Heavy Metal! - January/February - 2019

Aisin Warner AS68RC DTC P0796 Electrical or Performance

This is for the guys who work on the big boys: trucks of course. But you can apply this technique to any vehicle; this one is just a big truck. So we’re going to be looking at a 2012 RAM truck cab and chassis, equipped with a 6.7L engine and a 3500 heavy duty 4×4.

Here’s how this went down: Rusty Lorenzo from Rainbow Transmissions in Miami called the ATRA HotLine. He was dealing with a vehicle with code P0796: pressure switch 7. The problem was that the code was intermittent and wouldn’t set all the time. It didn’t even have a pattern: It could set in two days or drive for a week, and set without rhyme or reason.

THEORY OF OPERATION

The linear solenoid 3 adjusts input pressure to control valve 3. Control valve 3 uses the input to modulate line pressure to the appropriate supply pressure for the clutch.

The TCM verifies the operation of control valve 3 by monitoring pressure switch 7. If the TCM detects a fault, the transmission goes into limp mode. This disables the converter clutch, the gear it chooses will be based on the current gear, and the MIL will light.

CONDITIONS FOR CODE P0796 TO SET

  • PS6 closed and PS 1-6 working properly
  • PS7 closed for more than 160 milliseconds after the control valve switched from off to on 3 times. The TCM hasn’t detected an electrical fault from the solenoid within 10 seconds of seeing the condition.
  • PS7 is open for control valve 3 at maximum pressure. The TCM hasn’t detected an electrical fault from the solenoid within 10 seconds of seeing the condition. Possible causes:
  • PS7 open circuit
  • PS7 short to ground
  • Control valve 3 — If no oil pressure from the valve gets to the switch, the TCM may think that the circuit’s open or the switch failed.
  • Internal wiring problem
  • Transmission control module failure

I was invited by shop owner Rolly Farradas to come to the shop to put our heads together and see how fast we could figure this out. Not that this truck wasn’t going to get fixed, but how fast we could get it back on the road.

After the initial pressure test, and battery and charging system checks, we proceeded to check switch 7. We dropped the pan to check for obvious metal contamination or possible water intrusion. So far everything looked okay.

Then we dropped the valve body to remove the pressure switch and bench test it.

The switch tested fine through more than fifty checks, so next we checked the valve body, because sometimes performance issues can cause electrical codes to set. In this case, the computer is looking for an on/off signal at a specific time, measured in milliseconds.

If the valve doesn’t move on time, an orifice in the valve body separator plate is plugged with debris, or the switch itself is clogged, it won’t work properly… if at all.

These transmissions have been known to have wiring issues, so we disconnected the connectors: C1 at the transmission and C3 at the computer. Then we checked the resistance between pin 10 at the C3 connector and pin 1 at the C1 solenoid connector (figure 1). The wire had less than 0.5 ohms resistance, so it was okay.

Next, we decided to load the wire and put it through a real test. We pulled 7 amps through the wire, so it was okay too.

We repeated this test on the internal wires and they all checked out good. You can use the wiring diagram (figure 1a) for checking all the switches and connectors.

At this point, we decided it must be the computer: When it gets hot, it opens the circuit. One way to diagnose this type of problem is to soak a towel in water, stick it in the freezer until it’s almost frozen, and wrap it around the computer when the truck is acting up. But this was an intermittent problem, so it could take weeks for it to show up again. So that wasn’t likely to be very helpful on this truck.

RESIZE THE TERMINALS

Before we sprang for a new computer, we decided to resize the pins, the way we do on the A604. It was a nice thought, but the computer has T-shaped cavities and the pins are flat and thin. None of the wire terminal tools worked for removing the pins.

So Rusty came up with the idea of taking a damaged computer apart, cutting one of the male pins that protrude from the computer, and pulling it from behind to make a resizing tool (figures 2 and 3).

Once we had our tool, we resized the terminals (figure 4). It worked well in all the terminals except cavity 10; the blue/ black wire that went straight to switch 7. We found the problem, managed to remove the terminal, retighten it, and replace it.

With all the terminals resized, we took the vehicle on a test drive. The problem never returned, and still hasn’t, even after returning it to the customer.

So, if you run into a possible computer problem, don’t forget to check the terminals. A loose terminal can be enough to cause all kinds of electrical and performance problems.