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12 speed no reverse?


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Hello Fellow Truck Drivers, owners,

I own a 1990 Mack Ch613 and it has the mechanical 350 engine with a Mack 12 speed transmission. Four axle heavy haul truck with low rear end gears in 46,000 lb axles.

My dilemma is that occasionaly the truck won't shift into reverse? Reverse gear is actuated by an air solenoid (to the best of my knowledge) and I am able to shift through the gears in reverse mode. ( NIce feature). Like I said, from time to time, when I would move the selector at the top of the shifter to the far right, where reverse is located, the trans will not move into reverse? Now it won't do it again and I need to move some machines with my lowbed!! GRRRR... any suggestions what could be making this happen? Thanks you in advance. Brian/ Garden State Services LLC

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is it exhausting air when in reverse? there is a exh. line from the shift knob that is open to (open air) just below the cab. mine in cold weather will just exhaust air out the line until the shift selector warms up. if i'm in a hurry I can block the exh. line and make it shift to reverse. I tried kiting the shifter but no luck. I think I will have to get a new shifter valve.

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Yes, when the selector is in reverse position air is coming out of the bottom of the air solenoid mounted on the drivers side of the trans. there is a black rubber flat seal that air escapes past. Funny you should say that your truck does it when cold, usually if I drove around town for about 1/2 hour it would start working. Maybe I need to pour some brake line antifreeze in the system just after the compressor? Don't know if that would help? Thanks for the quick reply and your insight. Brian

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Sounds to me like you have an O-ring flipped in your shift cylinder on top of your transmission . Common problem with a 12 spd. A rebuild kit is pretty cheap for that it's just not in a nice spot to get at to do it.

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Ok, we have got reverse back again.. WOOOHOO!

WHat I did was, first I replaced the air filter under the cab next to the trans and filled it 1/4 of the way with air brake alcohol. Then I put a good amount of the same alcohol into the air line coming out of the compressor. Next I removed all of the air lines at the top of the shifter, poured more alcohol into each of them and reattached em to the shifter. (I think in the correct locations?)

Next I removed the square cover on the end of what I believe to be the shift cylinder on the top rear of the trans. When I took the four bolts out and pulled the cover away a fair amount of water came out of the bore. (Not good, I know) I did try to blow shop air into the airline fitting on the side of the cylinder to see if the piston inside would move, to no avail. Reattach the cover and start truck for about 5 minutes. shut off, and I had to leave this project till this morning. Started the truck again and I have reverse! NOw, the truck DID sit inside a heated shop for 3 days, that also helped. Hopefully wherever the ice was, it is no more.!?

I appreciate everyones' suggestions and help. Take care and be safe. God Bless. Brian Ostner/Garden State Services LLC

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