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Old 09-22-2011, 04:58 AM
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th350 Problem

I put a used 350 tranny in my 66 impala with a 400 SBC engine. When I installed it 7 years I put in a new convertor, filter and fluid. The tranny shifts fine it may need an adjustment because I feel it shifts too quickly but this is not the problem. I only have 4,000 miles since I restored this car.

When I deaccelerate it lugs like a standard tranny like its still in 2nd gear. If I manually downshift it downshifts and I can do that for the two gears.

When I drive it I can kick down also. (it is cable kickdown). It lugs and I feel this is causing an overheating problem and using a lot of gas. It's tri-power so it's only using a 2 barrel carb normally.

I used to build some trannys when I was a pro-mechanic but that's over 30 years ago so I still understand some things.

Any suggestions before I talk to a tranny shop.

Thanks
Frank

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Old 09-22-2011, 10:06 AM
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iv ben dealing with the th400 mostly but sounds like you could use a governer weight change. im not sure if theres a vacuum modulator on those but if there is, an adjustible one would help to change the shifts too. someone here will help out.
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Old 09-22-2011, 12:09 PM
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I'm not having a shifting problem. It's a lugging problem like it has a manual transmission in it.
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Old 09-22-2011, 12:10 PM
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I'm going to explain it again; Please forget the shift points and forget overheating. It's a lugging problem. Shift points and overheating have nothing to do with the lugging.
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Old 09-22-2011, 08:50 PM
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lugging? im not sure ik what u mean. is there another word to discribe it? i thought u ment shifting to early and bogging, sorry
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Old 09-22-2011, 08:54 PM
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ok, i reread what u said a coupple times, u mean is engine braking? like not coasting? im not sure what would cause that but interested in what it is. ill b keeping an eye on this one... good luck
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Old 09-22-2011, 09:13 PM
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Quote:
Originally Posted by fcompoccio
I'm going to explain it again; Please forget the shift points and forget overheating. It's a lugging problem. Shift points and overheating have nothing to do with the lugging.

yes they do... excessive heat and lugging can be caused by a faulty torque converter internal roller clutch. Often refered to as a sprag in the converter.

If the sprag inside the converter locks up (fails) , the trans lugs the engine and builds heat quicker
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Old 09-23-2011, 04:10 AM
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Ok guys for you that don't understand what lugging is:

Engine lugging is an automotive term used to describe what happens when the engine is used to slow down the vehicle. Used alot by truck drivers travelling down hill to help slow the truck without cooking the brakes causing severe piston damage. Race car drivers will often use it by intentionally downshifting into turns causing the vehicle to slow down.

Still don't understand? If you drive an automatic tranny vehicle, drive at about 40mph take your foot off the gas (don't touch the brake) then downshift, the vehicle will drag down.

For you manual tranny drivers: drive 40mph downshift take your foot off the clutch, what you are experiencing is engine lugging.

I'm sure us older mechanics out there who have built many engines in our time have seen pistons overheated and stretched due to lugging.

Don't believe me? Type Engine lugging into your browser.
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Old 09-23-2011, 04:11 AM
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I put a new convertor in when I installed the tranny.
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Old 09-23-2011, 04:14 AM
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I spoke to a tranny expert out in CA last night and he said if the convertor were bad there would be other problems when accellerating from a light and at slow speeds. He feels there is a problem with the modulator. For $15 I'll try that first. Will keep you posted.
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Old 09-23-2011, 06:15 AM
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Quote:
Originally Posted by fcompoccio
I put a new convertor in when I installed the tranny.

I had a converter sprag fail on the trans dyno in a few minutes last week... I have been in the transmission field of repair for decades myself



the vacuum modulator can be tested with a vacuum pump of the hand held design. An old mechanic as yerself may have one of these tools.

Also if there is fluid coming out of the nipple for vacuum hose hook up, the modulator is bad
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Last edited by Crosley; 09-23-2011 at 06:21 AM.
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Old 09-23-2011, 06:27 AM
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Thanks, yes I did test the modulator with a vacuum pump and it has no leaks. I hope you aren't right about my convertor, I have had that in the back of my mind. I don't have access to a lift anymore so I hope I don't have to pull the tranny. We will see what happens when I change the modulator, if can can get to it today with this lousy whether.
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Old 09-23-2011, 06:30 AM
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Quote:
Originally Posted by fcompoccio
Thanks, yes I did test the modulator with a vacuum pump and it has no leaks. I hope you aren't right about my convertor, I have had that in the back of my mind. I don't have access to a lift anymore so I hope I don't have to pull the tranny. We will see what happens when I change the modulator, if can can get to it today with this lousy whether.

IF the modulator is the adjustable type, try an adjustment... there would be a screw inside the nipple where the hose attaches... crank that in 2 turns, test drive.

2 turns on the screw adjustment should show a change


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Old 09-23-2011, 06:33 AM
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Thanks again Crosley I know about the adjustment but I figure if I have to jack up the car and get under it I mite as well install a late model modulator which I understand is the gold one with the red stripe. I never changed it when I put the tranny in with the new engine. For $15 what the hell.
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