Sorry if this post show's up twice. Apparently I have fat fingers .... anyway...
My truck is a '91 dually with 4.10s. While towing there are times were it "feels" as though the locking converter is stuck in the engaged position. The engine just seems to be struggling with the load. This usually occurs either during hot weather or long up hill climbs. When cold she starts the journey pulling just fine. Last year I installed a deep sump pan and external tranny cooler. Also changed the cap/rotor/plugs. Same issue still occured while towing the boat. Engine shows no issues when not towing - runs strong. So, is it possible that the TCC is engaged while in 2nd or 3rd?

Also, I rarely put the trans in OD while towing. Was told years ago this was bad. But the engine RPM's are pretty high for long hauls at 60mph. Is the ECM smart enough to keep from frying the tranny if I leave it i OD?

Thanks in advance for the help,
Virgil

What engine/specs are on that?

I wouldn't suspect something on the transmission, I would actually suspect something on the engine. Some emissions circuit that kicks in after the truck has warmed up. Like O2 or something like that.

If you think the TC is locked, tap the brakes a bit. You should see the RPM's jump a little from the TC unlocking.

I tow in OD. I have a gauge on my truck that measures the temp of the line going to the first cooler. (My stock cooler) This line comes from the transmission. I have been told that this is torque converter heat and should be the hottest the fluid is.

If I tow without the truck in OD, I can watch the temps slowly rise and just keep rising. If I tow with the truck in OD, the line temp is actually cooler once it all locks up and gets up to cruising speed.

I have watched this with a IH 856 with loader on a 32 foot flatbed. The transmission actually ran cooler in OD then without it.

I know that the newer trucks are smart enough not to fry the transmission. My dad's 04 GMC talks about this in the owners manual. If the computer sees temps too high it is suppose to downshift and start backing off the throttle to keep the temps down. Not sure if a 91 is this smart or not.

If you are sure its a 4l80e (17 bolts on the trans oil pan) then there is no problem to tow in OD, the 4l60e and 700 trans needs to tow in 3rd they are a much lighter trans., that is likely why you were told no towing in OD came from. I would get a trans temp gauge in there, the 4l80e will lockup the torque conveter if it get extremely hot in a effort to try to reduce trans temp..

I thought the 4l80 didn't come out till 93

91 was the first year for the 4l80e.

My L29 454 with 4L80E locks up TCC in 2nd, and a lot at WOT. Personally I don't tow often enough to know if I like it. But normal driving and punching it pass, it sucks. I don't like it. Slows you down a lot.