“Viva la Resistance” or long live the resistance seems to have a double meaning these days. Whether we are seeing anti-government or anti-establishment protests in the news or trying to diagnose today's modern cars, resistance always seems to get more attention than any of the other facts or information.
By turning each of the six solenoids in the solenoid pack on or off depending on its current state, the TCM detects an inductive spike. If no spike is found, that circuit is tested again to verify a failure. When a circuit fails, the code is set and limp mode is enacted as well but not until 22 mph if the code sets while cruising, which is the jerk the customers felt in this case while coming to a stop.
Curiously the trouble tree does not have the tech testing the solenoid directly. Rather it has you hook up the before mentioned special tester to the transmission and use the scanner to control the solenoid to see if a light blinks on the tester. If the lights blink as laid out in the test, then by process of elimination the solenoid pack needs to be replaced. A few things bother me about this. Not everyone has the Chrysler specified Transmission Simulator, and even the act of unplugging the main transmission harness and putting it into the tester could have wiggled or changed the wiring in some way to mask a wiring problem and lead to a wrong test result and the last thing anyone would want to do would be to change a solenoid pack on one of these transmissions for no reason as both vehicles it is necessary to remove the valve bodies to do so. While not bad on the Dakota with the 545RFE, it’s no small task on the 62TE.
On the Dakota, the easiest test points are right at the PCM/TCM on the fire wall, and on the minivan the main transmission harness is pretty exposed right on the pan facing the front of the vehicle. While testing it's always nice to know what a known good waveform looks like. In the case of these Mopar solenoid packs, a known good is just a wire or two over on a different solenoid if needed.
A few things can limit an inductive kick or “spike” on an electrical circuit. In a lot of cases, the internal circuitry of the module itself will limit these spikes to a lower level. High resistance anywhere on a circuit can limit the kick as well such as corrosion on electrical connectors causing low voltage or a bad ground or power feed of the circuit. Because we are observing the circuit live and in action, we can pause the recording and zoom in and look at our voltages in detail. In our capture above we can see the circuit being pulled within a half volt of ground and when the circuit is off full battery voltage is measured as well.
If we bring in the current waveform and look at the detail of the solenoid turning on we can see something does not look right.
The solenoid still has resistance within range of specification, however. Using ohms law with the 14.4 volts and the 10 amps of current we get about 1.44 ohms. That is technically within the 1 to 3 ohms given as a specification for solenoid resistance in the transmission unit repair manuals.
All that being said, what about a known good reading? Notice the known good reading has a ramp that starts at the bottom of the trace. Using ohms law based on the 12.1 volts in this capture and the 6.9 amps we can assume 1.75 ohms for this solenoid. Again, that is well within the 1 to 3 ohms specified.
After R&R of the valve bodies, the solenoid packs were replaced on both vehicles. After the relearns were complete they were both driven with no more issues and returned to their owners who were happy to not need a transmission.
While in these captures it is the solenoid itself failing that caused the smaller spike in the voltage traces had there been an actual resistance problem in the wiring that would have showed up as well in our testing. The load from the circuit under working conditions would have revealed a problem to us in the form of a voltage drop somewhere in the power or ground side. Again, if you were to ohm test a circuit with a bad wire or corrosion there is a good chance that if even 1 strand of wire is making good contact that the ohm reading would be good since there is no load on the circuit during a resistance check.