I placed an order for a couple of PCBs today from Southern Automotive. I held off on replying to this thread until after I verified my memory on a couple things with my buddy there. So here are some facts...
1. The circuit board, for those that have them, can most simply be described as a Go/No Go switch. On the M939 trucks it senses A/C voltage from the alternator to determine whether or not to send power to the starter. This protection circuit is there for the sole purpose of a battlefield situation. You might not hear the engine running with the noise of battle all around you. This only describes the M939 trucks.
2. There were several different manufacturers of these boxes over the years. Prestolite was the first and Southern Automotive is the current one. Southern Automotive has made 6 different revisions to their M939 box alone in an attempt to improve design and longevity. Most of the revisions pertained to the circuit board design. Enclosure design was one of the first ones. There is no way to tell what version you have without opening the box and knowing what to look for.
3. The boxes without a circuit board still had voltage sensing in them but used diodes and stuff instead of a circuit board. The most common killer of the PCB that they see is improper shutdown sequence. This sends a voltage spike to the board which eventually degrades the paths on the board.
4. The PCB design can't be changed by them. They can make small improvements to the design but the design must stay the same. It was agreed on both sides of the conversation that there were better ways to implement a system with these requirements, but they must make it the way it was designed.
5. (Personal observation) If nothing else impresses you about their improvements, the box design will. The box itself is a stamped corrosion resistant shell. It is then hermetically sealed to meet the current waterproofing ratings. Previous manufacturers used enclosures that looked like a 5 year old made with some tin snips. Moisture intrusion was an issue with those.
6. Southern Automotive is the current OEM and has been for a while. They continually improve on this design where they can. They also have the cheapest price on the most current revision. Anybody else selling new production boxes will have mark-up because they bought them for the same thing you could. You also will not know which version you are buying.
7. Individual purchases are being directed to Ebay due to some piss poor people and a couple of large vendors who can't seem to pay for things they purchase. Buying directly through them in small quantites without an account is a lengthy process now, involving fund verification and other protections. Their price is the same on Ebay and is much quicker for both sides, although the price has gone up a little due to their latest circuit board revision. Someone mentioned their Ebay username above. You're still buying directly from them but they are protected from deadbeats by Ebay and you get the box much faster.
I have no affiliation with them so this is not a sales pitch. I have no problem pushing a manufacturer that supports their products though, which is something rare in the surplus world. I only intend to share some facts, clear some confusion and find the best deal if a new PCB is the route you wish to take. Replacing solenoids is still a viable option.
I haven't seen it mentioned in a while so I'll say it again. Improper shutdown hurts the electrical system on these truck, and not just the PCB. Engine OFF, then Power OFF in that order. Not saying it's the case here but it's important on a system with the stock alternator.
