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Old 06-10-2010, 11:18 AM   #15
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Originally Posted by JimElliott View Post
Make sure the vacuum lines on the "manifold" and down below on the trannie are sealing 100%.

Jim

Thanks Jim - when the weather improves, I plan to crawl under the MH for a close inspection around the tranny.
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Old 06-10-2010, 02:30 PM   #16
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Gary,
I know it seems like a malfunctioning band should affect 2nd gear in both acceleration AND deceleration but I know from experience that it doesn't. It only affects the deceleration.
I hope you do find a solution that can be made without dropping the trans. The vacuum modulator and lines are worth checking but if those are leaking/bad, it will only give you late shifts (higher RPM) not lack of engine braking.
If it turns out to be an internal issue, it really isn't as difficult a job to remove as it first looks.
Let us know what you end up discovering and good luck!
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Old 06-10-2010, 02:59 PM   #17
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Second gear in a 400-425-475 under power is thru the intermediate clutch pack and intermediate sprag(one way clutch). Second gear braking is from the clutch pack , which has to be working because it shifts into second, and the intermediate band and servo. Broken band, leaky servo or valve body. I would vote band broke.

You can drop the valve body and pressurize the passages and you can find it that way.Throw away the filter and get a filter that is just a screen.

I've never seen a band broke, but I use to race these things not pull.Intermediate sprag and filter is the weakest point on these.

Use to rebuild these for $49 in parts. Long time ago.
Got 5 of them thats been sitting for 10 years.

Also,if it's the band, I believe they made a two piece replacement that can be installed without pulling the trans out. You have to drop the valve body. Long time ago.My mind is slipping.
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Old 06-10-2010, 03:34 PM   #18
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Found a parts breakdown PT# 30 is the front band
Attached Files
File Type: pdf th400_04.pdf (170.3 KB, 118 views)
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Old 06-10-2010, 09:04 PM   #19
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Originally Posted by Nick-B View Post
Gary,
I know it seems like a malfunctioning band should affect 2nd gear in both acceleration AND deceleration but I know from experience that it doesn't. It only affects the deceleration.
I hope you do find a solution that can be made without dropping the trans. The vacuum modulator and lines are worth checking but if those are leaking/bad, it will only give you late shifts (higher RPM) not lack of engine braking.
If it turns out to be an internal issue, it really isn't as difficult a job to remove as it first looks.
Let us know what you end up discovering and good luck!
Thanks to you - and AK...

Well, the general consensus seems to be a 2nd range band issue - something more than I could handle on my back from under the MH. Under the circumstances, and since engine braking in 2nd range so far is the ONLY symptom, guess I'll just live with it - it would be impractical to do anything other than a full rebuild or exchange, too little to gain for the cost involved.

No biggie - thanks to all for taking the time to reply - and I'll keep watching for any late arrival suggestions...
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Old 06-12-2010, 10:28 PM   #20
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OK, some confirmation on the 2nd range band as the primary cause of loss of 2nd range engine braking:

Quote:
The THM 400 has only one plug, for line pressure. The intermediate band is applied in "Manual 2" position of the shift quadrant (PRND21) and controls engine braking on deceleration only. The intermediate clutch provides second gear range. If it leaks pressure or the plates are burned, the result is "No upshift to second gear".
SO, remaining question is, can 2nd range band failure be the ONLY item of failure to apply engine braking - or could it be related band mechanical linkage or valve related as well?

This is a low mileage (59K) MH that has been well kept and maintained - yeah, "stuff happens", but I'd prefer to eliminate all easier amd lower cost causes and remedies, before I consider the high $$$ stuff...
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Old 06-13-2010, 07:55 AM   #21
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Quote:
Originally Posted by Gary - K7GLD View Post
OK, some confirmation on the 2nd range band as the primary cause of loss of 2nd range engine braking:

Quote:
Quote:
The THM 400 has only one plug, for line pressure. The intermediate band is applied in "Manual 2" position of the shift quadrant (PRND21) and controls engine braking on deceleration only. The intermediate clutch provides second gear range. If it leaks pressure or the plates are burned, the result is "No upshift to second gear
SO, remaining question is, can 2nd range band failure be the ONLY item of failure to apply engine braking - or could it be related band mechanical linkage or valve related as well?

This is a low mileage (59K) MH that has been well kept and maintained - yeah, "stuff happens", but I'd prefer to eliminate all easier amd lower cost causes and remedies, before I consider the high $$$ stuff...
I was looking at that parts breakdown pdf and it didn't show or list any intermediate band, only a front and a rear. How do you interpret that?
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Old 06-13-2010, 08:35 AM   #22
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We had this identical problem with our TH400 in our Suburban, and it turned out to be a broken band. I replaced the transmission with a shelf-rebuilt unit, and (obviously) the problem went away with the old transmission. Since the band that broke was toward the rear of the unit, meaning everything has to come out to replace it, you would be better off replacing all the parts as you go.
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Old 06-13-2010, 11:01 AM   #23
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I was looking at that parts breakdown pdf and it didn't show or list any intermediate band, only a front and a rear. How do you interpret that?
The PDF file pointed to earlier showed the intermediate band as listed - but unclear as to exactly how it is activated - by mechanical linkage, or fluid pressure, and whether there is any possibility that problem is in that area, rather than the band itself.

I can understand an outright failure of the band in a high mileage, neglected or abused tranny - but this is a low mileage, well maintained unit from all visible indications. The intermediate band sees extremely limited use in normal operation, as pointed out in this blurb from a related thread on another board:

Quote:
On a TH400 the int band is only used for engine braking in manual 2nd, it doesn't do much really. If you feel you need "more" band here, use a wider 4L80e band and the associated longer apply pin parts. I do this on rockcrawler setups that manually shift but otherwise I use a stock int. band.
SO, therein lies my puzzlement at the problem with mine - IF this was something that could be corrected with a relatively easy/inexpensive adjustment with the tranny still in the MH, great - but if it requires what amounts to a complete rebuild or replacement for this relatively minor issue, I'll just live with it.
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Old 06-13-2010, 08:29 PM   #24
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Quote:
Originally Posted by Gary - K7GLD View Post
The PDF file pointed to earlier showed the intermediate band as listed - but unclear as to exactly how it is activated - by mechanical linkage, or fluid pressure, and whether there is any possibility that problem is in that area, rather than the band itself.

I can understand an outright failure of the band in a high mileage, neglected or abused tranny - but this is a low mileage, well maintained unit from all visible indications. The intermediate band sees extremely limited use in normal operation, as pointed out in this blurb from a related thread on another board:



SO, therein lies my puzzlement at the problem with mine - IF this was something that could be corrected with a relatively easy/inexpensive adjustment with the tranny still in the MH, great - but if it requires what amounts to a complete rebuild or replacement for this relatively minor issue, I'll just live with it.
You have low mileage but the material on the band can breakdown and flake away. Also the accum/servo may be stuck and not fully applying. Did you buy this RV new? If not, in it's early life it could have been driven differently. I saw a new one this morning on the fwy being driven rather rough, even DW commented.
J
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Old 06-14-2010, 08:28 AM   #25
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.... Also the accum/servo may be stuck and not fully applying. ....
J
Let's hope that this is the case and exercising the coach loosens it up.
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Old 06-14-2010, 12:18 PM   #26
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Gary, Buy a pan gasket and filter kit for your turbo 400. Drop the pan and drop the filter. The servo has a cover over it, drop it and the servo will drop out by just pulling it down. This is the servo piston that activates the band with fluid pressure. Check the seals on the servo piston. They are made of metal not rubber like a engine piston. If they are worn and passing fluid then this is why your 2nd gear does not downshift. Turbo 350 transmissions use to wear the case out. Also check the spring at the same time. You have the part numbers and list in this board so you should be able to get the parts as needed. This is not an expensive procedure. You will also be able to look in the bottom of the pan for band flakes, if you find the pan to be particle free then most likely the band is good.
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Old 06-14-2010, 04:47 PM   #27
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Just keep in mind you will need a very large bucket to contain all the trans fluid that will come out if you decide to drop the pan. It's a major OH SH*T if you try to use an oil drain pan.
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Old 06-14-2010, 07:22 PM   #28
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Just keep in mind you will need a very large bucket to contain all the trans fluid that will come out if you decide to drop the pan. It's a major OH SH*T if you try to use an oil drain pan.
...That sounds suspiciously like the voice of experience speaking...
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