Just Another Little Known, and even less Cared-for, Fact of the Day - Transmission Digest

Just Another Little Known, and even less Cared-for, Fact of the Day

During the first road test after the installation of a 41TE (A604), the vehicle seems to shift fine, then you notice on the second takeoff it starts to act a little strange, like maybe it’s low on fluid. You might also notice a slight pump whine before you make it back from the test drive.

Just Another Little Known, and even less Cared-for, Fact of the Day

Shift Pointers

Subject: Fluid foaming after rebuild
Unit: 41TE (A604)
Essential Reading: Rebuilder, Diagnostician
Author: Mike Souza, ATSG

Shift Pointers

  • Subject: Fluid foaming after rebuild
  • Unit: 41TE (A604)
  • Essential Reading: Rebuilder, Diagnostician
  • Author: Mike Souza, ATSG

During the first road test after the installation of a 41TE (A604), the vehicle seems to shift fine, then you notice on the second takeoff it starts to act a little strange, like maybe it’s low on fluid. You might also notice a slight pump whine before you make it back from the test drive.

So you pull into the shop, and when you check the fluid level the reading just doesn’t seem to want to cooperate. What I mean is, one minute it seems to be correct and the next minute it doesn’t. You’re just not sure whether the level is too low or too high; you may also notice it seems a little foamy. This may be caused by the differential-lube-feed orifice in the pump gasket having been made too large. This orifice is in the pump-gasket output port as shown in Figure 1.

Some aftermarket pump gaskets may have this slotted orifice lanced a bit too large. The OE slotted orifice is about 0.025 inch (see Figure 2), but on some aftermarket gaskets this orifice may measure as much as 0.100 inch (see Figure 3); that’s quite a difference from the original.

To find out why this could cause the problem we needed to figure out why the slot was there and how it worked. With the gasket placed on the case this lube-feed orifice doesn’t really seem to do anything, as the casting of the case is just flat and it would seem as if the oil would not go anywhere at all (see figures 4 and 5).

When the gasket is placed on the pump and properly aligned, the picture becomes much clearer. The slotted orifice aligns with a groove cut into the pump (see Figure 6). This groove connects to the outer edge of the pump, which is beveled as shown in Figure 7 and, a bit closer up, in Figure 8 – something that you would never really pay much mind to, but nevertheless this beveled edge has now become a lube circuit.

A closer look at the case revealed the same type of beveled groove along the outer edge of the surface where the pump mates with the case (see Figure 9). When you follow it around the case (see figures 10, 11 and 12), it becomes quite clear that the opening in the case is where this lube oil can now spray onto the pinion gear at this opening shown in figures 13 and 14.

If the orifice on the gasket is too large, the excessive amount of oil now being sprayed onto the pinion gear will begin to foam when the gears are turning. This problem may become noticeable after only one short road test.

Be careful when rebuilding a 41TE, and check the size of the slotted orifice in the output port of the pump gasket found in your kit. Jim Dial at our ATSG office in Missouri was the first technician to become aware of this pump-gasket problem, which seemed to elude us on the tech line for a while on 41TE calls with the complaint of fluid foaming after rebuild. Thanks, Jim.

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