TH400 downshift problems - Pontiac GTO Forum
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post #1 of 16 (permalink) Old 07-11-2019, 09:01 AM Thread Starter
 
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TH400 downshift problems

Hey everyone it's been a while since I've posted meaning I've actually been out driving my car and not wrestling with it but one issue is finally getting to me. My th400 refuses to down shift when coming to a stop until I sit there for a full 3 seconds. And if I manually shift it down to first when I come to a stop it slams and lurches me forward an inch. It's rebuilt, has a 2200 stall converter and new vacuum solenoid and fill vacuum from the manifold. The only thing I didn't get hooked up was the electric throttle downshift that you put on the carb to the trans. I thought that only had to deal with full throttle down shifts so I didn't think it mattered but maybe it's part of it? Any help is appreciated!

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post #2 of 16 (permalink) Old 07-11-2019, 11:45 PM
 
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More info needed. Most important is what year?
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post #3 of 16 (permalink) Old 07-12-2019, 04:06 PM Thread Starter
 
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It's a 69 LeMans with a 73 455.i believe the th400 is from 73/74 but I'm really unsure. I don't know what info you need sorry. It shifts fine once taking off from a dead stop. The rebuild was done with new clutch packs and different springs for firmer shifts. My vacuum line tees off my intake manifold with the brake booster. Umm anything else?

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post #4 of 16 (permalink) Old 07-12-2019, 07:49 PM
 
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what carb are you running
if a q jet what is its part number 7043273 ?? or ?

how about a couple pics with the air cleaner off...
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post #5 of 16 (permalink) Old 07-12-2019, 08:46 PM Thread Starter
 
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So it's a quadrajet 7057256 at the back there you can see my tee fitting with one going to the brake booster and one to the hard trans line off to the left. I never installed a kickdown switch.
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post #6 of 16 (permalink) Old 07-15-2019, 09:52 PM
 
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The 69 used an electric switch mounted on the accelerator pedal bracket for downshifting when equipped with a two speed or TH400 transmission. The TH350 uses a cable mounted on the carburetor linkage. Not sure if that would be part of your problems though.
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post #7 of 16 (permalink) Old 07-31-2019, 09:45 AM Thread Starter
 
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So no one else has any thoughts on the trans not wanting to down shift or slamming into gear? I'm really at a loss. If anyone else thinks it might be the electric down shift switch let me know and I'll go spend the money and install one. Thanks!

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post #8 of 16 (permalink) Old 07-31-2019, 12:21 PM
 
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post #9 of 16 (permalink) Old 07-31-2019, 04:45 PM Thread Starter
 
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Yeah I guess when the transmission was refreshed I'm not sure the governor was replaced or even checked. Maybe I'll have my transmission guy look into that. Anything else to check guys?

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post #10 of 16 (permalink) Old 08-01-2019, 07:18 AM
 
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I've been watching this thread and I don't think you have a real unique issue. I know, it's easy for me to say, right??? I agree that a real good experienced automatic transmission mechanic, can figure this out and correct it. That's what I would do. Have him look at it. I have a similar problem, not identical, to yours with my '71 GTO TH400. Mine likes to upshift quite quickly from 1st to 2nd. It won't downshift again to 1st without being almost completely stopped. If driving in the 1st gear range, with it in 2nd gear, I have to really mash the pedal to get it to kick down to 1st gear. I have the factory electrical kickdown switch on the gas pedal. I tried making some adjustments with springs and other things, not successful, so I gave up on it. I might take my own advice and seek out a transmission mechanic. The car as a whole operates just fine. But, this transmission issue is irritating, Nicholas.
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