C6 modulator vacuum help

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Apr 6, 2012 | 11:12 PM

puroford

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Joined: Feb 2008

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C6 modulator vacuum help

Hello, my 78 has a c6 and I'm going to replace the modulator and the hoses. First off, my truck does not take off like it should when I put it in drive, once it gets going, it drives better, but at first I sound like I'm staging at the track then it gets going. The fluid level seems fine. So after reading other posts, I decided to try the modulator, it will be a cheap fix if it that's what's wrong.

I read that when they go out, I will pull the hose off the modulator and oil will come out the hose. Some said they didn't get any oil. I pulled mine and no oil came out, but the hose did have oil on it, so hopefully mine is messed up and will solve my problem.

So I have an idea of how to change it, I took off the metal line that runs along the trans to clean it.

My question is with the vacuum hose. I read that it should be hooked up to the manifold. The previous owner had the manifold vacuum plugged. I recently tapped it to get vacuum for the PCv valve. The way it was hooked up now was to the carb. Can I leave it like this or does it have to go to the manifold. If it does, where should I get a splitter to run the Pcv and the modulator? Or should I just leave the carb hook up. I'm running a Holley 4 barrel on a 460.

Thank y'all in advance

Apr 7, 2012 | 12:08 AM

devino246

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Joined: Mar 2011

Posts: 8,501

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From: Lynchburg, VA

The modulator only has an effect on shift points. It interprets vacuum level and adjusts the shift timing to suit. It sounds more like it's time to rebuild your transmission.

You should have a 'vacuum tree' attached to the manifold toward the back of the engine. Brake booster and trans modulator hook up to that. The PCV runs straight to the carb.

Apr 7, 2012 | 09:21 AM

mark a.

Lead Driver

Joined: Mar 2004

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When you take it out trans fluid will leak out so have a catch pan there. There is a pin that probably will come out with the fluid and end up in your catch pan. Make sure you put it back in the end of the modulator and like already said, get the vacuum line hooked up to the rear manifold vacuum tree, not the carb.

Apr 7, 2012 | 09:31 AM

skidoodave

Tuned

Joined: Nov 2010

Posts: 426

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From: Caledon ON

Your modulator valve isn't the problem. Sounds like the tranny is on it's way out or the torque converter is pooched. As stated above the modulator just controls the shift points. Are you sure the fluid level is correct? Have you dropped the pan and changed the filter, maybe it's plugged and it's not allowing enough fluid to pass through.

Apr 7, 2012 | 11:57 AM

400inatruck

Senior User

Joined: May 2011

Posts: 398

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From: Wrights community

I had the same problems as listed above, but I did say had. I haven't drove my truck since January, I've been fixing the floor pans,etc. and decided to change the fluid and filter while waiting on parts to be delivered. My filter was dirty and so was the fluid. I would recommend trying the easy/cheap stuff first before dropping $1500 into a rebuild.

Apr 7, 2012 | 03:22 PM

puroford

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Joined: Feb 2008

Posts: 72

Likes: 1

Thank y'all for the quick replies. I will change the fluid and filter. How many quarts will it take to fill back up? Also, should I just use the recommended oil or do y'all recommend something? Yeah I don't want to rebuild, so I will try this first. Thanks again.