Allison transmission shifting issue

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925a2Tim

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I have a 923a2 that I bought for the engine but found a machine shop that the owner needed a trade of services so a rebuild on my 925a2s engine became a no brainer. Only thing wrong with the 923 is the tranny doesn't shift right. It will go through 1st and 2nd and will try for 3rd if it is pushed but when I let off it drops back to 2nd. Reverse is fine. My question is what controls the shifting? Selonoids? Vacuum? Governor? The truck is to nice to part out.
 

simp5782

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I have a 923a2 that I bought for the engine but found a machine shop that the owner needed a trade of services so a rebuild on my 925a2s engine became a no brainer. Only thing wrong with the 923 is the tranny doesn't shift right. It will go through 1st and 2nd and will try for 3rd if it is pushed but when I let off it drops back to 2nd. Reverse is fine. My question is what controls the shifting? Selonoids? Vacuum? Governor? The truck is to nice to part out.
There are troubleshooting manuals on the web for the 600 series allisons. Also use the search feature on the site for previous issues and conditions similar to yours. It could be several things. You will get 1000 different answers and opinions. Also consult the TM for basic troubleshooting

I say that you need to read the M939 TM from page 1 till the end since you so not know that your truck doesnt have anyway to produce vacuum or how your transmission operates.

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925a2Tim

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Thank you for your reply. I have tried to seach this problem but the search feature seems to send me to Google and it isn't very helpful. I could read 100''s of pages of TM but I thought a knowledgeable person could help me narrow down the problem
 

simp5782

My safe word is Pork and Beans.
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Thank you for your reply. I have tried to seach this problem but the search feature seems to send me to Google and it isn't very helpful. I could read 100''s of pages of TM but I thought a knowledgeable person could help me narrow down the problem
Your talking about a lot of issues it could be with what you described. You need to do diagnostics first. Fluid level at hot? color? Type of oil in it? Spin filter condition? Modulator cable adjusted properly? Solenoid for the torque converter lockup properly hooked up and grounded? Pulled the pan to check for metal/clutch material? Internal filter condition? Valve body bolted securely to the transmission?
Pressure tested the transmission with a gauge?

Go down the list.

https://youtu.be/GEBD_v58vbk

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BKubu

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A trip to an Allison dealer is one of the first things I do when I get a truck. I let them service the transmission, check the shift points, and adjust/replace anything they suggest.
 

74M35A2

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Trans kickdown cable adjustment, located on the injection pump throttle lever. Lube the cable well while you are there.
 
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