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1979 Chevy Truck transmission is now sliping

Tiny
BSMICK69
  • MEMBER
  • 1979 CHEVROLET TRUCK
  • V8
  • 4WD
  • AUTOMATIC
  • 74,000 MILES
I resently had the moter blow up in my truck and changed it and put a moter in that is built up some the transmission did not slip befor it shifted great and had no problems but now that new built moter is in it is sliping very bad every thing is hocked up the same way with the vacume lines for the trany wat would make it slip could it be the biger cam in moter and a vacum issue because of cam and how do I solve the problem the moter is a 3504bolt main has manly 10 5 pistons hi end cam and new hi flow heads cam is suposto be mached to wats been done with moter and is as hi as I could go thay said to makenuf vacum to run brakes and trany I was told im lost and need help ty
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Wednesday, April 15th, 2009 AT 12:06 PM

10 Replies

Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
The more radical you go with a cam, the less vacuum you generate. You will notice this more in the power brakes and the shifting of the trans, BUT, it won't make a reans slip. What trans do you have, TH350, TH400 or the 700R4? Instincts tell me the transmission mat be just plain tired. Does the trans slip in all gears, or just 1st. You may have a converter problem, but I doubt it.
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Wednesday, April 15th, 2009 AT 4:52 PM
Tiny
BSMICK69
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  • 7 POSTS
Ok trans is a th400 the torkconverter was never taken off when we changed moters and trans worked fine befor I changed moters it shifted great now it slips in all gears the cam is not to radical but it is a few stpes above stock
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Wednesday, April 15th, 2009 AT 7:45 PM
Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
I've rebuilt many TH400 mostly used in plow trucks where they get a real beating. The buggers are just about bullet proof as long as you keep the fluid cool. The shifting pattern is controlled by the vacuum modulator. With less vacuum from a camed up motor it's going to change the shift pattern, but won't make it slip.
This may sound like a stupid question, but are you sure it's slipping or is the shift overlap off? There is an adjustable modulator valve available that allows you to adjust the overlap.
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Wednesday, April 15th, 2009 AT 8:06 PM
Tiny
BSMICK69
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  • 7 POSTS
Ya I agree bullit proof and yes its sliping im having to rev it up to even get it to move fluids are good dont no if thers a adjustment on the modulator valve or not and then its tuf to get it to move
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Wednesday, April 15th, 2009 AT 8:12 PM
Tiny
BSMICK69
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  • 7 POSTS
And when u do get to to move ia jumps in to gear solid then goes back to sliping if u let off the gas

[/quote:9cb0b91c0a]
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Wednesday, April 15th, 2009 AT 8:17 PM
Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
If you've got the OEM modulator it won't be adjustable. Normally, if you have modulator problems, low gear will be nice and solid. It's shifting into the upper gears where you'll see a problem.
From what you've posted, I would be real suspicious of the pressure regulator in the valve body. I'm going to see if I can find a diagram of the pressure taps and the pressures and will attach it to this post.
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Wednesday, April 15th, 2009 AT 8:30 PM
Tiny
BSMICK69
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  • 7 POSTS
Im not shur wat modulator is in it im gesen its factory
one I bought the truck and have only put about 6 tho mile on it when the moter whent outbut it alwas shifted great till I changed moters iv done these things several times over the years changen moters and trans jobs and this is first time iv ever had this prob o was thinking change trans filter and modulator valve and see wat happen but dont think I need to because of the way the truck ran but it cant hurt to change them but im shur im still gona be in the same boat when I change them
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Wednesday, April 15th, 2009 AT 8:40 PM
Tiny
JAMES W.
  • MECHANIC
  • 2,394 POSTS
I went through everything I have on transmissions and can find absolutely nothing on a TH400. I'm really suprised and dissappointed.
To get the specs you may have to buy a Haynes or something. I would go ahead and pull the pan and take a look/see. You can tell a lot about what's going on inside a tranny by what's in the pan. Let me know what you find. I'll keep looking on my end.
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Wednesday, April 15th, 2009 AT 9:02 PM
Tiny
BSMICK69
  • MEMBER
  • 7 POSTS
Ok ill let u no im disabled and the guy thats doing my work for me now wont be able to get back to look at it till next week early so well see wat I find out when we pull the pan then ill get back to u prob mon or tue depending on weather tywell talk next week
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Wednesday, April 15th, 2009 AT 9:18 PM
Tiny
BSMICK69
  • MEMBER
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Ok I had my guy come over and worked on the truck he checked the tork converter and it was seated in right so we got things back togather and changed the oil filter and mod valveand hose going from ther to moter and cerkulated the oil and changed it again and this time when we added the oil we still had slipageit was a little better but not much we also adjusted the screw in mod valve in and out to see if it made any diffwes a little better in than out but not much im still confused trany worked good when moter went bad and the vacum line seems to have plenty of vacum on it im lost

o and in the pan when we changed the oil smeled a little burnt but not to bad and ther was a little bit if like silver grafite particals thereforme the plates but wasent bad nothin I hadent seen befor in trany oil changes

[/quote:14a6f6a1a4]
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Friday, April 24th, 2009 AT 9:52 AM

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