Turbo 400 Tranny Problem

steve6568 · 11-10-2007, 10:29 PM

Recent professional rebuilt my automatic Turbo 400 tranny in my 1985 Chevey C30 1 ton Dually Crewcab pickup that has a 454 ci engine. Problem occurs between 55-65 mph after driving long distance on freeway for aproximately 1-2 hours. Tranny acts like it goes into neutral (similiar to pushing a clutch in on a manual transmission). Tranny re-engages once the vehicle slows down under 55 with foot off gas pedal or if you floor the gas pedal. Above 65 mph and below 55 the tranny stays in gear and shifts properly. It down shifts properly to pass cars or to climb a hill. The professional has went through the tranny twice, had new torque converter rebuilt, states everything looks good and is not sure what is causing the problem. Need expert help on this strange tranny problem. Does anyone have a solution?
Steve

brutis · 11-23-2007, 10:21 AM

just a guess but did you change the vacume downshift modual ? and check the line for a leak or hole , allso i noticed the fan clutch act strange on my 88 with a 400 w 454 it kicks in at about 55 mph or so and sounds like it's in neutral or i should say it used to till i changed it , the fan clutch i mean , you know they are reversed flo and diferant much biger , the new one i got lasted less than a yr. luckely it was under warrentee , im going to keep an eye on now for sure

436'd skylark · 11-23-2007, 12:19 PM

the 400 has a vacuum modulator, but utilizes an electric downshift solenoid that is activated by a switch in the gas pedal. Time to take the trans back out and take it to the repair shop.

75K30 · 11-23-2007, 01:19 PM

Joe:
That is a good call. It actually sounds as though the switch is activated at the wrong time. Most TH400's will act the same way when the thing thinks it needs to downshift.
Kinda like a real loose converter, but only at certain hrottle positions.
If the switch is positioned incorrectly the poor trans might be thinking it needs to downshift at partial throttle, and providing the above symptoms, especially when the throttle is released.
Disconnecting the kickdown wire would verify this.

11-10-2007, 10:29 PM	#1
steve6568 Junior Member Join Date: Nov 2007 Posts: 1	Turbo 400 Tranny Problem Recent professional rebuilt my automatic Turbo 400 tranny in my 1985 Chevey C30 1 ton Dually Crewcab pickup that has a 454 ci engine. Problem occurs between 55-65 mph after driving long distance on freeway for aproximately 1-2 hours. Tranny acts like it goes into neutral (similiar to pushing a clutch in on a manual transmission). Tranny re-engages once the vehicle slows down under 55 with foot off gas pedal or if you floor the gas pedal. Above 65 mph and below 55 the tranny stays in gear and shifts properly. It down shifts properly to pass cars or to climb a hill. The professional has went through the tranny twice, had new torque converter rebuilt, states everything looks good and is not sure what is causing the problem. Need expert help on this strange tranny problem. Does anyone have a solution? Steve

11-23-2007, 12:19 PM	#3
436'd skylark Senior Member Join Date: Mar 2007 Location: Buffalo Ny Posts: 661	the 400 has a vacuum modulator, but utilizes an electric downshift solenoid that is activated by a switch in the gas pedal. Time to take the trans back out and take it to the repair shop. __________________ Joe 89 K2500 Chevy, Wee-Oh-five 285K miles, completly rebuilt needs a motor, bad 1962 IHC Scout All Wheel Drive (Scout speak for locking axles) 152 ci, dana everything else

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11-23-2007, 10:21 AM	#2
brutis Full Member Join Date: Oct 2007 Posts: 36	just a guess but did you change the vacume downshift modual ? and check the line for a leak or hole , allso i noticed the fan clutch act strange on my 88 with a 400 w 454 it kicks in at about 55 mph or so and sounds like it's in neutral or i should say it used to till i changed it , the fan clutch i mean , you know they are reversed flo and diferant much biger , the new one i got lasted less than a yr. luckely it was under warrentee , im going to keep an eye on now for sure

11-23-2007, 01:19 PM	#4
75K30 Senior Member Join Date: Jul 2006 Posts: 2,992	Joe: That is a good call. It actually sounds as though the switch is activated at the wrong time. Most TH400's will act the same way when the thing thinks it needs to downshift. Kinda like a real loose converter, but only at certain hrottle positions. If the switch is positioned incorrectly the poor trans might be thinking it needs to downshift at partial throttle, and providing the above symptoms, especially when the throttle is released. Disconnecting the kickdown wire would verify this.