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Old 10-08-2016, 11:56 AM   #1
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Proper way to operate the Allison MT643?

Drove from about Des Moines to KCMO. Runs and shifts fine and water temp on 5.9 12v never got into white zone for the whole trip except when i got off I-49 and stopped at the bottom of ramp the started up a grade while in D. tOTAL RISE OF ABOUT 250- 300 FT over 1/8 mile.

Immediately i notice engine coolant gauge rising quickliy to middle of the whiteline operating range. Just started to pull over before it could get hot and it immediately cooled right back down.

Should i pull grades like that in 3rd and then move up to D?
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Old 10-08-2016, 03:07 PM   #2
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imo, when climbing a hill, if your rig gets slowed down to the shift point for 3rd (mid 40's?), its nice to keep it in 3rd to take advantage the power at the higher rpm. an 1/8 mile grade? that's pretty short. it shouldnt overheat that fast.

my guess is that it temp spiked because you stopped, the motor needed some extra cooling down to catch up. like climbing a hill and stopping. if you turn the motor off, you dont give the cooling system a chance to work.
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Old 10-08-2016, 03:17 PM   #3
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When I went over Monteagle in my Senator, I put it in 3. Went down in 3, too.
Before I put it in 3, my temps went up a bit. I let it idle for a while when I got to the top.
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Old 10-08-2016, 03:24 PM   #4
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be sure your fan works... im not sure what degrees your "middle white zone is" but the middle isnt out of the question for being normal... if you get to the edge of the normal then you need to think about why its getting warm.. esp if you dont hear the roar of a fan spinning...

every bus has a different trip on point for the fan.. if its electronic control its likely 208-210 coolant temp to start it,, if its viscous mechanical its likely 200 or so it should start to ramp up... dropping down a gear also lets the engine speed up and spin the fan faster if its engaging...
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Old 10-08-2016, 03:41 PM   #5
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Probably meaningless because of trans. difference but on a local interstate grade of 6% for 7 miles, our 545 equipped bus will run up to 217f (coolant temp) if I'm foot down and let the trans do what it wants (it will shift 2 to 3 and back as needed). If I slow a bit and shift into 2nd, keeping the Rs at 2300ish, the temp stays at 205.

I have a ScangaugeD that gives exact temps.
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Old 10-08-2016, 03:51 PM   #6
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at 217 id expect the fan to be blasting..

my DT-360 / AT545 used to run those kinds of temps before i fixed its fan... it would normal unless it was a hot day or I was pushing it hard.. after I fixed the fan, about 185-190 and I get heavy fan and it drops right back to 180-185.. even during the pull
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Old 10-08-2016, 06:58 PM   #7
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The fan goes on at 202f and goes off at 193. I have a 190 thermostat.
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Old 10-08-2016, 09:49 PM   #8
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Quote:
Originally Posted by 2kool4skool View Post
The fan goes on at 202f and goes off at 193. I have a 190 thermostat.
thats about perfect temperature!
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Old 10-09-2016, 03:48 AM   #9
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It also sort of depends upon which transmission is in your bus.

In 1990 the district where I worked took delivery of 10 Blue Bird TC2000 12-row buses with 190 HP Cummins 5.9L with Allison AT540 transmissions.

They were not in service a week before we had to start cycling them back to the dealer to have bigger transmissions coolers installed.

The district had tagged along on another school district's bid whose buses were used in relatively flat urban/suburban routes. Our district had a lot of hills and mostly rural/suburban routes.

I got one of the new buses. The first time up the hill on my route in the afternoon the trans temp almost pegged itself.

Our buses did not have fan clutches. Instead they had plastic flex fans.

I am sure that on an urban/suburban route where speeds were never over 30 MPH and hills were measured in blocks and not miles those buses would have had no problem. But going up an 8% grade for two or three miles with stops along the way can tend to really get things warmed up.

Once the new transmission coolers were mounted (they were so big that they almost completely covered all of the radiator core) we never had problems with overheating. On the other hand, once the ambient temps went below 45* you could never get any heat out of the heaters. Which required the addition of weather fronts that stayed on from about October to April.

The next batch of buses were ordered with 210 HP Cummins 5.9L engines with a thermostatically controlled fan and MT643 automatic transmissions. No overheating issues, the transmissions held the bus back on the downgrade without having to go to 1st gear, and none of those buses required weather fronts to keep things warm during the cold weather.

That is all to say, you may have a problem of not enough cooling on the transmission.
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