Does The HT740 Stay In The Selected Gear In Manual Mode?

HB of CJ · March 21, 2011, 04:43:37 PM

If you lock the tranny in lower gear coming down the Grapevine, will the tranny keep the engine in that gear, or will it automaticlly upshift to save the engine...IF the rpm is too high?

An extreme example would be letting a mechancial BC2 Cummins spin at 2500 rpm riding the Jake Brake. Would the tranny upshift, or would it hang in the manually selected gear? HB of CJ (old coot)

buswarrior · March 21, 2011, 06:27:53 PM

A properly working Allison will upshift, albeit at quite a higher RPM than normal.

happy coaching!
buswarrior

TomC · March 21, 2011, 10:57:10 PM

I turned my 8V-71 up to 2500rpm. With the V730 held in 1st gear, it will up shift to 2nd at 2400rpm. Yet, 2nd gear will go up and hit the governor and not shift to third. The bottom line here is to test out your equipment ahead of time so as not to have unexpected surprises happen at exactly the wrong time. Good Luck, TomC

JackConrad · March 22, 2011, 05:15:43 AM

Our 740 will upshift. I was told by someone, don't remember who, that is to prevent the engine from over-revving. Jack

thomasinnv · March 22, 2011, 05:55:24 PM

my 8v71 is turned up to 2500rpm and with selector in 1st gear it will upshift at about 2400 rpm. however with the selector in 2 or 3 it will hold it there past 2500 rpm if i'm not careful and watching.

Chopper Scott · March 22, 2011, 06:08:01 PM

Uphill or down? I think you are not answering HB's question. I can't answer it because I don't know but would be curious about the result. Drop her in low at the top and will she stay there?

luvrbus · March 22, 2011, 06:24:57 PM

If you down shift from 4th to 3rd or 2nd descending a hill they won't up shift

good luck

buswarrior · March 22, 2011, 07:03:39 PM

If the coach is allowed to run away on a downhill slope, while attempting to hold it in a lower gear, a properly working Allison will upshift, at much higher revs than normal, whether you like it or not, regardless of selector position.

Whether your transmission's internal controls are up to snuff in this regard, well, you have to let it spin up above governed speed a fair ways to experiment...

Or, maybe before starting, wonder what modifications or weariness exist that will inhibit this engine saving action...

Or, maybe best stay out of the game, keep control of the coach and never mind?

Don't let it run any faster downhill in a gear than it can push in that same gear on the flat.

So, roughly speaking, MC8, governed at 2100 rpm, 3.7 diff, nothing above 37 mph in 2nd, nothing above 50 mph in 3rd...

The brake pedal makes a good job of helping this...until it doesn't... a whole 'nuther thread!

happy coaching!
buswarrior

Joe Camper · March 22, 2011, 07:43:22 PM

On our 5 speed Allison in our 85 there is an ugly design.

It is a 8V92TA a mechanical one I do not know if the D-decks experience this phenomenon or not but I know there are others out there like ours because I was specifically told of it when we purchased it.

If you kill it in gear you can not get it back into neutral on the keypad and it will not restart till you manually click the linkage at the trans back into neutral. Than means you will be crawling under it at the rear to do it manually.

How can this happen well the oil psi is very low at idle and if it is very hot outside and the otr ac is on and your in stop and go the low oil lite will flicker if you let it lit too long it will turn off, in gear, no good.

Another way if for some reason say you overheat blow a hose or something, well before you see 210 or so on the temp gauge and your loookin for a spot to land you better kick her into neutral while you still can. The safety high water temp works very well it will shut you down in gear if you let it. Otherwise you will be lying on your back under the back of the bus doing it.

I have heard of some being able to restart, in gear, but mine don't

white-eagle · March 22, 2011, 07:46:38 PM

Quote from: buswarrior on March 22, 2011, 07:03:39 PM
Don't let it run any faster downhill in a gear than it can push in that same gear on the flat.

So, roughly speaking, MC8, governed at 2100 rpm, 3.7 diff, nothing above 37 mph in 2nd, nothing above 50 mph in 3rd...

The brake pedal makes a good job of helping this...until it doesn't... a whole 'nuther thread!

happy coaching!
buswarrior

BW, you're funny. nice explanation, no arguments here. i think mine has upshifted while i was trying to hold it in 2nd going downhill, before we got the jake. i don't think 3rd will jump to 4rth, but i do believe it will auto jump to 3rd from 2nd. guess i need to hit the break sooner if needed.

Sorry Clifford, i don't agree this time.

JC, i have not experienced ever having to crawl under the bus for transmission issues, but at least now i know where to find it.

luvrbus · March 22, 2011, 07:51:25 PM

I never owned a 740 that worked that way they don't have but 2 different transmission governors for the 740,it will upshift from 1st to 2nd

white-eagle · March 22, 2011, 08:03:29 PM

i'd be stupid to argue with luvrbus, so i guess i'll try to find a hill to try it on when we head up i77 thru NC and WV. i'll let you know how it turns out.

luvrbus · March 22, 2011, 08:17:26 PM

When you try it Tom don't let the 8v92 go over 2400 or you may be back in the shop for a engine expensive way to prove a point lol I just know how mine has always worked over the years

good luck

buswarrior · March 22, 2011, 08:19:49 PM

Well, where do we get someone to help us, cuz now I'm not sure either?

I miss the engineers who were posting on both bus boards a decade ago. If they didn't know, they knew someone who did.

It would be good if we had someone who could really explain the inner workings to us.

Not a paraphrase of the regular books we have, but an expert, academic and practical.

With the schooling and the dirty hands experience to know why each of us may have experienced the phenomenons we have.
And they might know how it is that any mythology got started.

"If this control spring rate was such, and that doohickey was stretched, and this was out of adjustment like this, hydraulic pressure would climb to... and this wouldn't happen."

I loved every confusing bit of it...and now they are mostly gone.

happy coaching!
buswarrior

JackConrad · March 23, 2011, 06:05:15 AM

Quote from: Joe Camper on March 22, 2011, 07:43:22 PM
I have heard of some being able to restart, in gear, but mine don't

When we re-wired our coach, I added a neutral start over-ride switch on the dash as a "just in case". Since our transmission control is via cable, we have have never had a problem with not being able to get in into neutral. I think there are several "official" Allison control configurations used, depending on application, so YMMV. Jack