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Discussion Starter · #1 ·
I have a 2008 6.4 F-350 that came with the Tow Boss package (4.30 rear end). I pull a 39’ 5th wheel camper that weighs about 14,000 lbs loaded. I have noticed that when using the Tow/Haul mode and going down steeper grades (nothing drastic) that it will gear down to the point that my RPMs will run up to 3,500 to 4,000. I HATE the way this sounds and it makes me feel as if I am over revving my engine. When this happens, I usually turn off the Tow/Haul so that I can “manage” my RPMs with my brakes. I do not “ride” my brakes, rather I slow the truck down with my brakes and then gradually build speed and then slow down again. My question is this - are there those out there that just leave the Tow/Haul on regardless, even if it runs your RPMs up to 3,500+? I just don’t see how this could be good for an engine.
 

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Tow/Haul can't overrev the engine. If it can't keep the engine speed in the safe zone it will upshift the transmission. The engine can run all day every day at 3950 RPM. If it goes to 3951 RPM the trans will upshift, slowing the engine.
 
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Discussion Starter · #3 ·
Wow, so I have never heard that explanation before. I suppose I am just not used to the truck winding that high. I will try it on our next trip out. Thanks so much for the response.
 

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Why not leave it in tow haul and use your brakes to also assist in slowing it to a comfortable rpm.
 

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Tow/Haul can't overrev the engine. If it can't keep the engine speed in the safe zone it will upshift the transmission. The engine can run all day every day at 3950 RPM. If it goes to 3951 RPM the trans will upshift, slowing the engine.
That's good to know. I've wondered about it too. Thanks for passing that along...
 

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Discussion Starter · #6 ·
The reason I have not left it in tow/haul is because it still revs really high even when using the brakes. I've always felt as if they system was "thinking" too much. However, the next trip out I will just leave it there and allow the truck to do what it was intended to do.
 

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It may not be able to hold the load in that gear. If so, you'll need brakes or to slow down enough to get into the next lower gear and see if it will hold
 

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On my 2nd F450--operates just as you describe: if you use the T/H without braking, it will drop a gear when it feels you gaining speed without the brake being applied. Unfortunately, by the time that happens, you probably are going faster than you want to. Then, you hit the brake and the T/H drops another gear and the revs climb above 3500. Glad to hear that it will upshift at 3951rpm, but mine must not have ever reached that point. However, the T/H itself is not an engine brake and is not sufficient by itself to keep from gaining speed on the steeper grades and higher loads. You need to be proactive in NOT starting down an incline at the wrong speed and with judicious use of braking, you can easily keep the rig under control. I have also used the gear selector many times when faced with long downgrade and use it instead of the T/H.
Another way to slow down periodically is to use the trailer brakes only with the manual lever. as long as you don't overheat them.
I got around 140k miles on the brakes on my '09 working the T/H and the trailer brakes and the gear selector--several times I had wiring harness problems and I had to travel the Rockies without trailer brakes--when you know you don't have trailer brakes, low gear is very useful.

And, if you happen to let yourself get in a bind enough that the engine revs past 3950 and upshifts--well, better hope your brakes are in top shape....you WILL have to use them, and hard.
 
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Discussion Starter · #9 ·
Yap...everything you are saying makes perfect sense. I have never really had a concern about gaining too much speed or losing control of my rig; my concern has only ever been the high RPMs that the T/H forces my truck in to. To be totally honest, this is my first Ford ever. I have always been a Duramax guy. I really like this truck, the cab room, stability, power, etc... But I have heard nothing but horror stories about these 6.4s and I just want to make sure I take care of this engine while I am under my extended warranty. The truck only has 59K miles, has never been deleted, and I plan to upgrade to a 6.7 as soon as my warranty runs out.
 

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Yap...everything you are saying makes perfect sense. I have never really had a concern about gaining too much speed or losing control of my rig; my concern has only ever been the high RPMs that the T/H forces my truck in to. To be totally honest, this is my first Ford ever. I have always been a Duramax guy. I really like this truck, the cab room, stability, power, etc... But I have heard nothing but horror stories about these 6.4s and I just want to make sure I take care of this engine while I am under my extended warranty. The truck only has 59K miles, has never been deleted, and I plan to upgrade to a 6.7 as soon as my warranty runs out.
Just wondering why you purchased a 6.4 if you heard so many horror stories about the 6.4 and are/were a Duramax fan?
 
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