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Discussion Starter #1
A plea for help from Scotland, UK.

Dodge Ram 2500, early 2004, 5.7 Hemi with 545RFE autobox and NGC controller.

So... about 4 years back my auto trans wore out. It was replaced with a remanufactured unit, shipped to the UK from the USA, and fitted by myself. I had an initial solenoid pack pressure switch fault which was repaired under warranty (new solenoid pack sent, I fitted it myself) and it's been perfect ever since.

3 weeks ago, I noticed a misfire and could hear HT "arcing" somewhere near no.3 cylinder's coil pack. Not wanting to go in there twice, I elected to just replace all 16 plugs, all 8 coil packs and all HT leads. Yesterday, I finished the job and the engine now runs smoother than ever - but I have a new, major problem.....

It drove up a fairly steep slope to the workshop door just fine 2 weeks ago, but today the trans is in limp mode, with no MIL lamp on! We've had some heavy rain this last week, but all ecu's and electrics are dry.

I have DRBIII scantool, so I had a play this morning.
One-trip fault, P0755, 2C solenoid circuit. (It will clear, but comes back as soon as Drive, 2 or 1 is selected with the engine running, fault does not re-appear when selecting P, R, or N from startup).
Unable to carry out solenoid function test due to Limp-Mode.
I've checked the electrical connections at the gearbox and NGC control module (ECU), and I'm measuring 1.7 ohms across the 2C solenoid at both the gearbox connector and the harness plug at the ECU, which is pretty much in the middle of the values I've found online of 1.3 and 1.9 ohms, so I can't see the fault being in the gearbox or wiring....
Am I looking at an ECU internal failure here? Anyone have any ideas or pointers that may help me? Please?
 

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Discussion Starter #2
Just to add, I find it really puzzling that selecting D (or passing through it going to 2 or 1) sets the fault code for the 2C Solenoid - because 2C is only engaged in 2nd, 5th and then when in Limp-mode.....
 

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Unlikely it's an ECU failure. Since this is a fairly generic code I'd suggest replacing the solenoid in question as a first and relatively inexpensive step. My experience is somewhat limited, but in every case where I've had a code indicating a faulty solenoid circuit, it has in fact been the solenoid that was faulty. The pack you received may have been described as "remanufactured", but in my experience regarding this kind of part, remanufactured means a used part that is cleaned, tested and given new seals or gaskets. No actual solenoids are replaced unless they are faulty or out of spec.
 

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Discussion Starter #4
Thanks guyonearth, my personal experience with vehicles in general is that the ECU itself failing is fairly rare, although not unknown. What's really confusing me here is the fact this fails on selection of D yet the 2C solenoid isn't even activated until 2nd or 5th.
I have the solenoid pack from my original gearbox, I shall try slaving that in (assuming there's enough reach on the wiring harness at the gearbox end!) and see what happens. I know it'll throw up all sorts of pressure switch codes, but if it eliminates the 2C solenoid code, then I think I might be getting somewhere.... I suppose it's worth a try at least?
I'm working outside though, so it's going to have to wait until the torrential rain here stops!
 

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Update. Slaving in the old solenoid proved nothing, just a whole heap of different codes because there's no range switch operation with it hanging on the wires! I decided to bite the bullet and replace the solenoid pack assembly with a new unit, which arrived yesterday via Rock Auto. I fitted it today, and I am still getting the exact same 2C Solenoid Circuit fault code, P0755. I've re-done all the harness continuity checks again, short-circuit checks and re-measured all solenoid coil resistances at the Powertrain ECU plug, and everything checks out just fine. When I turn the ignition on, the Transmission Control Relay in the IPM is operating, but then de-energises after 3 seconds, even if the engine is instantly started. I've checked all power feeds and earths, it's definitely the PCM that's shutting down the Transmission Relay coil power feed.
Could it possibly be, that I am in fact "Mr Unlucky" and I have a dodgy Powertrain ECU? I have heard of driver chips failing within them, but it would appear to be a rare fault.... I really don't want to go spending a LOT of cash on an ECU when it's only a guess....?
 

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Discussion Starter #6
Following a suggestion made to me, I removed the PCM plugs and from plug C4 I removed the terminals that feed the 2C and 4C solenoids. I swapped them over and reinserted them into the multiplug, then re-connected everything.
With the 2C driver chip within the PCM now connected to the 4C solenoid, and the 4C driver chip now connected to the 2C solenoid, I still get a 2C code - so the fault has to be internal within the PCM, as the 2C chip is now connected to a known good solenoid and wiring. If there was a 2C solenoid or wiring fault, the 4C chip would now be generating a code and it isn't.
Replacement PCM ordered, I'll update again when it's here and fitted!
 
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