AFter reading on line, and pondering the comments made over the last two weeks here on the forum, it hit me like the BIG DUH. Harnesses go bad because the voltage drops slightly. This can be either due to a poor ground, or bad charging system, or maybe even a bad battery that hasn't given up quite yet.

When the voltage drops, the amperage goes up accross the entire harness, and the wires are now all carrying amperage beyond their rating. This causes futher voltage drops, and more heat. The harness is now in a death spiral as all the crimp connections get cooked, and become high resistance joints. The longer it carries current in this condition, the worse it gets, and at some point, it is probably cooked too badly to work well at all, ever.

I am certain the harness on my Dart was run this way for years. When I started working on the wiring, all connections had brittle, browned insulation around the connectors. And when I stripped the wire back to put new ones on, the wire under the insulation was brown instead of shiny copper.

If the harnesses were designed with a saftey factor in the wire sizing, this would be minimized, but most cars are probably built to a budget, and heavier wire costs more money. If you sell 400,000 cars in a year, and the wire costs you $2 more per car, that is nearly $1,000,000.

Bottom line, keep track of the voltage on your system, and keep it up to snuff. If your old wires and connectors are not supplying full voltage through the system, replace it, starting with the heaviest wires first. Another early thing to check is the charging system. This would be the main wire to the battary, and the wire running from the regulator to the alternator.

Another find on the internet, according to one site, the Mopars after '70 had a seperate, parallel alternator feed to the battery that did not run through the ameter. So if this is right, you guys with later Mopars don;t have as mjch loss through the ameter when your wiring gets old.

Sam

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I believe they go bad due to corrosion and vibration. Moisture gets in there and turns the copper to copper oxide, slowly reducing the cross-section of the wire, and thereby increasing the resistance of the wire. Then, throw in some vibration so that some of the strands either break or at least start to thin out a bit, and resistance goes up further.

The insulation can go bad over time, but I've seen wires (well, coax at least) wick water long distances, despite having good insulation everywhere else other than the point of entry. Actually, I've read of cases (haven't seen, so take with a grain of salt) where a sensor in the coolant overflow tank allowed coolant into the harness, which was then able to migrate all the way into a taillamp!

Also, if the insulation cracks, the wire strength does go down a bit too--meaning it can flex more if there is vibration present. But I don't think insulation failure is the main cause of harness failure--corrosion and vibration would be bigger killers. That, and corrosion buildup on any connectors, causing a slow rise in resistance at interconnect points.

The external shunt type ammeter was phased in beginning in '71. A-bodies got them in '76. They don't work very well or very long, but they avoid full current through the ammeter.

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