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Topic: Allison HT 746 harsh upshift and downshift normal? (Read 703 times) previous topic - next topic

Allison HT 746 harsh upshift and downshift normal?

Hello forum,
I am in the process of purchasing a 1994 Foretravel U300 4000. Coach has about 160k miles and is in fantastic condition with records. I noticed on the test drive that the transmission seemed to have a pretty hard 1/2 upshift and 2/1 down shift that was a little worse with the exhaust brake engaged. The fluid looks good, seems to be at the right level and smells fresh.
Is this normal with the Detroit / Allison combo??
I appreciate any feedback y'all can provide.

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #1 –
Yes somewhat normal if memory serves me.  Recent owners I hope will join in
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #2 –
Yes, our 1993 U300 does shift a bit hard on certain shifts. If you are slowing down, it may shift abruptly into a lower gear. Heading down steep grades, I may stop at the top and then start down to avoid the harsh downshift if I select a lower gear and then slow. Third to fourth are smoother. Slow, smooth shifts in the lower gears promote transmission wear.

In shifting, you may notice that it shifts like a 5 speed. Part way up in second gear, it shifts into lockup and then accelerates quite a bit faster. At high altitude, it's a bit of a dog off the line as the DDEC limits the amount of fuel injected to eliminate smoke off the stop/traffic light. Once the ECU sees boost, more fuel is injected.

The 6 speed Allison has a 6.xx/1 first gear so racing a Cummins off a red light can be embarassing as first in the four speed is 4.xx to 1. At sea level with a rolling start, the 8.3 Cummins are easy pickings and the Detroit has a little better legs than the M11 at higher speeds until you shift to the next gear and the gap between ratios is quickly apparent.

Because of the four speeds, fourth gear (high gear) is not as tall as it could be for low RPM cruising but will still get good mileage as ours has recorded exactly 8.0 mpg over it's life, at least 10 mpg at 55 mph with no headwind and no grades to climb. 2130 rpm fuel cutoff is 82 mph. 

All in all, the HT746 is a very rugged transmission and has a very heavy duty transmission cooler not prone to failure. It's found in garbage trucks where a lot of start and stops are made in all temperatures and inclines.

As an automatic transmission hater, our Allison does a pretty good job. Not as good ratio wise as the later 6 speeds and not in the same league as the Detroit/Mercedes DT12 12 speed auto/manual transmission but nothing is.

Living in the mountains, I leave the Jake switched on all the time. Not noisy enough to ruffle feathers and excellent on the downgrades. Works best at higher RPMs.

I driven 2-cycles since the 1960's and that was the reason we bought the U300 otherwise it would have been a Provost, Eagle or Dina with an 8V-92TA.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #3 –
I really appreciate this input. What a wealth of knowledge.

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #4 –
Ours performs exactly as Pierce described.
Frank & Connie Williams
1999 U320 '36
Build: 5466
MC: 18335

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #5 –
Mine Also...... ^.^d
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Gerry & Brenda
CHARLOTTE HARBOR, FL
1994 Grand Villa - # 4466
U300 Unihome - 6V92 Detroit
4 speed Allison - Jake Brake

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #6 –
In have the Cummins M11. I find that if the engine is not at operating temp. and I'm driving slow between 1st and second gear and I slow so the trans downshifts to 1st I get a very violent downshift. I have the Jake brake.
When the engine and trans warm up the violent downshift  goes away.
1995 U320C SE 40'
Jeep 4x4 Commander - Limited - Hemi
"The Pack"  Yogi and Diesel our Airedales -  Charlie our Boxer/Akita mix. Gone but NEVER forgotten Jake our yellow Lab.
NRA Law Enforcement Firearms instructor - Handgun/shotgun
Regional Firearms instructor for national Armored Transp. Co.

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #7 –
Lon seen a few hard downshifts into first.  Tell me about your jake if you would?
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4

Re: Allison HT 746 harsh upshift and downshift normal?

Reply #8 –
Quote from: Caflashbob –
Lon seen a few hard downshifts into first.  Tell me about your jake if you would?

What type of info did you want on the Jake brake?
Works well with the Cummins M11 Celect engine, don't have the extreme service schedule that the trans retarder requires.
The Jake switch indicates 2 speeds, I really only notice one speed with all 6 cylinders braking.  I have read that in order to get 3 and 6 cylinder braking the switch needs to be wired for both modes.
1995 U320C SE 40'
Jeep 4x4 Commander - Limited - Hemi
"The Pack"  Yogi and Diesel our Airedales -  Charlie our Boxer/Akita mix. Gone but NEVER forgotten Jake our yellow Lab.
NRA Law Enforcement Firearms instructor - Handgun/shotgun
Regional Firearms instructor for national Armored Transp. Co.