Featured Case Study
PT Cruiser AT Code
Just about every ATG manual contains examples and cases using Lab Scopes, although we’re the first to admit that the Lab Scope is not often the first tool to grab. There’s usually a much shorter way to get the diagnosis done. But there are other times where you’re just not going to find the fault without taking the Lab Scope out of its bubble wrap. This case makes the point perfectly, and while the diagnosis involves transmission codes, it’s relevant to engine performance diagnostic strategies because the transmission is PCM-controlled, and because the PCM self-tests most engine and transmission actuators in a similar manner.
2005 PT Cruiser MIL ON & Transmission Limp In
Like most customers with older low-value vehicles, this one was only concerned with the shifting issue, not the MIL. For solenoid codes, Chrysler stops using the solenoids, which defaults the transmission to second gear.
The technician checked the entire vehicle for codes. The TCM is in the PCM, but there are some separate functions, so they are normally listed separately in an ‘All Codes’ scan. However, in a Generic OBDII scan they are listed together. The trimmed AutoEnginuity report (Fig. 1) shows them all in one place, although they were also listed under the appropriate modules in the full report. Even if your tool separates them, the PCM will have a P0700, which lets you know that the TCM has codes.
As already stated, the customer only wanted a shifting transmission, so they declined diagnosis of the P0420, and were warned that the P0340 would likely result in another visit to the shop soon.
The codes were cleared, the P0765 reset immediately after the key was cycled, and the transmission defaulted to second gear. Hard faults like this are great because they are easy to measure.
Initial Research and Diagnostic Path Choices
A quick check of calibrations and TSBs didn’t offer any direction. A quick check of the wiring diagrams showed solenoids by name, not number, so which one was ‘Solenoid D’? The flow chart and the Enhanced code definition from the code report (not shown) both agreed that it was the UD solenoid, so out came the Lab Scope.
Why not just follow the flow chart? Because it asks you to connect a Miller tool with LEDs to check the PCM drivers. Even if by some miracle you have one, if the test passes the flow chart says to replace the solenoids. As you will see, that mistake would be hard to explain to the customer.
You can make the Lab Scope connections at the transmission, at the PCM, or in the harness – whatever is easiest! On this application the NGC PCM is easy to get to in the engine compartment (Fig 2), so the pink UD solenoid control circuit was easy to access for both voltage and current testing (circled area in Fig. 3).
Note that current flow is the same throughout the circuit, so the current probe could also be connected to the power side (red wire). However, the waveform would show current flow through all transmission solenoids.
Lab Scope Test
Since the code sets KOEO, that’s when the circuit was tested. Fig. 4 shows that the PCM is cycling the UD solenoid right after the key is turned to ‘Run’. The PCM cycles all transmission solenoids 3 times at startup for diagnosis. This looks like a good solenoid and circuit to us!
- The open circuit voltage is good (all solenoids share relay power so most high side faults would have reset multiple solenoid codes anyway).
- The PCM pulls the circuit cleanly to ground.
- The ramp on the ground side is caused by PCM driver load and the rising current in the solenoid windings. This ramp proves that current is flowing, which proves that this circuit is not open.
- The flattening out proves that the current built to a point and then held at that level.
- This ground ‘float’ was under 1 Volt from true ground, which is great. Some Chrysler drivers can consume over 1 Volt, while most manufacturers’ drivers require only a few hundred mV.
- The inductive kick occurs each of the 3 times the solenoid is turned off. The peak voltage isn’t shown in this capture, but the presence of any kick proves that a field collapsed, which proves that the circuit had been complete and that current had flowed through windings.
Other solenoids were checked with the exact same results. All solenoids are working despite the PCM’s claim that the UD solenoid has a circuit fault.
The capture (Fig. 4) doesn’t show the peak of the inductive kicks, but they would only be lower or higher if the current flow through the windings varied between solenoids. Likewise, variations in current flow would have affected the shape of the ramp in the ground. Therefore, checking the inductive kick peak or current is not usually necessary. In this case, current was checked anyway just to be sure that the solenoids were all behaving the same before blaming the PCM for setting a false code.
The next capture (Fig. 5) shows the current flow through the UD solenoid and circuit during the same self-test. The current peaks over 800 mA, which corresponds to the section of the voltage waveform where the PCM initially pulls the circuit to ground. The current then cycles around 400 mA when the PCM cycles the solenoid. This kind of solenoid strategy is common – higher current to get it to work, followed by a duty cycle command to lower the average current to hold the solenoid in the open position. Like the voltage capture, the current was the same through all solenoids, once again indicating that the PCM was mistaken it its conclusion that there was a circuit fault.
Diagnosis & Repair
The P0756 is a circuit code, and it set when there was no transmission pressure (engine off). The only way the PCM could set this code is by watching the low side activity when cycling the solenoid driver. Sure, the solenoids may not be working mechanically, but that wouldn’t set this code, and no codes would set KOEO. Since these tests prove that the activity is good (by comparison to other solenoids), this fault could only be caused by a faulty PCM, which was replaced and reprogrammed.
Back to the flow chart: The Miller tool in the flow chart has LEDs that blink when the PCM cycles the solenoid at key-on. The problem is, if the test passes (which it would have), the next step is to replace the solenoid/pressure switch assembly because the flow chart never considered that the PCM could fail in this way. The advantage of the tests in this case study are that they test the PCM drivers like the Miller tool, but the waveforms also prove that the solenoids are electrically functional, preventing solenoid replacement per the flow chart.