Driving downhill: Pump or riding

[Boom Sounds]

Extracts from Motorhome chassis guide 1991 (may not apply to earlier models)

LOW2-This range is used when extra performance is required for hill climbing or it c
an also be used to provide "engine braking" to slow the vehicle when going down medium grades.The shift lever may be moved from "D" to "2" (and vice versa) under most driving conditions.

LOW 1 - This position is used to provide maximum braking when driving down very sharp grades or when maximum performance is required to climb a steep hill or run through deep snow or mud.You may shift into "1"at any speed but the transmission will not go into LOW until vehicle speed is under approximately 40MPH

CAUTION:TO REDUCE THE RISK OF PERSONAL IN- JURY, BEFORE GOING DOWN A STEEP OR LONG GRADE REDUCE SPEED AND SHIFT THE TRANSMIS- SION TO A LOWER GEAR .DO NOT HOLD THE BRAKE PEDAL DOWN TOO LONG OR TOO OFTEN WHILE GOING DOWNHILL .THIS COULD CAUSE THE BRAKES TO GET HOT AND NOT WORK AS WELL.AS A RESULT, THE VEHICLE WILL NOT SLOW DOWN AT THE USUAL RATE.FAILURE TO TAKE THESE STEPS COULD RESULT INLOSS OF VEHICLE CONTROL.

[RandyNH]

Originally Posted by GinaParmesan

I have the same question about maps for freeway mountain passes. Maybe Google Earth has an overlay?

Tried downshifting today. Slowed to 35 or 40 mph, then manually shifted to 2nd....and it was the same as being in neutral. So I guess the next hill I’ll slow to *25* before downshifting. Yeesh! Uphill I can go to 2nd at around 45 mph...

Thanks for the tips about RPM, I’ll keep it max 3,500-4,000 up or down.

PS for other responders, 290 is the smallest length of classic moho for this chassis so I’m not all that heavy. I went to a weigh station and I’m under GVWR even loaded up for full-timing. And lastly, this Chevy 454 purrs like a (tiger) kitten at 65 mph. :

While in Google Earth, create a point to point route, then right click on the route, you can then create an elevation profile for the route and moving your mouse across it will highlight it and tell you the grade for that spot, along with greatest ascent and descent. One caveat though, it does not recognize bridges, so if you pass over a ravine it will look very steep.

[GinaParmesan]

Originally Posted by Waipio Rim

My transmission doesn’t like to downshift as early as I think it should when going uphill, so I am always shifting in and out of second by hand when the roads get steep. Whenever I go from “D” to “2”, uphill or down, I give it a little gas to bring the rpm’s closer to where the trans wants them to be right when I am making the shift. Kind of the same thing that you would do with a manual transmission, to ease the shock to the clutch and the rest of the drivetrain. That might encourage yours to downshift when you want it to. I’’be been driving mine this way for the last nine years or so, and the trans works just like it did when I got the coach.

Wapio that’s a good idea bringing up the RPMs but if I’m going downhill in D and i hit the gas isn’t it the opposite of what I’m going for? ; But seriously, can I raise the RPMs like this in neutral and switch quickly into 2nd? I’m missing something. I know how to do this on a manual transmission but not yet getting it with the 3-speed automatic.

For other posters, I had the brake lines and fluid replaced after the first...fade? Lock up? The mechanic said there were a lot of air bubbles in the fluid. But no indication of the root cause of the problem. Not sure what type of fluid they put in but if I have the problem again I will switch to Dot4. Hot brakes seems to be the catalyst.

[Waipio Rim]

Originally Posted by GinaParmesan

Wapio that’s a good idea bringing up the RPMs but if I’m going downhill in D and i hit the gas isn’t it the opposite of what I’m going for? ; But seriously, can I raise the RPMs like this in neutral and switch quickly into 2nd? I’m missing something. I know how to do this on a manual transmission but not yet getting it with the 3-speed automatic.

I try to get down to 35mph or less one way or another before shifting, and it's just a quick bump of the pedal at about the time I am pulling the lever from D to 2. It may be more of a torque thing than an rpm thing, just a suggestion. After reading what Smartstream posted above, it may not work for you.

[Mollysdad]

I can't think of any reason to ride the brakes.
They get red hot. After all they're designed to create friction.
The heat boils the brake fluid, and surprise, no brakes.

[Olscot]

As to the original question, please forgive my longish answer. After driving several million accident free miles, a lot of them in 105,000 lb trucks in the mountains of So OR, IMHO no matter what type of vehicle you are driving if you find yourself using the brakes for very much more than the last few feet before coming to a stop, you are either driving too fast or not paying attention to the road ahead or the surrounding traffic. Like many of the others have rightly said use a low enough gear so that you do not need your brakes. I like to ask myself when going down a steep grade, if I round the next curve and traffic is at a dead stop, will I be able to safely stop without crashing/killing/hurting anyone. Those you love that are riding with you and those you may run into are depending on you. Almost all the hills/mountains that are very steep have a warning sign and usually the length of the downgrade. Most of us professional drivers like to get at least halfway down before letting vehicle speed pickup to posted limit. Then if you need to, apply very light steady pressure on the pedal. If you are going too slow down a hill for others they are welcome to pass and like the old fella that taught me to drive in mountains said 'you can only go down a hill once too fast'. If you are in a big hurry either leave sooner or take a faster mode of transportation. Airstreaming is all about having fun and enjoying life. For me the choice on brake application has always been light steady pressure when necessary. We routinely drove in excess of 100,000 miles before changing brake shoes/pads.

[Keyair]

Gina,
You say when you down shift to L2 the transmission feels like it is in N?
Do the engine revs drop to idle, and there is no engine braking at all?

If so, this is one I know about!
My 1987 Suburban 2500 had the same issue. 454 with Th400.
It’s not a good situation, and you should get this fixed ASAP.

Only figured it out when I stripped the transmission.
One of 2 issues.
1/ 2nd gear friction band is toast. Transmission needs to come out and be rebuilt... or..
2/ Plastic apply piston for the above friction broke. I think this can be done in the vehicle. The plastic part is weak, and gets brittle with age. Center part of it breaks(not Brakes... wink at Tony), and there goes control of the L2. Known and common problem, and it’s not a coincidence that your trans is also 1987. Replacements are all aluminum now for good reason!

I will post some pics and more info in a bit!

[GinaParmesan]

Originally Posted by Keyair

Gina,
You say when you down shift to L2 the transmission feels like it is in N?
Do the engine revs drop to idle, and there is no engine braking at all?

If so, this is one I know about!
My 1987 Suburban 2500 had the same issue. 454 with Th400.
It’s not a good situation, and you should get this fixed ASAP.

Only figured it out when I stripped the transmission.
One of 2 issues.
1/ 2nd gear friction band is toast. Transmission needs to come out and be rebuilt... or..
2/ Plastic apply piston for the above friction broke. I think this can be done in the vehicle. The plastic part is weak, and gets brittle with age. Center part of it breaks(not Brakes... wink at Tony), and there goes control of the L2. Known and common problem, and it’s not a coincidence that your trans is also 1987. Replacements are all aluminum now for good reason!

I will post some pics and more info in a bit!

Keyair, thanks for the reply! Going into 2nd on flat or uphill terrain is no problem at medium speeds of around 40 mph. 2nd works as expected. It’s only tricky when going downhill...even when slowing to 35. Does that still sound like the issue you described? When downshifting downhill, the revs do indeed drop to neutral and engine braking ceases until its shifted back into D in the downhill circumstances, but I haven’t tried slowing to 25 mph yet...done driving for this week, so I’ll test and report next opportunity.

[GinaParmesan]

Originally Posted by WayneG

You are almost the shortest Airstream Labeled MH, Mine is a 270, The argosy's go all the way down to 20'.
That said I tend to overload mine (with spare parts & beer) and tow a Trailer with a full size Polaris UTV.

Ah! Is the 270 also on a P30 chassis?

Love that spare parts and beer are getting equal weight in your travel prep. ;

[Keyair]

Sorry, busy day!

Ok, to clarify, my Suburban seemed to shift up and down in D just fine.
But if you were on a hill and pulled it down into 2 or L2, it was like you just put it in N and the engine revs would drop to idle and off she would go, down the hill like a bat out of hell!

So, the part is called the "2/3 Accumulator Piston", or just Accumulator Piston, and it is in the valve body.

The original GM part was plastic.

The suggested replacement part is aluminum, and is available many places
This is an example... its $3.50!
http://www.transmissionpartsusa.com/...-000046954.htm

Since you can remove the valve body with the transmission in the vehicle, it is my opinion that you can remove it and replace the piston.
But you should ask a reliable Transmission shop!

Here is how my 2/3 plastic piston looked....
Broken in 2, and cracked... this fits in the Valve Body shown, in the larger hole with the center pin... fluid pressure is diverted into the piston chamber pushing it and applying the brake band which holds the transmission in L2...
No piston seal... no pressure, no band application...

[WayneG]

Originally Posted by GinaParmesan

Ah! Is the 270 also on a P30 chassis?

Love that spare parts and beer are getting equal weight in your travel prep. ;

I think all Airstream classic's are on the P30 chassis, Actually P37 is a more accurate ID with variations according to the wheelbase length.
As for the beer vs spare parts weight, 2 reasons;
1st, I go with no alternate vehicle, so I need a weeks worth of supplies in the woods.
2nd, I need a few brews before I dig into the spare parts for repairs, and several after

Back on the downshift topic, I wonder about the plastic piston for 2-3 shifting. I had not planned on working on my tranny any time soon, how common is this failure? Is it a good PM part to buy?

[GinaParmesan]

Originally Posted by Keyair

Sorry, busy day!

Ok, to clarify, my Suburban seemed to shift up and down in D just fine.
But if you were on a hill and pulled it down into 2 or L2, it was like you just put it in N and the engine revs would drop to idle and off she would go, down the hill like a bat out of hell!

Thanks very much for the pics & further info, and no worries, Keyair. Still wondering if 2nd gear was totally out of commission all the time (uphill, downhill, flat, slow, fast) on your suburban when that part was broken, or only when trying to go into 2nd while going downhill at or below highway speed?

[Keyair]

I’m working on the assumption you have a TH475 3 speed! Anything else is different.

It definitely shifted up through the gears normally. Coming down, in D it was harder to say, but I think it went from 3 to 1.
The real obvious issue was moving the selector to L2. It would climb in L2 but no engine braking. I’m not expert enough to know or understand that difference and what happens inside the transmission between those two events.
I do know that if you lost engine braking on L2 the troubleshooters point to that piston OR the brake band that clamps that drum. I assumed because the PO used to tow a large boat with it, the band frictions were toast. Nope. A simple plastic piston.
If I were you, I would do a simple test. Find a deserted road and accelerate to maybe 40 or 50 in D. Then pull the shifter back to L2 and take your foot off the gas. When the road speed is right the trans will be triggered to downshift.
If the engine revs increase, and you feel a braking effect, it’s good. If the Motorhome continues along with no obvious engine braking and the Revs drop to idle, you have a problem.

Wayne’s question:
This piston is like $5, so cheap enough to buy. However, if your MH trans has ever been rebuilt its likely it was replaced with the aluminum piston as it’s a known and Common failure. The other thing is that it’s a PITA to replace, and will not really impact the functionality of the MH to get home or to a repair shop if you drive accordingly. I drove my Suburban over 50k miles with this issue!
However, there is a part I do suggest you carry.
There is a plastic gear on the governor that fails.
A Classic owner had this happen and it left him stuck in 1st gear.
The governor is at the back side of the transmission, and the gear is attached to the output shaft, so it reads road speed. This is what tells the transmission when it’s ok to upshift and downshift.
I will find a link to the part.

[Keyair]

Here is an example the Governor gear repair kit... its $15.
http://www.classictransmissionsoluti...free-shipping/

Here is a pretty good video of the governor assembly and rebuild, which covers issues, and replacement the gear I hear can strip.

https://www.youtube.com/watch?v=-0Ry1Z92CGk

[jcl]

Originally Posted by Keyair

I’m working on the assumption you have a TH475 3 speed! Anything else is different.

It definitely shifted up through the gears normally. Coming down, in D it was harder to say, but I think it went from 3 to 1.
The real obvious issue was moving the selector to L2. It would climb in L2 but no engine braking. I’m not expert enough to know or understand that difference and what happens inside the transmission between those two events.

I am not intimately familiar with that specific transmission, but I believe that the piston operates a friction band that holds the Sprag clutch. A Sprag clutch is a one way clutch, so without that band functioning you would have second gear (as when you are climbing) but on overrun it would freewheel, so no engine braking effect.

[Keyair]

There ya go!!!!
Thanks for the input!

[Isuzusweet]

I know the difference between a turbo 400 and 475, are the straight cut planetary gears in the 475, versus the helicut gears in the 400.......is the Govender gear the same?
Also, my 475 was rebuilt and hotrodded at 60,000 miles, supposedly at an Allison transmission shop. What are the chances they would have changed the governor gear?

Cheers
Sidekick Tony

[Ted S.]

When driving down hill I strategize the whole way, the goal is to get to the bottom without using brakes because for me they are all I have when all else fails.

I think uphill is overrated and downhill is underrated. Uphill is everyone's fear but generally the worst that can happen is overheating. Downhill is just plain dangerous, you can crash and die.

Downhill I use engine braking and try to never get over the speed where the engine brake can't check the speed. If I feel that happening, I quickly check the speed with the brakes to bring it back within the capacity of the engine break.

If traveling down grade where the engine brake is not sufficient to substantially control speed, I slow way down and put hazards on.

[Keyair]

That’s absolutely the right way to do it Ted!

Tony, as far as I am aware it’s a standard part, and no difference between Th400 and 475.

[Mike1997]

Thank goodness for a diesel with a Jake Brake!!! Works every time!!