M939 Series Protective Control Box Work Around

jaws4518

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I've got a bad control box on my M929/A1. I knew it was the control box, but I went ahead and did some electrical tests on the ignition switch to the neutral start switch etc... There is no 566 lead to the alternator for lock-out on this truck. So, I had a spare control box WSU 4001UT which came off a M809 series truck that I installed in place of the WSU 4004UT on the M939 series truck. Everything seems to be normal as far as dash electrical with battery switch on, but it's not letting me start the engine. Something is different with this unit that is locking out the start sequence though. No cranking power through the circuit? All electrical power to dash and protective controls seem to be energized. Parking break signal, and low air pressure warning lights are active etc. Everything looks normal. Starter switch is good too.

Any ideas on what is circuit or switch is keeping the start sequence locked out?

I opened up the WSU 4004UT control unit. The circuit board looks fine. I'm at the point of testing the solenoid and relay inside this unit. If those are good then something on the PCB is bad. Probably a diode or something? The failure symptom with the original control box is... battery switch on... short beep with warning lights and siren, then nothing. You can hear the relay click too. That's it!
 

simp5782

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I've got a bad control box on my M929/A1. I knew it was the control box, but I went ahead and did some electrical tests on the ignition switch to the neutral start switch etc... There is no 566 lead to the alternator for lock-out on this truck. So, I had a spare control box WSU 4001UT which came off a M809 series truck that I installed in place of the WSU 4004UT on the M939 series truck. Everything seems to be normal as far as dash electrical with battery switch on, but it's not letting me start the engine. Something is different with this unit that is locking out the start sequence though. No cranking power through the circuit? All electrical power to dash and protective controls seem to be energized. Parking break signal, and low air pressure warning lights are active etc. Everything looks normal. Starter switch is good too.

Any ideas on what is circuit or switch is keeping the start sequence locked out?

I opened up the WSU 4004UT control unit. The circuit board looks fine. I'm at the point of testing the solenoid and relay inside this unit. If those are good then something on the PCB is bad. Probably a diode or something? The failure symptom with the original control box is... battery switch on... short beep with warning lights and siren, then nothing. You can hear the relay click too. That's it!
M809 boxes will not work on the 939s due to the ABS. The 939 boxes will work on a 809 series but not vise versa

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ToolManTimTaylor

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I remember a thread on a simple 10 gauge jumper (emergency) from pin (D) delta to Bravo (B) (please verify pins) on the cannon plug itself to get you going.

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jaws4518

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M809 boxes will not work on the 939s due to the ABS. The 939 boxes will work on a 809 series but not vise versa

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That makes no sense. So, the ABS might be locking me out? That doesn't sound right? I wouldn't think that ABS would create a system fault to keep you from starting the truck? I'm going to look at the electrical schematics to see if I can find everything that might be in play on this. As far as I know, ABS works or it doesn't. Right now, the ABS initializes and the light goes off as normal. If the ABS keeps our trucks from starting then our military is in a world of hurt! Show me something that supports that? I haven't seen anything that supports what you are saying, but you could be right.
 

simp5782

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That makes no sense. So, the ABS might be locking me out? That doesn't sound right? I wouldn't think that ABS would create a system fault to keep you from starting the truck? I'm going to look at the electrical schematics to see if I can find everything that might be in play on this. As far as I know, ABS works or it doesn't. Right now, the ABS initializes and the light goes off as normal. If the ABS keeps our trucks from starting then our military is in a world of hurt! Show me something that supports that? I haven't seen anything that supports what you are saying, but you could be right.
The solenoids do not work in the same sequence to power up on an 809 box

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jaws4518

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That makes no sense. So, the ABS might be locking me out? That doesn't sound right? I wouldn't think that ABS would create a system fault to keep you from starting the truck? I'm going to look at the electrical schematics to see if I can find everything that might be in play on this. As far as I know, ABS works or it doesn't. Right now, the ABS initializes and the light goes off as normal. If the ABS keeps our trucks from starting then our military is in a world of hurt! Show me something that supports that? I haven't seen anything that supports what you are saying, but you could be right.
Can you think of any other protective switch points that could break the starter control loop? I can tell you right now that some N.C. switch is being forced open due to the differences in the 4001 control box. I'm not familiar enough with the 939 series trucks to have a mental picture where I can ID the problem and go straight to the source. All I can tell you is that "all systems seems to be go", but no start voltage to put it simply. Yeah, I can start tracing it all out.
 

jaws4518

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10/4! Can you think of a work around? I've got this 939 box open. It appears to be a Faraday protected box. The ends are soldered on with a copper gasket. It's like sweating copper pipe together.
 

73m819

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The 939 box has TWO safety start protection switches, one is for motor rotation (oil pressure) in another words a running motor , and a second one for trans natural to prevent starting in gear, the 809 box only has ONE safety start switch, the running motor start safety, the 939 safety starting switches are wired in series which mean BOTH HAVE TO BE CLOSED to start the 939, this can not happen using a 809 box on a 939.
 

jaws4518

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Thanks everyone! I plan to diagnose the control box fault this morning. I'm going to remove the 809 control box and put the M939 4004 back before I jump. It's either the solenoid or the relay that is bad in the box. Off the top of my head... it must be the relay because there is no power to my gauges and dash controls when you power on.
 

simp5782

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Thanks everyone! I plan to diagnose the control box fault this morning. I'm going to remove the 809 control box and put the M939 4004 back before I jump. It's either the solenoid or the relay that is bad in the box. Off the top of my head... it must be the relay because there is no power to my gauges and dash controls when you power on.

Just replace both solenoids. make sure one is continuous. Keep the spare one that is working on your Control box in case of future failure. Cost to replace both of them is around $50.
 

jaws4518

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Okay, that is exactly what is going on. I'm going to put the 939 box back and follow the jumper start instructions. But first, I'm going to NAPA to see if they have the solenoid or relay in stock.
 

simp5782

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Okay, that is exactly what is going on. I'm going to put the 939 box back and follow the jumper start instructions. But first, I'm going to NAPA to see if they have the solenoid or relay in stock.
Its a non stock part # ST88 at Napa
 

jaws4518

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You da man! Thanks for all the help. I will report back on what I find and how I fixed it. I'm very lucky that this did not happen in WalMart's parking lot. I noticed it failing several days ago. I would initialize power, then I wouldn't. Luckily, it had a few extra start iterations left before it went completely dead.
 

jaws4518

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Got both solenoids. Not sure about the part # on second one. Assuming it is an ST43 which is 12 volt. I'll have to cut the original out to see the part number. I'm going to review the schematic and test on the bench first to see if I can isolate which one it is. Hopefully it is one or the other? Luckily I have an old harness to test with. That will make diag connections easier.

If you know what the black solenoid part # is, please confirm.

Thanks!
 

simp5782

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

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jaws4518

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The ST88 is continuous. I believe the other one is too. If anyone knows more about the other solenoid and part # and voltage, sound off. I won't know until I cut it out. I bet it is the one that has gone bad, but I'm just guessing until I get busy.:confused:
 
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