Batteries not holding charge

[Geoff Weeks]

If you are going to go with a modern alternator, you'll need a pulley (most modern stuff uses a 7/8" shaft, the early L/N uses a smaller shaft) but most importantly, you'll need to make a new mount. The early L/N was designed to replace the normal generators of the day, and so came with a mount that would fit the same as a generator.

Modern alternators come in either pad mount (not what you need) or SAE j180 swing mount.  The SAE mount takes a 1/2" bolt and has a spool on the engine 1/2 of the mount, the two ears on the alternator fit over the spool and a 1/2" bolt passes through all.

The generator mount used 3/8" bolts and a much wider spacing between "ears".

To make the mount, you'll need some 5/16" or 3/8" plate and some cored bar or xx pipe (schedule 180) for the spool. It has a 1" or so OD and 1/2" ID. The mount will have to be fabricated carefully as it aligns the alternator (and therefore pulley) to the engines belts.

Alternatively, you could buy a mount for a Cat or Cummins (or other engine) and adapt it to your block.

Alternator will need to be either positive ground or case neutral.

[609albert]

ok ,we know that the pulley alinement  is most important , thank you for you wealth of tech advise 

[Geoff Weeks]

If your regulator is EXACTLY like the one in my picture (there were several different regulators used), with the side with the single terminal pointing away from you and the three facing you, the three are 

far left: output to batteries and/or ammeter

center: output from rectifier/ alternator

far right: field

the single terminal facing away from you is ignition feed.

Note the regulator is mounted in rubber bushings and need the case grounded to function. On most there is a tab on one of the bushing that grounds from the bolt to the case of the regulator.

If you are going to replace with a modern one wire alternator, you need to extend the wire from the left terminal on the regulator to the output stud on the alternator, and remove the regulator from the truck. You also remove the rectifier and assorted wires along with the L/N alternator.

Note: you can identify the main current carrying terminals on the regulator as they have 2 screws holding the wire terminals on the regulator, the field and ignition feed only have one. They are also stamped with letters (G or generator) F for field, ign for ignition etc.

Edited Wednesday at 01:49 PM by Geoff Weeks

[Geoff Weeks]

If you want to try and diagnose the present system, start at the regulator.

With the battery disconnect switch in the "on" position and ignition switch off, you should have power at the left terminal of the regulator when measured between it and ground AND measured between it and the case.

Next turn the ignition switch on engine off. You should see power at the ignition terminal of the regulator (the one by itself on the other side from all the other terminals), as well as power on both the generator (center terminal) and some on the field terminal. So all three terminals should have some power at least with the engine not running and the ignition on.

Start the engine and look for AC voltage between any two of the three big output terminals on the alternator.

If you don't have ac voltage, check the two small terminals on the back of the alternator (they are slightly staggered) one should be seeing power and the other should be going to the ground side of either the engine block or the rectifier. If you see power on one and none on the other, it is ok, if you see power on both, you have have a broken ground side wire, If you don't see power on either, you have a broken wire between the regulator field terminal and the alternator. With the engine running, the voltage at these small terminals (field voltage) will be less than the full 12 volts, but should be a readable voltage.

 The part most likely to fail is the rectifier.  It can fail open (no power gets through) or shorted (allows power from the battery to drain through the stator windings.

You can replace the rectifier with a set of diode blocks out of a modern alternator, you'd need to mount them on an insulated board and somewhere where they would get good airflow to cool them