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Recently my 2011 Ram 4x4 started shifting into 4th and staying there. I can reset it by switching off the truck and restarting it. After a few more miles it will select 4th again. I believe this is a "limp home" mode. Yesterday it threw on the Engine warning light and when I scanned it I got a P0750 code "LR Solenoid Circuit". Fluid level is fine and does not smell burnt. My biggest problem is that I am in Ireland and we have no Dodge dealers here so there are no specialists. Any advice would be appreciated.
 

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P0750 is an LR Solenoid Circuit fault, which indicates a problem with (duh) the LR solenoid circuit. This fault will put you in full limp-in mode (4th gear all the time with no TCC engagement), so truck will feel really sluggish when you launch from a stop, and will have high RPMs at speed. Do not drive it for extended distances in limp-in; it is easy to overheat the trans if you run in limp-in. As you've discovered, restarting the engine will clear the limp-in condition (as long as the solenoid circuit keeps working OK).

The PCM periodically (every 10 seconds, I think) checks the continuity of each of the main clutch solenoid coils. There are six of these solenoids (LR, UD, OD, 2C, 4C, and MS). To check each solenoid, the PCM briefly changes the state (turns it off if it was on, or on if it was off), and then turns it back to its original state. When each solenoid is on, it develops a magnetic field around the solenoid coil. When the current is turned off, this magnetic field collapses, which induces a short voltage spike on the solenoid wire. The PCM monitors the voltage on the solenoid wire, after it turns the solenoid off, and watches for this induced spike. If it fails to see the spike, it repeats the test. After I think 3 successive failures, it sets the related fault code (for the LR solenoid, P0750) and puts you into limp-in.

Now there are basically three different problems that can cause a solenoid circuit fault:
• Solenoid coil is open (broken coil wire)
• Wiring between solenoid and PCM is bad (open, or shorted into another wire)
• Solenoid driver chip in PCM is bad (either not activating the solenoid properly, or not recognizing the voltage spike)

The problem is in tracking down which one of these is the real root cause. I would suggest you first try simply disconnecting (and reconnecting) the trans wiring harness. Sometimes the connection on one of the pins gets flaky, and simply unplugging it and then reseating it fixes the issue. So disconnect the harness at both ends (trans and PCM), inspect the connectors (look for bent pins, pins that are pushed part-way back into the connector, water or other contamination, corrosion, etc.), and then re-connect.
Make sure the key is OFF when you pull the connectors.

The main transmission electrical harness is on the driver's side of the trans. The 23-pin connector is on the solenoid / valve body and sticks vertically up out of the trans, a few inches above the oil pan rail.

The harness connector has a locking lever and a secondary latch. The transmission side of the connector is square and has two small round posts sticking out of opposite sides of the connector (one towards the front of the vehicle, one towards the rear). The locking lever on the harness (which is probably gray in color) wraps around three sides of the connector. The latch is on the driver's side. The two sides that wrap around have slots that engage with the two posts on the transmission connector. When the connector is unplugged, the locking lever (latch side) is down (toward the open end of the harness). When you plug it into the trans, the slots engage with the posts. Then, you push the locking lever latch bar up, which rotates the locking lever and clamps the harness connector down into the trans connector (by grabbing and pulling on those posts). I know this is hard to visualize... sorry but I don't have a picture.

To disconnect the harness, you first need to release the secondary lock. This is typically a red plastic piece (on the driver's side of the connector, just below the locking lever latch bar, which will be at the TOP of the connector when it's locked). You'll need to slide this red secondary lock either up or down (I'm not sure which, probably down but I'm not positive) before you can unlock the lever. Once that's done, reach up above and behind the center of the locking lever. The latch that holds the locking lever is just behind the face of the center of the locking lever (there's about 1/4 inch space between them). Put your finger behind the face of the locking lever, and push it and the latch APART. You need to push the latch (the piece just behind the top of the locking lever) in, towards the transmission (away from the locking lever) in order to release it. Once the locking lever pops free, rotate it down as you lift the harness up and off.

When you go to re-connect it, make sure the locking lever is DOWN (towards the open end of the harness connector) so it will engage the posts on the trans connector, then plug it in and rotate the locking lever up til it latches. Then slide the secondary lock into position to retain it.

If unplugging the harness fixes things, you're home free. If you still have issues, post again and I can give you details on how to troubleshoot further.
 

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P0750 is an LR Solenoid Circuit fault, which indicates a problem with (duh) the LR solenoid circuit. This fault will put you in full limp-in mode (4th gear all the time with no TCC engagement), so truck will feel really sluggish when you launch from a stop, and will have high RPMs at speed. Do not drive it for extended distances in limp-in; it is easy to overheat the trans if you run in limp-in. As you've discovered, restarting the engine will clear the limp-in condition (as long as the solenoid circuit keeps working OK).

The PCM periodically (every 10 seconds, I think) checks the continuity of each of the main clutch solenoid coils. There are six of these solenoids (LR, UD, OD, 2C, 4C, and MS). To check each solenoid, the PCM briefly changes the state (turns it off if it was on, or on if it was off), and then turns it back to its original state. When each solenoid is on, it develops a magnetic field around the solenoid coil. When the current is turned off, this magnetic field collapses, which induces a short voltage spike on the solenoid wire. The PCM monitors the voltage on the solenoid wire, after it turns the solenoid off, and watches for this induced spike. If it fails to see the spike, it repeats the test. After I think 3 successive failures, it sets the related fault code (for the LR solenoid, P0750) and puts you into limp-in.

Now there are basically three different problems that can cause a solenoid circuit fault:
• Solenoid coil is open (broken coil wire)
• Wiring between solenoid and PCM is bad (open, or shorted into another wire)
• Solenoid driver chip in PCM is bad (either not activating the solenoid properly, or not recognizing the voltage spike)

The problem is in tracking down which one of these is the real root cause. I would suggest you first try simply disconnecting (and reconnecting) the trans wiring harness. Sometimes the connection on one of the pins gets flaky, and simply unplugging it and then reseating it fixes the issue. So disconnect the harness at both ends (trans and PCM), inspect the connectors (look for bent pins, pins that are pushed part-way back into the connector, water or other contamination, corrosion, etc.), and then re-connect.
Make sure the key is OFF when you pull the connectors.

The main transmission electrical harness is on the driver's side of the trans. The 23-pin connector is on the solenoid / valve body and sticks vertically up out of the trans, a few inches above the oil pan rail.

The harness connector has a locking lever and a secondary latch. The transmission side of the connector is square and has two small round posts sticking out of opposite sides of the connector (one towards the front of the vehicle, one towards the rear). The locking lever on the harness (which is probably gray in color) wraps around three sides of the connector. The latch is on the driver's side. The two sides that wrap around have slots that engage with the two posts on the transmission connector. When the connector is unplugged, the locking lever (latch side) is down (toward the open end of the harness). When you plug it into the trans, the slots engage with the posts. Then, you push the locking lever latch bar up, which rotates the locking lever and clamps the harness connector down into the trans connector (by grabbing and pulling on those posts). I know this is hard to visualize... sorry but I don't have a picture.

To disconnect the harness, you first need to release the secondary lock. This is typically a red plastic piece (on the driver's side of the connector, just below the locking lever latch bar, which will be at the TOP of the connector when it's locked). You'll need to slide this red secondary lock either up or down (I'm not sure which, probably down but I'm not positive) before you can unlock the lever. Once that's done, reach up above and behind the center of the locking lever. The latch that holds the locking lever is just behind the face of the center of the locking lever (there's about 1/4 inch space between them). Put your finger behind the face of the locking lever, and push it and the latch APART. You need to push the latch (the piece just behind the top of the locking lever) in, towards the transmission (away from the locking lever) in order to release it. Once the locking lever pops free, rotate it down as you lift the harness up and off.

When you go to re-connect it, make sure the locking lever is DOWN (towards the open end of the harness connector) so it will engage the posts on the trans connector, then plug it in and rotate the locking lever up til it latches. Then slide the secondary lock into position to retain it.

If unplugging the harness fixes things, you're home free. If you still have issues, post again and I can give you details on how to troubleshoot further.
Dude, that's awesome how much time you took to provide such a detailed, in-depth reply. That's why I love this forum, so many smart people that have so much knowledge. OP, I hope you figure out the problem! Good luck!
 

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Thanks for the reply. That's some good advice. Would you have a part number for the LR Solenoid ?
The LR solenoid is part of the trans solenoid module (or "solenoid pack"), that includes all of the solenoids. So it is only replaced as a module. Individual solenoids are not serviced.

For 2011+ trucks, the solenoid module is 52120000AA. List price is $434 US.

But in my experience, solenoid circuit faults are often caused by a wiring problem, or a bad PCM. So before you blow a bunch of $$$ on a new solenoid module, it makes sense to try to track down the real culprit.
 

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Discussion Starter #7
Update.
I cleaned the connections in the plugs at the PCM and the transmission. The problem came back. I isolated the individual wire from the PCM to the Trans and bypassed it temporarily - and the problem reoccurred. So I bit the bullet and bought a new solenoid Block and the problem has not returned. Hopefully that's the end of the problem. Thanks for all your help
 
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