I believe my transmission is overheating, when driving for some time let's say half an hour to an hour it starts feeling like towing a heavy load.

Checked ATF level found fine, color normal no burnt smell. This transmission installed on my 1992 4X4 came from a 1994 with 118K miles my truck has 208K swapped this tranny December 2012.

When it's cool it runs just fine. Pulled the codes and only shows 628 "Excessive converter clutch slippage - Transmission" This happen about a month ago, not driving truck much just for short trips and without issue but still haven't figured. The OD light not flashing.

First I though might have to replace the TC. I've been keeping the search for source of problem and I'm inclined to think it's due to overheating since it's after running for a while when problems start.

I found in the article section how to flush the transmission and even though the ATF is fairly fresh I was curious to check the flow of the fluid through the radiator. After following the instructions got no flow whatsoever.

I must say that the engine when tried this flush was not hot don't know is this matter to have a nice flow from the transmission to the radiator and back.

while researching found another TSB related to code 628:

Quote " ISSUE: Some vehicles may exhibit a flashing Transmission Control Indicator Lamp (TCIL) and Diagnostic Trouble Codes (DTCs) 62, 628, and/or 1728 may be stored in memory. These DTCs can be set by any internal transmission element slippage or potentially by torque converter slippage.
ACTION:
Refer to the following text when servicing these codes.
NOTE:
WHEN SERVICING A VEHICLE WITH A FLASHING TCIL WITH DTCS 62, 628 AND/OR 1728, DO NOT REPLACE THE TORQUE CONVERTER UNLESS PROPER DIAGNOSIS HAS PROVEN THE TORQUE CONVERTER TO BE THE CAUSE."

Quote "Some of the other causes that may result in DTCs 62, 628 and/or 1728 are as follows:
- Aftermarket modifications (i.e., performance enhancers, electrical modifications, etc.)
- Missing shifts (some/all)
- Transmission fluid leakage (internal and/or external)
- Erratic shift timing
- Valves, springs or retainers in the main control/accumulator body not assembled correctly, binding or sticking
- Check ***** missing and/or mislocated. Damaged, unable to seal/seat properly
- Higher or lower than normal line pressure
- Transmission fluid restrictions and/or level"

Transmission fluid restrictions is what caught my eye and that's why I tried the flush since the line where the ATF comes from the radiator is the one towards the rear of the transmission.

Not sure how to proceed now but I'm guessing will need to check the radiator cooler for blockage and possibly replacing and adding an additional cooler. At this point the TC might still be ok.

Your comments will be appreciated...

 

Do not use two auxiliar coolers, the flow will be affected by this, check the link in my signature (I've been repeating this a lot lately...) for a cooler and shift-kit install, there are also flushing and refilling procedures, both mine and from Mark Kovalsky, a contributor to the site and expert in these trans.

 

Well, finally was able to figure the problem. following the article about how to flush the transmission the line towards the rear of the transmission is the one coming back with cooled oil flow from the radiator. Got none whatsoever.

Disconnected the top line in the radiator, actually both. Blow some compressed air to check for flow within the radiator lines and air will not go in at all, completely blocked.

Put a piece of clear tubing to the top line coming from the transmission started the truck and voila, plenty of flow right away.

Guess the blocked fluid cooler in the radiator caused the transmission to overheat and to slip. Most likely was the cause of the old transmission to fail. I remember the lines were cleared before installing the new transmission at this point I guess is irrelevant.

Next on the to do list will be to install a new radiator this time I'm going to clean the fluid lines...a couple of questions since I want to do this myself:

1.- Is it too hard to replace the radiator?
2.- What chemical should I use to clean the fluid lines

Thanks for your advice....

 

that's not good.
this is why it's important to install a trans temp gauge.had you had one,you would have seen the trans temp climbing and knew to pull over road side and inspect fluid flow returning to trans.expensive transmissions to run blindly.
i would drop the pan and look for anything obvious.clean it all up and install new filter.drain the converter as well.get as much black/burnt atf out as you can.

i would then flush a little sooner next time too via that guide you read this time,and meanwhile,inspecting the color often.
just run new trans lines and replace the radiator.i would also ditch the aux coolers and replace those.
then i would run an aux trans filter pre- radiator (cus the question is,what caused the blockage in the first place? ) and install a trans temp gauge @ the test port.if she goes way up over 200F like 225-250 running empty,then Huston we've got a problem again.

 

Just blow out the lines with compressed air, you don't need chemicals. The air will blow the lines clear.
Craig