Shift Pointers: Focused DTCs - Transmission Digest

Shift Pointers: Focused DTCs

On occasions when a vehicle’s computer system detects an error, it can set an array of diagnostic trouble codes. The variety and quantity of them require the ability to diagnose diagnostic codes, so it’s nice when codes set that point directly to the problem. 

One interesting scenario that just recently came to us on the ATSG technical help line had to do with a 62TE transmission. It was in a 2013 3.6L Chrysler Town & Country sporting three DTCs after rebuild. Two of them were gear ratio codes: P0733 for third gear and P0734 for fourth gear.

Using Figures 1 and 2 to identify the possibility of a common component, you can see that the Underdrive is on for the first five gears while the 2-4 comes off and the Overdrive clutch comes on. So, there is nothing in the main center line of the gearbox that fits this “one component scenario.” But, when looking at the Underdrive Line where the compounder set is located, the Low Clutch becomes a strong candidate in this scenario. The third DTC P0792 that set confirms that this is the area of concern.

Shift-Pointers-December-Figure-1-500
Figure 1.
Shift-Pointers-December-Figure-2-1400
Figure 2.

P0792 is a compounder speed ratio error. By having three speed sensors located on this transmission (Figure 3), the computer system can monitor each of these two drivelines separately and collectively. P0792 is produced when ratios between the Transfer Shaft Speed Sensor and the Output Shaft Speed Sensor become out of range. The diagnostic steps for this code are simple and clear. If there is any Loss of Prime (low fluid), Line Pressure Sensor and/or Speed Sensor codes, diagnose these first as they will cause gear ratio codes. There were none in this scenario.

Shift-Pointers-December-Figure-3-500
Figure 3.

The next step points to the two clutch assemblies in the compounder set, the Low and Direct Clutch. This is where it gets interesting.

In using the application chart to identify a common component causing third and fourth gear ratio codes, along with P0792, it confirmed that the Low Clutch is the most likely candidate behind the problem. But it gets better. The application chart also points out that this low clutch comes on during coast for engine breaking purposes only. And this is exactly when these codes were set. They didn’t set during an upshift event. They set during coast downshift. Now those are some focused DTCs.

In working with ATSG’s technical field advisor Jerry Gott, David from Granbury Transmissions knew to investigate the Low Clutch circuit. Unfortunately, since we are all so accustomed to the worst-case scenario, the unit was pulled to inspect the low clutch assembly. The piston and feed pipe O-rings all looked good. While the unit was on the bench, David found the problem: The Low Clutch Accumulator in the case located under the valve body (Figure 4), had a pinch tear in the upper most sealing ring as seen in Figure 5. Too bad this wasn’t discovered first.

Shift-Pointers-December-Figure-4-500
Figure 4.
Shift-Pointers-December-Figure-5-1400
Figure 5.

These focused DTCs pointing directly to the cause made it easier to diagnose the problem. A good lesson here is to start with the simplest possible cause. It sure would be nice if problems always were this easy to diagnose!

Read more columns from our Shift Pointers series here.

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