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Junior Member
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Discussion Starter #1
My owners manual states that tow haul should be used when towing, hauling heavy loads or when used extensively in stop and go travel.

Accordingly, I've used tow haul when hauling the camper (gvw=14,000 lbs).
And since adding the ScanGuage II it's been easy to monitor transmission, engine and coolant fluid temperatures.

During my recent Mexico trip I quit using tow-haul while driving stop & go traffic through a city.
This resulted in lower temperatures for oil, coolant and transmission fluid temps.

After noticing the improved fluid temps, I continued NOT using tow-haul for most of the trip.
Not using tow-haul resulted in overall lower fluid temps.

The only time that I used tow haul extensively was while crossing the Sierra Madre Occidental mountain range.
Lots of up and down, twisty mountain road where having the lower range gearing was very helpful.

I always thought that achieving lower engine, coolant & trans fluid temps was a good thing.
The question in my mind is, is the owners manual recommendation to use the tow haul mode while hauling or towing. . mistaken?

Has anyone else noticed that cooler fluid temps are achieved by not using tow haul?
Your thoughts?
 

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Main thing I've noticed is when towing heavy in hilly areas with tow haul is it tends to keep the turbo spooled up better due to the longer delay in upshifting and results in lower egt's. Turn the tow haul off and it will upshift sooner resulting in lower rpm's and higher egt's. This is only in hilly areas that I use it, on flat or mild grades I leave it off.

The fluid temps can fluctuate dramatically, and depend on rpm and load and ambient temp conditions and heavy winds or not.

What kind of numbers were you seeing in fluid temps for comparison?

Harry
 

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3,533 Posts
I have a duramax and in the Alison the transmsion goes into lock up earlier and harsher, the increase in engine RPM is minimal at low power loads but at full power it will go to max RPM.
I hauled a load of top soil, up a hill for about 12 miles the first section is quite steep, in the 8 to 10% range, for about 2 miles. I came to the bottom of the hill at 60 mph in tow haul, and cruise. it slowed to about 52 mph till the engine got up to power and the transmissin went to 4th, and over 3,200 RPM, then it went back to 60 MPH and shifted to 5th to pull the rest of the hill. The temps never went up above normal. There is a free scale at Midway BC it grossed out at 24,552 lbs.
I know from testing that when pulling a hill, and not in tow haul it does get hotter, transmission temp because it stays in torque longer. I take it out of tow haul when on the flatter sections.
 
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Junior Member
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Discussion Starter #4
What kind of numbers were you seeing in fluid temps for comparison?

Harry
Outside ambient temps were often in the upper 80's driving at highway speed.
Oil temps mostly stayed below 220 and rarely made 230

In town traffic is where tow-haul keeps the engine in lower gear and builds heat in both oil and trans temps.
Keeping the truck out of tow-haul in town traffic kept tranny temps in the 160's compared to it normally going into the 170's

None of the Mexico driving that was anything that I've repeated.

A good comparison will be when I leave the Phoenix area going up Hwy 17 grade to Flagstaff - a trip that I've done before and will normally see EOT go to 240 very easily in outside temps of 80+ degrees.
This year I will keep out of tow-haul and see how it compares (next month).
 

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I've noticed the same, higher trans temp when in tow/haul. what I'm not sure of, is the max normal trans temp. as long as it doesn't cause it to exceed that, I don't see a problem. There definitely will be more torque load on the T/C lockup clutch when pulling heavy not in tow/haul, which could result in slippage. I'm with Harry not using it when towing hills etc probably will increase EGT.
 

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Pursuit Interceptor
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735 Posts
Tow/Haul does exactly what is supposed to do. It holds the trans shifting till it's a higher in the RPMs. Hence why you see the higher temps. Worth noting for sure, everyday use won't hurt anything. I for one find the shift points just a tad higher over what I would prefer while using Tow/Haul. Now if I'm towing Tow/Haul will be used.
 
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