# Two speed rear end



## TDunk (Jun 29, 2007)

I just bought an '83 GMC dump, decent truck but i'm having a problem with the two speed rear. At first, it didn't even want to go in to any range, but sometimes it would barely go into high range so it would move. I tore apart part of the rear end and lubed-up everthing, so now it stays in high range, still no low. But when i'm driving and take the RPMs real high to grab the next gear, it pops out. I let it go back to idle, grab the next gear, and it's back in gear. It's a vaccum system, so i understand why the rear ends popping out of gear ( High RPM, no vaccum) but is it normal???. And where does anybody find parts for it??? I called the local chevy dealer, lost cause. I can get parts for an electric system, but this is the first vaccum opperated system i've ever seen.


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## joesawer (Jul 4, 2007)

I have not seen a vacum 2 speed on anything newer than the late 60s and have never had to work on one.
Your best bet is to find an old run down looking truck repair shop that has some one that worked on trucks from that era. Or a logger or dump truck owner from the 60s. They used to have to work on them pretty regular.


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## Frank Boyer (Jul 5, 2007)

I used to have a trailer with vacuum over hydraulic brakes, what a pain. You need to run the correct hose and it must be in good condition. There should be a check valve to keep from loosing vacuum when under full power. I had to clean the check valve and control valves on a regular basis. Get a vacuum gauge and see if the system holds vacuum. Use a Mighty Vac to isolate components to see if they will hold vacuum.


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## TDunk (Jul 5, 2007)

I know the engine has good vaccum. I know the vaccum can on the rear end will hold vaccum. All of the hoses are in good shape ( they used plastic air brake hose like what they use on air brake systems now-a-days) But i also know there is no check valve anywhere in the system. (good idea) I do know that the vaccum switch is bad to switch betweeen high and low. Now i'm just trying to think of something that'll work to switch the vaccum from high and low, because GM can't get that either. Thanks


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## joesawer (Jul 6, 2007)

It is not GM part, they bought them from somebody and bolted it on the truck.
Call information and get the number for Porboy's (one O) in Oneonta Al, ask for Mr. Porboy, They will know who you are talking about. He has forgot more about old trucks and tractors than GM has entered into their computors about them. He is very freindly, helpful, and knows how to ask the right questions to get the info he needs. If he does not have the part in stock, he knows where to get all kinds of obsolete parts.


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## TDunk (Jul 6, 2007)

Perfect, i'll see what i can dig up. thanks


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## Frank Boyer (Jul 6, 2007)

It sounds like someone put a really old rear end in a newer truck. Have you looked at the ID tags to see what rear end it is? I checked the net for vacuum switches and many types are still made. Once you know what rear end it is, you might consider converting it to an electric unit.


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## TDunk (Jul 6, 2007)

I looked into converting it to an electric unit, but all the mounting hardware looks way off. I talked to a couple older guys who used to work on this old stuff (no offence to any old guys) they said put a bigger vaccum storage tank on it as close as you can to the rear end, then put an electric over air switch right on the back of the tank. They said it'll make it work alot faster. But now my only problem is trying to locate an electic over air switch. I know it's a GM rear end, the number is 3873551.


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