1991 K5 Transmission Overheating

[Nomadicflyer]

New here, and to the truck. Prior to this 91, the newest thing I have is a 72 C20, and then a 69. Read into that, I’m good with old stuff. I’ve acquired a 1991 K5, 350 w/automatic transmission. Everything is stock, and good shape (forest service rig). Zero frills, rubber floor, painted metal for headliner, not a stitch of cloth in the thing. Right down my alley, but this automatic transmission is stumping me and the local shop, and the local transmission rebuilder, the neighbor, my friends.... you get the picture.

The History is revealing itself as I move forward. After having an issue with trans overheating, a trip to a local shop found the rig in familiar hands. This same shop had replaced this transmission, before I bought the truck. They used a rebuilder that’s not local, but the best in the state. They would honor their work, and send the trans back to the rebuilder, who honored their work. Wow, I wasn’t sure about this move to a South Dakota, but that really frames up the folks I’ve run into, here.

The transmission was found burned up, and they sent it back, freshly rebuilt. Meanwhile, back at the ranch, the crew installed a new cooler, lines and an oil temp gauge for the trans. Put it all back together, and it overheats again. Dang. Out it comes, and a different trans goes in. Flush the lines/cooler. Same thing. When it overheats, it spits trans fluid out of the breather and down the belly of the Blazer. Really? Ok, the shop isn’t done with me, yet- these guys should have cut me loose, but they are stubborn, if nothing else! Trans shop sends out modules/valves and a new "RV" torque converter, and a new bigger cooler goes in. That’s GOTTA be it, right?

Around town driving is perfect. While working the trans on their dime, H&H was attacking Old Truck Syndrome on mine; suspension and drive train parts were flying out the back door into the scrap pile, replaced with shiny coolness underneath. This thing drives like a new truck. Tight, responsive and a real joy to run around in. The shifting was sewing machine smooth, the steering and suspension perfect, the eager growl of a nice engine/exhaust combo. Temps were spot on. Then I took it on a road trip, this week...

In South Dakota, it’s hours to get anywhere. Therefore, the main drag across the state doesn’t mess around. 80 MPH speed limit to help you get where you’re going. I have to avoid this highway in the 69 CJ, as anything above 60 is insane in a rock crawler. The 72 C20 isn’t much better, with no overdrive and towing gears. That’s why I was excited to have the K5- Overdive With a locking torque converter! That sounded perfect! However, perfection is still out of reach. The trans began its rapid temperature rise, once above 65 MPH. Dang.

Pulled back on the gas a bit, and it felt as if the TC locked up. Temps dropped, immediately. However, I couldn’t get above 65 MPH on level ground. Lean forward on the gas, the TC disengages, the temps climb like crazy. Lift off the gas, the TC locks up, the temps drop and all is well.
Downhill, or with a tail wind, I could get up to 75 MPH with the TC locked, and temps stayed low. Frustrating, with a statewide speed limit of 80!


I’m not familiar enough with how all the parts and players work, with this K5. I’m not even sure what’s in this thing, so I’m looking for some 101 on all things 91 K5. Not sure if it’s the 700, not sure what transfer case lives there. Not sure what the options are for torque converters, and what you experienced folks have found to overcome it. I could sure use a little help.

I’ve let H&H run with this, tried to stay out of the way. Now I think I need to pick up the reigns and do some work. What’s next? Is there issue with rear diff load up? Is there a better set up for the parts I do have, or do I need to start upgrading?

It looks like there are some very experienced voices on this forum. Excited to hear any suggestions.

Recap: On the third transmission. Second cooler, second torque converter. New temp gauge. TC seems to unlock at 60-ish, and then temps go skyward. Fluid vents when this happens. Need help, send cookies. Shop guys are heading this way with a gas can and a book of matches....

Nomadicflyer

[blakerj10]

Sounds like you are having fun...

Just traveled through SD to hunt near Watertown. Took my 90 K5. 350tbi, 700r4, 3.73 gears, 31x10.50s.

I have the same behavior on the interstate, although I don't have a temp gauge for the transmission so I am probably cooking it.

I think the thing just doesn't have enough power to pull those speeds and overcome wind resistance with the trans locked and in overdrive. Lower gears would help, but I think more power is the real answer.

At 65 it does great. Even at 70-75 if I have a tail wind. Coming back west, I just drop it into 3rd gear so I don't have to fight the trans shifting.

[Zoomad75]

What's the rest of the spec's on the K5. Specifically gear ratio and tire size. Countless 700's have been killed due to heat generated in OD due to large tires and crappy gearing. The larger tires act like an overdrive gear on top of the actual OD in the trans. The engine RPM is lower due to both and it's just not building enough torque to maintain at that given speed. Another way to look at it would be the load at that speed is enough that the TC can't stay locked up. A slipping TC clutch is an instant way to generate heat.

[SBTork]

Originally Posted by Zoomad75

What's the rest of the spec's on the K5. Specifically gear ratio and tire size. Countless 700's have been killed due to heat generated in OD due to large tires and crappy gearing. The larger tires act like an overdrive gear on top of the actual OD in the trans. The engine RPM is lower due to both and it's just not building enough torque to maintain at that given speed. Another way to look at it would be the load at that speed is enough that the TC can't stay locked up. A slipping TC clutch is an instant way to generate heat.

I absolutely agree with all of this. Seems like everything is working ok, but gear ratio is probably far from idea with your setup. What is the gear ratio and tire size? That info will be the key