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Turbo 400 Issues

Posted by Geoff <iframe src='https://members5.boardhost.com/impalass427/info/user=Hawkball' width='330' height='300' border=0 style='border:none;'> on 6/19/2017, 12:50 pm

Hey everyone,having some issues with my transmission and wondering where to start.

Some background... The TH400 was rebuilt approximately 8 years ago, has about 3000 miles on it. The engine is a 454 stroked out to a 496.

I have had no issues up until a couple days ago. The car was running rough (ie when I would step on it, it would kind of bog down ) then I lost 3rd gear. Not sure if these two issues could be related... perhaps a vacuum issue?

I did change the fuel filter, although not perfect the engine is running better... not bogging when I step into it.

Here is how the transmission now performs. I have first and second, (shifts normally manually and when in drive) and it will only shift into third when the rpm's get up there...at around 45mph. Makes no difference if I have it in drive or manually put it through the gears. Reverse works no problem. Fluid levels are good.

Vacuum issue? Kick down switch? Modulator?

Any thoughts on how to narrow down the issue?

Thanks in advance for any support.

Cheers, Geoff

Re: Turbo 400 Issues

Posted by Rob W <iframe src='https://members5.boardhost.com/impalass427/info/user=rjwetak' width='330' height='300' border=0 style='border:none;'> on 6/25/2017, 11:31 am, in reply to "Turbo 400 Issues"

I think I am in the same camp as Bigiron. Your issue seems to very much resemble a faulty modulator. Its a pretty straight forward project, should take less than an hour to replace.

Re: Turbo 400 Issues

Posted by BIGIRON <iframe src='https://members5.boardhost.com/impalass427/info/user=BIGIRON' width='330' height='300' border=0 style='border:none;'> on 6/19/2017, 10:33 pm, in reply to "Turbo 400 Issues"
VIP Member and Board Expert

TH 400's will shift WITHOUT vacuum around 5500 rpm. This can be due to a modulator valve or a leak in the vacuum line, ie: rubber hose connectors at intake or the modulator itself.

Possible there could be an intake fitting leak, would also check the vacuum advance hose from carb to distributor. This link can mimic a bad trans vacuum issue.(along with carb issues)

Have had vacuum connections slip off, crack or simply clog from rubber tubing deteriorating from the inside out.

Then again, as mentioned, it could be internal and require a closer look.

Interested to know what you find, and good luck!

BG

Check out the ImpalaArchive. Cars, parts and details.

http://s804.photobucket.com/user/impalaarchive/library/?sort=6&page=1

Re: Turbo 400 Issues

Posted by rcrusher68 <iframe src='https://members5.boardhost.com/impalass427/info/user=rcrusher68' width='330' height='300' border=0 style='border:none;'> on 6/19/2017, 7:44 pm, in reply to "Turbo 400 Issues"

It may very well be the sprag other wise known as a one way clutch

Re: Turbo 400 Issues

Posted by Dan <iframe src='https://members5.boardhost.com/impalass427/info/user=chevy327' width='330' height='300' border=0 style='border:none;'> on 6/19/2017, 5:36 pm, in reply to "Turbo 400 Issues"

I would check the governor and modulator. Worse case you may have a bad clutch pack.

Re: Turbo 400 Issues

Posted by Randy Kilgore <iframe src='https://members5.boardhost.com/impalass427/info/user=Randy+Kilgore' width='330' height='300' border=0 style='border:none;'> on 6/19/2017, 1:28 pm, in reply to "Turbo 400 Issues"

Geoff - I would check / replace the governor assembly. This is a mechanical part that is accessed from outside the transmission (there will be a tin cover rear of the modulator valve on the passenger side of the transmission). Pop the cover off and the assembly slides out. It will have weights on the outer end and a nylon drive gear on the inner end, sometimes the gear will self destruct ... Very easy to replace and usually cost about $50-$60. I hope this helps ! Randy

Randy Kilgore
Chattanooga, TN
ebay ID: impalas4ever
1967 Impala SS
1963 Impala SS

Re: Turbo 400 Issues

Posted by Geoff <iframe src='https://members5.boardhost.com/impalass427/info/user=Hawkball' width='330' height='300' border=0 style='border:none;'> on 6/21/2017, 10:27 pm, in reply to "Re: Turbo 400 Issues"

Thanks for all of your input.

I will start digging into this issue in the next few days and I'll let you know how it goes.

Cheers

Re: Turbo 400 Issues

Posted by Geoff <iframe src='https://members5.boardhost.com/impalass427/info/user=Hawkball' width='330' height='300' border=0 style='border:none;'> on 8/20/2017, 7:42 pm, in reply to "Re: Turbo 400 Issues"

Just an update on my transmission issues.... it turns out it was the governor.

Replaced and back on the road!

Thanks for your assistance!

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