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Discussion Starter · #1 ·
on my 2010 Ram 1500 5.7 Hemi auto, 41000 miles the trans. is stuck in fourth gear. I cant change it with the buttons on the shift lever and the check engine light is on. Any thoughts? I know it is still under warranty. Thanks, Dave
 

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Dave, sounds like it's in limp-in mode. I presume re-starting the engine doesn't help? Or will it be OK for a while, and then go back into limp-in?

The first key to diagnosing it is to find out what fault code(s) are set in the PCM. Your dealer can check those very easily using a scan tool. See if you can get the codes (like P0755) and post them here.

If it goes into limp-in as soon as you start it up, then you probably have an electrical fault (like a solenoid circuit fault). Your dealer may want to throw a solenoid module at it right away (without doing any diagnostics to find the real root cause), but I would discourage this. In my experience, solenoid circuit faults are usually due to a wiring problem (broken wire, short within the harness, etc.) or a bad PCM (bad solenoid driver chip in the PCM).

Get the fault code(s), and I can walk you (or your dealer tech) through how to diagnose it and find the real root cause.
 

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Full limp-in is 4th gear only. This happens in response to a number of different trans faults.

If you start it up fresh, let it idle in Park for a minute or so, and then shift into Drive, does the dash show "4" (instead of D) right away, or do you have to drive it a while before this happens?
 

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Discussion Starter · #6 ·
Its been going from ok to not ok since yesterday. I usually do start it up and let it idle while my wife is getting ready for work (I take her). Tonight we went out for supper and it ran perfect for about 40 miles, then it suddenly shuddered and went into 4th gear, there it stayed for the rest of the night. I took it to the dealer today and they put the code reader on it. It said solonoid "A" as being defective. It goes in next Tuesday for repair. Thanks, guys
 

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Did they give you the P-code? "Solenoid A" is a generic description. Are they using a real dealer scan tool? They should get a description referencing the UD, OD, 2C, 4C, LR, or MS solenoid.

I'm guessing you have a semi-intermittent Solenoid Circuit fault. This is a "strictly electrical" fault. 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 UD solenoid, P0765) 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. Have your dealer try this. For this example, I'll assume you're getting a UD Solenoid Circuit fault (P0765), but a similar procedure will work for others:

1. Swap the wire that controls the UD solenoid with the one that controls one of the other solenoids (such as 2C). Swap the wires at BOTH ENDS of the harness. So now the wire that used to carry the UD solenoid current is carrying the 2C solenoid, and vice versa. Now drive the vehicle and see which fault sets. If you now get a 2C Solenoid Circuit fault (P0755), then the problem must be the wire.

2. If you still get the original (UD) fault (P0765), then it's the solenoid, or the PCM. Now here's where it gets tricky. Does the fault set if you merely start the engine and idle it in Park? If so, then swap the UD and 2C wires again, but only at ONE END of the harness. Now the UD driver chip (in the PCM) is actually controlling the 2C solenoid, and vice versa. So you CANNOT drive the vehicle like this - you will blow various faults and may actually damage the transmission! But you can start it and let it idle in Park (neither UD nor 2C should be "on" in Park). So start it up and let it run for a minute or so. If you NOW get the same (original, P0765, UD) fault, then the problem MUST be the driver chip in the PCM (so you replace the PCM). If you now get the P0755 (2C) fault, then the problem must be the solenoid module (so you replace that).

If the fault won't set just idling in Park, then step 2 above won't work. In this case, carefully inspect the terminals in the connectors at each end of the harness (the terminals on the PCM, and on the solenoid module in the trans). If you see an issue (bent pin, corrosion, etc.) then fix that. If nothing is visually bad, then you must gamble and replace either the PCM or the solenoid module. At this point, I would bet (based on experience) that the problem is likely in the PCM, so I would replace that first.

Hope this helps. Using this procedure often avoids "Well, we replaced the solenoid and that didn't fix it. So we replaced the harness, but that didn't fix it either. So now we're gonna try a PCM."
 

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Discussion Starter · #8 ·
Wow, sure gets complicated doesnt it. My main concern right now is I have to make an emergency trip out of town to Pittsburg, PA about 5 hours away. The rpms are up when on the road but seems to run right along. Taking off from stop is a little scary. Should I just rent a vehicle or take mine. Will ask the dealer today as well. Dave
 

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Dave, I would use another vehicle for a long trip. When you're in limp-in, the torque converter clutch (TCC) will never engage. This means you are constantly generating extra heat within the trans/converter, and on a long drive you can actually overheat the trans.

If you do drive the truck, then monitor the trans sump temp on the EVIC display in the dash. When the sump temp gets up to 220°F or so, I would pull over, take a break, and let it cool down.
 
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