 | |||||||||||||||||||||||||||||||||
|
|||||||||||||||||||||||||||||||||
|
Corrosion Can Cause Hard-to-Trace Electrical ProblemsPosted 9/3/2002By Jeff Bach
Oxygen, moisture and pressure combine to form a perfect environment for growing corrosion on a conductor. Add an electrolyte such as salt water or leaking battery acid to the exposed copper and you have created a great fertilizer for the turquoise fur that eats wires from the inside out and causes some of the weirdest symptoms and hardest-to-trace electrical problems. I say hard to trace because corrosion-related high resistance in an electrical circuit can be as unpredictable as an opossum in the road with your headlights on it.
Now, this wouldn't be so bad except for the fact that the three-way positive cable on the Northstar system ventures so far from the battery - with one leg wrapping under the cradle and coming up the back of the engine to the power distribution box, one leg going to the nicely situated alternator, and the third leg heading under the intake manifold to connect to the starter.
I like to change them any time I have to take one of these gang cables loose. The lead lug is only good for one squeeze, maybe two if you quarter turn it the second time, and the zinc-coated bolt is almost always rusted in the middle. Another good preventative medicine is coating the exposed areas of conductor with a sealer of some kind. I like liquid electrical tape. I used to get it in 4-ounce cans and used it sparingly, but now I buy quarts and paint like a maniac. The last thing I want to do is create a green hole in a wire that will leak in atmosphere and ripen in a couple of years (see Figure 4). This piece of wire came from an A/C relay feed circuit that cost the owner a lot of aggravation and money trying related parts. I got it when all the parts on the list had been tried, and some twice. The A/C worked great when the weather was cool or wet; he just couldn't get the compressor to kick on when it was hot (Figure 5). Figure 5 shows the current waveform of the relay coil when the compressor was requested during output cycling but the relay contact bar didn't move and the clutch coil current didn't happen. Figure 6 shows the same circuit during a functioning compressor cycle. This one definitely falls under the "intermittent" category. With only 7 mA of current added, the relay works (although it takes too long) and the A/C cools. Notice the level that the current finally achieves is almost the same as the current it took to pull the contact bar down. This is a big tip that high resistance is likely playing a part in this circuit. Normal relay current peaks at about a third higher than it takes to operate it on most General Motors Corp. products and is done in under 10 mS (see Figure 7). Once the wire is repaired and the relay current is restored, the waveform takes on the more familiar shape at 2 mS of "half-an-elf." Figure 8 shows all three waveforms stacked for comparison. Showing these waveforms to a customer who has been parts trying usually generates a comment along the lines of, "I never would have found that." Or some with a sense of humor will say something like "I knew that's what it was." One of the best comments I remember hearing was when the wife said, "I'm so glad you guys found the problem," then turns to her husband and says, "Where would you like to take me for dinner, honey?"
|
||||||||||||||||||||||||||||||||
Copyright (c) 1996-2008. Automotive Service Association. All rights reserved.
 | |||||||||||||||||||||||||||||||||
The three-cable positive circuit used previously on the Cadillac Northstar system tends to have more than its share of corrosion-related electrical problems. When you see a car like the one in Figure 1 come to the shop you just don't expect to open the hood and see a bunch of corrosion around the battery cables, but all too often it happens. The battery cables are ganged together and bolted to the battery with a lead spacer separating them. The through bolt for the cables threads into the blind lead battery terminal, which is just deep enough to allow the terminal ends to squeeze their dimples into the spacer washer and make good contact before the bolt bottoms out in the battery terminal. This arrangement by the factory works pretty well until, for some reason, the battery cable gets taken off. Now I know that, usually, most good technicians aren't taking the cables loose to clear power control module (PCM) memories or reset air conditioning (A/C) systems, so it must be the do-it-yourselfers or rookies doing it.
The strategic inaccessibility of the negative cable bolt in its burrow under the low side A/C line makes the positive cable twice as likely a target for anyone wishing to disconnect the electrical power. Now the repair is complete or 20 seconds has ticked off the clock, whichever comes first, and it's time to reconnect the cables. Now anyone worth his buffalo wrench set knows that the battery cables should be tight enough not to swivel when you tug on them. If they do, you can usually just put a little "moretite" on them and they'll snug right up. The trouble is, too much "moretite" and the battery terminal starts letting out the green wire miracle grow (see Figure 2).
To do this job right you need to figure on a good cleaning and flushing of the whole area followed by a baking soda wash to neutralize the acid, then a coating of some kind to keep it from rusting. Add a new battery to this job and you can preface the estimate to the customer with the proverbial, "Are you sitting down?" It seems most people tend to equate a number in their head to what a job sounds like it's worth. You know, a ballpark figure range - such as tires, for instance; $200 to $300 for most cars usually gets the job done. Or exhaust systems: generally $150 to $250 will put you in the range. But a corroded battery cable? Most people tend to guess a number under $100. Sorry, no cigar.
If an ounce of prevention is worth a pound of cure, just think what 2 ounces can do. That's about what these weigh (see Figure 3).
;){kind=link}
;){kind=link}
;){kind=link}
;){kind=link}