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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...

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!

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...

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...

Okay, another member provided a schematic and some info.

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...

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...

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:

Actually, $100.00. The ST88 was $80.00 at NAPA.

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...

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...

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.

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...

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.

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...

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...

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...