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M939 Series Protective Control Box Work Around

jaws4518

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Pretty sure your 939 is 24volt unless you have swapped everything out

Sent from my SM-G965U using Tapatalk
The ST88 is the battery switch solenoid, and the SAZ -4401AA is a double-pole solenoid/relay with the (I) initialization & (S) start contact poles. NAPA does not have this exact relay and could not cross reference this part number. They did have the ST420 which is the exact same relay w/o the (I). So, in conclusion... my ST88 was bad. If the SAZ-4401AA was bad then I could tie the pre-start initialization routine to the battery switch and use the ST420 which NAPA has in stock. The only thing I'm not sure about is... does the PCB circuit board do a subsystem initialization check, then sets initialization "true", then cuts initialization power, then sets the start sequence flag "true". If you notice, the ABS light comes on, then you hears some relays or other noises, then ABS light goes off and everything goes quite. READY TO START! So... I'm not sure this ST420 without the (I) pole will work. There is more going on electrically than I initially thought.

Anyone concur on that?
 

jaws4518

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Hey, I went to the link you provided. I did not find any instructions or diagram posted. There is just a minor conversation about his solenoid getting hot after jumping. Nothing useful here...
 

jaws4518

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I was reading through some of the old posts and found some good information. One thing that caught my eye is the subject of grounding. I found through experience yesterday that the case of the control box has to be grounded to the truck. There is bare metal at all four bracket points. I just assumed that the unit would be grounded internally by some wire lead. I also found that the solenoids need to be bonded to the mounting plate. I guess that is why they are spot welded to the plate. I found this out through trial and error. Letting the box hang by the harness while testing won't work either. It wouldn't surprise me that the grounding issue is quite problematic if you don't pay attention to it. I'm going to drill two small weep holes in the bottom plate before I seal mine back up. Someone mentioned the control box being hermetically sealed. Yeah, I would concur with that for sure. Not any more though! Its got to breathe now, or everything inside will corrode quickly. I might even put a desiccant pouch inside to help keep the condensation level down. Hope my observations help the next guy.
 

jaws4518

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Where is "running motor start safety) switch? I'm still having a problem somewhere? Also, I just finished trying a "jumper start" with shunt across C & D. I get ignition power to low-air and parking brake light and audible, but no crank? I'm not getting any life from the repaired 929/A1 control box. Anyway, what am I doing wrong? Any other safety switch checks or connections I need to make?
 

jaws4518

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Yeah, I got it started using the jumper. I just need to go back through all the CB testing procedures again. It could be the circuit board. My switches and wire leads test good. I will start at the battery switch again and trace everything down stream from there.

Thanks!
 

Swamp Donkey

The Engineer
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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.
 

jaws4518

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Hey, thanks for the good info. I replaced my batteries yesterday, I should have done it last year when I noticed slow first crank. I probably put some stress on my control box and starter with weak batteries. I plan to trace through the control board circuit today. I cannot visibly see any diodes or other packages that look burned or bad. Lot's of diodes on that board. The electrical bus on the printed circuit board is large, so it won't be too difficult to de-solder and replace a diode. Maybe someone has done this and knows which area of the board gets degradates first.
 
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