A Twist of Fate - Transmission Digest

A Twist of Fate

Aisin Seiki's six-speed transmission used in medium-duty trucks is called by various names. It is the AS68RC in Dodge incomplete-chassis packages using L6 6.7-liter diesel engines. It is also called the AS6 in the Mitsubishi Fuso, the M036A6 in Nissan UDs and the A465 in Isuzu NPR trucks. One of the common failures with this transmission is the stator turning in the pump (figures 1 and 2). A similar problem occurs with Allison 1000/2000 transmissions, in which it causes a TCC overheat and/or C1-clutch failure. You can quickly determine this by pulling the return cooling line. Oil comes out of the case fitting instead of the cooling line.

A Twist of Fate

Technically Speaking

Subject: Stator turning in pump
Unit: AS68RC (AS6/M036A6/A465)
Vehicle Applications: Dodge, Mitsubishi Fuso, Nissan UD, Isuzu NPR medium-duty trucks
Essential Reading: Rebuilder, Diagnostician
Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor

Technically Speaking

  • Subject: Stator turning in pump
  • Unit: AS68RC (AS6/M036A6/A465)
  • Vehicle Applications: Dodge, Mitsubishi Fuso, Nissan UD, Isuzu NPR medium-duty trucks
  • Essential Reading: Rebuilder, Diagnostician
  • Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor

Aisin Seiki’s six-speed transmission used in medium-duty trucks is called by various names. It is the AS68RC in Dodge incomplete-chassis packages using L6 6.7-liter diesel engines. It is also called the AS6 in the Mitsubishi Fuso, the M036A6 in Nissan UDs and the A465 in Isuzu NPR trucks.

One of the common failures with this transmission is the stator turning in the pump (figures 1 and 2). A similar problem occurs with Allison 1000/2000 transmissions, in which it causes a TCC overheat and/or C1-clutch failure. You can quickly determine this by pulling the return cooling line. Oil comes out of the case fitting instead of the cooling line.

With the Aisin Seiki transmission, this stator turns, taking out the K2 clutch that is used for fourth, fifth and sixth, and overheating of the torque converter follows. The bigger problem with this failure is trying to replace the parts. The K2 clutch goes for $40 per friction, of which there are four. The pump is sold as a set with the valve body for as high as a whopping $3,100 ($300 dollar core charge on top of that with no exchange). A whole unit from Dodge runs around $3,500, so one might as well buy the unit. But here is the good news: I have heard there will be frictions on the market soon for quite a bit less money. The other is, if you have a talented machinist I have a suggested fix for the pump (many thanks to Buddy Scott and the good folks at TPS for making this article possible by providing a bad pump for me to use).

Press the stator out of the cover (figures 3 and 4).

Figure 5 reveals how the short splines on the stator shaft stripped out the splines in the cover, allowing the shaft to turn. This failure suggests that the spline in this area should be considerably deeper, longer or both.

Figure 6 reveals the critical passages that begin to cross-connect when this stator takes a slight twist, causing damage to the K2 and an inevitable converter-overheat failure. The circuit slots for the K1, K2 and TCC-in circuits are each about 0.472 inch wide, separated by about 0.175 inch of shaft surface that seals against the cover. Using the example given in Figure 2, the shaft has turned about half the distance of the circuit width (0.236 inch). Subtract from this the material width between the circuits (0.175 inch) and you find there is about a 0.061-inch-width cross leak of the K1 intruding into the K2, which is also mixing with the TCC-in circuit. Since the K1 is on in first through fourth, this cross leak causes the K2 to drag in first, second and third, resulting in rapid failure. The compromise to the TCC-in circuit eventually causes TCC-overheat conditions.

Figure 7 reveals the twist-of-fate fix – no pun intended, but the pump was! There is an area of the stator shaft and cover where a talented machinist could machine into it a keyway to hold the shaft in place. Since the stator shaft had a minimal turn in the cover as opposed to spinning, the interference fit of the shaft in the cover was not compromised to the point where it would not seal if aligned properly.

One other point: Since the pump is calibrated with the valve body, should you swap out pumps be sure to use the same number of shims on the PR valve that were used on the original pump. This will maintain calibration of the pump with the valve body.

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