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Old 04-11-2010, 03:34 PM
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T-400 modulator line

In the process of running new cooler lines today, the steel vacuum line going to the modulator fell apart. It was in rough shape.

I'm running short on daylight and I was wondering where the other end of this badboy usually hooksup on the motor...454 89 C3500 truck.

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Old 04-11-2010, 05:31 PM
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Should be a port on the intake manifold for full vacuum to the modulator.

Vacuum needs to react to the engine load as the throttle is opened.. the vacuum drops to raise line pressure in the trans
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Old 04-11-2010, 07:08 PM
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Quote:
Originally Posted by unstable
In the process of running new cooler lines today, the steel vacuum line going to the modulator fell apart. It was in rough shape.

I'm running short on daylight and I was wondering where the other end of this badboy usually hooksup on the motor...454 89 C3500 truck.
On an unrelated but nonetheless important note, if the vacuum lines and cooler lines are that bad, for heaven's sake, do the fuel AND BRAKE lines as well!
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Old 04-11-2010, 07:16 PM
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@cobalt...I absolutely plan to!! I found the vacuum line no problem, but I've got a problem.

After I replaced the cooler lines and refilled the trans-fluid (it was down about 2 quarts), I took it about 100 yards down the block and she wasn't shifting. I figured it was because of the modulator, so I ran to the parts store and picked up some vacuum hose and steel line...I rigged up something I figured would produce enough vacuum and took it around the block again...still no shifting.

I could be mistaken but, I tried putting it into first and then I manually shifted into second and it didn't seem like it shifted. I did some research and the only suggestions I found pertained to the modulator or the governor.

I just got the truck on Friday from about 80 miles from my house. Checked fluid before I took it on the road and it was good...was stuck in traffic for a long while and it drove fine with stop and go traffic and highway speeds up to 65ish.

Saturday morning I took my son out for a ride in the truck, made a few rounds in town and when I got home I noticed she was leaking and I stopped driving it and replaced the lines. She had been shifting fine...didn't seem like I was encountering any slippage or anything of that sort.

Any suggestions?
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Old 04-11-2010, 07:29 PM
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I'm going to assume the modulator valve is in the correct way. The governor will cause the tranny to not shift from first to second, but I doubt that the gov. has gone bad just sitting there.

The modulator would cause the shifts to come later, but they would be there. Did you run the RPM up pretty good in "DRIVE" when you did your short road test? If not, you might not have gotten a shift- and I'm not suggesting you go blowing through the neighborhood at 5000 RPM in first, either!

You should check to be SURE there's vacuum at the modulator, and that the vacuum is plumbed to manifold (NOT ported) vacuum. And that the modulator valve is in correctly.

You should be able to draw a vacuum w/your mouth or a mityvac to get the modulator to retract. You can't see it retract (unless you're in front of a mirror), so stick a straw into the end to see that it works. And it should hold a vacuum, as well.
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Old 04-11-2010, 08:13 PM
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I believe I have the modulator plumbed correctly. I traced the line and it goes into the intake manifold and the steel tubing looked to be 1/4" tubing but on the end that went into the manifold it had a reducer that narrowed the I.D. to about the size of a needle.

In any case, I salvaged the little deal that went into the intake and ran a short piece of rubber vacuum hose to a new piece of 1/4" tubing and tossed a new piece of rubber hose between the steel line and the modulator.

I did not really push the truck too hard around the neighborhood. Prior to me working on the lines it did seemed that it shifted sooner though. I will do the vacuum check tomorrow at some point.

I've always heard that TH400's are "bulletproof" and while it's not good to run it low on fluid, I really hope I didn't do any damage to the internals. The ironic thing is that I have a 99 mercury sable that had leaky lines and I toasted the tranny in it...but that doesn't surprise me. I seen it was leaking and I totally neglected the leak...because I hate Ford/Mercury And when I was driving home from work, the RPM's went really high...but the car wasn't going forward...I knew I had problems. hahaha...

really I hope the same didnt' just happen on this truck...
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Old 04-11-2010, 08:28 PM
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Quote:
Originally Posted by unstable
I believe I have the modulator plumbed correctly. I traced the line and it goes into the intake manifold and the steel tubing looked to be 1/4" tubing but on the end that went into the manifold it had a reducer that narrowed the I.D. to about the size of a needle.
I've seen this- albeit rarely. Lose it, it is not needed, nor wanted.

It was probably there to delay the modulator enough that the tranny wouldn't "hunt" between gears, and/or keep the modulator from functioning as quickly w/the application of throttle. But discard it, just the same.
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Old 04-12-2010, 01:04 PM
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ok I will ditch that little needle-crap, and I'm going to replace the modulator today just for a good measure. I'm going to try to get my hands on a vacuum gauge to see if I'm producing vacuum from the intake. Just seems odd that I'd suddenly encounter problems after switching these lines and that section of the vacuum line disengrating...but I will keep my fingers crossed and hope for the best.
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Old 04-12-2010, 05:30 PM
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I asked my buddy to borrow his vacuum gauge and he didn't have it on hand today...so I picked up a modulator and the correct size steel line from napa...autozone is horrible...

Came home, installed the modulator, bent the new line, clamped it...ditched that needle crap and fired her up and she's perfect...omfg I'm in love.

Oh...also found out that my neighbor who lives across the street works at the local chevrolet dealership, so I chatted with him for about an hour...cool stuff.

Thanks everyone. I really appreciate the help.
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