Allison Run Hotter on Downgrade?

[joed]

When towing, I've noticed my 06 Allison tends to run higher trans temps on longer downgrades using the grade braking feature.

Is this normal/typical? I know the TC is locked during this. I've had it creep up to 250F or so, and even had the "Transmission Hot" warning once.

It doesn't happen right away, but on a longer (several miles) downgrade, after climbing to the summit. It will warm up climbing, but doesn't seem to get as high.

I'm wondering if I need to look into additional trans cooling or is this not something to worry about?

I don't think I'm overloaded ~12K of trailer.

Any thoughts/opinions.

Thanks. Joe.

[DmaxMaverick]

It isn't normal, but it is (??) to some degree. Your tranny has 2 ways to shed heat. One is the external fluid-air cooler (behind grill), and the radiator tank fluid-coolant heat exchanger. On a downgrade, the engine isn't generating enough heat to open (or keep open) the thermostat(s), so there is little to no coolant moving through the radiator (thus, no coolant flowing around the tranny heat exchanger). This condition removes the majority of the heat regulation built into the system. If your external cooler is obstructed (air), or your ground speed is low enough to prevent a positive airflow through it, there is little heat exchange going on. This condition should effect the tranny temp very little (toward the hot side), especially approaching 250°. With the TC locked and only a compression load on the tranny, it should stay relatively cool. Either your TC isn't locked, or your cooling system has obstruction(s), or any combination of things. 12K is a significant load, but should not, in almost any situation, cause this excessive heat. Something is amiss, but is probably something very simple. Perhaps all that is needed is a routine tranny service, with some fresh fluid and an external filter change (filter restriction can reduce fluid flow, and cooling ability).

[joed]

Good info.

I'll try a trans service first, and go from there.

Thanks. Joe.

[richp]

Hi,

I had a similar situation develop coming out the west side of Death Valley last November.

I was going down an exceedingly long but moderate grade, with the Allison locked in second or third gear and moving maybe 25-30 mph. External temps in the 70's, so that wasn't a big contributor. Holding back the weight of the truck and a 12k fifth wheel meant that the tranny was absorbing a whole lot of energy, and that it had to be going into the trans fluid.

I never got an indication on the instrument panel that it was overheating, but at some point I recall getting uncomfortable with the buildup. So about 2/3 of the way down, I stopped in a pulloff and let it rest. It did cool down some, but not as fast as I was expecting. Of course with the engine cool, the fan clutch would not have been pulling anything through, so that's the reason for that. Natural convection was all I had going for me.

But it did drop, I used the brakes a bit more the rest of the way down, and once the bottom was in sight and I let the speed creep back up, the gauge started to move back into a more normal range.

FWIW.

[joed]

On mine, it seems to be a function of how fast the engine (and transmission consequently) are turning - the faster the more heat build up. The lower the gear it uses to hold back the speed, the worse it is, heat-wise. I guess this make sense, as more rpms would equal more friction/heat?

I run this stretch of highway loaded a lot (I-70, west of Denver) - if the condition seems to persist I'll try using a higher trans gear and see if that helps.

Thanks, Joe.

[DmaxMaverick]

There shouldn't be a functional difference between the trannies and the cooling system in T/H mode, within all the model years. I can't find a descriptive difference, anyway. Something is amiss. If a tranny service doesn't help, I would add a larger cooler, perhaps one with an electric fan.

That said, I've had mine heavy (at GVWR) in just about every condition imaginable. Extended up/downgrades of 6%+ from freezing temps to 120°+. I always maximize grade braking, very often in the "red zone" (3500-4500 RPM). The tranny temp was never an issue. I've seen/heard the same from friends with similar conditions and later model trucks. Never a complaint of tranny temps, except extreme hot climate up steep grades at lower speeds (open converter and less than 25 MPH up 10%+). It can get hot, but hangs in there long after any other auto tranny would have died (these roads are stained with ATF, coolant, engine oil, miscellaneous parts, etc.).

[kaylabryn]

I would start looking between the coolers; if you have never done it you might be surprised. Mine shortly after I got it was completely plugged, at least from appearances. Cleaned all 4 coolers separately and thoroughly, probably saw around 4 cups of bugs alone come out or more. M truck was 6 years old and 160K on the clock. It made a huge difference in all situations.

Mike

[joed]

I'll definitely check the coolers as well.

Quick Fluid Change ?, how many quarts are typically needed for draining the pan and filter?

Thanks, again.

[DmaxMaverick]

The pan should drain about 8 quarts, or less. The amount in the filter is negligible.

It's always a good idea to keep an eye on the cooling stack, but I don't think it has anything to do with your issue. If it did, you'd have heat issues under heavy load, not necessarily during no-load conditions, and it should indicate on the coolant temp gage. The tranny cooler is in front of the stack (gets fresh air, first), so it shouldn't be effected much, if at all, by a dirty stack.

[joed]

Sorry, another ? regarding the cooling stack.

Can you clean between the coolers without removal? I'm wondering if you can get down between them by just unfastening the top mounts, rather than complete removal (a/c condenser, especially).

I've been using compressed air, blowing from the engine side and am getting quite a bit of dust.

Should I follow up with water from a garden hose, flushing from the engine side?

Thanks.

Joe.

[DmaxMaverick]

You can. It's less effective, but much better than doing nothing. If you are blowing/washing from every angle you can access, getting between them to remove accumulated debris isn't that difficult. Once the top mounts are loose, they can be moved apart enough to get a narrow shop vac tip in to pick up what remains (blowing usually forces debris into the fins). Be careful with the hard lines/plumbing and the fins. The soft lines are fairly flexible. While you're in there, check the A/C line running across the top for chafing (this was an issue on early models, I don't know about later, but it's free to check). If you find some, insulate it with something. Depending on where it is, you can use a piece of split hose or plastic/vinyl tubing, or wrap it with tape.

[Mark Rinker]

When towing, I've noticed my 06 Allison tends to run higher trans temps on longer downgrades using the grade braking feature.

Is this normal/typical? I know the TC is locked during this. I've had it creep up to 250F or so, and even had the "Transmission Hot" warning once.

It doesn't happen right away, but on a longer (several miles) downgrade, after climbing to the summit. It will warm up climbing, but doesn't seem to get as high.

I'm wondering if I need to look into additional trans cooling or is this not something to worry about?

I don't think I'm overloaded ~12K of trailer.

Any thoughts/opinions.

Thanks. Joe.

(Experienced this a couple times, although not a 'typical' kind of thing...) IIRC my remedy has been to downshift one gear lower than the Allison wants, which keeps the RPMs a little higher - reducing ground speed during the descent even further.

Higher RPMs help by circulating the tranny fluid a bit faster, keeping the fan moving more air through the cooler stack, helping to shed additional heat.