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Ruminations from electrical study/why harnesses go bad.
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Author:  Sam Powell [ Sun Dec 16, 2007 12:47 pm ]
Post subject:  Ruminations from electrical study/why harnesses go bad.
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.

Sam
Author:  Dart270 [ Mon Dec 17, 2007 6:58 am ]
Post subject: 
A voltage drop in itself will not cause heat, but bad connections that have higher resistance cause voltage drops that create heat at those junctions. That can cause harness breakdown. If the battery has lower voltage, that will not cause heat in your harness. Maybe I am repeating you here, but just clarifying...

Lou
Author:  KBB_of_TMC [ Mon Dec 17, 2007 10:53 am ]
Post subject: 
I've repaired about a dozen harnesses for our Mopar club's giveaway cars (www.tidewatermoparclub.com), and I'd like to add a few comments.

The #1 bad spot has **always** been the right at the +&- ammeter studs, followed by

#2 the bulkhead feedthroughs for the high current leads, then

#3 the headlight switch connector and hi/lo beam switch (especially in rustbelt!) and

#4 the fusible link had been replaced with any handy heavy wire

Other damaged areas were almost always due to somebody with more enthusiasm than knowledge trying to change things.

I've always been able to fix the harness using pins from NAPA & Waytek and correctly colored wires and keyed connectors from donor harnesses from other years and models. I got the special vinyl wrap (NOT electrical tape - it gets nasty with age!) from Eastwood. I don't even think of beginning w/o the full factory schematics.
Author:  SlantSixDan [ Mon Dec 17, 2007 11:14 am ]
Post subject: 
Quote:
A voltage drop in itself will not cause heat
Unless the voltage drop is caused by an underspecced/overloaded wire, in which case there will indeed be heat. And whether the resistance and resultant heat is at a connector, at a junction or splice, or in a wire itself, it's a spiralling chain reaction. Hotter conductors have higher resistance than cooler conductors, so the heat increases, increasing resistance, which increases heat, which increases resistance, which increases heat, etc. Eventually a failure of some kind will occur, but until that happens you've got electrical gremlins and operating faults.

But you're right, if the line voltage is reduced by e.g. a weak battery or whatever, that in itself won't cause extra heat.
Author:  Sam Powell [ Tue Dec 18, 2007 4:55 am ]
Post subject: 
I guess I need a little more education here. If the battary voltage drops, does not the load device wattage remain constant? In which case the amperage would rise. What am I missing here? I don;t doubt either Dan, or the good Doc, so I bow to your superior training and wisdom. After all, I am still the electronic Dunce in the group. :wink:

When we had a new heat pump installed in the house fifteen years ago, it kept blowing fuses because the service was not heavy enough to handle the load. True, the voltage dropped because the outside wire was not big enough, but inside the house, all the wires saw was low voltage, which popped circuit breakers of all kinds because the ampere load went up with the reduction in volts. The stove still was trying to pull 3000 watts, which became 35 amps under the low voltage condition. This then blew the 30 watt curcuit breakers for it. I just need some basic theory here to straighten out my thinking. Tell me where I went wrong in drawing a general principal from this specific situation. Inductive, or deductive, I can never remember which is which.

In a similar situation, my daughter's apartment building was blowing fuses because the transformer that supplied it fromt he road was deffective. How is this different from the battery being down? All the appartment saw was low voltage. Nothing was wrong with the wiring inside. And finally, if the fuses are blowing, would this not, in a marginal case, make the wires get hot, if there was not quite enough heat to blow the curcuit breakers?

In fact when she called me and told me the problem, I said "look outside and see if there is not a truck working at the junction box by the road". Sure enough, he was there, and when I went over to see what was up, he told me the transformer was bad. I have seen this in our shop where a meter in the meter room was deffective, and it blew all the breakers on phase C. We wired around it for awhile until they could change the meter.


We did a heavy up on our house from 60 AMPS,(this is an old house) to 250. No blown fuses now.

At the very least, when the connections start to go bad, then the situation deteriorates from there because of heat build up.

Sam
Author:  supton [ Tue Dec 18, 2007 9:48 am ]
Post subject: 
I'm not an expert on this, but some loads will draw more current if the voltage is not up to par, while others will draw less. Light bulbs will draw less current--the output gets dimmer--and therefore less power is used.

Inductive loads, on the other hand, tend to go up in current. I don't believe it is because the load is trying to pull the same amount of power as much as it is related to the back emf of the motor windings. Less power is applied to the motor initially, it spins slower, so less back emf is generated--while the resistance of the windings stays low. Resulting in higher current.

While resistive heaters tend to be inductive, I believe those circuits draw more power due to feedback mechanism like what you think--less voltage means more current, in order to have the same power output (as heat). As heater wire heats up, the resistance goes up, giving some level of output power regulation.

Electronics, though, can go either way. Old school stuff may continue to draw the same current until the voltage drops too low. New school stuff with switched mode power supplies and high efficency will tend to draw the same power though.
Author:  SlantSixDan [ Tue Dec 18, 2007 10:17 am ]
Post subject: 
Quote:
If the battary voltage drops, does not the load device wattage remain constant?
Well, yes, but in general this phenomenon is self-limiting: Line voltage might range from 12.3 to 14.3 or so under normal conditions with no restrictions (everything from "ignition off, cold day, multiple loads" to "engine running fast, few loads"). But usually we don't have a bunch of loads running with the engine off — not for very long, anyhow. So now our range is between about 13.2 and 14.3 (engine running, various speeds and loads). The increase in current draw from a device operating at 13.2v rather than 14.3v isn't enough to make any problems, and is accounted for in the design of the circuit. Obviously, anomalous conditions (such as a dead cell in the battery) can throw a wrench in this self-limiting mechanism, but an electrical fault of that nature gives obvious problematic symptoms and we tend to fix it promptly.

You're quite correct on the concept; you understand it just fine. It's just at the implementation level, it doesn't make a problem under normal/anticipated conditions.
Quote:
At the very least, when the connections start to go bad, then the situation deteriorates from there because of heat build up.
Yes, definitely. Corrosion → resistance → heat → more corrosion + more resistance → more heat → more corrosion + more resistance → more heat → more corrosion + more resistance ⇒ failure.
Author:  slantvaliant [ Tue Dec 18, 2007 11:47 am ]
Post subject: 
In my experience - mostly three-phase industrial applications - it's rare to find a lead itself to be warm (relative to the other phases) over its length. That is usually caused by a significant phase imbalance.

Here are some thermographs. Sorry, I couldn't locate a good phase imbalance shot on short notice. Note that white is hottest and black is coldest in these thermographs.

This is different: It could be a bad lead in phase B. The middle lead between the starter and fuses is hottest in the middle. Note that the leads at the bottom of the starter are much closer in temperature.
Image

A much more common fault is a bad connection. Not necessarily loose, but inadequate contact to carry the load properly. The damage done to the wire in this situation is usually localized near the bad connection. Like this:

Image
Author:  SlantSixDan [ Tue Dec 18, 2007 12:24 pm ]
Post subject: 
That bottom thermograph is closely representative of a lot of the damage I see (or hear of) when people put overwattage bulbs in their headlamps.
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