I've been getting my 1969 F250 CS with a 390 up to snuff mechanically, so I can eventually depend on it to pull an RV. I don't know the history of the truck. It has 104K miles on it and was basically a ranch truck originally from North Dakota, then W. Texas. Don't know when the transmission was last serviced.
I've replaced the water pump, 195 degree thermostat, valve cover gaskets, rear transmission seal, U-joints, aftermarket gauges so I can track Voltage, Water temp, Oil pressure, and a few other things.

The engine seems to be running well, and if I putt around town without getting the engine very hot I have no transmission trouble. However, yesterday when the engine came up to running temperature to about 200 degrees the transmission overheated and started having trouble moving my truck down the street, so I pulled over quickly and killed the engine.
I could tell the transmission was hot, so I got under the truck to have a look. The cooling lines near the transmission looked like they may have leaked a little, but everything was still connected. Checked the fluid level - looked good.
I let the engine/transmission cool for about 20 minutes, and I was able to make it home about 3 blocks away with nothing feeling unusual.

I'm trying to develop a plan of action to correct this. Any help is appreciated. I can do everything except rebuild the transmission. I've never done that.
My immediate plan is to make sure the lines aren't plugged and can flow through the radiator OK. I don't know anything about the modulator or if it should be replaced.
Maybe disconnect one of the lines, pull the coil wire and turn the engine over to see if fluid is flowing.
I do need a new donut for that side of the engine, so that might be heating the tubing even more than the exhaust already does.

Beyond the above, the only other things I know to do is to change the transmission filter, install a Temp sensor on the inlet line from the front port to the radiator. (I've read that I probably have NPS fittings, and I shouldn't use NPT when buying a sensor port.)
I probably need to install an aux cooler and filter before I put it into service to pull an RV, since I'm in South Texas.

I know I'm probably missing something.
Thank you in advance!

 

Transmission pics - not sure how helpful they will be.
I can see that I need to clean the area so I can tell what is what.

 

How did you know the trans overheated exactly? Does the fluid smell or look burnt?

The 'filter' in a c6 is just a screen, it will pass metal chunks. Doubt that is the issue.

If the shifter is in between detents it can do what you observed.

Some ghetto repair there with AL line and compression fittings. Suggest running new 5/16 steel lines, adding additional air/air cooler after radiator (or skip radiator cooler).

Good luck

 

Thanks for the response!

When I pulled over and had a look under the hood, it was smoking a bit when I was looking down beside the engine near the torque converter. It could have been other liquids burning off the exhaust from me spraying degreaser, etc.
Before I pulled over I stopped the truck and tried first and second gear, and reverse - no movement of the truck. The water temp was fine, so I assumed the transmission overheated. Especially after letting it cool and I was able to drive it again.
I'll certainly consider any other possibilities.

I thought there were a lot of connections for the tubing. I'll fix that. Might go with braided cable if I can afford it. :-) BTW - what's an AL line?
I've already ordered a temp gauge, Hayden 689 cooler and a Derale filter.

The fluid didn't smell burned, but I think it was a bit darker. Will look carefully again.

 

Does your truck have a transmission cooler on it? My truck with a C6 automatic has an aftermarket transmission cooler attached in front of the regular radiator. These are designed to help keep the transmission fluid cooler.

 

In my experience, an overheated transmission will puke fluid out the vent. It doesn't sound to me like that is what you are describing.

But transmission problems that only show up at different temperatures usually indicate a transmission that is failing. Some have problems when cold, others when heated up.

A problem like you are having could be why the truck was parked in the first place.

 

AL = aluminum. I'm assuming the short chunk of tube that is a different color is aluminum. Comp fittings are a poor choice on anything that vibrates.
You can put any sort of pressure gauge in the cooler loop and observe pressures when ok and when failing.

 

Ah, ok gotcha.

I’ve ordered braided lines, heat shields, aux Hayden cooler, and an inline port for the temp sensor.

Will also replace the modulator and check the vacuum line.

Then I’ll be able to tell exactly what is happening with the temp. Not sure what temp it should run at.

 

You can likely pop a pressure gauge in the in-line temp port to test line pressure. Should be around 40-50 psi and go up when you accelerate. Would isolate a pump issue vs a worn band.

 

X2
You did not say how many miles are on the truck / transmission.

On the lines it could be the trans was pulled for some reason and because the fittings on the trans are hard to get to for most people the lines were cut and the trans dropped. When going back in the fittings were used to join them back together.

If me and everything looked good, fluid level & color, and I plan to pull an RV I would check around to see who could rebuild it.
I seen it posted some shop dont have people that dont know how to rebuild the older transmissions.
If you can pull the trans and take it to the shop will keep the $$ down and may help the shop not tie up a bay.
A bench top rebuild should not take that lone once out.

Also if you tell them you plan on pulling a RV they can "beef it up" and maybe a shift kit to firm up the shifts.
The last thing you want is to break done pulling the RV.

Yes run the radiator cooler, check if it flows and flush it before putting back in service.
Then run the largest axu cooler you can maybe even 2 because of the heat and the extra load the trans will be under.
Run the trans thru the radiator first then the exu cooler(s)
A ddeper trans pan will also add more ATF and help keep it cooler.
I seen posts that the 4x4 pans are deeper but I dont know how true this is?
Good luck
Dave ----

 

Thanks Dave!

The truck has 104k miles on it. It’s my understanding it was mostly a ranch truck.

If my upgrades don’t fix the issue I’ll definitely drop the transmission for a rebuild.

 

You definitely need to fix those cooler lines. On a C6 the most forward line is fluid coming from the converter relief valve, going to the cooler, the rear line coming out of the cooler is your rear lube that is the only lubrication the bushings on the output shaft get.
Changing the modulator might help but you need to connect a 300-psi gauge to the line pressure fitting. Thats the 1/8 pipe plug drivers' side in front of the band adjustment screw. Set the line pressure to around 80 psi at twice the idle rpm.
Good luck but I fear you are heading for a rebuild.

 

Thanks Crop Duster.

Today I’ve started replacing the cooler lines, adding an aux cooler and heat shield.

I’m hoping that restores the transmission for dependable trips without pulling anything. Then I will plan on a rebuild before RV pulling.

If it still has issues I’ll take it for a rebuild.

 

Be sure to put a new modulator in while you're at it even if you don't have the gauges and make sure the vacuum line is good. If it doesn't help you are one part ahead on the rebuild.

 

Will do! Thanks!