4R70W transmission overheating / failed cooler bypass valve?

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Vehicle history: 2005 F150 XLT 2WD with a 4.6 and 4R70W. 172k miles. Has both radiator end tank ATF cooler and small aux cooler piped in series. Boss bought it used with ~40k miles; original transmission gave up around 100k miles and got replaced with rebuilt unit. Truck had some other issues at around 171k miles and got parked for a few months until I was hired and started fixing it up on the side since the boss wanted to try first instead of buying another truck immediately. Transmission would flare badly or entirely miss 2nd gear on kickdown, but would shift fine up though the gears under WOT acceleration. Fluid was dark red-brown but not gritty and didn't smell very burnt, just overdue. After a single drain and fill plus new filter it began shifting smoother and 3-2 kickdown became closer to a flare than a straight to neutral missed shift. Also installed a pan with drain plug for easier fluid changes. But after a mild 30 minute drive half on freeway and half in town, the "O/D Off" light began to flash indicating overheated ATF and I got a vague hot oil smell at stoplights. I was only a mile from destination so I limped it there letting it idle in neutral at stoplights and coasting whenever possible. Infrared thermometer showed transmission pan temp of 285, but the aux transmission cooler was ambient temp as if no fluid was flowing. All subsequent trips were short with half hour or more cooldown time so I got it back to base without any further overheat warnings. Boss remembers his other worker mentioning the warning light when he drove that truck a while ago. In tracing the cooling lines, I found a little inline filter piped after the aux cooler on the return line. Since it may well have been in place since the rebuild, I deleted the filter and replaced the hose to start. To confirm that the coolers weren't clogged, I left the hoses pointing into a bucket and started the engine. It pumped about two quarts out in 15 seconds, so there doesn't seem to be a blockage, but it seemed like a fair bit of fluid came backwards out of the return line going TO the thermostatic bypass valve. Did another drain and fill, and the cooler lines downstream of the bypass valve seem colder than I'd expect even as pan temp gets up to 180. System has a lot of thermal mass so I'll have to see after another long drive on Monday or Tuesday, but I'm suspecting the bypass valve may be jammed or lazy, so coolers don't get enough flow as fluid gets hot. Anyone else seen a problem like this on Ford transmissions with the external cooler bypass valve? I found a few anecdotes but I suspect this problem may be getting misdiagnosed as regular old transmission failure, with the bypass valve getting flushed or replaced when the the new unit goes in and never investigated as the cause of failure.
 

CT8

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172,000 miles gets into the trans being worn. There is always room to find a reputable trans pro to get an other diagnosis.
 
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Originally Posted by CT8
172,000 miles gets into the trans being worn. There is always room to find a reputable trans pro to get an other diagnosis.
Except this trans is the rebuilt unit with only about 70k on it. If it was dying and slipping and roasting fluid, I'd expect the coolers to be blazing hot trying to keep up. Instead the coolers seem to be doing nothing as the trans overheats.
 
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OD Flashing on that model means it has a trouble code for the transmission. You can pull it with a decent code reader. If you want to throw a few bucks into it, while you have it apart put a deep sump pan on the trans and a proper cooler...
 
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No codes stored. By the time I had scanner available the overheating had stopped. Subsequent driving hasn't had the OD light flash again, but also haven't been long enough to get it super hot again. There was significant flow through the coolers when I had the hoses disconnected. Maybe try back flushing them to be sure ? This is a recent problem that didn't exist for a long time after the replacement then started happening sometime before the truck got parked.
 
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Originally Posted by VeryNoisyPoet
but it seemed like a fair bit of fluid came backwards out of the return line going TO the thermostatic bypass valve. Anyone else seen a problem like this on Ford transmissions with the external cooler bypass valve?
These valves almost always end up going into the crusher with the rest of the vehicle, so if a wrecking yard charges more than $10 for one, its a rip off. Just REPLACE it already - you have already done all the troubleshooting and found out what it was!
 
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Fort Worth, Texas
There are other things that can cause low cooler flow besides a faulty Thermal Bypass Valve, But are internal to the transmission. Temporarily bypassing the thermal bypass is the most logical next step. While I won't say they are a common problem.....I have seen them fail. If the unit overheated, Exceeded 275°F.....It should set DTC P1783. A flashing O/D lamp indicates that a transmission related DTC did set. *I question capability of your scan tool! You need to make sure the Torque Converter Clutch is Applying & Holding at steady state cruising speeds! Ford doesn't always include a TCC Slip RPM PID in the data stream......If you can't find it, Compare Engine RPM to TSS/ISS RPM with O/D locked out. *With the TCC locked-up.....Both RPM readings will be identical. TCC feed, Converter feed, & Cooler feed are all connected!
 
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Torque converter is holding at cruise as far as I can tell. It stays locked under mild acceleration and there is a definite unlock feeling under moderate acceleration where the revs jump up. Letting off the gas will have the revs drop back down and then a slight bump as it locks again. I'll bring the scan tool along next time so I can look at the data to confirm. There were five key on cycles before I got the scan tool on it so not sure if that would reset any codes. Flashing light did not reappear after I shut it down the first time and it didn't get that hot since. Scan tool simply said no stored or pending codes found in module. I admit it's old and cheap but I've pulled transmission codes from other cars before (just not on a Ford).
 
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Does this one use an external thermostat/bypass? I did one some years ago (it may have been an 4R100 not sure of the year) that had one that was failing, more fluid was bypassing the cooler and going directly back to the transmission instead of through the cooler. IIRC I checked it with a multi meter that had a thermocouple, the output line from trans was hotter on the thermostat/bypass input that the output. Just a thought. +1 Clinebarger, check your scan tool, it doesn't seem to working with trans codes.
 
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Originally Posted by Trav
Does this one use an external thermostat/bypass? I did one some years ago (it may have been an 4R100 not sure of the year) that had one that was failing, more fluid was bypassing the cooler and going directly back to the transmission instead of through the cooler.
Yes it has external thermostat. Back flushed cooler and lines today. A fair amount of black sludge with chunks came out crzy sick, but no improvement to flow or cooling performance. Transmission pan reading over 200F with aux cooler barely above ambient temp except at its inlet. New parts on the way for installation this weekend. Despite the cooling system problem, transmission is shifting like new now that it has two rounds of fresh fluid onboard. Flawless 3-2 downshift with no hint of flare or slam. Holds every gear strong even at WOT. Torque converter locks without shudder at cruise.
 
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Repair successful! Removed old thermostat and used two stubs of flex hose to bypass it, then drained and filled pan with fresh fluid. Unhooked return line at cooler and started engine to flush out cooler circuit. Plenty of flow through coolers and pushed a few more quarts of fresh ATF into system in the process. Next I installed a Derale 13011 fluid control thermostat up front and rerouted the lines. Had to cut one of the hard lines, so used a flare tool to make a lip to prevent the hose from slipping off and double clamped it to be sure. Transmission cooling system works properly, and the truck shifts like new, so I can drive it with confidence again. Considering adding an external transmission filter, especially with all the crud I saw coming out of the coolers when I flushed them. Probably will do one more fluid drain and fill, but maybe in a few weeks time.

derale thermostat.jpg


thermal cooler view.jpg
 
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