Peculiar Truck Problem - Montana Owners Club

rames14 · 06-08-2005, 01:56 PM

I am wondering if anyone has had a similar problem to this before. I have the Chevy Silverado with the Diesel/Allison package. I was pulling the Montana through town and it sounded like I had downshifted from OD to 1st. RPM's stayed at about 2000, temp okay on engine and tranny, no idiot lights, gages okay. Seemed to really struggle like it was in super granny low, even at highway speeds. Took it to the dealer today and they didn't find anything but recommended a new fuel filter and the external trans filter. $170 bucks, but won't know if it fixed the problem until I pull the Monty in a few weeks. I'm heading into the mountains of Colorado and don't want to have any issues there. Also noticed a drop in fuel economy from about 12 down to 9. As I recall last year I was getting about 13.

Bill and Ann · 06-09-2005, 01:40 AM

Ron: Change the fuel filter. I think you will find that is the problem. John and Donna(OntMont) and Ormee and Ginette and ourselves had the same type of problem this spring. The truck acts like it is starving and won't shift properly.
Change the tranny filters at the regular mtce. schedule.

rames14 · 06-09-2005, 01:06 PM

Thanks, Bill. It was so loud you couldn't hear each other talk. I guess I will find a source and learn to change myself. It would be expensive doing that every 18k miles - $147. I did some looking on line and found the filter for the tranny is about $7 compared to $25 installed and I think I saw the fuel filter for about $17. I will find out in a little over a week if that helped. Actually, I will be towing a Uhaul this weekend so that may be a small test. Should improve fuel economy, also.

mobrownies · 06-09-2005, 07:15 PM

I think the fuel filter is supposed to be replaced every 15,000 miles. I bought one to carry as a spare, but it was about $35 at O'Riely's. Where did you find one for $17.

tweir · 06-13-2005, 07:57 AM

If you were in Tow/Haul mode, the torque converted is locked through 4th. gear---always locked in 5th.. If you were slowing down, the unit did downshift. There are two ways to "fix" the problem: one, when it downshifts like that, take it out of tow/haul and give it some fuel (overcomes the de-fueling mode)then put it back in Tow/Haul; #2:---and you already did this; change the fuel filter. If you have the LLY (04.5--05 model years)there have been some issues with the fan comming on too soon. That noise will really scare you the first time it happens. LLY's run a little hotter than the LB4's, so watch your EGT's if you have boost/EGT guages.If the engine still runs 2000 rpm @ "highway speed" ( I assume 65 or so), its probably the fan; don't think the truck will hit 65 in first gear...!. Should have noticed an increase in fuel economy "empty" after changing the fuel filter if as you say, your economy was dropping off. For more specific info, go to the "Diesel Place" website---lots of GM mechanics answewr these sorts of questions

tweir · 06-13-2005, 08:03 AM

Ron: I stand corrected, the older style GM diesel is the LB7---up through model year '04. Here is the link to the Diesel Place: http://www.dieselplace.com/forum/index.php?

RKassl · 08-01-2005, 03:15 PM

Quote:

quote:Originally posted by tweir

If you were in Tow/Haul mode, the torque converted is locked through 4th. gear---always locked in 5th.. If you were slowing down, the unit did downshift. There are two ways to "fix" the problem: one, when it downshifts like that, take it out of tow/haul and give it some fuel (overcomes the de-fueling mode)then put it back in Tow/Haul; #2:---and you already did this; change the fuel filter. If you have the LLY (04.5--05 model years)there have been some issues with the fan comming on too soon. That noise will really scare you the first time it happens. LLY's run a little hotter than the LB4's, so watch your EGT's if you have boost/EGT guages.If the engine still runs 2000 rpm @ "highway speed" ( I assume 65 or so), its probably the fan; don't think the truck will hit 65 in first gear...!. Should have noticed an increase in fuel economy "empty" after changing the fuel filter if as you say, your economy was dropping off. For more specific info, go to the "Diesel Place" website---lots of GM mechanics answewr these sorts of questions

I had this happen to me and I thought that the truck was going to explode. I was probably going 50-55 mph, slowed down for an intersection and there was the noise. Is this a problem that needs to be fixed? I was in the Tow-Haul mode, did the tranny make a mistake? I never was told to take the truck out of the T/H mode if you could help me understand this i would appreciate it.

tweir · 08-02-2005, 02:51 AM

Actually, be grateful the fan did engage. That unit really moves the air through the stack in an effort to cool down the unit. The transmission did not make a mistake as the fan is regulated by a temperature sensor. By taking the truck out of tow-haul you disengage the lock-up feature (designed to minimize "slip" and slip equates to friction, equates to heat)and although permits a little slip, you now have the ability to allow the computer to enter into the "de-fuel" mode used in regular ****ing. This allows a reduction in torque to the transmission thus eliminating hard shifts and elevated shift points. What I do when necessary is "shift out of tow-haul then when I hit third gear, re-engage tow-haul. Again, I watch the guages and when the engine temps get to 210 and transmission temps around 180 I know the fan will soon engage and if necessary, do the engage-disenge tow-haul thing. I always use the tow-haul feature when towing, especially in hilly terrain. If I'm on cruise and towing in level terrain, I disengage tow-haul at times. Again, its how your specific truck "behaves" and no real worry to "fix" it. If though, your guages show abnormally high temp readings, then you have a concern. Check out the website I posted a few messages back---they have a whole section devoted to this issue under the "LLY" section I think. Really some good info there; and these guys know a heck of a lot more than I.

tweir · 08-03-2005, 03:30 AM

Here is an addendum to my last post. Hopefully, this will "clean-up" my previous post:
The O/D lockout is very handy if you do a lot of towing. Most of the time the Allison is smart enough to know what gear to be in and when. But, it is blind and can’t anticipate changes in terrain. That’s when I take over. On a stepped climb, for example, where steep grades that would force a downshift are followed by short, less steep grades where the trans would upshift only to be followed by another steep section, and so on, I’ll hit the O/D lockout and just hold the gear for the duration. As for T/H, I use that whenever there is something attached to the hitch. I learned a long time ago that in hill country even a light load of around 3k lbs can be enough under certain conditions to cause the trans to withhold a T/C lockup on a long pull, which adds heat. With the T/H engaged the trans will downshift, lock the T/C and run cooler at about the same rpm as it would with the T/C unlocked in a higher gear.

padredw · 08-03-2005, 05:32 AM

I expect it has to do with the fact that we have one of the smaller and lighter Montana models and a fine towing vehicle, but I never use the "Tow/Haul" except on mountain passes (up and down). Otherwise the transmission seems to handle everything just fine and has never run hot at all, and has no tendency to 'hunt'.

I hope my practice is OK with my particular setup.

You may also like: