Transmission temps
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Damage would have to be defined in multiple ways ...
component wear
seals
fluid degradation
Slightly to moderately overheated fluid will not make gears destroy themselves over a few minutes. Whereas the wear rate may see an uptick, having fluid at 275 for a few minutes won't make gears grenade inside the case. Over very long exposure periods of sustained high fluid temps, the gears would wear out faster because the viscosity breakdown would have a negative effect on face-to-face contact. To give perspective on this, having high tranny temps with a conventional fluid may make a 150k mile tranny only last 100k miles or less. It won't self-destruct in a matter of a few minutes pulling uphill upon the first "hot fluid" pull.
Seals can harden if the fluid chemistry isn't right and the temps are very high. After hardening, the seals will lose their ability to "seal" and leaks will develop externally or internally.
Fluid degradation can affect cooling, lubrication, contamination control, viscosity, etc.
My point is that a short moment spent at 275F isn't going to make a trans explode. Do what Ford says (pull over, reduce load, keep engine running at idle) and the temps will cool and all will be OK. These type of infrequent, intermittent, limited duration events may alter the lifespan of the fluid, but it won't make the trans fall onto the ground.
If you were running a conventional fluid (such as a fluid previously known as Dexron III or Mercon) and you hit 250F repeatedly it would affect the lifespan of the fluid; you'd need to change fluid more often. But the trans would be OK as long as you had a good fluid maintenance schedule. Running that same fluid at 275F for sustained periods may well cause a problem in true wear-rates. Running those fluids at 200-225F isn't going to have wear-rate effects much at all. Running today's fluids (LV, Dex VI, and many of the "synthetic" choices) is just moot at temps below 275F. It's the combination of temp and duration of exposure that defines the net effect. Most any of our trucks today can safely sustain 250F over long periods, and 275F over short periods, with little concern.
Mark would be able to add info specific to his experience.
I've seen a LOT of UOAs on tranny fluids; generally I would not lose sleep over anything under 250F, and if it hit 275F but was immediately dealt with (pull over and cool it down) I would not sweat that either. Anything less than that is no cause for concern. The old school fluids (Dex IIIh and Mercon) typically had a starting vis of 7.0+ cSt but would often shear down quickly below 6.0 cSt and oxidize. But UOAs that showed the wear-rates went unaffected because the high temps were not sustained, but short-term. The "heat" was enough to shear and oxidize the lube, but that effect didn't directly manifest into altered wear because the TIME of that exposure wasn't great.
Further, the more upscale premium products (full PAO syns for example) scoff at 275F. But the OEMs have to predicate their alarm sensors on the "least case" scenario, and so 275F for an alarm setting is the safe bet.
Like I said before ... anything below 250F does not even get my attention. Hitting 275F would make me pull over and cool down, but not panic. If I hit 275F often, I'd consider swapping to a PAO fluid. But then again, if I were hitting 275F often, I'd probably also look at adding cooling capacity via a supplemental cooler. In fact, if my rig were hitting 275 repeatedly, I'd rather add capacity to the cooler than try to rely on chemistry. The only time you really "need" a PAO is if you hit high temps often, and have no ability to add cooling capacity due to a limited compartment area situation (such as a E-350 motor-home chassis with small front area and tight engine compartment making for difficulty to improve the heat-exchange rate).
component wear
seals
fluid degradation
Slightly to moderately overheated fluid will not make gears destroy themselves over a few minutes. Whereas the wear rate may see an uptick, having fluid at 275 for a few minutes won't make gears grenade inside the case. Over very long exposure periods of sustained high fluid temps, the gears would wear out faster because the viscosity breakdown would have a negative effect on face-to-face contact. To give perspective on this, having high tranny temps with a conventional fluid may make a 150k mile tranny only last 100k miles or less. It won't self-destruct in a matter of a few minutes pulling uphill upon the first "hot fluid" pull.
Seals can harden if the fluid chemistry isn't right and the temps are very high. After hardening, the seals will lose their ability to "seal" and leaks will develop externally or internally.
Fluid degradation can affect cooling, lubrication, contamination control, viscosity, etc.
My point is that a short moment spent at 275F isn't going to make a trans explode. Do what Ford says (pull over, reduce load, keep engine running at idle) and the temps will cool and all will be OK. These type of infrequent, intermittent, limited duration events may alter the lifespan of the fluid, but it won't make the trans fall onto the ground.
If you were running a conventional fluid (such as a fluid previously known as Dexron III or Mercon) and you hit 250F repeatedly it would affect the lifespan of the fluid; you'd need to change fluid more often. But the trans would be OK as long as you had a good fluid maintenance schedule. Running that same fluid at 275F for sustained periods may well cause a problem in true wear-rates. Running those fluids at 200-225F isn't going to have wear-rate effects much at all. Running today's fluids (LV, Dex VI, and many of the "synthetic" choices) is just moot at temps below 275F. It's the combination of temp and duration of exposure that defines the net effect. Most any of our trucks today can safely sustain 250F over long periods, and 275F over short periods, with little concern.
Mark would be able to add info specific to his experience.
I've seen a LOT of UOAs on tranny fluids; generally I would not lose sleep over anything under 250F, and if it hit 275F but was immediately dealt with (pull over and cool it down) I would not sweat that either. Anything less than that is no cause for concern. The old school fluids (Dex IIIh and Mercon) typically had a starting vis of 7.0+ cSt but would often shear down quickly below 6.0 cSt and oxidize. But UOAs that showed the wear-rates went unaffected because the high temps were not sustained, but short-term. The "heat" was enough to shear and oxidize the lube, but that effect didn't directly manifest into altered wear because the TIME of that exposure wasn't great.
Further, the more upscale premium products (full PAO syns for example) scoff at 275F. But the OEMs have to predicate their alarm sensors on the "least case" scenario, and so 275F for an alarm setting is the safe bet.
Like I said before ... anything below 250F does not even get my attention. Hitting 275F would make me pull over and cool down, but not panic. If I hit 275F often, I'd consider swapping to a PAO fluid. But then again, if I were hitting 275F often, I'd probably also look at adding cooling capacity via a supplemental cooler. In fact, if my rig were hitting 275 repeatedly, I'd rather add capacity to the cooler than try to rely on chemistry. The only time you really "need" a PAO is if you hit high temps often, and have no ability to add cooling capacity due to a limited compartment area situation (such as a E-350 motor-home chassis with small front area and tight engine compartment making for difficulty to improve the heat-exchange rate).
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jim henderson
Clutch, Transmission, Differential, Axle & Transfer Case
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Jun 9, 2004 12:21 AM
