| Author | Message | ||
| Timnvt (Timnvt)
Rating: N/A |
Who is a busnut’s HT-740D Guru? I’m having troubles up here in the northeast and I need a busnut friendly Allison transmission guy out there, retired or otherwise, who knows their stuff to talk to. My situation is DDAllison has sent a traveling tech to my place and taken my valve body out of my coach. The original problem I was experiencing was a harsh if not violent downshift from 4th to 3rd. The tech can’t tell me what’s wrong with my transmission other than it has a weak spring in the 3-4 relay valve. It is about 6-7 lbs and it should be 18-19lbs. He can’t say whether that is causing my problem or that replacing the spring will help. Rather, he wants to replace my separator plate and trimmer valves and springs throughout the valve body because they do not match the assembly number of the transmission. (The transmission originally was set up for a fire truck application.) If that doesn’t satisfy the problem they will happily charge me to take the valve body back out of the transmission and try tweaking it with different springs and such, put it back in the transmission and try it again. They also recommend having the “soft shift” kit installed for about $700, but again offer no guarantee that it will satisfy the problem. For the record the transmission has exhibited no other symptoms and there is no evidence of damage or trouble in the oil pan and the fluid is clean. At $91.00/hour we’re already at $1600 in labor and other than adjusting modulator, throttle, and shift linkage, no repairs have been made. Not a happy camper, Timnvt. PS. Because I’m dissatisfied with their performance they are offering to discount the bill and put the valve body back in unrepaired (gee thanks). | ||
| john w. roan (Chessie4905)
Rating: N/A |
One option would to get a used bus trans from one of the sellers like bernhard bus and transfer the valve body. Unless there is a bad unit inside the trans, the valves and their timing control the shift quality esp. when only one gear change is giving you trouble.What does Allison or one of the major trans rebuilders get for a rebuilt exchange valve body? | ||
| John MC9 Rating: N/A |
For whatever it's worth: The manual's here: www.tpub.com Tons of more here, too. I had been told by two bus companies that the 740 normally shifts with a bang, and if it doesn't...worry.. Don't know if that's true, but they could have $whacked$ me good by telling me different. | ||
| Jim and Myrna Lawrence (Daffycanuck)
Rating: N/A |
Be careful...I was just $whacked$ for a rebuild by Detroit Diesel Allison B.C. for a MT650...$10,000.00 after core refund.... They had me by the short hairs.... After install, another three days in another DD shop to correct shifting and oil leak problems.... it still leaks.... If it aint broke...leave it alone!! | ||
| Earl-8-Ky Rating: N/A |
My transmission shifts a little rough in the 3-4 and 4-3 shift pattern. I shift mine by hand. Mine would shift from 3-4 at to low of a speed. I hold mine in 3rd. till I am at about 50 mph. I then go to 4th. It has helped a lot on smoothing out the shits. I shift down to 3rd at about 50 too. I also added a gallon of Lucas to the transmission. This has helped a lot. | ||
| Buswarrior (Buswarrior)
Rating: N/A |
Hello Timnvt. OH RATS.... Timnvt, where are you? Perhaps an HT740 busnut is nearby and you could try out his/her bus to see if there is any difference from yours. I'm in Toronto, if that helps. I hate to suggest it, but maybe nothing is wrong? The HT740 does not like to downshift smoothly on its own... Earl-8-Ky described the practice: you shift an HT740 yourself if you want smooth operation from it. You might want to just get those people to put your tranny back together before it get$ wor$e... Just have a fresh set of proper springs installed and be done with them? For the benefit of all, I’ll describe what mine does. I’m currently governed close to 2100 RPM, good for ~70mph. The path to smooth operation from an Allison HT740 and its various siblings is to place the shifter in 2, letting the tranny take care of 1 and 2. It manages to do that well. Use 2 until just before the governed speed, near 37 mph/60 kph, go for 3rd. 3rd is good to 52mph/80 kph, and go for 4th. The trick is to go for the upshift just before the governor kicks in, for maximum performance and smooth ride for the passengers. You also want to know where your tranny will downshift on its own, so you may plan your smooth operation. When shifted manually, mine will grab 3rd at around 55 mph on the way down, second seems to overlap at much the same 37 mph as on the way up. In contrast, if left to itself, the tranny will downshift from 4 to 3 at 47 mph and from 3 to 2 at 30mph. Generally, on any bus of this vintage, the shifts are usually smoothest if you catch them right up at the governor. Of course, each bus has its own character, and once we become familiar with it, we may take great advantage of its particular quirks to suit our driving style. When hill climbing, there is a bit of a trick to timing a throttle lift in conjunction with a downshift. You want to break torque for a smooth shift and then get back on the fuel to help bring the revs up to match the third gear engine speed required, as well as to maintain momentum. This is not unlike a portion of downshifting a manual tranny. Really smooth, and no loss of hill climbing momentum if you get it right. Also, it won't hurt the tranny to move the shifter before you have slowed sufficiently, the tranny is smart enough not to shift until the road speed has slowed enough. (smart in an analog way!) The term "automatic" is a bit misleading. Even the newest Allison World B500R 6 speed automatics in the new coaches can get into situations where the driver wants something from it that it isn't programmed to do. They are prone to upshifting to too high a gear in the city, and then holding it, so you have no acceleration. (Partially opposite to a HT740, which loves to shift up too far too, but then bang back down again when you push harder on the throttle) When driving the new stuff, I follow the same logic as with the HT740, only difference, you can let a B500 take care of 1,2 and 3. I select 4,5 and 6 on the key pad when I want it, not when it chooses. Timnvt, let us know how things work out! happy coaching! buswarrior | ||
| Mike (Busone)
Rating: N/A |
buswarrior, I assume the HTB748 also does this? | ||
| Buswarrior (Buswarrior)
Rating: N/A |
Hello Mike. Is the HTB748 electronically controlled? Keypad instead of a shifter? I am not familiar with the shift strategy of the electronic 740 family. I expect that they upshift at lower engine revs and hold the higher gear better under throttle at lower revs than the mechanically controlled ones, if my experience with V731 is any indicator. happy coaching! buswarrior | ||
| Mike (Busone)
Rating: N/A |
Yes it is the electronic version of the HT740 and also has a retarder. | ||
| Richard Bowyer (Drivingmisslazy)
Rating: N/A |
I completely concur with buswarrier. I almost always shifted mine manually, in the same manner as buswarrier and I always got relatively smooth shifts. In climbing grades, I typically shifted down to the next lower gear about 10 mph before the tranny would shift. Then a slight decrease on throttle pressure would allow it to downshift when I wanted it to. I never let it downshift automatically. The rpm's were much too low, in my opinion. Richard | ||
| RJL Rating: N/A |
Check the "Articles of Interest" section of this website for one about shifting. Might find it helpful. | ||
| Timnvt (Timnvt)
Rating: N/A |
I appreciate everyone’s comments on this so far. The DD Allison folks want $900 for a rebuilt valve body specific for the coach application which includes the “soft shift” kit. A complete rebuilt transmission is somewhere in the $5000 range. I do have the service and parts manuals for the HD740D but my manuals being 1975 edition didn’t list the build or assembly number for the valve body I have. That number is evidently the means of knowing just what you’re supposed to have for valves and springs. Without an updated manual listing the specific assembly number, there’s no way to know how the valve body is supposed to be set up. I'm in northeastern Vermont and would have appreciated having another coach owner drive with me to try the manual downshift trick you fella’s are describing. I did try to shift manually 4 to 3 without satisfactory results. Having someone who HAS practiced the manual downshift show me precisely how to do it might have helped (Silly me, and I though automatic MEANT automatic). However, I’m not convinced it would have satisfied the problem. My transmission downshifted appropriately at about 55mph without the loss of rpms that some of you are describing. It seemed nothing I did to try manually could keep it from SHOCK going from 4 to 3. According to the service manual, that’s what the trimmer valves are there for. Buswarrior and Richard (or anyone else needing to manually downshift their automatic), Do you know whether your transmission has the so-called “soft shift” kit? I think it would be interesting to know whether there is a dramatic difference with or without the modification and whether folks still need to manually down shift the automatic despite the upgrade. Anyone know when the “soft shift” was standard equipment in MCI’s coaches having the HT740D transmissions? I’m getting an education about transmissions. An education can be an expensive thing to have. Then again, it can be even more expensive to be without. Thanks guys for ALL your input. | ||
| H340 Rating: N/A |
For what it's worth... Ever since I had my 740 serviced at Jack's rally, it has never shifted better! I put it in 4th and let it do the rest. I never HAVE to shift it manually since being serviced. It upshifts and downshifts very smoothly! Ace | ||
| Richard Bowyer (Drivingmisslazy)
Rating: N/A |
Ace,have you ever had it in the mountains? Richard | ||
| H340 Rating: N/A |
Richard only time I had it in ANY mountain region was when I purchased it in Bassett, Va and then it appeared to be only down hill on the way back to Florida. It didn't shift good then and I knew something was wrong with it but I brought it home anyway trying to get away from an oncoming freeze! Once home, I researched the problem and had the guys in Arcadia to test drive it. Almost if not immediately, they diagnosed the problem. They told me it needed a skinner valve. Once replaced, it never worked better. Manually or Auto! Very smooth and very positive in all gears up and down! Ace | ||
| Richard Bowyer (Drivingmisslazy)
Rating: N/A |
Ace, Based on my perceived perception of you as being somewhat of a performance nut (as well as a Bus Nut), I think you will want to do some experimenting once you get her out into some real mountains. DML was just too late in downshifting when climbing grades. It would usually be down around 1400 rpm before shifting down. Once the shift was completed it would be in the 1750 1800 rpm range. This was at least 300 rpm below what I really wanted it to be for maximum climbing power. By manually shifting I could force the shift to occur in the 1650-1700 rpm range and maintain the engine above 2000 rpm much of the time. I had my governor set for about 2300 rpm. Richard | ||
| H340 Rating: N/A |
Thanks Richard but I'll be in the flat land of Florida for a while yet and by the time I HAVE to go into any mountains, it will be too late, if you know what I mean?  Ace | ||
| Doug McCartney
Rating: N/A |
Timnvt, Shift quality problems on HT740s can be a real challenge. I have work as a trans tech on Allisons in transits and know these units pretty good. The shift quality is adjustable using different trimmer valves and springs. The problem is that harsh shifts can be caused by too high of apply pressure or too low. Too short of apply time or too much apply time one. Generally I like to move to softer springs and slower valves first. If that does not work, move to faster valves and stiffer springs. I agree with the tech that the soft relay spring may or may not be contributing to the problem. I would put a new one in before I did anything else to the valve body. Also worn clutches and other mechanical problems can affect shift quality. An engine that is low on power can affect full throttle shift quality. The bottom line is….. it is a real art and I have only see a few guys that were really good at it shift quality problems. I have often had to take the pan off several times before I come up with the right combination. It seems like there are no two transmissions respond to the changes in the same way. We had busses that were exactly the same and they needed different timmer packages to make them shift right. The Allison tech will try a set of valves and springs that should give you softer shifts and often times that works. If it still has undesirable shift quality..... off with the pan and some good ole educated guesses plus a little trial and error will be needed to get it right. I know this isn’t a lot of fun for the tech and can be pretty aggravating for the guy paying the bill but this is the way these units are. Is it harsh full throttle downshifts or closed throttle downshifts that you are having problems with? Do you know the valves and springs that you have? The trimmer valves have a four-digit number on them. The color will identify the springs. I would try to only adjust the trim on the clutch that you are having problems with and leave the ones that are working fine alone. A soft combination that I like to start with when I have a harsh shift is: 6686 trimmer valve Orange 0045 external spring (primary) Blue/white stripe 0118 internal spring (secondary) If this combination cause a flare I move to a 5920 valve and see what happens... Good Luck, Doug | ||
| John MC9 Rating: N/A |
Trial and error at $100 bucks per hour, while the rest of us peons shift it manually. (sorry for that outburst) | ||
| Buswarrior (Buswarrior)
Rating: N/A |
Hello John MC9. We get a professional contribution, with specific suggestions as to a strategy for parts replacement.... Any idea what this sort of advice might cost if you had to pay for it? And you mouth off at him??? Hello Doug. Thank you very much for your detailed post. Most of us here really appreciate when a professional chimes in and shares. Please ignore the other ones, they don't know any better. happy coaching! buswarrior | ||
| John MC9 Rating: N/A |
buswarrior- Well.... I did say "sorry for the outburst". I had just washed my hands and couldn't do a thing with them. Doug - While I too, greatly appreciate your input concerning all the hows and whys, and the process needed to isolate and possibly repair the problem. You were more than candid in stating that it: "can be pretty aggravating for the guy paying the bill ". The poster stated: "At $91.00/hour we’re already at $1600 in labor and other than adjusting modulator, throttle, and shift linkage, no repairs have been made. " If I were an expert who has spent half my life repairing older Allison transmissions.... and I knew that the high cost of repairing a transmission of that vintage will be near as much as a replacement.... I personally would feel a bit guilty in recommending that the owner continue his pursuit and dump the remaining .05 from his 401k into the hard shifting transmission. But - I'm glad you did give your input, since there are many here that do all their own work, and do not have to hire and pay a mechanic. My comment was only to indicate the frustration that might be felt by the rest of us; those of us who can not afford to pay for the repairs, and found that careful manual shifting can not only cure the problem, but keep us from starving to death as well. (I hope you were not offended, that wasn't my intention). | ||
| Timnvt (Timnvt)
Rating: N/A |
Thanks a lot Doug for taking the time to spell this out. In some measure it is reassuring that what I hear from the service manager is essentially the same as you have related. It’s tough as a customer to hear that ultimately it comes down to experimentation. I’m lead to believe the electronically controlled transmissions have an advantage for remedying these troubles. Knowing who a few of those individuals are who have the knack for coming up with a winning combination would be well appreciated among private coach owners I’m sure. As these older coaches become more the exception than the usual, technicians with the experience to know how to efficiently fix the problem will be few and far between. Thanks for the trimmer and spring combo numbers. Timnvt | ||
| Timnvt (Timnvt)
Rating: N/A |
Hi John MC9, If you can shift manually and produce satisfactory results, more power to you. However, I’m not persuaded that the trouble I’m having downshifting from 4-3 is rooted in the same characteristic that so many of you successfully manage by manually controlling the shift. I’ve put about 4000 miles on my coach since I purchased it and I’ve tried every which way I could to minimize the 4 to 3 shift shock. My decision to have a professional check it out was not for comforts sake but because the shift was so severe I feared doing damage to other components of the driveline as well as the structure of the coach. My MC5A was “professionally” re-powered with a 6v92 and auto transmission from the original 8v71 and 4-speed manual about 25 years ago. I know that since then both the engine and tranny have been changed out. The tranny in it now is a fire truck tranny and doesn’t match the serial number of the first HT-740D. Not only that, but when we dropped the oil pan the valve body in it didn’t match the build number on the transmission tag. It may very well be that this transmission was not set up properly by the last person who had it opened up. I think it’s great if some of you guys have the wherewithal to tinker with your transmission. I didn’t know enough about transmissions to begin with and didn’t figure this was a good area for me to experiment. Tim. | ||
| Doug McCartney
Rating: N/A |
No offense taken………I know it is difficult to pay a guy big money to “tinker” with something until it was right and I sympathize with the dilemma that it puts you guys in. As a technician it is difficult also. A guy with a lot of experience will be able to hit the right combination most of the time but I have seen the best of the best get his butt kicked a time or two. A service manger telling you this up front is being honest. So as a customer you have to decide what you want to do or what you can afford to do. Living with the problem is an option that I can appreciate knowing that you can get into some money fixing it. You can also have them try to fix the problem knowing that they may have to take a couple of shots at it and there is a good chance that it my be fine after the first attempt. Heck a new relay spring could possibly be the fix in this case. I know from my experience that you can’t guarantee that you will be able to put the right combination in the first time. We had groups of busses that were exactly the same. They were ordered at the same time, were one serial number off and some needed different trim packages. The shop that I work in is a fleet shop so there is much less pressure than when you are working on a bus for a customer. We have the luxury of chassis hoists, a tank full of ATF and a parts room that has all the valves and springs that a guy could ask for. Even then, there were occasions where I would get asked how many times I was going to take the pan off of that bus. My response was usually “I will let you know when I am done”. Not because it didn’t want to fix it right…… this is just way it seems to work out. So good luck with this…. Doug | ||
| John MC9 Rating: N/A |
Doug- How much (approximately) would a used transmission cost, installed? Since Tim's transmission has obviously been knuckled together by some ape in a previous life, wouldn't it be less expensive to get a decent one rather than play in someone else's septic tank? | ||
| Doug McCartney
Rating: N/A |
I’m sorry, I can’t help much with the price of replacing the transmission. We don’t have any hydraulic HTs in our fleet any more. Friday they were taking the decals off of one of our last units with a HT-ATEC so they could send it off to auction. We only have a couple of units left with V-731s in them. Otherwise, we have all Allison WTs in our fleet. Additionally we do all of our transmission work in-house so we don’t buy many transmissions. As far as replacing the transmission with one that has not “been knuckled together by some ape”, It may be best to deal with the transmission that he has. Most rebuilders put the trim package that was in the transmission back in it. This works most of the time if it goes back in the same unit or one that is close to it. If he got a smooth shifting one out of a similar bus it would probably work fine but there is a chance that he could still be in the same boat that he is in now. If he went to a standard trim package based on the serial number of a similar bus, it wouldn’t matter if he did it on his transmission or a different one. That is assuming his transmission is in good condition. The problem is that external factors can affect shift quality and in some cases the shift quality needs to be fine-tuned to the vehicle. If you have a transmission out of a high horsepower engine and you place it in one that has a lower horse power engine it can have an effect on shift quality. Different vocations also have different requirements. Firm shifts in a dump truck that is hauling rocks works well but in a transit bus hauling people around, it may not be acceptable. Certainly a transmission that came out of a fire truck is going be set up different than one for a bus. Another aspect of this problem is the possibility that worn clutches or other mechanical problems inside the transmission could be causing harsh shifts. Generally if we can’t solve a shift quality problem in any transmission (HTs, B-500s, B-400s) we will ultimately pull it out and send it to the rebuild area. If this was the case with Timnvt's trasmission, another transmission may be a better choice. Unfortunately, no matter how you look at it, at this point, it’s a judgement call. Doug |