Allison 750 shifting issue

HudsonMarine · July 22, 2016, 10:07:20 PM

1983 MC9 that was originally a manual trans but now has an Allison 5 speed (HT 750?) (converted before I bought the bus).
Noticed the other day that it has an issue when shifting from third to fourth.
The best I can describe this is when it tries to shift, it seems to hesitate for a couple of seconds, and acts like it's partially engaged in fourth but then it fully engages and the gear is solidly there.
All other shifts are fine and crisp.
Praying I don't have to pull this trans out, it looks a tad heavy, LOL.

luvrbus · July 23, 2016, 06:24:15 AM

What is the shift point rpm when going from 3rd to 4th ? what is the 750 model you have

TomC · July 23, 2016, 07:58:50 AM

Sure you have a 750? The 740 4 spd feels like a 5 spd with the torque converter lockup. My HT740 has what's called the soft shift. While 1st converter, 2nd converter, 2nd lockup is the same, when shifting to 3rd and 4th, the torque converter comes out of lockup during the shift to make it smoother then locks back up again. Almost sound like that with yours. Good Luck, TomC

luvrbus · July 23, 2016, 08:17:16 AM

He doesn't have any problems if the transmission is not slipping in 4th lockup if you get the shift point setting to low between 3rd and 4th they will have the same symptoms

HudsonMarine · July 23, 2016, 01:15:04 PM

No folks, I'm not sure it's a five speed or, a 750 (where wouild I find a tag or numbers to verify?).
Driving it, I can count 4 distinctive "shifts" so I assumed it's a five speed.
Having no tachometer, I cannot answer at what rpm it shifts however, it does seem to shift sooner than I perceive it should.

I know it's hard to describe to other people but, when it shifts from third to fourth, it ALMOST feels like there's 2 shift taking place, one right after the other.

HB of CJ · July 23, 2016, 01:55:16 PM

Sounds like it is momentarily after going into 4th gear, engaging 4 converter mode before immediately going into 4 lock up mode? I do not know what would cause this.

If you have the usual modulator your cool 5 speed should shift in the following manner. 1 converter, 2 converter, 2 lock up, then 3 lock up, then 4 lock up, then 5 lock up.

The shifts should be firm and quick.

luvrbus · July 23, 2016, 02:10:26 PM

They set 1 up to shift like that, C=converter L= lock up they shift 1C,2C,2L,3L and 4L yours probably set with a kit to go from 3L to 4C then to 4L look on the drivers side about the middle of transmission above the pan there should be a plate with the model and number of the transmission.You could have a 750CR Allison the CR stands for close ratio fwiw

HudsonMarine · July 23, 2016, 03:45:22 PM

All of you guys are seriously appreciated.
I was unaware of the lockup mode!
I guess I need to properly ID what the things really is and go from there.
Are filter/fluid changes worth the effort or, best to find a shop to give it a look over?

DoubleEagle · July 23, 2016, 04:58:10 PM

Timely fluid changes are important. The best trans fluid for wear and cooler temperatures are ones that meet the Allison TES-295 spec. TranSynd was the original, but there are many other versions by other companies that either meet that spec or come close to it (in their opinion). Price is a key factor; TranSynd is the most expensive (don't ask how much), Mobil 112810 Delvac Synthetic ATF can be had for $41.83 a Gal., CITGO QuatraSyn, $177 for 5 Gal., and Schaefer's 204SAT-All Trans Supreme for $28.83 Gal (shipped-Amazon). Shell, BP, Mack, John Deere, and Fleetrite have versions. At our usage rates, this level of fluid will last a long time and only need filter changes and periodic testing.

luvrbus · July 23, 2016, 05:48:43 PM

Don't waste your money on synthetic oils for a 700 series Allison it won't extend the life of a 700 series, one will run for a million and half miles on 10/30 or 15/40 C rated engine oil with changing it every 50,000 miles and changing the external filter at 25,000 miles

They do fine and shift just as smooth as any with synthetic oils you don't need the TES 295 or 389 spec oil in a old Allison 700 series unless you have money to waste JMO

DoubleEagle · July 23, 2016, 07:52:34 PM

Quote from: luvrbus on July 23, 2016, 05:48:43 PM
Don't waste your money on synthetic oils for a 700 series Allison it won't extend the life of a 700 series, one will run for a million and half miles on 10/30 or 15/40 C rated engine oil with changing it every 50,000 miles and changing the external filter at 25,000 miles

They do fine and shift just as smooth as any with synthetic oils you don't need the TES 295 or 389 spec oil in a old Allison 700 series unless you have money to waste JMO

That is probably quite true, but the fluid change interval with TES-295 will be stretched out to a point where most of us won't have to change it for the rest of our lives. We will feel warm and cozy knowing that we gave the Allison the very best that has been created for it, and if we want to waste money, what better place than our buses? If I had to pay TranSynd prices, I would not do it, but with the lower prices of the alternatives, it's an easier choice

TomC · July 23, 2016, 08:08:22 PM

Easy to tell if it is an HT740 or HT750. At the rear of the transmission, the bolted on tail shaft of the 740 4spd bolts directly to the case of the transmission. The 5spd has the extra gear bolted onto the rear of the transmission. So you have the tail housing bolted to about a 3 inch space (the 5th gear) and then the transmission case. Good Luck, TomC

luvrbus · July 23, 2016, 08:23:02 PM

Walter, you still need to dump the synthetic oils every 3 or 4 years where you drive your bus or not you cannot just leave it for ever.I have friends with the B500 in buses they don't run enough miles per year to justify the cost of the synthetics and changing it every 3 or 4 years so they just use Dexron.I believe synthetics are better for a Allison with the retarder though

DoubleEagle · July 24, 2016, 09:08:06 AM

Quote from: luvrbus on July 23, 2016, 08:23:02 PM
Walter, you still need to dump the synthetic oils every 3 or 4 years where you drive your bus or not you cannot just leave it for ever.I have friends with the B500 in buses they don't run enough miles per year to justify the cost of the synthetics and changing it every 3 or 4 years so they just use Dexron.I believe synthetics are better for a Allison with the retarder though

Gee, we are in our seventies, can't we get a break in fluid change intervals? How about dumping in 5 years? They work better with retarders because there is less heat generated, right, shouldn't that also help the transmission in general? $:-\$

eagle19952 · July 24, 2016, 09:10:32 AM

Quote from: DoubleEagle on July 24, 2016, 09:08:06 AM
Gee, we are in our seventies, can't we get a break in fluid change intervals? How about dumping in 5 years? They work better with retarders because there is less heat generated, right, shouldn't that also help the transmission in general? $:-\$

the HT 740 was bullet proof long before Transsynd/synthetic oils...