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09-20-2010, 11:51 PM
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#1 |
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Senior Member
 
Join Date: Jan 2002
Location: Williston, ND
Posts: 2,999
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th400 shift question
My gmc has a th400 in it behind the 427. The previous owner stated the tranny had been built for towing. the fluid and filter are getting changed this week. My question is the tranny will stay in first until 65 mph or roughly 6000 rpms. That isn't even close to normal compared to other th400 trucks I've drove. Seems like it is sticking in first. Is there anything else I can do to make it shift lower?
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68 Chevy CST/10, 50th Anniversary, 327/th400, 373 posi, buckets 68 GMC Shortbox Stepside, 402/th400, Tilt, Tach, Bucket Seats, 3.08 Positraction 69 Chevy Camaro SS, 383, 4 speed, positraction 70 Chevy El Camino SS396, Triple Black, 454/th400, Tach 70 Chevy Shortbox Stepside 4x4, 350/th350, Tilt, Tach, Cab Lights, Bumper Guards, Tow Hooks 70 Chevy 3/4 Longbox, 350/auto, 70k, SURVIVOR 71 Chevy Blazer, CST, 350/auto 71 GMC Sierra K2500, 454, 4 speed, 4.10 posi, 37s 71 Custom/30, Open Road, 350/th400, tilt 72 Chevy Shortbox Fleetside, 496 Stroker/th400, Tilt, Tach, Bucket Seats, 3.42 Posi 72 GMC Sierra Grande 2500. 427/th400, tilt, a/c, tool and stowage, buckets/console, 3.54 posi |
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09-21-2010, 05:14 AM
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#2 |
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Senior Member
Join Date: Jul 2001
Location: Muskegon,MI,USA
Posts: 6,026
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Re: th400 shift question
The vacuum modulator and /or vacuum to the modulator would be the first thing to check out.
Jim
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1971 Shortbox step side 4x4, 350 sbc, 3:07 rear end 1965 Impala SS 400 sbc, Muncie rock crusher 1966 Impala SS 396 bbc, TH 400 1969 El Camino, 350 sbc, TH 350 1971 Snowplow built on a Blazer frame 1972 GMC Short bed, stepside, TH 400, 427 BBC |
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09-21-2010, 07:15 AM
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#3 |
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Redefining LowBudget
Join Date: Mar 2009
Location: lebanon Cow Hampshire
Posts: 4,538
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Re: th400 shift question
also check the down shift switch...not sure where your is but mine is on the side of the carp...theres a plug on the DS near the speedo cable try pulling it off and running it...if that fixs it then its the switch
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1970 C10 CST fleetside 472 ....big dreams little cash... SunShine Syndicate..  Mikes Sandwich Fair Run OCT 8th 2011
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09-22-2010, 04:31 AM
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#4 | |
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Registered User
Join Date: Mar 2003
Location: San Francisco, CA USA
Posts: 2,454
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Re: th400 shift question
Quote:
Ray |
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09-22-2010, 09:47 AM
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#5 |
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Senior Member
Join Date: Jan 2002
Location: Williston, ND
Posts: 2,999
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Re: th400 shift question
raycow, yes this is under full throttle. then when you let up it shifts. if you floor again though(under 55 mph) it downshifts right back into first.
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68 Chevy CST/10, 50th Anniversary, 327/th400, 373 posi, buckets 68 GMC Shortbox Stepside, 402/th400, Tilt, Tach, Bucket Seats, 3.08 Positraction 69 Chevy Camaro SS, 383, 4 speed, positraction 70 Chevy El Camino SS396, Triple Black, 454/th400, Tach 70 Chevy Shortbox Stepside 4x4, 350/th350, Tilt, Tach, Cab Lights, Bumper Guards, Tow Hooks 70 Chevy 3/4 Longbox, 350/auto, 70k, SURVIVOR 71 Chevy Blazer, CST, 350/auto 71 GMC Sierra K2500, 454, 4 speed, 4.10 posi, 37s 71 Custom/30, Open Road, 350/th400, tilt 72 Chevy Shortbox Fleetside, 496 Stroker/th400, Tilt, Tach, Bucket Seats, 3.42 Posi 72 GMC Sierra Grande 2500. 427/th400, tilt, a/c, tool and stowage, buckets/console, 3.54 posi |
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09-22-2010, 01:25 PM
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#6 |
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Registered User
Join Date: Mar 2003
Location: San Francisco, CA USA
Posts: 2,454
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Re: th400 shift question
Well then, I don't know if you could call it a malfunction. I am thinking the transmission may have been deliberately modified for racing. It could certainly be returned to stock configuration by any competent shop. If you want to try it yourself, I would recommend looking at the governor first.
Ray |
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