Allison HT748 High Oil Temp Warning !!

[NeoplanAN440]

I started yesterday morning again,going trough NM and the bus felt like it has less power.
my trailer was only loaded at 8k and i was wondering whats going on,as i still have tp pick up other stuff!!
i checked on fuel pressure readings and everythink else was normal.
after 5hours of driving the temps here got to 102F and i noticed my engine running about 5to 10F hotter then normal.on a grade while pulling temps where raising faster then normal.(i was going three weeks ago to vegas at 108F with 16k on my trailer,and temps stayed low as allways)

on a long uphill pull my temps raised to 195 and before i could backup the throttle i got the transmission light.
checked codes and have set the 24 for transmission oil to hot.
let it cool down in high idle,went on my way,and getting the same again and again,when trying to pull.

no burned fluid,normal oil level( not to hight)
running rotella 15W40 on the tranny for years now without problems!!

any ideas what to look for!!! we are thinking if we turn around ,as we have to pick up a load in CO (mountain driving)and will then have close to 16k or more on the trailer.

trans does not feel like slipping and lockup feels normal!!!

cooling system is clean and intact (i had it all cleaned,rodded out and bypass filter installed)
cooling fan is locking up and working.

so my guess is that the power loss and higher coolant temps are comming from the transmission.
as said coolant temp is raising as soon as i have to pull.normal i would see a 5 degree raise after pulling a step grade for miles,and it would drop when i took my foot out of it!!!


thanks for any input

[neverlearn]

I have no knowledge of what on the bus could cause this, however for a trailer capable of the capacities you mention I assume that it has brakes.  Do you know if the trailer brakes are dragging?

[Greg Roberts]

It sounds to me like you may well have an issue with the retarder. It could be engaging partially or even fully and this would account for reduced power and increased heat. I believe there is an electric solenoid and other items switching air pressure to control the retarder action but your unit might be different. Take a hard look at the retarder. It is the only thing I can think of that fits your symptoms. Good look!

[NeoplanAN440]

Ok,here is the update of our Problem so far!

we did some more testing,and the problem is getting worse!
when taking of the trans shifts normal into 1 and 2.
when going into 3rd i will loose power and speed and if i keep it in third it will overheat with check trans light and code 24.
i have to speed up in second up to the governor,let it shift in third,and then by letting loose on the throttle force it to shift into fourth!!
fourth is normal again,and we can cruise init with no problems and heat build up!!!

oil still looks normal!!

im guessing a planetary gear is going out!! but could it be something else??
if there is a chance for this situations that i only have a bad valve ?
asking as i have one more trip,before i was going to swap to a cummins,so if there is a chance for an easy fix,let me know.

otherwise i will have to cancel the trip and start with the swap!!!


thanks

[Len Silva]

Sammy, we miss you!

[hargreaves]

Hook up a prolink reader, while going through the gears check to see if the lockup clutch drops out when going into third. If the lockup clutch drops the trans will create heat and it will seam like less power.

Good luck,     Gerry

[NeoplanAN440]

thanks for the reply!!

i dont think the lock up clutch is dropping (slipping)
when i step on the throttle i let it shift at 2100rpm (on my tach)from 2nd into 3rd.
rpm drop is around 500rpms.i can then step on it till 1800-1900 rpm but beginning at 1700rpms,it feels like im going up a grade.at 1900rpms in third it starts it starts slowing me down,and even if i stand on the throttle it will drag me down under 1600rpm till i drop back in second.if i keep it in third to long ,i get the overheat code.
temps at the filter confirmed over 275F.

if i go from second to third at 2100 ,foot on the throttle.then in third i have to step down to force it into 4th.
in 4th i have all the power back as normal.
if i step on it in 4th,that it downshifts into 3rd,i will loose power and rpm,as soon as 3rd engages.
have to let it down in 2nd,to start over again.

feels like somethink is binding(sticking) in third and gets worse if i stay longer in it!!!


pretty wierd

i try to get home as is,as im stuck here in nowhere.
then i will have to drop the pan,and i guess that will show me to pull the whole deal.

i was hoping that there is a combination of solenoids or valves that could cause the tranny go on the wrong path.as it feels like reverse is acting against it!!
from the manuals i have on hand ,i still guess that the planetary gear on the third clutch is going bad.
i just cant hear any noise or see or smell real burned oil.
the oil shows some abuse after the last trip,but still good to me!!

thanks for any input

i was hoping somebody who rebuild one can chime in,if there is a weak link in these.
even it says the 748 is allready stronger on 2nd and third then a 740

[hargreaves]

The only thing that I can see happening is the snap ring on the second gear side of the centre support is broken or not in it's groove. When the trans shifts into third it moves the centre support towards the second gear clutches partially applying them.  This will cause a binding situation and create a lot of heat. You won't notice it in any other gear because third gear clutches only come on in third gear. The only other thing that may be happening is the piston seals on the third gear side of the centre support are leaking, partially applying the second gear clutches.   Hope this helps.

Good luck!      Gerry

[NeoplanAN440]

thanks hargreaves!!!

that makes sense to me ,and i was ready to get it home and pull the tranny!!

we decided to get the bus home as far as we can get.after another 50 miles the problem got worse.
the power loss in 3rd was so great that i couldnt get it anymore to shift into 4th.
we made it onto a wal mart lot,to get some rest,and think about our plan how to get the bus home.

i only checked the oil,and it still looked not too bad ,for what we are doing to the tranny right know.
somehow i checked the connector above the oil pan,but it was clean and dry inside.
we decided to leave in the morning,to stay out of traffic and drive only in 2nd home .
taking little steps at a time.

the next morning when we started,the bus shifted all gears and lockup normal!!!!!
no power loss,no heat,temp stayed low even pulling long grades in third!!!!

am i crazy now?


50 miles from home it got wild!!!
we stopped for a check on the trailer,and when i started again,the tranny wouldnt go out of 1st gear.
i tried kneeling,to turn off the ecm,and even a restart.i could select all gears,no code.
when starting the tranny would stay in first,even running for some time on the governor.
i then stopped,selected reverse to push into a side road.
reverse engaged,but it felt like the clutch is slipping and i had a rattling sound.
stepped on the throttle and reverse clutch engaged with a bang.
no slipping ,all normal.
i tried D and the tranny went into gear normal.
we started again all gears normal with lockup and everything!

made it home as is no problems
at home i tried again.
if i stop and go to N ,i have first to select R .slipping clutch that sets with a step on throttle.
after that all forward gears normal!!!


getting crazy??

[NeoplanAN440]

ok

here is the update in my tranny problem story!!!!

after some chatting with  hargreaves  (thank you for all your help) we came to the conclusion,that the transmission connector on the bus harness side is bad.i ordered one and the day it arrived ,changed it.
the old one had a broken grommet,and i measured that some of the pins had resistance to each other,if i wiglet on the wiring.

we just passed about 1500miles on the new connector.i changed the tranny fluid,but stayed with the rotella 15w40. this way i could make sure,all changes are only due to the connector.
so far the transmission shifts, like never before.all shifts are on point.lockup works all the time at same shift points.automatic downshifts on throttle and no hunting or harsh shifts.
i dont recognize my bus again.
more power, 1 mpg better fuel mileage.

this shows me that im running around with this problem for a long time.i never had a set code or a check tranny light!!

now i have one problem left

if i go into reverse when the tranny is hot,i have a slipping-grabbing clutch.
to back up uphill pushing my trailer ,i have even to step on the throttle to get over the slipping part.

i think this is a leaking clutch or valve body!!???

any hints one this one

i guess i put a lot of stress on the tranny,with all the years towing with some electrical problems!!!!

[edroelle]

Transynd trans fluid was engineered for higher temps.   It may be a good investment.   Ususally, 7XX series transmissions, unless pulling big loads, do not need it.   World transmissions are not as robust as the 7XXs such that it is more desireable.

I see it as some insurance against overheating the trans.

Ed Roelle
Flint, MI