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Engine harness swap / fried ECU


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5:08 PM
Aug 13, 2020
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71 roadrunner, electronic ignition. May 71 build, so not sure whether.it was converted, or if original? Carging system failed, checked out, dead alternator, sent out to be repaired. In the meantime, discovered that my engine harness was very sketchy.... degraded connection insulators, cracking wire sheathing, etc., bought a new engine harness from Megaparts. Old harness is four post ballast resistor, new harness is for two post. I've seen several cars using one side of the two sided BR, so I ran it that way. It has now killed two ECUs within a few minutes of running...... does switching to a two post BR solve this problem, or do I need to modify this harness to the prior configuration??.....it was running fine before the harness swap, however, I did have the alternator repaired too, so I broke the "one thing at a time" rule. I do still have the old harness, and it isn't butchered up, just very weathered...... any help is appreciated!!
do you have a 4 or 5 pin ECU?

The four post ballast has different ohm ratings for each
How does a 5 pin box function with a single ballast resistor? That what the dual ballast was for, the 5 pin Box.

4 pin box only uses 4 of the 5 wires at the box.
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I did find some posts discussed that addressed exactly that, and I think that's where my issue is...... apparently I need to switch to a 4 pin ECU for the single ballast, which is now the norm...... it seems they're internally regulated for the 5th pin function.
Both ECUs I fried were old 5 pinners...... I'm a hoarder! Lol
If you are using the five (5) pin ECU, then you need the four (4) terminal ballast resistor. The resistor plug is oriented snd will allow the wires to be connected one way. The origional two value, 4 terminal ballast resistor is: 5.0 ohm section to provide to the ECU operation thru the 5th terminal and 0.5 ohm to limit the coil current.
Later designs of the ECU, eliminated the 5.0 ohm section of the ballast resistor, as the internal circuitry changed of the ECU, but the coil and the external transistor on the ECU still requires a ballast resistor, which can be from 0.5 ohms to 0.7 ohms or even 1.0 ohms.....to limit the coil current and protect the ECU . Failure to use use the correct ballast resistor with the correct ECU will result in damage to the ECU......and use only the recommended coil by the manufacturer......NOT what your "buddy" says........just my opinion of course....
Thanks Bob. The true root of the problem here is that I don't truly know what I have. I'm not certain if my car came with solid state or point system....... very well could have been either, given that it's a May 71 build, but it was working fine until I switched the wiring harness and ended up with a 2 point BR plug instead of the 4 point plug it had before. The coil is chrome, so definitely not stock, but I don't know what brand..... no markings on it or either ECU..... probably cheapo stuff from over the years. I guess worst case scenario is I start over with a matched set of all new ECU, coil and BR, but I think I'll try a 4 pin ECU first...... maybe that'll work with this harness. I'd rather not modify the new harness, but it does seem logical that would restore the system to what it was before the harness swap.
The electronic ignition was standard equipment starting in '73. Some High Performance engine options 340, Hemi, 6 barrel cars it was optional, but '71 cars incl 383 Road Runners used single point Mopar distributors and 2 wire ballast resistors and a std Mopar coil...the prior owner must have converted to the MOPAR electronic ignition system, as the first conversion units were as described OR a '73 and newer system may have be adapted. IMO....in order to obtain reliable operation going forward, perhaps you should consider starting from scratch....suggest you contact Ray @HALLIFAXHOPS, on this site, as he may have exactly what you need....parts and advice...
Yeah.... after further study I see that the 71 models that came with solid state were manual trans cars, and mine is auto, so for sure, it was converted. Thanks for that contact info!
I thought I'd come back to this in case someone else runs into this in the future. This problem turned out to be my alternator putting out way too much voltage. I did something one should never do, and trusted that the alternator shop had properly repaired my alternator....... NOPE!! It is putting out over 28 volts, so it murdered 4 ECUs. I put one of my old ugly pull off ones on there, and voila!...... smiles all around! ....... now it's time to find out if my alternator shop is going to make this right.