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  #1  
Old 01-22-2011, 07:17 PM
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Unhappy Trashed Tranny?

74 J10 with a TH400

My sons truck. Drove fine one day and then the next time he drove it wouldn't shift out of first until he hit 40mph. He took me for a test ride and sure enough winding the motor out to 40mph it shifts.
So I try a few simple things out to see if it'll be something easy.
Pulled the governor, checked the weights and gear on it, all appears to be intact.
Switched out the vacuum modulator and checked all the vacuum lines leading to it, all ok there.
Dropped the pan, put a new filter in it, the old fluid is nice and red and not stinky. The bottom of the pan has a thin thin layer of real fine grey sludge. No bits or chunks of anything. I suppose it could've looked like metallic paste if it tried real hard.
Put her back together and go for a test drive.
Same thing is going on, shifting at 40mph. This time I drove it instead of him. You know I believe it's shifting directly to 3rd gear at 40mph and skipping right over 2nd. Take it on up to freeway speeds, no more shifts and the motor sounds right for the speed we are going. No tach in there to tell how fast the motor is spinning.

Does this mean I need to drag the tranny out for a rebuild? Or could it be something silly I could fix in the truck?

The boy goes back to school next week and I'm not really wanting to teach how to drive a stick so he can take my 68 J2000 Selfish I know.


Any suggestions?
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  #2  
Old 01-22-2011, 07:32 PM
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Crazy_Jeepman Crazy_Jeepman is offline
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I would check for Vacuum at the modulator again, be sure it is pulling good Vac. Buy a new modulator try that. Sure sounds like that is the problem.
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Old 01-22-2011, 07:37 PM
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Pull the governor out of the side of it, I bet the gear is chewed up.
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Old 01-22-2011, 08:25 PM
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REDONE REDONE is offline
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Quote:
Originally Posted by babywag
Pull the governor out of the side of it, I bet the gear is chewed up.

Doubtful, the governor is what is making it shift at 40mph, it's a design of the TH400 to protect itself and drives drag racers with the TH400 crazy. But I do agree that it is a known common problem for shifting with TH400s.

Make sure the modulator is getting vacuum, the hose popping off, or cracking or getting chewed by a ninja marmot would do this overnight.

If it's not a lack of vacuum then it's the 2nd gear apply solenoid not getting a hydraulic signal to apply, or failing to apply when the signal is given. Failures like these happen during use, not sitting overnight, so check that the modulator is getting vacuum first! If not I'll post some pics from a dissassembled TH400 out in the shed to show you what to look at for failure and how easy or hard it is to replace.
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Old 01-22-2011, 09:55 PM
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I checked for vacuum at the manifold and it was good but I will check at the actual modulator itself tomorrow. The hoses are nice and pliable but not new. I did nip off a half an inch at each connection due to minor cracking. The modulator is new.

The plastic drive gear on the governor is not chewed up, it's intact. When I put it back in I gently tried to turn it once it was seated and it wouldn't turn so it felt like it had positive engagement.

Thanks for the replies guys. I'll check out the vacuum out tomorrow.... gotta find the gauge
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  #6  
Old 01-22-2011, 10:42 PM
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The Colonel The Colonel is offline
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If it's not the modulator vacuum, take it out and drive it in manual low to about 25MPH then shift to second and keep the gas pedal steady...what happens?
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Old 01-23-2011, 12:17 AM
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Yes, it would be good to know if you have 'manual-second'.
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Old 01-23-2011, 12:49 AM
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It's times like these I wish Gsmikie wasn't banned. He is the trans guru and knows FSJ trannies inside and out. It's entirely his fault that I know anything about auto transmissions and what I know is maybe a 1/10th of what he knows.

I know the governor will shift at ~40mph. You can put your selector in "1" and get up to 40mph and it'll still shift out despite the position of the selector. So I was thinking that the solenoid could be bad, but if the solenoid was bad, then the governor wouldn't shift it either, it wouldn't shift no matter what. It's got to be the modulator not functioning, either from lack of vacuum or being broken. I'm going to go stare at an empty TH400 housing and drink a beer now thanks to you!

I know it's understood, but I'm going to say it again anyways, new parts aren't necessarily good parts. It took three dizzys from the zone before I got one where the vac advance canister wasn't hanging off the side by it's operating rod, and I once bought a set a calipers from O-really?s where one's piston had already rusted into the bore, in the shrink wrap, in the box. Could still be a bad modulator...
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  #9  
Old 01-23-2011, 01:37 AM
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Agree with Redone ... could be a bad "new" modulator off the shelf; or remanned unit. But also could be the 2nd gear solenoid too. Also peeved that GSMIKIE got himself banished from the kingdom. He's one cool customer and a great guy to get to know!
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  #10  
Old 01-23-2011, 07:51 AM
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Sounds familiar to me. Krista's 76 Wag has similar symptoms. I found we had manual 2nd if you shift to N then back to 2. Same with 3rd, N back to 3. It makes for a screwy way to drive. We have a new tranny sitting on the shelf and as soon as I am up to crawling around under a Jeep again, we are going to swap it out.

check:
http://www.ifsja.org/forums/vb/showt...highlight=T400
there may be some hints there
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Old 01-23-2011, 01:31 PM
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you said the vac line is nice and plyable.it could be that it is calapsing thus not giveing vacume to the mod.
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  #12  
Old 01-23-2011, 01:38 PM
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Quote:
Originally Posted by The Colonel
If it's not the modulator vacuum, take it out and drive it in manual low to about 25MPH then shift to second and keep the gas pedal steady...what happens?

Yeah tried that. Nothing happens just stays at 25mph in low.

I have good vacuum down at the modulator. Took both the old modulator and new one, hooked em up to the hose and ran the engine so I could see what was happening inside the modulator. The plunger inside pulls in and out depending on what the vacuum/rpms are. Both the same. No vacuum leaks on the modulators themselves either. Un plug the hose and the rpms change of course.

One other thing I didn't mention before was once it did shift and lets say we're doing 50mph you let it coast down to below 40mph and then re-apply the gas to maintain 35-40mph, the tranny doesn't engage until you wind the motor up, still in first gear. Seems to miss second gear going up and down in speed.

Puzzled
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  #13  
Old 01-23-2011, 01:41 PM
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Quote:
Originally Posted by talntar
you said the vac line is nice and plyable.it could be that it is calapsing thus not giveing vacume to the mod.

I thought of that, then tried holding the mod in my hand while the engine was running to see what was up. The plunger moves in and out depending on the engine rpms.
Don't think it's collapsing.
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Old 01-23-2011, 07:46 PM
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gsmikie got banned??? what the heck, i drive some old chevy crap for a while, come back and the powers that be ban a tranny guru? mods u do realize these old trucks all have transmissions dont ya... oh my, im gonna have to go have a beer
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Old 01-25-2011, 12:48 PM
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I wonder if the downshift switch is stuck 'on'.

Unplug the electrical connector on the side of the transmission and see how it drives.
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Old 01-25-2011, 03:04 PM
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If the govenor is gummed up, it'll act like this.
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  #17  
Old 01-25-2011, 08:09 PM
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Has the new modulator been adjusted after install?
J20
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