Check engine light on m1088a1 (limp mode?)

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iKent82

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So I am new to the FMTV world and recently took delivery of a 2000 m1088a1 fro GovPlanet. That is a whole story in itself but I now have the truck and I am trying to diagnose what I think is an electrical issue. The truck is very low on power and only gets up to about 15mph on flat ground. Just doesnt get the RPMs up enough to shift. On a downhill it can get going and shift. Uphill I have to manually select gear 1 and barely move. I used a cheap code reader that tells me that there is high voltage. I have checked each battery and get about 12.3v and 24.5 or so when linked. I have replaced and checked the alternator and regulator. I have checked everything with a volt meter. What I do notice is that even though the batteries are topped off the alternator outputs 28v. I am thinking somehow it senses the batteries need a charge. I am thinking the problem lies with the "reverse polarity protection box". Not sure how that works though. I am guessing my other two options would be checking the ECM (requires a CAT mechanic) and replacing the really board in the cab (expensive). Does anyone have any experience with an issue like this? Or ways to trouble shoot?
 

snowtrac nome

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you don't need et to check out a 3126 you can do it with a prolink and cartage you can buy inexpensively off e-bay. that would be the way to go to retrieve codes, likely your total cost would be the same as having a cat tech plug in with e-t. there are no trim codes to put in when replacing injectors on a 3126 which means you don't need et to work on it. first thing I would look at check the turbo, see if it turns I plugged into a 3126 the other day with et just to find there was no boost, and the turbo was locked up. it was doing the exact same thing yours did .
 

Suprman

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2000 should have the older style polarity box. Just a bunch of diodes that protect against reverse polarity. If you have 12 and 24 volts at the power panel then its working. The green pcb power panels go bad the soldering is lousy on some of them and you get intermittent electrical issues. But I dont think it would cause your specific issue. Take off the turbo intake and see if the turbo spins.
 

iKent82

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I don't think it is the turbo as no black smoke. Also, I have some sort of an electric issue I would assume since that it the only code I get. No indication but I will look at it. The polarity box thing is confusing (to me) anyone have tech manuals on it? I have also heard the PCB panels go bad and have considered replacing it.

I know the truck is over charging the batteries I just don't know why. And I am guessing because of the overcharging it is in limp mode.
 

Suprman

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If its the 4 post first gen one its just 12 in 12 out and 24 in 24 out. Only cab power goes thru it. There is a separate 24v lead to the starter from the batts. If you have 12 and 24 at the panel then its good. Just a simple diode array to prevent someone from hooking up the batteries backwards and cooking the cab electronics. The panels go bad the military used eco friendly lower lead solder in some of them and you can get bad solder joints. I have the new boards. Use a digital meter measure at the slave port with the truck running and then truck off and post the results.
 
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