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Discussion Starter #1
Hi all,

My friend has a problem with his 71 El Camino with the TH 350 Trans. The problem is that somewhere in the driveline, it seems to shudder mainly when it would upshift on a flat area of road or when you gave it gas on a hill going upward on the hill. The trans fluid is clean and red and does not smell burned at all. The torque converter is new made by Draco if I remember right(unknown what stall speed but not stock stall). (have to double check that but I think its right) The rear is a posi rear also. The old torque converter was the stock one and I do not remember this problem happening. What could the problem be? Could it be the new torque converter going bad or could it be the clutch discs in the posi rear going bad? The problem seems to be mainly happening in first gear but when you are on the highway doing about 75 and punch it, the RPMs jumps up to 5000 like it is downshifting but then goes to 3000 then jumps back up to 5000 and so on. Any help would be greatly appreciated on this as it is driving my friend nuts as well as me.
 

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This problem sounds like a vacuum modulator and/or the hose gone bad. I assume you guys know what I'm talking about, that little canister on the passenger side of the tranny. When this deal goes bad, all sorts of funky things happen, especially under load (when stabbing the throttle, taking off from a dead stop). The vacuum modulator is a load sensing device for the tranny, so if the valvebody is getting questionable signals from the modulator, everything goes haywire. This is just my first thought, for a proper diagnosis a test drive is necessary with a person experienced with transmission work.
 

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Yeah, I would check/replace the vacuum hose and replace the modulator to see if it corrects his problem. This could just be a quick and cheap fix. The car is 33 years old and if thats the original hose and modulator he got his moneys worth.

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Discussion Starter #4
We replaced the modulator about a year ago cause someone said that might have been a problem with when we punched it from a dead stop, you felt a shudder possibly coming from the posi unit. At that time, we also drained and replaced the rear axle lube and put in some new additive for the posi unit from GM. The problem seemed to have been fixed after that. Maybe the modulator went bad again. The vacuum hose that connects to the modulator is good and the steel tube seems to be in good shape with no holes in it. I guess I will try to find a new vacuum modulator and see if that is the problem. When I replace it, should I get a fixed one or an adjustable vacuum modulator? I believe there are two types out there.
 

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An adjustable vacuum modulator is usually not required unless the engine has a long duration camshaft (creating very little vacuum) or if you want to experiment with shift points at different RPMs. If the modulator checks out to be ok, my next step (and perhaps the first) would be to check the kickdown cable to see if it's broken or unhooked. This rpm hunting when stabbing the throttle sounds like a valvebody issue more than slipping clutches. Maybe the valves are sticking, a lot can happen in 30 years. I believe the differential is innocent, I work on differentials where I work and a malfunctioning clutches in a posi will not cause the engine to jump up and down in rpms.
 

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Discussion Starter #6
"This rpm hunting when stabbing the throttle sounds like a valvebody issue more than slipping clutches. Maybe the valves are sticking, a lot can happen in 30 years. "

If the valvebody was sticking, wouldnt the fluid be getting burned? The fluid is nice clean and smells ok. The color is fairly clear still with a tint of reddish pink to it. The downshift cable is in excellent condition. It moves freely and is not broken or misadjusted. I was thinking it would be more the torque converter if anything since the tranny fluid is still clean.
 

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Discussion Starter #7
Ok, we replaced the vacuum module on the trans. No change. We also replaced the vacuum hoses going to the vacuum module. The metal line was also in good solid condition. Any one have any ideas as to what it might be?


Any help would be greatly apprecaited. Thanks.
 

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Discussion Starter #8
Well.... I yanked the tranny today and swapped out the high stall torque converter with the stock one that was still good. Survey says.....Torque converter was the problem. The car now has no problem launching as well as spinning the tires in first and second. Big improvement from the higher stall torque converter. Now the big question is if Dacco will do anything about a refund or exchange since it has been about a year and a half old but with under 3000 miles on the car since it is show and cruise only driven.

Thanks for everyones input with pinpointing the problem.
 
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