TH400 bangs into gear

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chengny

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With the TH400, a common cause of harsh engagement (especially when moving the selector to reverse) is an insufficient vacuum supply to the modulator.

This can be due to a couple of things:

1. Not enough vacuum existing in the intake manifold in the first place

2. Leakage in the interconnecting tubing - causing vacuum to be lost


Intake manifold vacuum can be increased by dropping the idle speed and eliminating any air leakage under the throttle plates.

If sufficient vacuum exists within the intake manifold - but there is leakage in the interconnecting tubing - then full manifold vacuum will not be available to the modulator. Inspect the rubber couplings at either end of the steel tubing. Check for looseness, cracks and splits. The steel tubing rarely develops any leaks but it may occasionally become plugged (especially if either of the rubber sections has been broken for any length of time). If the tube is plugged, manifold vacuum will be unavailable to the modulator.



The following is common knowledge to most members of this forum but I will state the obvious since the OP may not be familiar with it.

Connection point to intake manifold:

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Disconnected from the fitting:

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The intake removed:

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Where the tubing heads down along the firewall:

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The lower end - where it connects to the modulator:

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The steel tubing is specifically shaped to suit the particular vehicle application.
 
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mistaake

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Wow... okay, thank you posting the obvious, because it sure wasn't obvious to me!

Will investigate that.
Now, before I go to order a filter and get a bunch of ATF to do a transmission service since the last one was done over 15 years, ago, can somebody confirm for me that I actually have a TH400 in my truck? It's an automatic with three forward speeds and no overdrive.

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HotRodPC

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Yep, that's a Th400 with an added funky looking air cooler. It probably does a little bit of good though.
 

mistaake

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Yep, that's a Th400 with an added funky looking air cooler. It probably does a little bit of good though.

It looks weird but added cooling is always great for an automatic :) I think the previous owner was into fabrication because of the bumpers and that transmission pan thing.

Now that I look at my own picture again I think I see an exhaust leak in the "corrugated" bendy part of the exhaust pipe... might explain why I always smell exhaust a little bit when the truck is running.
 

wight14

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I've got the same thing going on with the "bang" into reverse. I was having some work done on the engine last month, and the mechanic noted that there was a lot of play in the drive shaft. I'm debating if I can live with it, or whether I want to take it to a shop and have them - I dunno, would they have to rebuild the rear differential?
 

HotRodPC

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Not likely they need to rebuild the rear diff. I can't imagine any damage has been done to the diff. Not saying it can't because that bang is very harsh. Usually the driveshaft and U joints are going to take the brunt of that abuse.
 

wight14

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Thanks for the reply HotRod. My mechanic didn't get too descriptive on what was going on back there. We were in the midst of a simple 350 swap, and just couldn't seem to get the new engine to run right - I don't think he wanted to take on anything else at the time:). Now it's showing signs of spring around here, and I'd rather drive it all summer than have it in the shop. I'll have to take it back when snow falls to see what the diagnosis turns out being.
 

HotRodPC

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I may be late to the party. So is your Th400 really banging hard going into R due to a bad U joint and it just seems that way, or is it really banging hard and has caused a U joint to go bad?
 

Joshua Keith

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The actual automatic 1-2 and 2-3 upshifts and downshifts are fine, but P/N to R/D are pretty hard. Is this normal? Yes, I brake completely before shifting in/out of gear.

Did you ever figure what was wrong?
 

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