Plastic Classic Forum

I was looking at a maintenance manual for a USCG diesel engine and noticed that the electrical schematics showed that the negative leg of circuits was switched, not the positive, as I'm used to seeing. My Westerly was wired that way when I acquired it and I seem to remember somebody telling me that "they" had found that there was less corrosion in systems that are switched on the negative side. I don't know why it would make a difference, but I can only imagine it's in response to the more recently popular electron theory of electricity.

Can any of you help me better understand the advantage to switching the negative side of a circuit?

so all the wiring and components of the whole circuit (right up to the switch) are forever energized, just waiting for something to chafe through or otherwise short out for an instant catastrophe?

No thanks.

Wouldn't that depend upon whether you've got a negative or positive ground system? I recall that some cars were positive ground(!); why, I cannot imagine. Either way, you want to switch or fuse the side that will spread amperage through daintier things.

I think I've seen fuses recommended for both sides of the battery; I think that might be good for a boat that's got its battery at an unreasonable distance from the ground. Quetzal has about 8' of wire from both banks to busses, both sides. And the wires are bundled in close proximity.

The most exciting failure I've witnessed was in my '83 Volvo 242 GL Turbo. The heavy wire from the hot side of the battery was tucked neatly and snugly against the engine as it led from the right front down to the starter solenoid at the right rear of the engine. I was doing a conservative 60 or so, on the Schuykill Expressway, in rush hour traffic, just going under 30th Street Station, when the engine quit, all the electrics failed, and smoke began billowing up from under the hood. I coasted off at South Street, jumped out, got the extinguisher out of the trunk, got the hood open in time to see that the smoke had diminished. It was such a thorough short that the battery was destroyed and there was no more juice to heat things anymore. Just as well, there was no way that I could have gotten a wrench on the terminal quickly enough!

My Atomic 4 is wired the same way, its perplexing.
Afraid of getting shocked I always turned off the main switch before touching on the engine. One day I forgot and did not get a shock.
I guess if it’s good for the Coasties its good for everyone else, but like you I don’t understand.
Antonio

Quetzalsailor wrote:... my '83 Volvo 242 GL Turbo.

Ooh, classic .... metal :)

How can you get a perceptable shock off of 12 volts? Something else must have been getting you, like the coil.

I really work to avoid shocks! I could never understand how some mechanics would unplug one sparkplug wire after another, looking for the malfunctioning one.

Loved that car; 242 means it had two huge doors, unlike the 244 sedans and 245 wagons. I could get in and out of that car in comfort, and I could look to the side and not have a doorpost in my face.

How can you get a perceptable shock off of 12 volts?

Its subtle but its detectable. For me its more of a tickling feeling. Takes a few seconds to register usually and depends on how wet/ oily my hands are to begin with.

I really work to avoid shocks! I could never understand how some mechanics would unplug one sparkplug wire after another

Keep your hands off the metal parts. No path to ground = no shock. After a couple of jolts most of the better mechanics figure this out.

Can any of you help me better understand the advantage to switching the negative side of a circuit?

I have no idea. Wait five years and the new theory will be just the opposite.

In the case of the engine schematics, perhaps they did that to reduce the length of wire runs? If the switch is on the "negative side" of circuit then attaching the switch directly to the block migh alleviate a ground wire completely. The switch would be internally connected to the switch housing which in turn is connected to the block which in turn is grounded. Assuming a negative ground circuit of course.

As Mike suggested, I prefer the switches to be on the opposite side of the ground because of the reduced chance of a chafed wire causing a problem. A chafing ground wire that is headed for a ground connection anyway doesn't cause any problems with the new short cut. Avoiding conditions that cause wires to chafe is a good idea too.

Postive ground, negative ground... as far as the electrons are concerned it doesn't make a difference. Sticking with convention means no one is going to get a surprise. That's a good thing I think. Hook up a heavy amp hour battery backwards once and you will forever afterwards check to confirm the proper ground.

This is good. I imagined somebody would come back with the "duh" reason for switching the negative leg, thereby making me look dumber than I really am. While I always like to learn new things, it's comforting that it doesn't make much more sense to my fellow forumites than it does to me.

Without going nuts looking it up and confirming, what pops into my mind when considering negative vs positive ground is the effect of "depositing" material with poorer connections...

Electroplating uses DC current flow in poor conectivity (acid bath) and I believe the polarity has a role in deciding which material gives and which material gets plated... In the process the Anode is the possitive electrode and the thing to plating will function as Cathode, or negative electrode, which will receive metal electron from the solution, if we're plating with chrome, then this negative electrode will cover with chromium layer.

Also, think of DC stick welding - where a poor, sparking DC connection (striking an arc) causes the rod to give and deposit metal deeply into the weld surface - reverse polarity and the rod still gives, but the weld is a lot flatter and shallow.

Aluminum and other special metals specifically need to be reverse polarity welded...

So, I believe it is no incident and the polarity definately has an effect on material movement through media (ionized gas, acid, etc.) True, with sound, solid same-metal connections there is probably no effect at all from polarization, but as soon as there is a jump that needs to be made, there is a chance we'll move, or otherwise compromise, the positive side of the conducting metal..

If this compromise makes holes in car frames or sets up stray currents elsewhere, perhaps it's better to reverse polarity (to negative ground) and sacrifice the metals of the small components attached to said frame, instead.

Then again, I might be completely wrong.

My favorite car fire was a volvo too!! 245 wagon. Ground strap to engine failed. So now the engine was grounded though...wait for it....THE CHOKE CABLE!!!

Did you know that choke cables glow red hot when energized? They do...

They also run under the dashboard...

Which is how I also got my first ride in a side car when I got the fire out (dry chemical extinguisher...bad idea!) a guy with a sidecar offered to give me a lift home. SO that was cool!

Of course I was a dumb kid and cut and spliced in 6 inch wires where everything was burnt..using crimp connectors after cleaning out the dry chemical extinguisher. So now I had 4 dodgy connections per wire all snugged under the dashboard of the car.

Needless to say the electrics were sketchy after that.

The car eventually holed a block in a catastrophic failure around 300000 miles. When we pulled the head for fun...it still had crosshatching in the cylinders.

I'd like to find another one now.

Ok, with two Volvo dead short stories already in the thread it can hardly be considered hijacking, right?

Mine was a '68 142S with a pair of amplifiers in the trunk that I had wired (I use the term loosely) in. The main power supply cable ran direct from the battery through the firewall and under the interior carpeting, then out into the trunk. It was fused, but only once it arrived at the amps (wonder why they say to fuse as close to the battery as possible?). The chafing was under my (driver's) seat, caused by passengers feet, and the failure was immediately noticeable as the car filled with smoke so quickly that we were all three coughing and spluttering as we got out despite my pulling over immediately (on a university campus street where we were only traveling 25 mph to begin with).

I popped the hood once we were out and breathing normally again and found a glowing red snake going through the engine compartment, and immediately went to the trunk for wire cutters (old Volvos need tools permanently residing in the trunk). Just as I was about to cut the wire, my girlfriend (Sue) cried out, "no! don't! I don't know CPR!", then my friend said, "I do!", and Sue said, "oh, ok - go ahead". Ha! Of course we all know (even if only from this thread) that there is very little or no risk of anything bad from 12 volts - but I sure razzed Sue for awhile about having been ready to see me collapse and need resuscitation! Sheesh!

Perhaps more meaningful for boaters is the issue of chafe.

Dad had a wire chafe through in the engine space of the Morgan 27. It caused the entire bundle of wire to burn which of course shut the engine down and gave off a great deal of smoke. As is normal, the failure occurred in tight quarters, so tending the fire and raising sails and steering was quite the adventure.

I rewired my 1953 Studebaker Champion from the engine back to the taillights, back in 1969 when I was in the army in Hawaii. Driving back from the north coast of Oahu, in the very dark night that was normal in that clean atmosphere and unlit world, the lights began to flash off and back on. There would be a 'clink' from under the dash, the lights would go off, stay off for a moment and then you would hear another 'clink' and the lights would come back on. Gradually, the 'off' portion became longer than the 'on' portion of time. We stopped at a fast food joint and considered the problem. We realized that a wire must be 'bouncing' against something to short as the car traveled along. And we realized that the 'clink' was a breaker opening and closing the circuit. Obviously it was my new wiring, and sure enough there was enough slop in the wire across the back of the car, and in the hot exhaust system, to cause the occasional short. Very cool that the lights on Studebakers were protected with a breaker that would reset! (My 1959 Hawk had the same sort of breaker.)

Some of us have noticed that corrosion can cause interesting problems. Only Bosch would know why, but late '60s and early '70s Volvos and VWs had grounds that were connected under the battery (instead of the usual braided ground strap between engine and body). When the grounds would corrode and loose contact, circuits in my 1968 Volvo 145 S would operate in very different ways, for instance, pull the headlight switch on and the wipers would come on, but not the headlights. The house ground and all the electric fuel injectors were grounded under the battery in my '72 Volvo 164 E. Since the injectors ran at 2 Volts, that was wonderfully problematic.

Fuses are great fun in a corrosive atmosphere. Marine fuses are commonly the metal-tipped glass tube type; they often are carried in a neat, sort of sealed fuseholder and they do pretty well. My Balmar regulator uses the GM type fuses which are not so bad, either. Old Bosch stuff, on the other hand, were boat- or shuttle-shaped pieces of ceramic with cheesy lead-coated tips which were mounted in a multiple fuse holder with brass clips. The 200-series Volvos had this type and the fuel pump fuse habitually ran warm; with a little corrosion, this fuse would run very hot. Soon there would be an open circuit and the car would quit for lack of fuel. Coast the car to a halt, climb out, uncover the fuse holder which was by the driver's left foot, clean the contacts and stuff it all back together. After a number of years and episodes, the fuse contacts and adjacent holder would be quite burned and you could not get it clean enough. Go to a junk yard and find that all the wrecks had burned contacts for that fuse.

Hint!: Don't try to clean the contacts with steel wool without disconnecting the battery! The steel wool will flash into fire in your hands since it will short between the fuse holder contacts. Very exciting!