Back on the Bench - June - 2017

Gear Ratio Codes vs Solenoid Performance Codes

You’ve no doubt heard the old saying: “Don’t judge a book by its cover.” That old saying can be revised for the modern diagnostician: “Don’t judge a code by its description.” Many code descriptions can be very misleading, and jumping to conclusions by what’s in front of you can become a costly mistake.

Here’s a story about a technician who was working on an AS68RC that was displaying a P0756 shift solenoid B performance code. He had a few of these valve bodies on the shelf from cores, so he grabbed a solenoid, installed it… and the same code came back.

On these units, the computer controls the solenoid; it strokes shift valve two, which is monitored by pressure switch four. The problems can be a solenoid mechanical failure, a sticking valve, a faulty pressure switch, or even a problem with the pressure switch circuit.

I recommended he drop the pan, and, with a scan tool still connected, remove the wire to pressure switch four, ground it, and check for change on the scan tool display.

There was no activity on the scan tool and no voltage on the wire. He traced the external wiring to where the wire loom had fallen out of a bracket and rubbed against the body. The wire was cut and several others damaged. He repaired the wire harness and out the door it went.

A couple days later, he was checking a 6F35 with a P0761 SSC solenoid performance code. He assumed it could be a stuck valve or a solenoid mechanical problem, so, without any further diagnosis, he pulled the valve body. All of the valves were working freely.

So, assuming the problem was in the solenoid body, he replaced it, reprogrammed the solenoid strategy, and guess what: the same code came back.

P0761 is pretty much a gear ratio code: The computer commands the solenoid and, if it doesn’t see the gear ratio it was expecting, it sets the code. This can indicate a solenoid mechanical problem, a stuck valve, a failed clutch, or a problem in the geartrain.

Since he eliminated the valve body and solenoid with “parts replacement diagnostics,” I recommend he pull the transmission, which he was reluctant to do since he told the customer the solenoid would fix the problem.

After disassembly, he found the 2-6 piston molded seal was peeling off. So he built the transmission and out the door it went.

Two weeks later a 6T40 rolls in the door with system performance code P0965 for the CPC solenoid 2. P0965 is generated when the TCM detects an invalid voltage in a pressure control solenoid circuit for more than 4.4 seconds, so this is generally a solenoid circuit code. That would usually mean replacing the TECHM with no need to rebuild the transmission.

Solenoid-stuck-on or stuck-off codes for GM have essentially replaced gear-ratio codes. The computer commands the solenoid on or off and looks for a specific gear ratio. It sets this code if it sees the input shaft speed is at least 60 RPM higher than it should be for 2.25 seconds.

Diagnostic trouble codes are very helpful in narrowing what possible problems exist in the transmission. But, as technology progresses and changes, so do trouble code descriptions and definitions.

Gear ratio codes have been vanishing, and now we’re seeing solenoid performance and solenoid stuck-on or stuck-off codes replacing them. It’s always a good idea to learn exactly what a code means, what the computer is monitoring, and why it sets that code. Never assume by its description that the problem will be automatically be the same as another manufacturer’s code.

When working in a transmission repair shop, two main things are always top priority: customer service and making money. It’s all too common for management to rush you into getting things done quickly to satisfy their customers, but sometimes you just have to slow down and open the book to see what’s inside, and not go by what’s on the cover.