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Topic: 4 Speed Allison Transmission (Read 1898 times) previous topic - next topic

4 Speed Allison Transmission

 ???  My 1994 U300 has the 6V92 Detroit mated to a 4 Speed Allison Transmission. While driving I have noticed that it actually shifts 4 times, which would mean this Coach has Five (5) forward gears plus reverse.  Is that correct for a 4 speed transmission? or possibly do I have a Five (5) speed transmission?  The Model is;  HT 746 ATEC  and it defiantly shifts 4 times.  ::) I'm not complaining! this coach runs great and I like the extra gear...
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Gerry & Brenda
CHARLOTTE HARBOR, FL
1994 Grand Villa - # 4466
U300 Unihome - 6V92 Detroit
4 speed Allison - Jake Brake

Re: 4 Speed Allison Transmission

Reply #1 –
Gerry,
What you may be sensing is the lock-up of 2nd gear (I think...others may correct this) before going to third gear.
Peter & Beth Martin
No Forrest? What have you done?
MC# 15890 until Dec 2016; FMCA #F329677
Cincinnati, OH

Re: 4 Speed Allison Transmission

Reply #2 –
Correct, the Torque converter lockup feels like a full gear shift, that is one great transmission. Enjoy :)

Re: 4 Speed Allison Transmission

Reply #3 –
BTW. those with the Allison 6 speed transmissions, both 3000 and 4000 series will feel the same "mini-shift" as the torque converter locks-up in 2nd gear.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: 4 Speed Allison Transmission

Reply #4 –
Gerry, that fooled me too for a long time. I actually experienced the same thing with my U225 and would have bet that it was a 5 speed, but alas it is the lockup.
The selected media item is not currently available.Kent Speers
Locust Grove, OK
1993 U300 SSE 40' (Restored at FOT 2009) Build 4323
720 watts Solar
6V92TA DDEC Silver Engine
2014 Subaru Outback

Re: 4 Speed Allison Transmission

Reply #5 –
NOTE for Jake Brake owners;
The Jake will only operate when the converter is locked up.  Why it does not operate in 1st gear.
There is a port in the transmission that has a pressure switch that when in lock up, it completes the circuit to  operate the Jake Brake.

Re: 4 Speed Allison Transmission

Reply #6 –
Quote from: Dave M –
NOTE for Jake Brake owners;
The Jake will only operate when the converter is locked up.  Why it does not operate in 1st gear.
There is a port in the transmission that has a pressure switch that when in lock up, it completes the circuit to  operate the Jake Brake.

Dave, there has been some conjecture that my lockup might not be working causing my lower fuel mileage. Can I assume that if the jake is working the lockup is also working properly?
The selected media item is not currently available.Kent Speers
Locust Grove, OK
1993 U300 SSE 40' (Restored at FOT 2009) Build 4323
720 watts Solar
6V92TA DDEC Silver Engine
2014 Subaru Outback

Re: 4 Speed Allison Transmission

Reply #7 –
Kent, I can not tell you all the posswibilities of what or why, but I can tell you there is a port on the transmission that has a oil pressure switch in it,  The porpose of it is to complete the electric circuit to the jake brake WHEN then converter is locked up. That port only gets oil pressure when the converter is locked up.
If the jake operated without the lockup condition, the jake would kill the engine and do nothing to the transmission or speed.  In other words, it would be like engaging the jake in neutral, same effect on trying to stop. And would kill the engine or atleast reduce its speed to where not enough oil to operate the jake, IE not be able to open 2 of the 4 exhaust valves, would run very poorly indeed.

Re: 4 Speed Allison Transmission

Reply #8 –
Feels exactly like a 5 speed. Fooled me the first time I drove it. Sure wish I could go slower than 48 mph without it shifting down into third.

Heading down a steep grade (the twisty two lane kind), especially with toad, I pull off the road, punch in first and then head down hill ready to switch on the Jake. I like to know exactly what gear I am in and be in control the situation rather than other other way around. That second to second lockup shift can be a real surprise in some conditions.
 

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

Re: 4 Speed Allison Transmission

Reply #9 –
Pierce, I wonder how the jake brake works when you have it locked in 1st gear. If the converter would lock in 1st gear, it would be exactly the same as a manual transmission and in 1st gear, no clutch, so would stall engine trying to come to a stop.  This is why Allison prevents the 1st gear lockup of converter.
Some firetruck transmissions have very different programing from highway units, Maybe they have a lockup condition like when in pump mode etc.  Too many variations but for RV, no lockup in 1st.
Maybe you can explain how this works, lockup in 1st?  Always learning new views.
Thanks

Re: 4 Speed Allison Transmission

Reply #10 –
Dave,

I have never driven a RV or fire apparatus automatic with first gear lockup. I have driven a GM car with a bad torque converter lockup switch that did indeed stall when you came up to a stop sign. Strange feeling to have an automatic do that. So, there are a lot of automatics that do lockup in all gears. I'm sure Allison could have installed a like system but must have had a good reason not to. In fact, a clever programmer could reprogram the existing transmission firmware to do a lockup part way through first and all of second... I think.

 At the top of a steep two lane grade, I like to wait until I have a clear road behind, start off in the gear I figure will work well with the Jake and then head downhill where I can relax and pay 100% attention to the road and any traffic coming up behind me with a glance at the tach every few seconds. Too distracting to have to add down shifting to the equation on a really twisty, steep road. An example was the steep, switchback downgrade into Taos, NM out of over 10,000 feet. This makes for a stress free non-event and allows moments for taking in the views. Using this technique, I have never heated the service brakes and never worry about searching for a pull off if I think the service brakes might be getting hot. First gear and the Jake make for easy 10% or steeper descents. Heading downhill in second, the RPMs will build and then you get a "shift" into second lockup. This can be a little disconcerting if not ready for it. I like to have in the back of my mind the RPM when each shift is made. If first is overkill, easy to just select second. Note that the Jake brake works best in lower gears. Does work well on the interstates in fourth gear on the easy downgrades.

The fire apparatus were 90% manual with crash boxes, twin countershaft and all syncro transmissions. We had a couple of gas Internationals automatics with retarders. They worked well until the retarder overheat light came on and then we had to stop using it for a few minutes. This is understandable as we could have almost 10,000 lbs of liquid on board. The non-supression agent in charge of purchasing was too cheap to fit our 8V-71s with Jakes when we ordered the new trucks. That proved to be a huge mistake. On one campaign fire in Malibu, we were following the lead jeep down the side of a mountain trying to stay ahead of the wind driven blaze. We were in second gear when we started to get brake fade.  I downshifted to first and hit the "anchor locks" (like Foretravel parking brake) but no effect. I finally put the right side of the rig down off the road into the dirt shoulder and got it stopped after bumping along for a couple of hundred feet. I never have forgotten that helpless feeling so now I make sure my ducks are in a row before starting down steep, windy roads.

A Jake brake or retarder don't make up for badly adjusted slack adjusters and/or worn brake pads so when doing that under chassis fluid level/lub check (making sure the rig is blocked) use a strong flash light and follow the instructions in the brake section of the factory manual for checking slack adjusters/pad wear.


 

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