4l80e hard shift from 2-3 gear.

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Mildphil

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I have to wonder if it isn't in the valve body, accumulator or something? Those are moving parts that wear out over time and not the end of the world to replace.
If it comes down to replacing the valve body I'll do it. Hopefully the reverse boost valve fixes my issue. Either way, anything is better than having a trans shop charge me an arm an a leg to perform a swap-nosis.
 

Mildphil

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Also, I'm assuming the correct reverse boost valve is the first one (3420001k)? That was the one in the sonnax article Andy linked.

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MIHELA

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The piston is under that big can looking thing next to the valve body. The 3420001K is the valve I used on my trans.
 

Mildphil

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The piston is under that big can looking thing next to the valve body. The 3420001K is the valve I used on my trans.
Yeah the can looking thing with a few bolts holding it right? Did you happen to have an issue similar to me?
 

MIHELA

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I rebuilt my trans straight from the junkyard. The 95 trans is much less likely to break that piston, however I did have a broken accumulator spring. That could cause a harder shift and would be another thing you could look for. Valve body has to come out for that however.
 

Mildphil

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I rebuilt my trans straight from the junkyard. The 95 trans is much less likely to break that piston, however I did have a broken accumulator spring. That could cause a harder shift and would be another thing you could look for. Valve body has to come out for that however.
Just ordered the sonnax piece first to see if that works. How mich did it cost to rebuild a junkyard unit? The thought has crossed my mind a few times in the past just in case if mine ever died.
 

Mildphil

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Boost valve came in, I'll install it once the rain subsides and update the thread with the results.

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Mildphil

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Ok so I'm a little confused here. The kit has a small aluminum balance disc that is supposed to be inserted from the TOP of the pump, where as the rest of the kit (sleeve and valve) go in at the BOTTOM. Is there any way to insert the piece from the bottom? Is the piece absolutely necessary? The sonnax instructions online are kind of lacking.

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andy396

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I guess it depends on the condition of the old plug. I’m not sure exactly which part is to blame for the leakage that causes high line pressure. I guess worse case you don’t replace it and the problem still remains and you have to then pull the trans.
 

Mildphil

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I guess it depends on the condition of the old plug. I’m not sure exactly which part is to blame for the leakage that causes high line pressure. I guess worse case you don’t replace it and the problem still remains and you have to then pull the trans.
Yeah I couldn't think of any way to get that plug in without ripping out the pump, so I just did the valve. Time will tell how it goes. I have noticed though that after dropping the pan and replacing with fresh fluid each time, the 3rd gear shift has gotten more tame. It's more of a firm shift than a harsh thud that makes all the interior plastic creak. I also noticed that around sharper turns, it shifts into 3rd butter smooth, very weird. The harsh shift in general doesn't seem to be as consistent. I'll keep the thread updated on how it drives with the valve and sleeve in.

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