Technically Speaking
- Subject: Stator shaft turns in pump cover
- Unit: Allison 1000/2000
- Essential Reading: Rebuilder, Diagnostician
- Author: Wayne Colonna, ATSG, Transmission Digest Technical Editor
The Technically Speaking article in the June issue of Transmission Digest was an article called A Twist of Fate. The subject matter was the effects produced when an AS68RC transmission’s stator shaft turns in the pump cover. The article briefly mentioned a similar issue with the Allison 1000/2000 transmission. This comment prompted several people to call asking for more information about it.
In case you didn’t read this article, it described how when the stator shaft turned in the pump the converter would overheat and/or the C1 clutch would fail. In some instances this can be determined quickly by pulling the return cooling line. If fluid under pressure comes out of the return cooler-line fitting on the transmission (Figure 1) instead of the cooling line from the cooler, you have a turned stator shaft.
As you’ve probably surmised by now, this is not an article about bass player Al Turner (aka “The Burner”) and his album Sunny Days. This is about the Allison transmission with a stator shaft that turns in the pump. It is the turned stator shaft in the pump that I call Al Turner aka “The Burner” of torque converters and clutches. Al has produced a hit single called “Not Such a Sunny Day” that’s appropriate for those of you who didn’t know the cause of the problem when you rebuilt the unit the first time. It’s a totally different song when you do know what caused the problem before doing the job!
If you have the June issue of TD to refer to, you could compare the spline on the stator shaft from the AS68RC with that of the Allison shaft shown in Figure 2 of this article.
The comparison will quickly reveal that the splines on the Allison shaft are much longer than those on the AS68RC shaft. The problem with both transmissions, however, is not with the spline on the shaft but the spline in the cover; that is where they strip out (Figure 3).
Once the spline is stripped, the converter-charge passage moves toward the converter-out passage when the shaft turns, causing a crossing of circuits (Figure 4). In some instances the stator shaft moves forward toward the converter instead of turning. This can restrict the converter-charge circuit so severely that it will cause no-move condition.
To assist in verifying a compromised pump cover, figures 5 and 6 provide a visual way to determine the proper shaft-to-cover alignment by using the two converter-in or charge passages in the shaft.
So the next time you meet Al Turner aka “The Burner,” you can play the hit song Sunny Day when you build this one right the first time.