Stuck in 3rd gear (limp mode), P0882 - Trans 12 volt supply relay control circuit low

[fahrvergnuugen]

I thought I would document my experience trying to solve the following issue with my 2005 Dodge Ram Cummins with 28RE automatic transmission:

Check engine light on, P0882 - Trans 12 volt supply relay control circuit low DTC code.
Transmission stuck in 3rd gear (limp mode). Manually shifting from Drive -> Neutral -> 1st will put the transmission in 1st gear. You can then manually upshift to second and then drive (which puts it in 3rd). Overdrive doesn't work and the truck won't down shift on it's own.

The service manual indicates that this DTC is caused by a failure of the power circuit to the transmission control solenoid (indicating faulting wiring, relay, fuse, etc), however after a lot of digging and testing, I believe mine is being caused by a bad Governor Pressure Solenoid in the transmission.

How it works:
The ECM activates a relay (located in the fuse box/IPM) which then provides power to the solenoids in the transmission. The ECM can control each individual solenoid by providing ground to the other pin on the solenoid. I'm guessing that the ECM can detect faults in these three solenoids by measuring current drawn on the ground pins that it has for each solenoid.

Here's the wiring diagram for the 48RE Cummins Transmission Control:

Heres the 2005 fuse box (integrated power module) layout and relay map (2006 is different from what I understand, even though both are Gen 3):

How I tested:

• First I needed to make sure that relay is working and eliminate an issue with the relay itself or the Fuse Box / IPM.
  1. Remove the wiper relay for easier access.
  2. Pull the transmission control relay slightly out to expose the blades.
  3. Connect a test light to the battery ground and using an alligator clip, clamp the other lead to pin 87 on the transmission control relay (the relay's pins are labeled on the bottom side of the relay).
  4. Start the truck and make sure the test light turns on with the same brightness as it does off of direct battery.

This checked out okay. My ECM is toggling the relay and the relay is outputting voltage. By using a test lamp I know it's also able to deliver amperage too. I was disappointed at this stage because I had just repaired my IPM and was sure I must have screwed something up or overlooked an issue (it had a ton of corrosion in it).

• Next I need to verify the solenoids & the wiring between the ECU and the transmission
  1. Unhook the batteries
  2. Get a T25 torx driver and remove the C2 plug from the ECM on the side of the engine. C2 is the plug closer to the firewall.
  3. Using a multimeter set to measure resistance / ohms, measure
  4. pin 87 of the transmission control relay to C2 pin 14 (Governor Pressure Solenoid) - 4.2 ohms
  5. pin 87 TCR to C2 pin 15 (Solenoid Control) - 32 ohms
  6. pin 87 TCR to C2 pin 25 (Torque Converter Solenoid) - 32 ohms

Here's what C2 pinout looks like:

A google search tells me that normally 12v solenoids should have between 20 and 40 ohms of resistance. Some quick math tells me that 13 volts at 4.2 ohms is over 3 amps. That's a lot of current for an ECM to handle, and I do wonder how long it would last like this before burning something out.

Further research indicates that these Chrysler solenoids are prone to failure, and indeed my father's truck (identical to mine) has the same limp mode problem. I have ordered an upgraded solenoid from ATS Diesel which adapts a solenoid from GM to work with the 48RE. The installation seems pretty easy (drop the transmission oil pan, unbolt four bolts and 2 screws, unplug two wires, install new solenoid, change filter, gasket & reinstall oil pan).

Will update this thread once I have confirmed that this fixes the issue.

 

[Ozymandias]

Did you change the pressure transducer also at the same time? That fails also very often.

 

[fahrvergnuugen]

Did you change the pressure transducer also at the same time? That fails also very often.

I didn't order one, but I suppose I should. Is this the one you are talking about?

https://www.rockauto.com/en/catalog...tomatic,pressure+control+(epc)+solenoid,17897

 

[Ozymandias]

Nope, not the solenoid, the sensor. Officially named pressure transducer.

https://www.google.com/search?q=rostra+hd+transducer&ie=utf-8&oe=utf-8&client=firefox-b-1-m

Take the HD version from Rostra.

 

[p-Bar]

Sometimes the wiring harnesses go bad in the trans

 

[Dynamic]

The ground side of the governor pressure solenoid is Pulse Width Modulated (PWM) to regulate the current through the solenoid. It is not simply grounded like the other two. The more current through the solenoid, the more it closes. 4.2 ohms is just about right.

 

[fahrvergnuugen]

Well that's bad news. I just opened up the transmission pan yesterday and plan to reinstall everything today.

If it's not the solenoid and I have power at the solenoid connector, what else could it be?
How does the ECM actually detect the condition for P0882?

 

[Dynamic]

Code P0882 is detecting low voltage on the output circuit of the transmission relay. Have you measured the actual voltage on the circuit with the circuit intact and under load (back probe)? I see you were using a test light, and while it does provide a small load, and requires a bit of current flow to illuminate it (provided it's not an LED type of light), you'll really want to quantify what voltage the PCM is seeing, since it is a code set by a voltage discrepancy on that circuit. You will need to have the circuit intact and functional, and then measure the voltage at the transmission connector (back probe the connector).

Governor pressure solenoids do fail, so it's not a complete waste of time and energy to change them, but yours appears to be functioning correctly electrically. The much larger failure rate item is the governor pressure transducer. They are typically replaced as a pair. I see that you bought a GM conversion, which do work fine, but are just expensive, and many end up having the same problems the solenoids had in their GM applications. I prefer and use the Borg Warner HD solenoid and the Rostra HP pressure transducer. But, no matter. You've got them, so you might as well use them.

 

[fahrvergnuugen]

Thanks for the insight.

The test light was one I made using a blinker socket and a tail lamp bulb - so it was drawing approximately 1 amp. Now that I have everything apart, I will backprobe and check the voltage with the solenoid in place.

So should I assume the ECM detects X volts as reference, and Y volts from the solenoid ground circuit and if the difference is too high, it throws a code?
If so then I should check the voltage of that circuit under load and compare vs the battery voltage I guess?

And yeah, the solenoid adapter was expensive, but I went that route due to the number of failures I read about and the fact that my father's '05 has the same exact symptom as mine. Since I was sure the solenoid was bad due to the 4.2 ohm reading, I jumped to conclusions.

 

[fahrvergnuugen]

I just got done installing the solenoid and the transducer. I cleared the codes and the P0882 code (along with all the other transmission related codes) stayed off. Before it would come back instantly. I did a short test drive and it shifts through the gears like its supposed to.

So whether it was the solenoid or the transducer is a mystery, but considering the cost of these parts I think it would be foolish not to replace both considering the labor involved.

 

[Ozymandias]

Let us know if your fathers had the same outcome.