Restomod Question - 2012 5.7Hemi & 2005 545RFE

[Mavrick_7]

Hi all, I've exhausted all my resources locally (rural town) and am looking to this forum for some help with my restomod project. I have a 1974 Challenger that I've swapped a 2012 5.7 Hemi from a Challenger (stock with no modifications) and 545RFE automatic transmission out of a 2005 Ram 1500 into. The wiring harness and computer came from HotWireAuto, I believe the computer is from a Jeep Grand Cherokee as Chris told me this is the computer that's compatible with the engine & transmission .

I completed the swap recently and have put ~150 miles on it going down my private road around 20-40 mph. I finally got my car alignment done and have started taking it on the highway for the first time going 60+ mph. I have issues with the car going into a "limp mode" where once I get above 50-60 mph the computer will be completely unresponsive (car still running, but no gas pedal response). I will pull over, turn the car off, and turn it back on. It fires right up like nothing happened until I get above ~60mph and then it'll do the same "limp mode" again. My computer went into SKIM mode after this happened multiple times so I sent it back to HotWireAuto to clear it and fix the code. I talked to Chris with HotWireAuto during this issue who stated it's likely a brake light switch, which I've replaced 3 times now and I'm still having the same issue. Yes, I hooked the brake light switch up correctly. 

I was told the engine and transmission are simply not compatible, however I've also spoken with several people who have done the same swap with similar year engines (2009+) with the 545RFE and don't have a problem.

Am I missing something here?
Do I need to use a different transmission that's more compatible with my 2012 motor (65RFE)?

I want to avoid spending more money on a new transmission, wiring harness, computer, etc. if it's a simple fix that I'm overlooking.

[rebelyell]

How are you getting speedometer signal? Using the stock sensor in the trans tailshaft?

Are you getting any codes?

[7E-Bodies]

Hopefully @anlauto will be able to help? I know he does some gen 3 stuff. I just saw a beauty he did over on the farcebook site.

[Mavrick_7]

How are you getting speedometer signal? Using the stock sensor in the trans tailshaft?

Are you getting any codes?

I'm running a GPS Speedo. I need to pull my wiring to see where I placed the Speedometer Signal, however I believe it's wired to my Dakota digital interface.
Yes, I'm using the stock speed input and speed output from the trans.

I've gotten a handful of codes throughout the build and testing, however I believe the main one is P0571 (Brake Switch "A" Circuit).
I've also gotten the following:
P0300 - Random/Multiple Cylinder Misfire Detected (fixed by installing new spark plugs and coils)
P0306 - Cylinder 6 misfire detected (fixed by installing new spark plugs and coils)
P2317 - Ignition Coil 6 Secondary Circuit Insufficient Ionization (fixed by installing new spark plugs and coils)
P2181 - Cooling System Performance (Fixed by installing a new thermostat and thermostat spacer)
P0934 - Hydraulic Pressure Sensor Circuit Low (Fixed by getting trans oil to full)
P2504 - Charging System Voltage High

[Finoke]

One possibility with any code is your wiring. For example, I owned a 2008 Viper that kept going to limp mode for crankshaft position sensor. I thought bad sensor or engine issue. Turned out it was a wiring issue. The female part of the connector got pushed in and wasn't contacting the pin.

I found my issue with the wiggle test. I know it sounds crazy but I started to wiggle all the wiring from my crank position sensor and the engine stumbled. If tech thinks it brake switch, it could be your connection, not the switch.