Viva la P0750 Resistance

“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.

Why is that? While I can't speak for the news reporting end of things, I feel as far as diagnosing vehicles goes resistance usually is the only specification given, and the means of testing the resistance of anything is usually right at hand in most technicians toolboxes. But just because it’s easy does not mean that's the way it should be done or that it is accurate. A bad resistance test is always easy to interpret assuming there is no operator error in the way the test was conducted. But does a good resistance test mean everything is good, too? The appropriately named "trouble trees" when followed certainly have you thinking that.

A couple of very similar cases come to mind that I would like to show you as to why an ohm test might not be enough. Both vehicles in question come from the Mopar world and shared the same P0750 L/R (low reverse) shift solenoid electrical fault DTC. Both vehicles had different transmissions. One vehicle, a 2005 Dodge Dakota, has a 545RFE Transmission, and the other car is a 2009 Chrysler Town and Country with a 62TE 6-speed automatic. Both vehicles were decently maintained and had the customers complaining of jerks when coming to a stop followed by no shifting after that with a high gear start. Re-starting the vehicles sometimes would clear things up for a short time, but in general both vehicles were worse after driving through a warm-up cycle.

Naturally the customers were describing the "Limp Home Mode" effects caused by the TCM setting one or more of its many DTCs. When we scanned the TCMs, the only code stored in memory was the P0750 L/R solenoid electrical fault. The initial test drives went smoothly as well. All the shifts were there and felt good. Upon the return leg of the test drive back to the shop, we felt a jerk, a no shift and confirmed a high gear start, and it was clear the P0750 set again.

Diagnostic time always is easier to sell to a customer knowing a certain problem is as repeatable, as both these cases seemed to be. The first step I like to do after confirming the complaint is to look up the DTCs to see what the code setting criteria is for the trouble code being diagnosed. In these cases, it was interesting to see how the TCM is monitoring the circuit for faults.

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.

Now a few things come into my mind when reading the code description. They do not give a specification on the inductive kick, only saying that if the TCM does not see one the circuit is bad. Also when scrolling through Chrysler's trouble tree, it doesn't mention testing for an inductive kick, merely just hooking up a transmission simulator (flashing light box). Depending on those test results, it suggests doing a few resistance checks to rule out shorts to power and ground and then either fixing a wire or replacing the solenoid pack or TCM/PCM.

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.

So how should we test the circuit? We can do resistance tests that depending on temperature might vary, or we could observe the circuit in action and test it the same way the TCM does as per the code setting criteria. The best way to see the circuit in action is by means of a lab scope and observe the solenoids voltage and amperage under operating conditions.

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.

Once hooked up the vehicle is driven and the voltage and amperages are recorded. The nice thing as well about a vehicle that doesn't act up right away then does later is you can observe the change in the circuit as well. At first the voltage trace as seen from the Dakota was showing some nice inductive kicks when the L/R solenoid was turned off. Later in the drive we could observe these spikes get lower and eventually the transmission defaulted to limp home mode again. So what is causing these diminishing inductive kicks?

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.

So what else can cause an issue on this circuit? In our case, the inductive spike is the induced voltage of the collapsing magnetic field of the shift solenoid. If the magnetic field is weak when the circuit is turned off, then the induced voltage will be less. While we can't measure the inductance of the circuit or its magnetic field without expensive lab equipment, we can see its effect in the amperage of the circuit.

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.

Instead of having a nice even ramp up the amperage trace shoots up briefly and then ramps up. This brief “short” is a sign of a lack of inductance in the circuit. Inductance causes a form of resistance to the electrical circuit as the magnetic field is increasing and this is what causes the current waveform to ramp up. The lack of a ramp in the beginning is a sign of a lack of inductance within the circuit.

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.

Unfortunately, the Dakota's owner actually works for an automotive parts supplier that manufactures solenoids and wanted their old parts back. Fortunately, the minivan's owner did not want a chunk of aluminum with solenoids in it, so when it was on the bench I decided to try the ever so popular resistance test. The two resistance readings shown here are the values from the L/R solenoid and 2-4 solenoids. Can you tell which one is bad? If you had to rely on those readings what would you test or replace next?Using the resistance readings shown, you would have no choice but to assume the solenoid is good, if this is the only test you are using. How the solenoids work under

load is unknown under open circuit conditions. The TCM does not test the resistance of the circuit, rather it infers a good circuit based on the inductive kick generated by the collapsing magnetic field of the solenoid as laid out in the code setting description.

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.