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Old 11-11-2018, 09:52 AM   #41
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Join Date: Dec 2017
Location: Camden Maine
Posts: 3
Year: 1977
Coachwork: Gillig
Chassis: 426-13
Engine: DD 671N
Rated Cap: 52
I've driven a lot of DD's with Allison automatics in San Francisco ... I used to shift them when necessary to a lower gear just to keep the rpm's up ... and to keep them from bucking from one gear to the next. I normally would shift them based on feel, most of the different DD's I've ever driven (including my bus) somewhere in the neighborhood of 200+ didn't have tachometers ...

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Old 11-11-2018, 10:01 AM   #42
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Join Date: Jul 2018
Location: Just south of Dallas.
Posts: 169
Year: 1999
Coachwork: Thomas
Chassis: 40' MVP-ER
Engine: Cat 3126
Congratulations! Good Luck! I was looking at Phantoms too!
W I D E &TALL

high ceiling AND a basement!!!
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Old 09-22-2021, 09:15 AM   #43
Mini-Skoolie
 
Join Date: Aug 2016
Location: Battle Creek, MI
Posts: 32
Year: 1997
Coachwork: Gillig
Engine: Detroit Diesel 6V92
Quote:
Originally Posted by Iceni John View Post
It has a Detroit 6V92TA with DDEC II ECM. If it's the 253 HP engine it could have an Allison MT6xx transmission, but if it's the 277 HP engine it should have the big HT740 transmission to cope with the extra torque. If it has a Felsted T-handle shifter it will have half a mile of Morse push-pull cable terminating at a Quadco connector on the side of the transmission. Some Gilligs used an Allison touch-pad shifter or a Stone-Bennett air shifter instead, but then they could be using an electronically-controlled MT or HT transmission.

The 6V92 is a true heavy-duty engine; actually it's classified by the EPA as a heavy heavy-duty engine (it weighs 2100 lbs.). The 6V92 and HT740 combination is what was in Greyhound's MC9s, built by MCI as 3-million-mile buses. They would routinely rack up 100,000 miles a year with GH, then be sold to second-tier operators after ten years of GH revenue service, so a drivetrain like that is good for many hundreds of thousands of miles if cared for and driven correctly. Just DON'T LUG IT and DON'T OVERHEAT IT!

Nice buses, but check for frame cracking. Is that why it's being sold?

John
Hey John, I wondered if you could clarify lugging an automatic. Sorry if that's a SUPER dumb question, but I just bought an 88' phantom with the HT740 tranny. It's empty so there isn't hardly any weight in it (relative to what it would be with the seats in it). I noticed a few times when driving home that the pedal was down but I was getting almost no power... eventually the tranny would kick in (this was mostly when I had pulled off the highway and was driving slower to find a gas station). It just seemed a little odd... I'm wondering if I should have manually downshifted in that situation. I mean, I kinda assumed the transmission would just do what it is supposed to do... but that seemed slightly odd that is did that driving home. Any suggestions on driving the bus in general to keep from lugging? That's a new term to me
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Old 09-22-2021, 11:53 AM   #44
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Join Date: Jun 2016
Location: Orange County, CA
Posts: 1,066
Year: 1990
Coachwork: integral
Chassis: Crown Supercoach II (rear engine)
Engine: Detroit 6V92TAC, DDEC 2, Jake brake, Allison HT740
Rated Cap: 37,400 lbs GVWR
Quote:
Originally Posted by lanegordon View Post
Hey John, I wondered if you could clarify lugging an automatic. Sorry if that's a SUPER dumb question, but I just bought an 88' phantom with the HT740 tranny. It's empty so there isn't hardly any weight in it (relative to what it would be with the seats in it). I noticed a few times when driving home that the pedal was down but I was getting almost no power... eventually the tranny would kick in (this was mostly when I had pulled off the highway and was driving slower to find a gas station). It just seemed a little odd... I'm wondering if I should have manually downshifted in that situation. I mean, I kinda assumed the transmission would just do what it is supposed to do... but that seemed slightly odd that is did that driving home. Any suggestions on driving the bus in general to keep from lugging? That's a new term to me
Lugging isn't only with automatic transmissions - one can lug engines with manual transmissions just as badly. Lugging is usually defined as the engine's inability to increase revs due to it being in too high a gear for the road and load. Detroit 2-strokes will suffer more than most 4-strokes if lugged: at lower RPMs their oil pressure drops quite a lot, and if the crank bearings are being hammered hardest just when there's least oil pressure it's going to shorten their life. Also, 2-strokes make a LOT of heat, much more than 4-strokes of comparable size, and most of that heat must be transferred to the coolant as quickly as possible; keeping the engine's speed higher means that the water pump is circulating coolant quicker, hence the radiator is able to dissipate heat into the air better.

Detroits need to be kept at higher RPMs when climbing grades. Even if it can pull down to 1400 RPM in 4th gear, it's always better to manually downshift to 3rd and keep the engine at about 1900 or 2000 RPM without the accelerator pedal all the way to the floor. With a 4.1 rear axle ratio and a direct-drive top gear, I manually shift down to 3rd at about 45 MPH and let the engine stabilize at just under 2000 RPM with a 3/4 pedal, also keeping the turbo boost to between 18 and 20 PSI. Doing this, I can climb 6% grades all day without the coolant exceeding 200 degrees (as long as it's not crazy-hot outside; if it is, I drop another gear).

Driving a 2-stroke Detroit is a bit like flying - you must be constantly watching the gauges, especially the coolant temperature's, and driving according to what they're telling you. You wouldn't fly a plane without regularly scanning its instruments, and driving a 2-stroke is no different. Manually shifting a 2-stroke's automatic transmission is just part and parcel of the ownership experience!

John
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Old 09-22-2021, 12:08 PM   #45
Mini-Skoolie
 
Join Date: Aug 2016
Location: Battle Creek, MI
Posts: 32
Year: 1997
Coachwork: Gillig
Engine: Detroit Diesel 6V92
That all helps a ton. Especially the info on climbing grades. Would you say what I was experiencing driving home was an issue of me needing to downshift? Do you need to worry much about downshifting when driving fairly flat roads?

Also, again sorry if this is a dumb question, but does this tranny have stopping power when downshifting going "down" a grade?
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Old 09-23-2021, 08:21 PM   #46
Bus Crazy
 
Join Date: Jun 2016
Location: Orange County, CA
Posts: 1,066
Year: 1990
Coachwork: integral
Chassis: Crown Supercoach II (rear engine)
Engine: Detroit 6V92TAC, DDEC 2, Jake brake, Allison HT740
Rated Cap: 37,400 lbs GVWR
Quote:
Originally Posted by lanegordon View Post
That all helps a ton. Especially the info on climbing grades. Would you say what I was experiencing driving home was an issue of me needing to downshift? Do you need to worry much about downshifting when driving fairly flat roads?

Also, again sorry if this is a dumb question, but does this tranny have stopping power when downshifting going "down" a grade?
The benefits of manually downshifting is when you're in the mountains, for descending as well as climbing. In flatlands the transmission can be left to fend for itself. A good rule when driving in the mountains is to descend in a gear no higher than what you ascended with (assuming the gradients are the same), or even descend in one gear lower than when climbing. Without getting drawn into the perpetual argument of whether stab braking is the best technique down hills*, the overall intention is to get to the bottom with the brakes as cool as possible.

Transmissions have no stopping power per se (unless they have an integral retarder): only the engine has some. If the transmission has not locked up, which for a HT740 is in 1st and the beginning of 2nd, then there's no direct mechanical connection between engine and wheels, so any engine braking effect will be negligable at best. When locked (in 2nd, 3rd and 4th), there's better connection between engine and wheels, but remember that diesels have little inherent braking ability compared to gasoline/LPG/CNG engines that create a vacuum on their inlet side. It's that vacuum that slows your car down when you take your foot off the throttle pedal. Diesels have no throttle because they create no vacuum. In effect they are always running at full airflow, with only the quantity of fuel injected determining their power and speed. Stoichiometric they ain't!

The only way to add braking power to a diesel vehicle is to have a compression-release brake such as Jake brakes that all big trucks use, or a transmission with a retarder, or an eddy-current retarder such as a Telma, or an exhaust-restrictor brake if it has a smaller engine. If your Phantom were a transit bus it won't have Jakes, but maybe it could have a Telma if it were from Colorado.

John

* Yes, it is, especially with Jakes!
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Old 09-24-2021, 09:07 AM   #47
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Posts: 15,818
Year: 1991
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Chassis: International 3800
Engine: DTA360 / MT643
Rated Cap: 7 Row Handicap
a perfectly programmed automatic should never lug... its easier with newer electronic transmissions to avoid it as the computer creates an engine load factor of 0-100%. when you hit 100% then exceed it by a little more throttle the trans knows to drop down..



the HT740 has a mechanical spool modulator.. the cable should be adjusted such that when you are crowding the engine it calls for a downshift , howerver its sill possible to lug the engine to a degree when you are floored and a downshift would mean it goes beyind the Rev limit of the valve body .. at that point the modulator can call all it wants for a shift but it wont happen..



if you find that you are shifting up way early regardless of throttle and having a hard time punching it and getting downshifts then its time to check the modulator cable adjustment.. they can and do stretch over time which results in earlier shifting and less downshifting..
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Old 09-24-2021, 09:24 AM   #48
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Posts: 1,126
We have two Phantoms which are equipped with retarders and they make a huge difference when activated. John's comments, above, are probably the best brief summary of how diesels brake (or don't) without a retarder. Our Sterling truck has Jake brakes and, for a while, they weren't working...and going downhill without Jakes, even with a manual transmission, did little to keep the truck's speed down even when in a low gear. It was a good demonstration of what John described.
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Old 09-24-2021, 11:13 AM   #49
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Join Date: Sep 2019
Location: Bly Oregon
Posts: 284
Year: 1986
Coachwork: Crown
Chassis: Supercoach
Engine: Cummins 350 big cam
Rated Cap: 86 passengers?
My "new Crown" has the Alison HT740 transmission in it. I haven't driven it a huge amount but when I drove it home to Oregon from Salinas Ca. I did well with it. The original engine had more than sufficient power in the Siskiyou mountains even on the grade coming out of Dunsmiur. I believe it stayed in 4th gear up the grade as I was going over 60 mph at the top. At that time It had 4.10 gears in it. Now I have 3.42 gears to provide for freeway speeds without overreving the engine.
The replacement engine is a Cummins big cam 400 that can handle the taller gears.
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