Th 400 up shift problem
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Joined: Jul 2006
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Th 400 up shift problem
Under normal driving conditions my 73 L82 TH 400 shifts from 1st to 2nd with no problem. On medium to hard acceleration if I slightly let up on the accelerator before the shift, the transmission missses 2nd gear and the engine rpms go wild. Once the engine rpms decrease, the trans shifts ok.
gm73vette
gm73vette
Burning Brakes
Joined: Nov 2006
Posts: 893
Likes: 7
From: Orange Park Florida
On my '72, there is an orange wire that enters the rear of the tranny at a connector. When the gas pedal is pushed to the floor, there should be 12volts on this lead - notice that this voltage is present only when the pedal is fully depressed. This operates the detent solonoid inside the tranny - if all is well, you can hear the solonoid operate when voltage is applied. On my car, the solonoid was defective/weak, allowing fluid to flow thru a valve body keeping the tranny in 2nd, or kickdown gear.
Before you tackle the solonoid, just be sure you are getting the proper vacuum (14-18 lbs) to the vacuum modulator.
To replace the solonoid, drop the tranny pan, replace the solonoid with a new one, just two bolts, then connect the solonoid lead the the inside portion of the connector to the pedal lead. Very simple. I purchased my new part from NAPA, less than $20. Good luck.
Before you tackle the solonoid, just be sure you are getting the proper vacuum (14-18 lbs) to the vacuum modulator.
To replace the solonoid, drop the tranny pan, replace the solonoid with a new one, just two bolts, then connect the solonoid lead the the inside portion of the connector to the pedal lead. Very simple. I purchased my new part from NAPA, less than $20. Good luck.
Last edited by JimT; Feb 4, 2010 at 11:47 AM.
Melting Slicks
Joined: May 2002
Posts: 2,431
Likes: 52
From: Florida
Check that the restrictor in the vacuum line at the manifold is not partially plugged, remove it if it's there. It's a compression of the steel line and clogs with carbon. See that you have a strong vacuum to the modulator. Replace the rubber hoses at each end of the line, they crack. A loose/defective kickdown solenoid will also give this problem. Under acceleration vacuum decreases and you may be barely getting the vacuum you need at normal throttle and not enough upon acceleration.
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
I had the exact same problem as JimT with my brand new th400. Mine had very similar symptoms as the OP except it would even shift to 2nd....or if it did, it would slip real bad almost like the converter was toast or clutches were toast. I called the builder, he told me to pull off the plug on the kick down solenoid. I did this and it shifted fine after that. My problem is when he rebuilt the tranny, he used the wrong solenoid. Stock is a 2 prong, he installed a 1 prong so there was no contact for one of the wires. For now it is just disconnected. I really don't need it anyway with th converter I am running.
Definetly try disconnecting that plug on the kick down solenoid. I bet it's sticking in passing gear.
Definetly try disconnecting that plug on the kick down solenoid. I bet it's sticking in passing gear.
