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Transmissions and Rear Ends Three on the tree or four in the floor? Shift it all here.

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  #1  
Old September 18th, 2020, 04:56 AM
Crosshairs6 Crosshairs6 is offline
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Default GMC 4000 rear axle vacuum shift

I have a 2-speed T150 rear axle and a SM420 transmission.
I have to maintenance manual for the subject truck and I don't see anything written regarding to required vacuum pressure that is optimum for shifting the truck. I have traced to vacuum lines and identified where the rubber connectors are to separate and test with a vacuum gauge. I just don't know what the pressure should be.
Picture attached of the T150 rear axle with the shift chamber installation.
Thanks in advance for any information.
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Old September 18th, 2020, 03:19 PM
James James is offline
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Default Re: GMC 4000 rear axle vacuum shift

This is what I found in my 62 manual. Hope this help.
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Old September 18th, 2020, 05:00 PM
Crosshairs6 Crosshairs6 is offline
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Default Re: GMC 4000 rear axle vacuum shift

Thanks James. This same information is in my manual as well. The bench testing procedure for testing the shift chamber is there but it doesn't mention what the standard operating PSI should be with the engine running.
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Old September 18th, 2020, 07:10 PM
James James is offline
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Default Re: GMC 4000 rear axle vacuum shift

While you're shifting, the clutch is pushed in so the engine should be at idle, the manifold vacuum should be around 15"Hg (with a healthy engine). This vacuum is applied to the control valve then passed on to the shift chamber (shift chambers should see the same as manifold vacuum). I see no regulator in the system. As long as the is no vacuum leak in the system it should work properly. Also make sure the air filter for the control valve is good. Any restriction (dirty filter or kink lines) would make the shift slower or not at all.
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Old September 19th, 2020, 04:26 AM
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LEWISMATKIN LEWISMATKIN is offline
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Default Re: GMC 4000 rear axle vacuum shift

the higher the engine vacuum, the better the shift mechanism will function. vacuum leaks generally will occur first at the diaphram on the slave chamber bolted to the chunk, and secondly the vacuum shift valve on the frame. seems i remember dad using mineral oil from an apothecary to lube the seals the vacuum valve.
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Last edited by LEWISMATKIN; September 19th, 2020 at 04:27 AM. Reason: misspelling
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Old September 19th, 2020, 05:13 AM
Crosshairs6 Crosshairs6 is offline
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Default Re: GMC 4000 rear axle vacuum shift

LEWISMATKIN,
Thanks for the advice. I will be looking at the system again tomorrow. I do have mineral oil on hand.
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