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Author Topic: 87 MCI MC-9 Allison HT 748 4 Speed Auto Trans issue  (Read 4957 times)
thepiratebus
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« on: May 22, 2011, 06:03:11 AM »

Any help would be greatly appreciated.  Last winter we hit something leaving a parking lot that pierced our tranny pan.  Luckily we immediately shut down, but it did run us dry on fluid.  We had the unit towed to where we work on the bus (removing the axle to avoid any more issues).

We were able to weld the pan and so far no leaks.  We put the pan back on, refilled to proper level and it would not go in gear (any gear).  Could cycle through all gears on the auto shifter.  We drained it again and removed the pan only to realize that the filter/screen was bent and not sitting correctly.  A solenoid (closest to the front) was also loose.  We reseated the solenoid and put a new screen in that we got from the Allison dealer.  While it now would now go in gear occasionally it still is random.

We parked it for the winter and tried to get it out this spring.  Fluid levels seem OK (although instead of the level rising when hot it went down) so we added a little more, careful not to overfill.  It randomly goes in (more often in Reverse), but still is not right.  When it is in gear, it seems to go fine.  Once going down the highway 2-4 gears work perfect.  Odd part is it seems to go into gear when we cycle through the shifter and play with the parking brake.  This could be just random luck too (wishful mojo).  My thinking is it is the solenoid that was loose.

Questions: Could this be it?  Other suggestions?  Is there a way to order just that one solenoid?  Any help would be appreciated.  Don't want to have the tranny guys have to make a house call.  Thanks in advance.
« Last Edit: May 22, 2011, 06:35:55 AM by thepiratebus » Logged
luvrbus
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« Reply #1 on: May 22, 2011, 06:21:32 AM »

Doesn't sound like a 740 to me are you sure about the model ? maybe a 741 or 748 electronic a 740 has no electronics in the pan 

good luck
« Last Edit: May 22, 2011, 06:29:29 AM by luvrbus » Logged

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Chopper Scott
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« Reply #2 on: May 22, 2011, 06:24:51 AM »

Electric shifter? My 740 has a manual type lever.
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thepiratebus
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« Reply #3 on: May 22, 2011, 06:32:54 AM »

Sorry - this should be the 748 for the auto.
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luvrbus
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« Reply #4 on: May 22, 2011, 06:47:31 AM »

My condolences to you guy but more than likely a bad solenoid or bad ground you need a guy with a reader or laptop to check it for you it will be money well spent unless you have plenty of time on your hands.
Buy some electronic cleaner and clean every connection going to and from the transmission maybe just corrosion on the plugs after cleaning use a good electronic grease on the plugs then go from there  


good luck
« Last Edit: May 22, 2011, 06:58:32 AM by luvrbus » Logged

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NeoplanAN440
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« Reply #5 on: May 22, 2011, 07:14:38 AM »

try reading the transmission codes first!
there should be a test switch near the dash. hold it , with the engine running in idle, transmission in N and park brake engaged.
code will be shown by a a transmission test light. counting how often it flashes.
like the old obd one systems.

the ht 748 is very sensitive to wiring problems or bad connectors.
maybe a cracked connector plate on the valve body (from hitting by the pan)

mine gave me a thousand issues, due to a bad pan connector, that had insulation issues between the locking tabs!
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thepiratebus
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« Reply #6 on: May 22, 2011, 07:19:48 AM »

Thanks guys!  Is there a code index to match the flashes to?
« Last Edit: May 22, 2011, 07:39:12 AM by thepiratebus » Logged
luvrbus
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« Reply #7 on: May 22, 2011, 07:35:40 AM »

That transmission fluid is checked hot while running make sure it is full hot not cold and not running should have taken around 7+ gals to refill depending on the pan

good luck
« Last Edit: May 22, 2011, 07:37:53 AM by luvrbus » Logged

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buswarrior
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« Reply #8 on: May 22, 2011, 07:58:50 AM »

Perhaps a coincidence, how old is your filter?

Proper functioning is dependent on fluid level and flow.

happy coaching!
buswarrior
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thepiratebus
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« Reply #9 on: May 22, 2011, 08:10:28 AM »

Good question - probably need to check that at the same time we are under there.
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trailblazer2
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« Reply #10 on: May 22, 2011, 09:53:42 AM »

I have the exact same type of coach ,as you!I would read the codes off of the lights (located in the back baggage bin,drivers side)This will tell a lot.The conectors are sensitive & might be loose? However start with the tale tale light in that back bin. I might have a translation book,just let me know.
 Bill
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« Reply #11 on: May 22, 2011, 12:15:12 PM »

if you read any  codes, i can tell you the problem and troubleshooting guides by the manual that i have.

if no code is stored, there will be no flash

check if the light will come on for 3 sec when you turn the master switch on.

as soon as you select r   it will erase any codes!!!
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NJT5047
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« Reply #12 on: May 22, 2011, 08:17:46 PM »

Your coach probably has an electronic high idle and kneeling system.  It might also have a lockout switch that prevents shifting when the door is open.  
If the red indicator lights work when you push the touch pad, the shift pad is probably working.
If this problem began when the oil pan was damaged...probably something related to the repair...or the big trans electrical connector on the PAX side of the trans housing.   If the problem existed before the trans pan was holed, some sort of electrical problem exists.  
As Bill said, most '87 DDEC 1s have code reading capability in the upper drivers side rear bay.  There are two lights on a panel.  The green light is the ATEC reader (red light reads the DDEC ECM).  The lights might still have the factory cover over them which can be openedd with two zeus buttons.   The engine doesn't have to be running to read codes.   Turn on the battery disconnect, turn on the ignition master, and go to the LH rear bay and hold the switch next to the green light and count the flashes.   It will flash on set, pause, and flash the second digit.  
Other signs of electrical problems outside the trans are no "Check Transmission Light" in the dash for about 3 seconds whenever the master ignition is turned on, and "Do Not Shift" indicator on the touch pad...or no pad indicator lights.
There are ATEC fuses in the battery box that are famous for corroding.  They should be checked.   Once the fuses are pulled, any codes will be erased.  DDEC II and up will store codes, but DDEC 1 requires B+ in order to store codes.  
Unlikely that you damaged the trans due to idling once the trans fluid leaked out.   Once air hits the hydraulics it won't go, yet the trans is still lubricated.  For a short period of time.  
If any other work has been done on the coach that preceeded you trans problem, consider what might have been moved or dinked with.  
There's a large multi pin connector on the bottom of the touch pad. The'll get loose and cause problems, and the touch pads will eventually fail.  
Take a look as all of your CELs, touch pad lighting, and DNS light and see that they are functioning.  The ATEC harness is one piece (originally anyway) back to the ATEC.  There are connections at the front and in the rear bay.  
BTW, if you have codes set, post what you read and I can ID the code (s) for you.
Good luck, JR  
« Last Edit: May 22, 2011, 08:20:37 PM by NJT5047 » Logged

JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.

Ayn Rand
thepiratebus
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« Reply #13 on: May 23, 2011, 09:31:22 AM »

Thanks - I will get in there this week and see what is going on with the light.  The shifter lights up and the bus in fact will go in gear 1 out of every 6 times.  Reverse, when it goes in, actually works as normal.  When drive kicks in it usually has very little power (like stuck in first), however last time we were bold enough to take on the highway after our pan fix and screen install it hesitated in first, but 2-4 worked fine both up and down shifting.  Check engine light does come on occasionally (not every time) when you put in drive.  It did not do this before the pan incident.

Thanks for all the help and we will see what comes out of those codes.
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NJT5047
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« Reply #14 on: May 23, 2011, 10:22:39 AM »

The transmission should drop firmly into drive and stay there.  Shouldn't "slip" or jump...however, unless you are familiar with large equipment transmissions, the feel may seem abnormal when it isn't. 
Another thing is that the park brake may not be releasing completely and it would seem as though the coach wouldn't pull when in gear.   If the park brake releases, the coach will usually move slightly forward (if on flat ground) when "D" is selected.   If it doesn't....recheck the park brake.  Even with the park brake applied, you'll feel the transmission engage when "D" is selected.  If you don't feel the "engagement," then the transmission system has an issue. 
An '87 MC9 DDEC1 is slow on takeoff.  The transmission will 'hesitate' or 'hunt' when shifting if you slack up on the throttle or use a light touch on the throttle.   First gear just rolls off slowly.   
The best way to drive the bus is with the throttle almost on the floor when you want to accelerate, and off when you want to slow down.   The transmission will 'hunt' around if you lighten up and then slightly accel.   It'll shift into 4th lock at something close to 35MPH on a light throttle, and then if you speed up, it'll downshift 2 or 3 gears down as it looks for the best map.   
Often if you want to cruise along at 35 MPH or so, selecting "3rd" gear will work great.  Forcing 3rd will keep the trans from hunting back and forth between 4th and 3rd. 

HTH, JR
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JR Lynch , Charlotte, NC
87 MC9, 6V92TA DDEC, HT748R ATEC

"Every government interference in the economy consists of giving an unearned benefit, extorted by force, to some men at the expense of others.

Ayn Rand
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