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Seleting reverse on a 12speed


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Reverse is becoming more troublesome to engage lately on my RS600. I have a 12 speed with the air selector reverse. Im having to try far too hard to get it to engage reverse. I'll inch it around in forward gears a bunch, in and out of the clutch and other range selections until finally it will grab reverse. I don't hear any air leaks. So where can I start looking for an issue, I have just ordered a new air line filter, Mack# 2MD3131. After that, im lost. I have attached a picture of the air solenoids/valves on top of the tranny. Which one activates reverse? Does anyone have any tips in engaging reverse?

Yesterday I had to free wheel down a small hill because I couldn't get it into reverse. Other times it goes in as easily as it used to. But those times are becoming fewer and further between.

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The round cylinder on the driver side is the direct/lo cylinder. The square one on the passenger side is the neutral/reverse cylinder. The air fitting on the side of the neutral/reverse (passenger side) is the reverse supply air line. You could remove the airline from the reverse cylinder and select neutral/reverse and see if the supply is quick or slow. If the air supply seems ok, you may need to remove the top cover and replace all the orings in the top cover shift cylinders. It looks like you have plenty of space above the top cover to remove it. There is a oring kit that comes with all the rear top cover orings. Also replace your little circle stone breather thats on the bottom of the direct/lo cylinder (round cylinder on driver side)

If the air supply is slow or intermittent, you could have a bad inversion valve or shifter. The airline from the reverse fitting runs back to the inversion valve.

Alot of problems with air selection are the top cover orings.

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Ok so I replaced the air filter/breather and removed the air line from the round reverse cylinder and found lots of pressure from the air line when reverse was selected on the shifter, didn't even put a gauge on it as I no longer thought that pressure was an issue. I sprayed a little (not much) WD-40 down into the reverse cylinder in the air fitting, hooked the air line back up and reverse is now engaging first try again. Did one small job with the truck and it worked fine the entire time. So, im not sure if the breather may have been clogged or the WD-40 lubed things up a bit or both; but its working again. Im still going to order the o-ring kit just in case.

Thanks again. I will stay posted with how it works over the next while.

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The round cylinder on the driver side is the direct/lo cylinder. This cylinder will have air to both direct and lo fittings while selecting reverse, this puts them in neutral so reverse can be selected.

The square cylinder on the passenger side is the reverse/neutral shift cylinder. The fitting on the passenger side is reverse and the one on the rear is neutral. Just wanted to clear that up.

If the breather is clogged it could cause a delayed reverse because the direct and lo wouldnt go into the neutral position correctly.

Sounds like you're good to go.

If it happens again replace the orings.

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  • 2 weeks later...
  • Solution

When you buy your oring kit ask the Mack parts guy to print out the rear top cover illustration page. You can use this as a guide for reassembling. Or just mark every piece in order to help you while reassembling. Its not that difficult, but sometimes I have to break out the book because I forget which way parts face. Time seems to be important for you, so every bit helps.

Once you have the cover off, there is an interlock pin inbetween the cylinders. This allows only one cylinder to engage. The pin has to hold one cylinder shaft for the other to come out. So if you cant pull a shaft out, its because the other shaft isnt in the correct position. Just a heads up so you can save time.

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  • 2 weeks later...
  • 2 weeks later...

Ok so im finally getting around to working on it. Pulled the top cover off no problem, lots of room. I noticed that on the reverse cylinder, the air fitting on the square top cover is loose. I have attached a picture of it. I can wiggle it to the point where it will come right out. Could this have been my issue all along? I understand that this is not the supply line for reverse but still wondering. I could not hear air escaping at any time however. If this is a problem, does anyone have the part number for these fittings? I suppose it may snug up with the line attached; but thought id ask anyways.

And I started on the hi-lo cylinder and im REALLY struggling with this internal snap ring. I have attached a picture of it also. I had to leave it for today due to frustration. Are there any tricks for that one? Im feeling like I want to leave the remainder of that cylinder alone as the hi lo was working fine. I have neatly placed all parts that I have removed, in order on a clean table but im not yet seeing all of the o-rings that I got in the kit. They must be down deeper in the hi-lo cylinder im guessing. And am I safe to use brakeclean to clean everything up in there? ...if all the orings are getting replaced that is..

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