It seems every other transmission problem that comes into the shop today has its roots in another part of the vehicle. Sometimes it’s the engine; other times it’s the computer system. In this case, it was the cooling system.
A 2011 Ford F450 with a 6R140 and a 6.7-liter diesel engine truck showed up at the door. It had 310,000 miles on the clock and the fluid was burnt. The unit appeared to be overheating, so out it came.
The technician assumed the unit had been incinerated due to the smell and color of the fluid. But once he got it apart, he didn’t find the damage he was expecting: Some of the clutches were worn out and some discolored, but there was no destruction of any hard parts at all.
He refreshed the unit with new clutches and steels, gaskets and seals, and a new torque converter. He installed the transmission and took the truck for a road test.
Everything seemed to work fine, so they delivered the truck. The customer left happy.
But the next morning, there was the truck, parked right in front of the shop. The transmission was overheating after driving about 10–15 miles, losing high gear and lockup. According to the customer, the truck ran fine until it got hot; then started acting up.
First, they put the truck on the lift and connected the cooler flow machine. Cooler flow was low — less than a quarter of what it should have been. So there’s no question about why the transmission was overheating. But that left the question of why cooler flow was low.
Since there were no issues with the valve body, they pulled the unit and tore it down again. Nothing obvious showed up.
After going through the unit with a fine-tooth comb, the technician called the ATRA HotLine. We examined the hydraulic diagrams to see if we could shine any light on the problem.
We had the technician backtrack the hydraulic system to the pump, air-checking the gullies and port holes; nothing was amiss (figure 1). Then we had the technician take the pump apart once more. After a thorough examination, there it was: The steel sleeve in the pump had spun and was blocking the converter-out passage (figures 2 & 3).
The technician replaced the pump body, reassembled the unit, and down the road the truck went. Everything seemed fine.
Fast forward 7500 miles and the truck was back with the same problem: After driving for 10–15 miles, the transmission started to overheat again.
The sleeve in the pump is where the sealing rings for the PTO gear rides. The PTO gear spins with engine RPM because it’s splined to the torque converter. Non-PTO units don’t have this sleeve.
Upon further inspection, they discovered the secondary cooling system was low on coolant. Secondary cooling system? Yes, these Ford trucks have primary and secondary cooling systems. The primary system is just for the engine.
The secondary cooling system has a separate coolant circuit (figure 4). It cools the exhaust gasses for the EGR system, the transmission fluid, the fuel, and the air charge entering the engine.
A secondary, engine-driven coolant pump provides coolant flow, and a secondary, two-stage radiator is mounted in front of the primary engine cooling system radiator. Two thermostats, one mounted on each side of the secondary radiator, operate independently to regulate the temperature of the coolant flowing to the various components.
Our original issue was repeated transmission overheating due to the sleeve spinning in the aluminum housing, which occurs only on PTO-equipped units. The secondary cooling system ran low because one of the hoses was leaking, and that caused the secondary cooling system to overheat.
Because of the different expansion rates between the steel sleeve and the aluminum pump body, the sleeve spins and blocks the converter-out passage. That shuts down cooler flow and causes the transmission to overheat.
The repair was simple: the technician corrected the leak in the secondary cooling system. Then he replaced the pump body again. That took care of the problem: The truck is still on the road.
One more instance of the integration between the transmission and the rest of the car… or in this case, truck. These days it isn’t enough to fix the transmission: You have to be able to repair the entire vehicle to make sure your transmission work remains working right and on the road.
Special thanks to Roger Rodriguez of Payless Transmissions for providing additional information and photos for this article.