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Allison 755 questions…
It’s been a rough week with my transmission. This is on my 8V92 and 755 transmission. I’ve had a trans leak for a while and have asked the guys who work on my bus to fix it several times. It never got done. Finally, it got worse and I pressed the issue. The shop owner suggested to do a trans service too. The leak turned out to be just a hose leaking.
First trip it after the service, the guy driving called and said that “the trans is unlocking in high gear.” He got it back home and I put in a work order on it. I had two weeks off. A few days later, some friends from England were here and I took them to lunch in Hendersonville and had no issues – 30 mile round trip. I mentioned this to the shop owner. He said he’d taken it on an hour long test drive and also had no issues. I said I didn’t trust it and still wanted it hooked to a computer and checked out.
So the day comes for me to leave. Spoke to both the shop owner and the foreman. In the past, I’ve asked them about if work had been completed and they got a little offended, as if I was suggesting they hadn’t done their job. So, I made my mistake of figuring they would have told me if it weren’t fixed since they both knew I had a five day trip ahead of me.
I got about 100 miles out when the transmission started acting up. As the driver said, it seemed to unlock in high gear. My first sign was that my cruise control wouldn’t stay engaged. Also, when I would coast down a hill, instead of having the engine hold RPMs against the trans, the trans would disengage and RPMs would fall to idle speed even though I was running 63 mph. At the time it seemed to be connected to heat. If the trans was running 180-190, it seemed to be okay. I could achieve that by staying around 63 mph. I got to my destination and rechecked everything. Fluids were all good. Nothing seemed out of shape. Called the shop and left a message. It was then that I was informed that they “didn’t get time to check my transmission.” You can imagine that I hit the roof. I took it by a local shop and was told that I should have never let them do a service. Apparently, the belief is that you should never service an older trans unless there is something major wrong.
Anyway… There’s a lot more I could tell, but I’ll cut it short. Finally got back to the shop and they did some checking. They replaced the speed sensor. I test drove it last night and it doesn’t disengage anymore, but I feel like it still gets a little too hot and the RPMs are wrong I think. At 70 mph, I’m turning about 1925 RPMs. Maybe I’ve just never paid attention before, but I think that’s high. Temp stays around 180-195 on level ground, but goes to 210 on even slight grades… And even just idling. Thoughts?
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26 Replies
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When I bought my 91 XL (755CR) a few years back it had been sitting for about 7 years with 50,000 miles on it. First thing I did is drive to Prevost Ft Worth and have all fluids changed including trans with Synthetic Trans Fluid. Don’t know if it hurts to open them up if you know what you’re doing. Don’t have a heat gauge on my 91 XL. Seems like mine runs about the same rpm’s and mph. Good luck.
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Thanks. I may just be paranoid. I know it will be very expensive if I have to replace the transmission. Plus, I have some long trips coming up and don’t want any problems.
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Dale just looked on my Build Sheet from Prevost and mine has a 3.58 gear ratio in the rear end. Mine runs about the same down the road for mph and rpm’s. How many miles on your coach. Mine is hitting around 80,000.
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My coach just passed a million miles a few months back. The engine has about 80k on it. Not sure about the trans. Runs like a top, but this thing has me spooked. I imagine mine has a similar gearing. So I may be okay on RPMs. Still a bit paranoid about the heat. I don’t like heat in these things! Thanks for looking it up. I appreciate it.
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Dale, As was mentioned the road speed at a given RPM is dependent on the rear axle ratio. Once you know the ratio you can determine your speed at a given RPM.
I cannot be sure but I seem to recall a previous coach I had with the 8v02 and the 755 was turning 1800 at 59 MPH. Going by memory so it might be wrong.
The current and previous coach was at 61 or 62 MPH at 1500 RPM.
Your transmission is very sensitive to overfilling, If you are experiencing higher transmission temps start by insuring the fluid level is exactly right. I believe on your coach the transmission oil pan has a 1/8 pipe plug on the driver’s side. If you start the coach cold, let it idle in neutral and remove the plug no oil should come out. If you begin to add oil, even a little bit and oil begins to come out the level is good. Add oil slowly until it comes out. That is when it has the correct amount. Do not try that with the transmission warm, the engine off, or on unlevel ground. Over filling can make the transmission run warm.
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Dale, I don’t have the Allison manual, but the Country Coach manual for my 1990 XL40 with the 755 trans says that the normal temp range for the transmission is 160-250 degrees. Mine always ran on the high end of that and I had it checked a few times at Prevost Mira Loma. They always reported that it was fine. Unless it is significantly different than before, I think your temp is OK.
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So, the shop replaced the speed sensor in the transmission. They said it about fell apart in their hands when they removed it. Hopefully, that was going to be the end of it but my luck doesn’t run that way…
I took it out for a test drive the night before I was supposed to leave on a 1200 mile trip. The transmission no longer disengaged, but now it seems to hold in gear too long and then really shift hard when it does shift. The ship says there is no adjustment for this. So I’m wondering why it changed. It was always smooth as silk before.
So I left on the trip and immediately saw that the engine was running a good but hotter than normal still. It had done that prior to changing the speed sensor, but I hoped that was part of the symptoms of that. Both the trans and engine temps are getting high. It got so bad that I was running 190-195 on flat ground at 65 mph. Luckily, I got in the same traffic carguy did on Monteagle because I was up to 205 within the first mile of the climb. As soon as I got into traffic and stopped, temps fell. They only rise when pulling. So I called the shop and without looking at anything, they wanted me to order a new $4000 radiator.
Now, I may need a radiator. But prior to this transmission fiasco, my normal temp was 167-170. I have crossed Monteagle many times at 60mph and my temp only got up to 180. The last time was just over a month ago, prior to the trans issue. So I believe, even if I buy a new radiator, this issue will not be fixed. I’m supposed to leave for South Dakota on Friday and could use some guidance. Thanks.
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My radiator does have a little rust in the fins… front edge. But I don’t think this developed in the last month and started this issue. With that in mind for preventive maintenance, does anyone have a good source for these, or a good one lying around for sale?
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Dale my 1991 XL 8V92TA runs about 190 to 195 all the time in summer on dash gauge. It’s about 10 to 15 lower on engine compartment gauge. Have tried and check with Temp Gun and out back is closer to real temp. It will hit 200 or a little above on dash gauge if you hang in at 65mph pullig a F150 Supercrew Ford on hills around Austin area.. Don’t know if this is good or not. Winter or cool months down aroung 180. Good luck on that radiator. I know mines a coming closer to replacing with age. One other thing make sure the air valve is all the way open on belt tensioner on fan. A couple oil changes ago somebody closed mine a bit and it would run hot at higher RPM and okay at lower RPM’s.
My trans (755CR) with synthetic fluid shifts pretty solid.
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My first reaction is did the shop change or add transmission fluid. I mention that because the HT755 is very sensitive to overfilling. When cold the fluid level is easy to check. With the transmission cold, level, slow idle in neutral, remove the small plug on the (drivers?) side of the transmission pan. If fluid comes out, let it drain until it stops. If none comes out add some until it starts to come out. Like the differential plug that plug is the correct level for fluid.
As to the radiator….they don’t just quit working. If you recently under similar conditions were able to maintain lower temps you might have an issue with the cooling system but it just doesn’t seem likely the radiator has plugged that quickly. It could be a water pump impeller, or some restriction such as an old hose blocking flow. But if the temps have tracked the rapid rise in the outside temps don’t rule out the radiator. It could be in need of a new core, but corrosion over time could have caused the fins to disintegrate which ruins the ability of the radiator to reject heat.
There is one guarantee…….if you keep seeing high temps (175 to 185 would be the normal range, with up to 195 on a hard climb) I would urge you to find and correct the problem. Our engines turn into junk very quickly if overheated. I think DDEC will shut you down at 215 to 220, but that is dangerous and still puts the engine at risk.
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UPDATE
Just as no one could ever explain why my bus ran so cool (averaged around 170 from the DDEC), no one can explain what changed. The shop kept wanting me to buy a new radiator and I was sure that wasn’t the issue. I did check the trans fluid as Jon suggested. It was fine. Finally settled on changing thermostats. It’s still not down to where it was, but now I’m running about where everyone else is, I assume. On my recent trip to SD, I stayed in the 184-187 range most of the time. Got to 190-191 a couple of times on long pulls. Still wonder if I might be getting a little too much heat out of the trans sometimes. Sometimes it will go to 200 when the engine coolant is still 185. Maybe I’ll consider adding an external trans cooler later.
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Dale, I think you are running temps that as I recall were the same as my 8V92. Depending on how much work the transmission is doing will determine its temp, but 200 degrees is nowhere near a temperature that would get my attention.
If you are approaching 250 you have a problem.
I surmise when the coach is driven in the rain you see the transmission temps drop well below that of the engine so those temperature swings all sound pretty normal to me. Any 8V92 guys willing to share the temps they are seeing?
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I agree with Jon…200 is nothing to worry about.
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I agree, nothing wrong with the temps you are running.Mine runs 175 on easy runs and maybe 190-95 on really long, steep grades. Transmission goes up to 200 if you are working it on grades and then drops right back on the down side. Like Jon said, hit rain & it drops like a rock. My 8V92 has 175000 on it.
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Got a couple of trips under my belt now. Feeling better about it. I wish Thanks for all the insight and experiences. Now I can move on to other minor annoyances. It’s always something with a bus! haha
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