Many transmissions by various manufacturers use pressures switches to monitor the presence of fluid in a circuit. Some early GM’s used them for determining position of the manual valve, some used them for determining position of the lock up valve. Late GM front wheel drives and many other manufacturers used them to monitor shift valve positions. For most Chryslers, these pressure switches are used to verify pressure in a circuit that is being operated by the solenoids.
In most Chrysler transmissions, a solenoid is turned on or off to control fluid flow in a circuit to a clutch, and the pressure switch monitors the existence of pressure in that circuit, to verify the solenoid has done it’s job. The relay provides battery voltage through a three hundred ohm resistor to the pressure switches in the solenoid pack (figure 1), this voltage is fed back to the computer to monitor the state of these switches. When the switch is activated, it provides ground and the computer sees the change in the voltage, and determines the switch has closed.
One of the most common codes that appear in many Chrysler transmissions is the pressure switch rationality code. This code means that either the switch is not responding, or the switch is activated when it’s not supposed to be on. There’s a few different things that can cause these codes, hydraulic leaks in the valve body or case, the switch in the solenoid pack, the connectors, pins in the connectors, shorted wires, or even a failed computer.
Recently I had a 42RLE come in with an overdrive pressure switch rationality code set. I hooked up a scan tool, put it in graph mode to monitor the switch. At first, everything was fine, but as the vehicle warmed up on the test drive, I noticed that the overdrive pressure switch would activate in 2nd gear. My first instinct, this seems like a hydraulic problem, since the switch is staying on in 3rd and 4th.
At this point, I hooked up a pressure gauge (figure 2), and test drove the vehicle again. In 1st and 2nd, there should be zero pressure on the overdrive tap. When the vehicle was warm, I found about 10 psi, which is enough to activate the pressure switch, and set the code. This confirmed my suspicions of a hydraulic leak.
Leaks from the overdrive clutch circuit into the 2nd clutch circuit can be narrowed down to the valve body or case damage. Cross leaks due to worm track damage, from operation is extremely rare, how ever mishandling of the valve body or the case causing damage to the worm tracks, can lead to these kinds of problems. Most of the time, I have found the cross leak is the switch valve plug bore worn through vacuum testing, which requires an oversized end plug to fix. As was the problem I found with this transmission.
Though this was an easy fix, the overdrive pressure switch rationality code can be set due to various other issues. Wire harness damage is not uncommon. Wires pinched, or damaged by looms falling out of brackets and rubbing up against other components. Oil contamination, causing the wire insulation to deteriorate, allowing contact with other wires. Terminals in the connectors damaged, or corrosion in the connectors at the transmission or the computer can also interfere with operation. The couple ways to verify this, is by cutting the wire at both ends, and tying in a jumper wire. And rechecking operation of the switch. Or back probing into the pressure switch wire and grounding it while checking operation on the scan tool.
Inside the solenoid pack, a solenoid mechanical malfunction can fail to allow fluid through to operate the corresponding clutch and activate the switch. Also the seal in the switch can fail due to heat or contaminates.
And finally, the computer. Though rarely, the computer is the problem, it’s always a possibility. I have had one computer problem related to a pressure switch out of a hundred times seeing these codes. I back probed into the pressure switch wire at the computer (figure 3), and monitored the voltage going into the computer while monitoring the switch on the scan tool. The voltage was changing on the meter, while the switch state was not changing on the scan tool, this would indicate a possible computer malfunction, which led me to replace the computer to fix the problem.
Though diagnostics can be tedious at times, especially trying to back probe into a hard to reach connectors, it can often be worth it. Replacing parts can get expensive, fast. And taking the time to perform these test, can often keep these transmissions from showing up back on the bench.