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Old 06-16-2014, 04:41 PM   #1
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Towing in OD 4r100

OK....here we go. I have a 2000 F250 7.3 4X4 crew cab short bed with a Ford remanufactured heavy duty 4r100 and 6.0 cooler.

I tow a 13,300 lb 5th wheel twice a year and do it in overdrive. I worried about it so I called my transmission guy who I would trust with my life. He said as long as the temps were fine (it never got over 160) and it wasn't shifting back and forth from OD to drive it was fine. On hills kick off the OD.

I got home and was talking to my mechanic, who I also trust with my life, and he said NEVER pull in OD because the clutch plates are half as thick as drive plates are and I would smoke my transmission.

So...........
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Old 06-16-2014, 04:53 PM   #2
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Check on the power curve of your engine. If you're tooling down the highway in OD and the engine isn't lugging, I'd think you'd be fine. If out of OD your engine is winding up RPMs that puts you outside of torque and HP sweet spots, again OD would be fine. Hill climbing in OD of course puts extra strain on engine and drive train so switch it off as you start to climb. The info about the clutch plate thicknesses is new to me, I'd think that the rating for the transmission would be good for all gear ratios it's designed for.
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Old 06-16-2014, 04:54 PM   #3
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If your tranny is in distress, it'll downshift out of overdrive to a proper gear. In any case, the temperature tells the tale, and you're running very cool. That 6.0 tranny cooler saved a bunch of transmissions over the years.

It is fine to run it in overdrive.

One thing you might do is send your ECU to D-P Tuner in Houschton, GA. For $110, they'll reflash your engine horsepower which includes a matching John Woods transmission program. Or, you can get a bunch of programs on one of their tuners with custom transmission programs for the intended use.

I'm running the D-P 80 hp Econo program, and my truck is incredibly fast. When I get my new fifth wheel shortly, I'm going to have to go to a 40 hp tow mode ECU program.
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Old 06-16-2014, 05:25 PM   #4
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I would call these guys and ask. Brian builds really strong 4R100's and knows what fails. http://www.brianstruckshop.com
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Old 06-16-2014, 06:03 PM   #5
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The first guy is right and the second one could not be more incorrect. The OD clutch's are huge. They are big enough to just fit in side the front part of the case. I believe that transmission is rated up to 25K in towing weight in OD. You figure motorhomes and big box trucks (F550's) have this transmission and they last hundreds of thousands of miles driving in OD. The PCM will dictate if the transmission needs to be shifted out of OD. No reason for you to do so unless you find it shifting back and forth a lot within a small distance.
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Old 06-16-2014, 06:14 PM   #6
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I'm glad to see some BTS fans on here.

BTS (and Ford tranny engineers) say that overdrive in the 4R100 tranny is just another gear. With a good program in your truck's computer (PCM), there's no reason not to tow heavy in overdrive. The main problem with the 4R100 was inadequate tranny cooling capacity for towing heavy, so it was very easy to get the tranny temp over the 225 redline. But the 6.0L tranny cooler should have fixed that problem for you.

The tranny shifting program in the PCM will not allow "hunting". However, in rolling hills the tranny will downshift and upshift often, causing the tranny to heat up. So in Hill Country, I usually locked out OD and ran in 3rd gear.

One time as a test I towed the 5er plus a 2,500-pound tag trailer 300+ miles from Midland to Texas Motor Speedway with OD locked out. Then I allowed the OD to work when it wanted to on the return trip. I got less than 9 MPG going slightly downhill with the wind behind me to the track, but I got 12 MPG on the return leg using OD. That was the last time I ever locked out the OD when towing that rig.

I had a '99.5 F-250 PSD with 4R100 tranny. At 106,000 miles the torque converter began slipping, so I drove to BTS and had him do his thing to bulletproof the tranny. I sold the truck with 197,000+ miles on it and no tranny problems. But two things I did that Brian Thompson at BTS said were good ideas. I changed the ATF every 30,000 miles using Mark Kovalsky's DIY procedures linked to below, and refilled with Mobil 1 ATF. (Mark was one of the Ford tranny engineers that helped develop the 4R100 and 5R110 trannies.) And I had an excellent tranny temp gauge with the sender installed in the port on the side of the tranny. (Install instructions are on the DieselManor website.) The tranny temp red line is 225, but with my 8,000-pound 5er I never saw over about 210 in over 100,000 miles of towing coast to coast, along with another almost 100,000 miles not towing.

Here's that link to the DIY ATF change procedures:
Changing ATF: 7.3L PowerStroke Engine and 4R100 Automatic Transmission. - Diesel Forum - TheDieselStop.com
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Old 06-16-2014, 06:35 PM   #7
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Thanks for all the replies. I failed to mention I have a TS 6 position chip, open air intake and 4 inch turbo back exhaust. I got 10.7mpg towing with OD into the wind.

This was my first time towing with cruise. (The fuse Ford installed in the recall had blown several years back.) So I noticed that cruise wants the truck going 60 no matter what. I quickly learned that cruise can make EGTs go sky high fast. So it worked best to shut off cruise heading into a hill to keep speed gradual instead of mashing the pedal to the floor like cruise wanted to do. Then kick out OD to finish the climb.

It's OD towing for me.
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Old 06-16-2014, 09:51 PM   #8
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Originally Posted by Rolfsted View Post
So I noticed that cruise wants the truck going 60 no matter what.
On '99.5 7.3L with with stock-size tires, 4" performance exhaust, AIS intake, and a DP-Tuner 60-horse towing tune, minimum speed was 62 MPH. Slower than that and the engine didn't make enough horses to not downshift for every little bump in the road. But faster than 62 MPH cruising speed and the MPG fell off. Towing 5er cross country on the plains while grossing about 16,000 pounds:

2000 RPM = 70 MPH = 9 MPG
1900 = 67 MPH = 10 MGP
1800 = 64 MPH = 11 MPG
1700 = 61/62 MPH = 12 MPG

So most of my towing was with the cruise control set on 62 MPH and overdrive turned on.
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Old 06-17-2014, 05:53 AM   #9
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Wow.... I appreciate the homework you did.
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Old 06-17-2014, 07:07 AM   #10
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That is all some very good info. I just wanted to add one thing. I let my tranny shift as it needs to for the most part, but sometimes I will take the OD off if I am climbing a long grade. One thing I noticed that helped big time with temps is that when the trans downshifts from 4th to 3rd, it also unlocks the converter. I climbed one hill with the tranny like this and the temps climbed from 125 to 165 by the end of the grade. Next time, after it downshifted, I lifted off the accelerator slightly, the RPM dropped a few hundred as the converter locked, still in 3rd gear, then finished the climb. trans temps didnt climb more than 10 deg that time.

Just shows how much heat is made when the converter is unlocked. OD or 4th gear is always locked, but 3rd can be locked or unlocked, its like an extra gear.

Im by no means an expert, so feel free to correct me if im wrong, this is just how my truck operates.
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Old 06-17-2014, 08:36 AM   #11
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Just shows how much heat is made when the converter is unlocked. OD or 4th gear is always locked, but 3rd can be locked or unlocked, its like an extra gear.

Im by no means an expert, so feel free to correct me if im wrong, this is just how my truck operates.
You're learning fast. Us old guys that had the early 4R100s behind 7.3L engines learned back in 1999 and 2000 that the unlocked torque converter was the cause of a volcano worth of heat going to the tranny cooler. And the small oil-to-air (OTA) tranny cooler was overcome with that extra heat, thus the tranny sump temp soared.

But you're wrong if you think the torque converter is always locked in OD. With a stock tune, the torque converter locks at about 37 MPH as you are climbing, and unlocks at about 37 MPH as you come back down the grade, regardless of which gear you are in. Of course you're probably not going to be in OD either climbing or coasting down the grade, but that's the logic in the tranny shifting tune.

There are two fixes for that unlocked torque converter. One fix was to significantly increase tranny cooling capacity, perhaps by replacing the OTA cooler with a much bigger cooler, including a fan to suck more air through the heat exchanger. That was later fixed by Ford's development of the 6.0L engine with its much bigger OTA tranny cooler.

For those that kept up to speed with info posted on Ford-Diesel.com, Ford had a TSB (technical service bulletin) that told dealers to replace (under warranty) the radiator of 7.3L automatics with one that included an oil-to-water (OTW) tranny cooler. The gotcha was only that the owner had to complain about overheated tranny, then the dealer could do the TSB work under warranty. Of course I complained loudly to my dealer and he had the OTW cooler installed on my truck.

But even with the OTW cooler, the tranny could overheat on long pulls up a crooked mountain trail at less than about 40 MPH - because the torque converter would be unlocked under those conditions. But a simple fix was to install a switch that you could flip to keep the torque converter locked when climbing steep grades where you couldn't maintain a speed of about 45 MPH or more. Instructions on exactly how to install that switch were frequently posted in the 7.3L forums on Ford-Diesel.com, which later was renamed to TheDieselStop.com. You can probably still find those instructions by searching on TheDieselStop, or maybe by looking through the articles under the "contents" button off the home page.
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Old 06-17-2014, 09:32 AM   #12
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Good point smokey, I did know that about it unlocking around 37 MPH, (45 on my truck due to the tire size) but yes, I agree completely. I guess I was more so referring to highway speeds, I usually maintain 60 to 65 on uphill grades, so the converter is always locked in my situation. But I'm only pulling around 6000, so those speeds might not be as easily met for others, in which case you would have to worry about it unlocking like you said.

Its always great to keep learning stuff, thanks.

P.S. I did also add an aftermarket cooler right to the front of the grill. Not only does it keep it cool, but once I hit the top of that grade, that temp rapidly came down from 165 to about 130 in 5 minutes of driving.
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Old 06-17-2014, 09:22 PM   #13
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I did the TC lock up switch complete with diodes and such to keep the OD light from flashing. LOVE it. You learn how to drive with the TC locked and keep heat down.

Used it once climbing a very steep grade with a roughly 9k-10k lb fifth wheel. Had it manually in 2nd, locked TC and foot to the floor, literally. Feathered right around 1225*F-1250*F EGTs. With an unlocked TC, it probably would've melted right there.
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Old 06-18-2014, 07:53 AM   #14
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How difficult to do the lock up switch?
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