View Full Version : Transmission question

01-27-2008, 06:08 AM
Hello All
I have a transmission question. I have an Allison 6 speed behind a Cummins 300hp turbo. The engin rpm at low speeds has started dropping to low before shifting. For example if I slow for a stop light and the light turns green and I do not need to stop. Or if I'm cruising along a back road and start up a grade. The rpm's will drop below 1500, as low as 1200. This is at halve throttle. To get it to shift I must push the throttle on down to the floor or shift it manually. At 1500rpm at halve throttle it starts to smoke. As the rpm's drop below 1500 and I'm pushing the throttle down, the smoke gets quite heavy. It is somewhat embarrassing. I serviced the engin last summer, replacing all filters including air filter. I have not serviced the transmission. I have just over 50.000 miles on the coach. This all started at the same time I had to switch to ultra low sulfur diesel fuel. I do not think this has anything to do with it, but thought I should pass on that info. I first thought that the program parameters needed reset. but the transmission preforms flawlesly other than this problem. So now I'm thinking that the transmission filter may be clogging and dropping the internal fluid pressure confusing the computer. I'm by no means a mechanic. However I have driven diesel bus's for 28 years. Our fleet mechanic's will not touch our transmissions before 100.000 miles. I was thinking that I would change the trans filter without changing the fluid, this is much less of a mess and I can do that myself. Your thoughts on this would be helpful. I would like to resolve this issue myself without taking it to Freightliner.
Thanks Kevin

01-27-2008, 10:40 AM
Hi Kevin:
This sounds like your engine, not the transmission.
The Cummins was designed to produce max hp and torque at very low rpm(s).
However, the smoking, and lack of engine response lead to the injectors, or injector pump as the first source of your problems.
You can service the transmission yourself, and this is not a bad idea. I have mine serviced once a year, regardless of miles driven. Cenex in Conrad Montana does this for $85 (I supply my own Amsoil synthetic trans fluid, but many of my trucker friends run 15-40 in theirs).
Bottom line, I think your first look should be at the injectors.
Safe Travels

01-27-2008, 01:11 PM

I think that Freightliner would be your best bet. There should be problem codes stored either in the engine computer or in the Allison program showing what the problem is.

Most times that is cheaper and less headaches that just throwing parts at it in a trail and error approach.

I too believe it sounds like an engine problem that a Allison problem.

If it was my coach, I would change the fuel filter(s) before I did any thing else. Restricted fuel flow does some crazy things with loss of power and response being one of the first.


01-27-2008, 05:41 PM
Something similar to what you described happened to us in my FIL's coach a few years back and all it amounted to was a broken hose clamp for the turbo. We limped about 40 miles in to a diesel repair center and took the family out to eat while they found and fixed the problem.

It did as you describe with lots of smoke and little power. We actually had top speeds of 35 or 40 mph! It is worth a look.

Good luck.

02-01-2008, 04:30 PM
Whenever my exhaust brake sticks closed it does the same thing. Might want to check it.

02-03-2008, 03:17 AM
Thanks for your help. I was not thinking engin, I was thinking transmission. I have thought about this for a few days now. I think you are right. After you changed my thinking I have talked with a couple of mechanics this week. They suggested that since this is a relatively minor problem, and only beginning to smoke, and at very low rpm's. That I try a couple of things before taking it to Cummins. First they told me that maybe I should have thought twice before buying a diesel due to the lack of miles I drive. I only ran about 400 gallons of fuel threw it last season. And one tank always set's in it for six months threw the winter. They told me that I should ALWAYS use an additive. It is possible that bacteria may be starting to grow in the fuel. I thought the refineries put somthing in it to control that. One other suggestion was to bleed the fuel line at the injectors. I was told to start the engin, take a wrench and crack open the fuel line fitting on the injectors. Sometimes air will get trapped at the injectors. I don't understand how that could happen, but I had just replaced all three fuel filters, (pre-filter, filter, water separator). And after they had told me this I remembered back several years ago they had to do this to our bus's after filter changes. So anyway it is worth a try to see what happens. One reson I do not want Cummins to work on it just yet is that I'm still getting an average if 10mpg. (9 worst, 12 best ) I know this is not the norm, and I think if Cummins works on it, that would be the end of the great MPG.
Thanks Kevin

02-03-2008, 09:40 AM
Hi Kevin:
You have a few things that may be going on, in addition to the injectors/injector pump that I mentioned earlier.
Water is a big destroyer of diesel engines. You were wise to replace the fuel filters. There is a big debate about fuel additives, and which ones to use if you use one.

I like lucas, power service, howes, and stanadyne. You cannot go wrong by purging your fuel tank, then filling it with fresh fuel, and using an additive.
If you have deposits blocking the free flow of your fuel this may produce the symptoms you described.

The other thing that comes to mind is your turbo. When you travel short distances, that do not fully warm up the engine and its intake/exhaust lines, the rapid heating then cooling of the lines draws moisture.

That moisture can destroy your turbo. You should check the turbo's gate valve, bearings and (if present) waste gate valve.

Last, perhaps your model has an uptake pump in your fuel tank. This "primes" the fuel system by directly pumping fuel at the fuel tank into the fuel line, allowing it to reach the engine fully pressurized.

Whew, sounds like a lot but not impossible to test yourself.
Safe travels

02-03-2008, 10:18 AM
If you bleed your fuel lines at the injectors be very careful. Fuel at that location could be at up to 25,000 psi pressure! When you change your fuel filter inspect it, bacteria will show up as white/off white specs/pieces of material.