What a difference °100+ makes!

I towed my father's 10k RV from Rockport back to SA for him today. I have towed his RV for him before, but usually earlier in the year when it is still °80-°85 out. My truck hardly even breaks a sweat in those temps with the coolant being in the high °190's, the trans in the °170's, and the oil temp about °206.


Towing the same trailer on the same 170 mile route in the °105 Texas heat.....
(Note: The coolant temp was °208, but dropped by the time I took this pic)


I have seen hotter temps when towing heavier stuff. I am just shocked of how much of a difference °100+ makes especially with the trans temp.

I'd be curious if locking out 6th would benefit trans temps.


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I locked out 6th for about a quarter of the trip and it made no difference besides lowering the EGT's from high °900's to high °800's. The trans temp gradually increased over the 170 mile trip which leads me to believe that the heat was more from how hot the outside temps were with the trans cooler not able to get cool air rather than any slipping.

I've seen higher pulling 7-8% grades and even steeper off highway stuff getting into nat'l forest campgrounds and such, but never on flat lands towing. I almost exclusively lock out 6th. We are heading for the bighorns Saturday. It's supposed to be in the mid nineties around here, 4,000' climb, but it's at 20-40 mph.


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Yeah, if I were pulling any long grades or towing at 65 mph or slower then I generally lockout 6th. There are a few short uphill climbs along the route that would have required 5th when my truck was stock and all the emissions equipment on, but it easily handles them in 6th at 1,500 rpm doing 70 mph now that it is tuned and deleted without the EGT's getting to high.

Update: Well, after doing some digging and talking with a Ram transmission specialist on another forum, I think the cause of my trans temps slowly rising was more than likely due to the thermal bypass valve being partially open keeping fluid from the cooler. He said it should partially open at about °160 and fully open at around °180. He also said it could also be the cooler, but most likely it is the TBV since they are notorious for staying partially open

Luckily it is something I can remove and just bypass since it is mainly used to get the fluid up to temp quickly for better fuel efficiency and removing it will not harm the transmission. In fact, he said it will probably be better for it given the temps I commonly drive in since all trans fluid will go through the cooler at all times.

SouthTexan said:

thermal bypass valve

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68RFE? If so, where is this valve located?

BossHogg said:

68RFE? If so, where is this valve located?

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I haven't gone under there to see it yet, but it is an H looking block on the passenger side between the trans and cooler from the way it was described to me.