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Old 05-30-2017, 07:47 AM   #1
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1991 7.3 diesel with AT-500 tranny

I can't seem to find much about the AT-500. Imwondering about the specs, highway speeds and MPG.

My bus is only a short bus so I would think that with less weight than a 40 ft but with the same engine etc it should get a little better mpg?

It seems like I get 8.45 mpg if I am careful. How does that compare?

On the highway the rpm is about 3000 at 65/70 mph. In a car, at highway speeds it would drop down....do buses not do that?
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Old 05-30-2017, 08:16 AM   #2
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Originally Posted by TobinHagler View Post
I can't seem to find much about the AT-500. Imwondering about the specs, highway speeds and MPG.

My bus is only a short bus so I would think that with less weight than a 40 ft but with the same engine etc it should get a little better mpg?

It seems like I get 8.45 mpg if I am careful. How does that compare?

On the highway the rpm is about 3000 at 65/70 mph. In a car, at highway speeds it would drop down....do buses not do that?
Try searching "At545". It doesn't "drop down" because you don't have overdrive like most cars have. 4th gear is 1:1.
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Old 05-30-2017, 08:46 AM   #3
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Over drive

Thanks, I will search for the 545, is there a way to add an overdrive? I'm sure that would make a huge difference in the gas mileage correct?

Or is there another transmission that has an overdrive that I should swap out?
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Old 05-30-2017, 08:49 AM   #4
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Thanks, I will search for the 545, is there a way to add an overdrive? I'm sure that would make a huge difference in the gas mileage correct?

Or is there another transmission that has an overdrive that I should swap out?
Probably easier and cheaper to just buy another bus thats got OD.
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Old 05-30-2017, 09:44 AM   #5
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What ECCB said. Getting harder and harder to find parts or service for the 545's. Any 1000, 2000 series would be vastly superior and have OD or can be unlocked. And having lockup will save both fuel and the tranny.
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Old 05-30-2017, 11:44 AM   #6
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Originally Posted by TobinHagler View Post
Thanks, I will search for the 545, is there a way to add an overdrive? I'm sure that would make a huge difference in the gas mileage correct?

Or is there another transmission that has an overdrive that I should swap out?
ha!! I am embarking on that very project you speak of.. my AT545 transmission is about dead..

so im UPGRADING to a Billet 1000..

here is a BORING video of some dude(me) telling you how a Busted AT545 transmission sort of doesnt work..

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Old 05-30-2017, 12:42 PM   #7
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Thanks for the bus ride and the description of bad signs from the 545.

All this time I thought my transmission was ok, but I've had to learn to drive it differently than I normally would. As you say, with high pressure my 545 shifts well. I had assumed it was necessary to keep the pressure high to make the transmission work correctly, because as you say at low pressure it doesn't always shift correctly. I'd written the odd shifting off to simply having a large automatic in a diesel.

So is there a test for the TCM other than slightly inappropriate shift patterns? My 545 is acting better since changing out the fluid, but as I said I've had to learn to keep transmission pressures higher for proper shifting.

Are all TCMs for 545s the same model or do they vary from one bus to another?

There had to be something wrong with this bus. The bus barn was very upset at this bus because of the top hatch leak, but that turned out to be just a loose knob. Odd shifting would be a good reason for the bus barn to shuck this bus.

Possibly related;

Seemingly the same issues but with references to a bad valve body.
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Old 05-30-2017, 12:53 PM   #8
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theres no TCM on an AT545 it is a mechanical trans except possibly the electronic modulator.. this video you posted is where i finally figured out my modulator was no good...

on your bus if you have a cable-type throttle you may have the old style modulator that is a cable connected to the mechanical modulator on the side of the trans.. and not this electronic type.. I dont know your bus..

my trans acts exactly like a valve body related issue and not so much burnt clutches..
when you changed your fluid did you change the spin-on and internal filter? there is one in the pan..

quite possibly I could have a clogged pan filter.. it does shift as little better when the fluid is cooler and thicker.. I didnt want to waste 20 qts of fluid in a transmission that quite literally in a few days will be on the ground..

I'll take a video of my other bus which has quite possibly the crispest shifting AT545 ive ever driven.. it is much more "whine / whistle" than this bus but it shifts dead perfect.. i can kick it down at any speed, it actually modulates, adjusts shift points perfectly to the throttle.. that one is all mechanical.. no electric modulator.. just a cable to the throttle linkage.. I fine tuned its adjustment when I first got that bus..
I'll be really surprised if that transmission goes anytime soon.. it looks to have been replaced when that bus's engine was replace.. the engine / trans / and dash cluster were replaced in 2004.. if all is correct that drivetrain is less than 100K still.. even with all the miles ive run on it..
-Christopher
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Old 05-30-2017, 01:21 PM   #9
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Quote:
Originally Posted by TobinHagler View Post
I can't seem to find much about the AT-500. Imwondering about the specs, highway speeds and MPG.

My bus is only a short bus so I would think that with less weight than a 40 ft but with the same engine etc it should get a little better mpg?

It seems like I get 8.45 mpg if I am careful. How does that compare?

On the highway the rpm is about 3000 at 65/70 mph. In a car, at highway speeds it would drop down....do buses not do that?
It most likely is an AT545 transmission. Not only is top gear 1:1; it's also not a "lockup" type transmission (the "drop down" you are probably talking about).

Honestly, at 65-70, I would not expect much more fuel economy than this, regardless of transmission, gearing and tire pressures. At speeds above 50 or so, you're expending a lot of energy simply overcoming wind drag in something not terribly aerodynamic. And yes, it is common for these diesel engines to run wide open at highway speeds, that's how they are designed and engineered.

Diesel engines, by their very nature, are "lean burn" engines, which is one reason they get better fuel economy than their gas burning counterparts. Basically, a gas burner needs a specific fuel-air mixture to run. A diesel compresses air to make it hot, then injects the fuel which self-ignites (assuming the air is hot enough); it works by only injecting enough fuel to meet the current need.
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Old 05-30-2017, 01:31 PM   #10
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I concur on fuel economy.. at 65-70 my busses are about the same.. actually my DT-360 carpenter is a little less at 65.. but 65-68 is absolute flat-out on flat ground..

the red bus has smaller tires and onstead of being nearly 11 feet tall its only like 9 3/4 feet tall and has a bit of a sloped windshield.. so it does a little better on fuel at 65-70.. but 8.5 is in the neighborhood for that speed.. I'll know if I do any better here soon after I go to an Overdrive trans
-Christopher
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