7 months ago. Alternator replaced, new pigtail. Another alternator last week, also replaced the dash guage cluster. Still no charging output. His inside guage would never really go over 11.7 volts after starting the truck.

So I researched it here, found the diagnostics chart with the proper output voltages for each wire to include ko/koff position. Got the schematic for all the fusible links.

I was suspecting a blown fusable link or a rat chewing a wire in two?

His truck ran out of power just driving it to my house, had to do all this on the side of the road.

I used my voltmeter and checked the circuits. B+ wire reading batt voltage, A circuit reading battery voltage, but here was the problem. I circuit sense wire that feed the dash/battery light. It should have read 0 volts with the key off. It read .01 volts. Now I know thats not much but it was off according to the chart. With the key on it read 2.49 volts, right on the money. Then with the key on engine running it was supposed to jump to 8-9 volts but did not. It stayed at 2.49 volts.

So even though this alternator was only days old I suspected it to be defective.

Took it to auto parts store, it tested BAD! He was fit to be tied. They unboxed a new one and it tested good. We put it on and it fixed the truck, all is well and working properly.

A point of interest is the relationship of the glow plugs and the charging system on this truck.

He was worried the pcm on his truck was bad and not letting the alternator charge. Some on this forum thought that was true but others correctly stated that it is not connected on this truck. This truck has two batteries and one alternator.

We watched after starting it up and initial voltage while the glow plugs were on was hanging around 11.7....after they started turning off, somehwere around 1 minute, the voltage started rising and came up to 13.7 and I knew we were on the right track! So it does appear that the alternator can NOT keep up with the 180 amp draw of the glow plugs for the first two minutes, and when they shut off all is warm and fuzzy again.

I want to thank this forum for all the info available here and the diagnostics chart posted to help me diagnose this thing.

 

That's partly the reason that it has two batteries. These trucks will start in the summer on one battery, but when it gets cold and the glowplugs are on for a long time, the oil is thick, and it takes more cranking, you need the reserve of both batteries to get it going. Ford usually puts a official temperature limit on these trucks as to how cold it can get before they are required to be plugged into 120v. I think my truck is 10 degrees. I know they will start lower than that, but it will probably get to a point when they all have to be plugged in to start.