After a long saga of transmission issues, one rebuild with new triple lock torque converter, clutch packs,etc. followed by a sprague gear cratering and a second rebuild, adding a Tru Cool cooler, and then changing the OEM 7.3 cooler for a 6.0 cooler I still had temps running a little warmer than I thought was good.

Then a couple of the guys on this forum, including Kwik, suggested I check the cooler bypass valve. Today I changed it out and put a new bypass valve and tube on the tranny. When I took the old one off I discovered that someone, somewhere had taken the tube and put it in a vise and pinched about 4 inches of it so that the tube was closed - thus no fluid circulation when the valve was sticking towards the open side.

I put on the new one and went for a drive, some city, come highway, about 35 miles (85-90*) getting on to dark and temps ran 140-150. It'll be interesting to see what it runs in tomorrow's Texas Gulf Coast heat and humidity. Before the change with the 6.0 cooler and Tru-Cool in line I was running 165-170 earlier today with temp fluctuating.

Here's hoping I've finally got this issue resolved without going to another tranny. Thank God for temp gauges.

 

There's a theory going around that the trans will actually run cooler without the bypass valve, but I don't buy into it. Plus, BTS puts new ones on all of their rebuids they do. My guess, is someone was attempting to accomplish the same thing as removing the bypass tube by pinching it off. Glad to hear that it looks like you've got your cooling issues resloved though.

 

I don't know about it running cooler without the valve but when the valve starts sticking open or partially open and the fluid stops going through the cooler or circulating through the tube, the tranny sure runs warm. I'm hoping this gets mine back to cool like before.

If not I start saving for the ole BTS tranny, too.

 

Yeah, but Mike, if the tube was pinched off, that would be the same as the valve closed. How come it is cooler with the new valve and tube? Maybe it wasn't pinched all the way? I'm also still bothered by the fact that it runs cooler with the radiator cooler bypassed. If the radiator cooler is not plugged, then the only way that could happen is if the water in the bottom radiator tank is so hot that it was keeping the tranny fluid hot. You should do a check on that with a non-contact temperature guage. You sure can't depend on the original guage to tell you anything.
Warren

 

Wow, I hope your troubles are over!

 

Warren, on my truck the tranny has always run hotter when going through the radiator. I have posts on this forum over a year old and prior to two tranny rebuilds because of tranny heat. I do run a 203* thermostat in the truck but the truck has never shown any sign of running hot. I know other guys on the forum have encountered similar issues and bypassed their radiators.

I have checked radiator temps and blown and cleaned the cooler in the bottom of the radiator. It is clean and clear.

The valve sticking open means that the fluid was NOT circulating through the cooler properly, that it was attempting to bypass the cooler and circulate through the tube back into the tranny. With the tube pinched shut the fluid was neither circulating through the tube or through the cooler, thus hotter temps. In high heat and humidity, stop and go traffic I am currently running 150 in the tranny.

Mike

PS Kwik, thanks for the hint on the bypass valve. That sure looked to be the problem and after reading several other posts found a couple of other guys who solved their tranny temp issues by replacing the bypass valve that was stuck open.

 

Sounds like you got it there Pastor. PTL!

 

I'm running with a stock thermostat. I've read on this forum that many are running with a 203 degree thermostat. I've also read here that the 7.3 PSD runs most efficiently around 200 degrees. Have I just talked myself into converting over to a 203? Is my thinking on the right track here? What is the advantage of a 203? Comments appreciated.

 

The argument is that the 7.3 was designed to run at around 200 but Ford used a stock long neck thermostat at 193*. Running at the slightly higher temp is supposed to be more efficient. I've been running the hotter thermostat for a year and didn't notice any significant difference.

It does seem that the motor is a little smoother when its warmed up and a little more responsive. But that could just be wishful thinking as the temp difference is not a lot. Some guys here swear by the change, others have had more of my experience. When I did mine I changed the top radiator hose to not go through the serpentine belt and changed the thermostat housing to a more secure housing. I will keep the hotter thermostat cuz I like it but I don't recall any night and day difference in performance.

So, you're on the right track and I think it was worth it to do.

 

Mike:
1: With the bypass tube pinched off, the oil is FORCED to go through the cooler, or at least try to. If the cooler or piping is obstructed, then with the tube pinched off the oil might not flow fast enough, but it certainly can't bypass the cooler. Thats the reason for the flow test, where you check the flow rate of the oil RETURNING from the cooler(s) to verify adequate flow. If the fluid temperature is higher with the built-in radiator bottom tank cooler in the circuit, then ONE reason is that the cooler is restricting the flow. Air blown through the cooler can come whistling out, even though there is a major obstruction to oil flow, which is much more viscous.

2: Under most service conditions, the radiator coolant temperature in the bottom tank is far below the thermostat setting; up to maybe 80 degrees lower depending on the outside air temperature. Only under heavy loading will the bottom tank temperature rise much. Thats why the primary transmission oil cooler is in that bottom tank. If you reduce the transmission oil temperature by bypassing the oil-to-water cooler, then either a: it was obstructing the oil flow (detectable by the flow test) or b: the bottom tank water temperature was (is) higher than it should be, suggesting a radiator or engine coolant circulation problem.

You have successfully solved the tranny cooling issue with the auxiliary cooler. Keep an eye on the engine coolant temperature and circulation to be sure you don't have a problem there.

Warren

 

Thanks Pastor. Would you by chance have a part number for that top rad hose that doesn't go thru the belt?

 

Sorry, I don't have that part number but it is the upper hose designed for trucks with a second alternator. I believe I got mine locally with that information. If not, I'm pretty sure some of the forum sponsors carry it.

Warren - I did the flow test and it was with limits but barely. I really can't figure out what the issue as the truck runs great and I have had no heating problems I am aware of. I probably should get a coolant gauge since I'm relying on the factory idiot gauge and I know how unreliable that is. Transmission seldom gets to 155 without a load now. Generally stays around 140 but on really bad days (mid 90 ambient temp, high humidity) it gets up to 155 or so. Highway driving in OD at 65 runs about 150.

thanks,

Mike

 

I've kinda lost track of your cooler configuration. Mine is: Radiator bottom tank, then OEM oil-to-air in front of radiator, then tru-cool in front of radiator, then return. I just made a run on the freeway, 30 miles, 70 mph. indicated tranny temp was 100 with the outside air at about 60. My flow test was about 130 percent of the Ford spec, and confirmed that the bypass valve works. The highest I've seen with this configuration was 190, which was city stop and go after coming off the highway, towing 12000 lb 5th wheel. I've very happy with those results.
Warren

 

Mine cooler configuration is 6.0 oil to air, tru-cool, back to transmission. I haven't done a flow test since before I added the tru-cool. It was at the bottom of Ford specs. I suspect the bypass valve was sticking intermittently causing circulation through the cooler to come and go as the temp would occasionally spike for no apparent reason. The valve sticking plus the pinched bypass tube was resulting in highly impeded flow. Since replacing the bypass valve I've had no more problem and in high heat never run more than 60 or 70 degrees above ambient temp. The other issue is high humidity which definitely slows down the cooling process. When not on the Gulf Coast but in high temps, other parts of Texas, the ambient temp to tranny temp ratio is even lower.

 

Do all the trucks seem to run tranny lines through the radiator? Mine 00 does not run through the radiator, but my buddies 00 dually does. Make any sense?