Pressure switch systems have been around for a long time now, and in case of confusion, the main purpose is to signal the controller when a specific clutch pack has hydraulic pressure. So, let’s dig in and explore a key feature of the Dodge RFE pressure switch system that will make our lives easier.
I’m not sure if it’s purposeful, but the key on engine running (KOER) reverse test feature with all pressure switches in an open state in the reverse position makes it much easier to diagnose pressure switch (PSW) codes and problems. When all PSWs are in an open state in reverse, that is a sign that wires and connections are functioning correctly and that the control system is completing the basics of providing system voltage from the battery to the transmission components and the controller. This feature makes diagnosis easier by viewing scanner PSW data while gently moving connectors and wires to check for faulty connections on a vehicle. If any faults are present, they must be dealt with first before other diagnostics.
Dodge RFE units use a five-pressure-switch system. Each PSW has its own individual electrical circuit with a single signal wire fed with system voltage (12.6–14.4V). Inside the solenoid pack, the current is reduced to 50mA or less by 300-ohm resistors for each PSW. Each PSW is naturally open with no hydraulic pressure at the switch.
When the PSW pressure reaches 22 psi, the switch goes from an open state to a closed state. Going the other direction, PSW releases the closed state at 11 psi. A closed state means the PSW signal wire is brought to ground, and 25–50mV is present at the controller for that specific switch.
PSWs can malfunction because of either physical, hydraulic, or electrical causes. However, if they pass the wire manipulation KOER reverse test, chances are it is not an electrical fault. But be careful and pay attention to detail.
Going past the KOER Reverse test and into the code P0871, you may find problems within the valve body, such as faulty solenoid packs, mixed-up parts, check ball mistakes, cross-leaks, and switch valve faults. During the KOER reverse test, external connector and wiring issues are sometimes present as well. So, keep an open mind and pay attention when diagnosing code P0871. Rare solenoid faults also happen (such as drivers failing in the controller), but most PSW problems are simple.
PERFORMING KOER REVERSE PRESSURE SWITCH TEST
First, set KOER with the selector in reverse. All switches should be in an open state. The open state verifies that wires and connections are functioning properly now, and the electronic basics provide system current/voltage to all components, including the controller (Figure 1).
Note: Any discrepancy during this test must be corrected first.
- An open PSW state means that system voltage (12.6-14.4V) is present at the controller on the signal wires.
- A closed PSW state means a low voltage of 25–50mV is present on the signal wires at the controller. The hydraulic closing of the PSW brings the circuit to almost full ground.
Note: Any “absence of voltage” to the controller on a PSW signal wire is the same as the switch’s closed state. If the signal wire is shorted to ground, or if the signal wire is open, it is interpreted the same as a closed switch at the controller.
Notice that the KOER Reverse test cannot be performed if system voltage (12.6–V to 14.4V) is not present at the transmission solenoid pack from the relay in the TIPM or a vehicle has a code that results in a transmission failsafe condition. In a failsafe condition, the relay cuts voltage to the transmission.
OD PRESSURE SWITCH OPEN IN 1ST, 2ND, AND 3RD GEARS
On a road -test in 1st, 2nd, and 3rd Gears, OD PSW should be in an open state. The OD clutch is not applied until 4th, 5th, and 6th Gears.
Let’s look at the Dodge 68RFE OE solenoid switch valve assembly in the valve body, with an intermittent OD PSW closed condition in 2nd or 3rd Gears but no issues during the KOER Reverse test. Worn-out valve body bores and outer switch valve plugs will cause code P0871 about 90% of the time (Figure 2). This is so common that many shops automatically ream and oversize the solenoid switch valves on every build.
You must also check for erosion and porosity inside the bore (Figure 3). The aluminum alloy Dodge uses nowadays for valve body castings is not the highest quality. In fact, some valve body solenoid switch valve bores are beyond repair, so keep your eyes open and inspect physically and with a vacuum test.
There may be conditions in which there is doubt the problem is hydraulic. When diagnosing an intermittent code generation condition while on the road, you may need a scope sometimes. Preferably, a quality scope with a minimum of four channels and a 100V capability is needed. The more modern the scope, the better, as most recorded scope files nowadays are viewable in an app with unlimited configuration.
A transducer and proper tooling should be set up on a scope channel. PSW signal wire is a simple voltage channel. Needed fittings, tubing, gauges, taps, and drill bits are available from various resources.
If you are viewing a scope file you have recorded, you are looking for one thing: whether the PSW in the valve body is receiving pressure and closing before the corresponding signal wire changes state. That is proof it is a hydraulic issue in the valve body. If you do NOT see hydraulic pressure on the transducer, but the signal wire suddenly goes low or has no voltage, the problem is electrical.
When you suspect a solenoid or controller is at fault, you may want to add a scope channel for the corresponding solenoid, especially when you see signal wire change but no pressure on the transducer. The pressure control solenoid signal is PWM, so you would measure the PWM percentage. Watching the initial voltage spikes on PCS solenoids with an amp clamp for each key cycle is also a good idea. When cut off, a solenoid circuit reaches 42–-46V momentarily, so the scope must have 100V capability.
OD PRESSURE SWITCH CLOSED IN 4TH, 5TH, & 6TH GEARS
A closed PSW and OD clutch in an applied state is normal for 4th, 5th, and 6th Gears. For the record, an open state during OD clutch apply is an unusual condition. There could be leaks in the OD clutch that drop the clutch and apply pressure below 22 psi, such as the pressure port plug missing in the valve body or solenoid pack bolts loose. An open state here could also indicate a wiring issue, such as the OD signal wire having a short to voltage after the solenoid pack. Valve body check ball mistakes can cause a neutral condition, which we will discuss later. Remember that, electronically, a broken or grounded wire is the same as a closed switch, or there is no voltage at the controller.
Speaking of a code P0871? In 4th, 5th, and 6th Gears, code P0871 causal would now be because of an open state rather than a closed state. So, if that were the case, what does that mean? Well, it would be a neutral condition and noticeable immediately. Any condition with PSW returning to an open condition or losing hydraulic pressure would be very physical in nature. Kind of crazy, huh? Yes, this does occur.
SOLENOID PACK FAILURE: OD PRESSURE SWITCH OPEN IN 4TH GEAR
What if you have a good 4th Gear and OD applies normally, but the pressure switch stays in an open state? That is a good example of a failed switch in the solenoid pack. Think about it for a second: If it makes a gear change, it can’t be a solenoid or command issue, right?
This is not as common as a switch that fails in a closed state. What typically causes this open state to occur is a small hole or crease in the pressure switch lip seal (Figure 4).
COMMON VALVE BODY MISTAKES & CODE P0871
- It is not relevant to the valve body, but about the only cause of code P0871 that is internal is in the input drum. If a chunk is out of the red O-ring that separates UD and OD oil, or if it’s misplaced in the snap ring groove, that can be a code and a bind-up as well, immediately when moving.
- Number 6 check ball installed in a five-ball valve body will cause P0871.
- Number 6 check ball left out in seven-ball valve body will cause that Neutral 4th Gear we mentioned earlier and P0871.
- Number 4 check ball missing will bring TCC on in the Drive range and can cause P0871.
Note that other common pressure switch codes are commonly the result of wrong separator plates, and check the ball configuration. Keep in mind a 2019 channel casting on an early unit can create havoc with a 3rd Gear ratio code. Even aftermarket valve body gaskets on the wrong side of the plate cause pressure switch codes. Finally, solenoid pack gaskets are not all the same, so pay attention. Remember that if the electrical is checking out normally, don’t pull that unit so quickly, as it’s almost always a bottom-end issue. Enjoy. We are always learning new things about these RFE transmissions.
