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Old 06-08-2010, 06:36 PM   #1
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Downshift TH400 into 2nd for downhill compression braking?

On our recently purchased '88 Winnie - GM P30 Chassis, 454/TH400 combination, the engine functions perfectly, the tranny upshifts perfectly and cruises nicely - the passing gear kick down works fine. But what DOESN'T work properly, is the ability to drop the range selector back into 2nd range for controlling MH speed on a downhill grade - all it does is freewheel in 2nd range, regardless of road speed. Low range is fine, and will retard speed as I would expect - but of course, is not usable at higher road speeds.

SO, why doesn't 2nd range engage the tranny for compression braking as it should, why does it freewheel - and where might I look for the cause and correction?
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Old 06-08-2010, 07:11 PM   #2
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Do you have a second gear when driving and manually shifting from low range to second gear and then third gear. If so, I would look at maybe the shift linkage needs adjusting.

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Old 06-08-2010, 07:35 PM   #3
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Gary,

Does this happen just on downshifts? For example, if you just drive down the street from a standing start with the selector in 2nd, get up to speed, and let off, does it freewheel under those conditions as well?

Steve
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Old 06-08-2010, 08:08 PM   #4
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A couple of things come to mind: front servo/accumulator (worn out seals) and the front band (burned out or broken). Servo/accumulator can be accessed by dropping the valve body. The band is inside the trans. Everything else works ok because that band is only applied in the manual second gear selection.
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Old 06-08-2010, 09:04 PM   #5
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Gary,

Does this happen just on downshifts? For example, if you just drive down the street from a standing start with the selector in 2nd, get up to speed, and let off, does it freewheel under those conditions as well?

Steve

AS I said in my first post, EVERYTHING works and shifts perfectly in normal driving, ALL ranges work and engage as they should from 1-2-3 on acceleration, and low range works as it should if you pull the selector into low to decellerate while moving at an appropriate speed.

All shifts are firm and normal, and at the right moving speeds. ONLY the 2nd range is abnormal in that moving the selector to 2nd range on a downgrade does NOT engage 2nd range, and the MH simply freewheels as tho' the tranny is in neutral - and yes it will freewheel while decellerating at ALL TIMES if I let up on the throttle with the selector in the 2nd range position - but if I then apply throttle, once engine RPM increases above current road speed, 2nd range engages, and the MH accelerates properly...

IF the shift linkage is indexed properly in 1st and 3rd ranges, It's hard to imagine that 2nd could be off...

Obviously, SOMETHING is disengaging in 2nd range when DEcellerating, and ONLY in 2nd range - and I dunno what it is, or how to correct it!
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Old 06-09-2010, 12:54 PM   #6
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Allright guys, surely there must be a FEW TH400 tranny experts out there - speak up!
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Old 06-09-2010, 02:32 PM   #7
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Goldwinger2 has the most likely answer.
My old Suburban with the TH400 did the same thing. I replaced the main band and I regained some compression braking in 2nd.
If you can rule out other issues, like a bad/stuck servo then the band is the likely culprit.
The servo can probably be pulled and inspected with the trans in place. I don't have a TH400 to look at but as I recall it is under a round 3" cover set in the pass-side of the trans case (external) if memory serves me. You'll also want to drop the pan and you MAY be able to see the actuator pin that the servo pushes against the band to tighten it.
To replace the band, you'd have to drop the trans and pull the front pump. You can then "sneak" the band out and the new one in. Takes a bit of finnesse, but that's how I did it (many moons ago).
You might want to pick up a TH400 repair manual. Helps to see what you are getting into before you get into it.
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Old 06-09-2010, 02:34 PM   #8
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Quote:
Originally Posted by Gary - K7GLD View Post
Allright guys, surely there must be a FEW TH400 tranny experts out there - speak up!
Your issue is internal, there is not an external adjustment to be made.
J
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Old 06-09-2010, 02:59 PM   #9
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Goldwinger2 has the most likely answer.
My old Suburban with the TH400 did the same thing. I replaced the main band and I regained some compression braking in 2nd.
If you can rule out other issues, like a bad/stuck servo then the band is the likely culprit.
The servo can probably be pulled and inspected with the trans in place. I don't have a TH400 to look at but as I recall it is under a round 3" cover set in the pass-side of the trans case (external) if memory serves me. You'll also want to drop the pan and you MAY be able to see the actuator pin that the servo pushes against the band to tighten it.
To replace the band, you'd have to drop the trans and pull the front pump. You can then "sneak" the band out and the new one in. Takes a bit of finnesse, but that's how I did it (many moons ago).
You might want to pick up a TH400 repair manual. Helps to see what you are getting into before you get into it.
OK - but would that condition ONLY affect engine compression (DEcelleration) braking in second range? Seems like any 2nd range band engagement issues that severe would affect both acceleration as well as DEcelleration?

If there is a stuck servo, or mis-adjusted band, why would it only affect DEcelleration engagement - the 2nd range functions perfectly under ALL other conditions - no slip, no grab, etc...
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Old 06-09-2010, 08:29 PM   #10
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does it decellerate (4e's one word lol) fine in first gear ?,as in hold a load? i do alot of hot rods and if one has a high stall or "loose" converter it won't engine break for crap,,,just a thought ,a second opinion never hurts........................... fabricator john '73 d-18 brave
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Old 06-09-2010, 10:23 PM   #11
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does it decellerate (4e's one word lol) fine in first gear ?,as in hold a load? i do alot of hot rods and if one has a high stall or "loose" converter it won't engine break for crap,,,just a thought ,a second opinion never hurts........................... fabricator john '73 d-18 brave
NOT to appear cranky, but...

C'mon guys - pay attention. so we don't keep running around in circles...

Here's what I said in my FIRST post:

Quote:
ALL ranges work and engage as they should from 1-2-3 on acceleration, and low range works as it should if you pull the selector into low to decellerate while moving at an appropriate speed.
As clearly as I can make it, the tranny works PERFECTLY in ALL aspects, EXCEPT for lack of engine (compression) braking in downhill or coasting situations when the selector is manually pulled into 2nd gear/range.

I'm certainly no auto tranny expert, but I'd sorta think that anything of a significant nature, such as a bad 2nd range band or servo, would ALSO affect the rest of 2nd range function - not just the deceleration function. I guess I hoping for some sort of external issue that is relatively easily located and corrected, such as the modulator or governor - or even a vacuum hose leak. The kick-down works fine for dropping into 2nd range for passing maneuvers - so that seems out of consideration...

I wonder if it's possible for a sticking check valve in the fluid control portion of the tranny to cause this effect? I had the tranny serviced shortly after buying the MH late last summer, and all seemed well inside the tranny according to the shop doing the work - and the freewheeling in 2nd range deceleration was there before the work was done, so not likely the shop screwed up...

I'm baffled!
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Old 06-10-2010, 08:26 AM   #12
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Quote:
Originally Posted by Gary - K7GLD View Post
On our recently purchased '88 Winnie - GM P30 Chassis, 454/TH400 combination, the engine functions perfectly, the tranny upshifts perfectly and cruises nicely - the passing gear kick down works fine. But what DOESN'T work properly, is the ability to drop the range selector back into 2nd range for controlling MH speed on a downhill grade - all it does is freewheel in 2nd range, regardless of road speed. Low range is fine, and will retard speed as I would expect - but of course, is not usable at higher road speeds.

SO, why doesn't 2nd range engage the tranny for compression braking as it should, why does it freewheel - and where might I look for the cause and correction?
Hi Gary,
These old warhorses can be frustrating at times, can't they? I hope you checked with your transmission shop as to the possible causes.... I am not a transmission expert either, but I do know that the biggest enemy of these coaches is lack of use. The worst thing we can do is park 'em for long periods of time. Now I don't know the history of your coach, or whether having compression braking is is an absolute priority where you drive, but if it were me I would just start putting some miles on it, especially stop and go miles. It's possible that if something like a pressure valve or a governor is sticking, that the exercise may loosen it up. One of three things will happen:
  1. It will get better.
  2. It will stay the same.
  3. It will get worse.
If it gets worse it will be easier for a tech to diagnose the problem. By the way, you may have a TH475. That's what our '85 has. If I wasn't working on the brakes I'd take it out and see if I've got second gear compression braking; the trans has just been completely rebuilt during our engine swap.

Here's my wife rolling it out from under "Titanic".



Anyway, if all other functions are as good as you describe I'd just "Ride 'er hard and put 'er away wet".
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Old 06-10-2010, 08:48 AM   #13
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Make sure the vacuum lines on the "manifold" and down below on the trannie are sealing 100%.

Jim
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Old 06-10-2010, 11:15 AM   #14
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Hi Gary,
These old warhorses can be frustrating at times, can't they? I hope you checked with your transmission shop as to the possible causes.... I am not a transmission expert either, but I do know that the biggest enemy of these coaches is lack of use. The worst thing we can do is park 'em for long periods of time. Now I don't know the history of your coach, or whether having compression braking is is an absolute priority where you drive, but if it were me I would just start putting some miles on it, especially stop and go miles. It's possible that if something like a pressure valve or a governor is sticking, that the exercise may loosen it up. One of three things will happen:
  1. It will get better.
  2. It will stay the same.
  3. It will get worse.
If it gets worse it will be easier for a tech to diagnose the problem. By the way, you may have a TH475. That's what our '85 has. If I wasn't working on the brakes I'd take it out and see if I've got second gear compression braking; the trans has just been completely rebuilt during our engine swap.

Here's my wife rolling it out from under "Titanic".


Anyway, if all other functions are as good as you describe I'd just "Ride 'er hard and put 'er away wet".
Thanks - I'm hoping ours is the slightly stouter 745, but no way I know of to verify without opening up the tranny - I wonder of the 745's might have a slightly different shifting/downshifting characteristic? If you manage to give yours a try, I'd appreciate hearing the results. By the way, your wife is a dead ringer for my sister in law!

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