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Old 02-12-2012, 09:36 AM
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THM400 detent (downshift)

Hello hotrodders, i use this site anytime i have an issue with my 86 Chevy C10 so i want to help give back to our "community" as well. Recently i have been convinced that my turbo 400 wasn't operating correctly. It upshifts just fine, 1st to 2nd at around 50mph and 2nd to 3rd upwards of 70mph at WOT. The downshifts were what i was concerned about. It would downshift to 2nd at 45mph or lower and NEVER downshift to 1st unless i came to a complete stop.
this concerned me when it came to passing cars over 55mph because it would hang in 3rd and lag. I did my research and found the thm400 uses an electric switch to down shift, which is actuated by the accelerator pedal, so i checked to make sure i had 12v to the switch and 12v out when i depressed it and the switch worked. So i unplugged it from the switch and put a fuse in between the terminals, (fit perfect) so i could test the solenoid in the tranny. Solenoid "clicked" when 12v was applied so that works too. Last thing i checked was to see if the acc. pedal was activating the switch. It WASN'T! for the 3 years that I've had my truck it wasn't "detenting" correctly! For a cheap fix i used some 3/8 fuel line on both the switch and pedal,cutting about an inch length and slitting down the side, followed by 1' and i taped it all up so it wouldn't move. After that i took my truck out, let it warm up fully, then tested it. At about 30mph i floored it... For the first time since i got my truck, it dropped into FIRST and boogied! I thought for sure it was supposed to just stay in 2nd and lag, but not anymore! Plus the 3rd to 2nd downshift, as in for passing, now works where as before it'd stay in 3rd and lag! I sure hope this helps anyone else who may have this problem with the THM400

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Old 02-16-2012, 05:56 PM
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After further investigation into my problem, i came up with a better solution than taped up fuel line. I took the accelerator pedal bracket down which also holds the switch and cut exactly 1inch out of where the switch fits so it would be closer to the pedal. I used the furthest pre existing hole, and drilled 2 more to attach the switch back and that solved my problem!
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Old 02-16-2012, 07:44 PM
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even though i understand what you did in order to get kickdown out of the vehicle, others may not be able to do the same. if you can, could you take an illustration pic and post so others can see what and how you did it so they could possibly do the same? good job BTW!
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Old 02-17-2012, 08:59 AM
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Yes i was thinking the same thing but hadn't had time, but i will definitely upload some pictures this weekend.
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Old 02-17-2012, 06:14 PM
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This is the first fix i came up with, the fuel line used as spacers. I took the whole accelerator deal out to get a better view
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Old 02-17-2012, 06:29 PM
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This is my permanent fix, when i cut out the slot to move the switch closer to gas pedal
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