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This is A Very Good Thing. Better yet, ground the radiator and heater core together to the same good body ground. In the marine world, they call this "bonding". No, it's not a touchy-feeley thing. The idea is to make sure all parts of the cooling system are at the same electrical potential. If there's no voltage difference, there's no galvanic corrosion. (It's NOT "electrolysis", folks!)
Ok, interesting: here are two paragraphs that I find key from my Google search.
Fix that one so the voltage in the water goes away, but then continue checking the other circuits until all the fuses are re-installed. Then start your engine and check again. This time you have the charging system in the loop (alternator).
One thing to watch for is the main power return paths. In a 12V negative ground vehicle, the vehicle chassis is assumed to be the ground return path - every thread where electrical problems are discussed, you will see at last one poster mention that you need to check your grounds.
WARWAG: Having 0.22V of the power (on a 24V system that's 1%), come back through the cooling water is REALLY bad.
Nathanater: Check your engine to battery ground strap, and make sure your ballast resistor (if you still have the 24/12V system) isn't corroded or shorted to ground. Then check that the glow plug controller and contactor/relay is properly grounded.
For an electrical system, we want to keep all currents in metal that was intended to carry current - this goes for any electrical system; Car, Boat, Truck, Airplane, or Building (Home, Office Building, Industrial Plant, etc...). You get a voltage in water if there is a voltage potential from one point to another, and this indicates that there is a resistance, or something that is preventing the flow of power between those two points which causes the current to find another path of least resistance (and if it uses the water, that means the water is more conductive than something else). Galvanic corrosion is caused by dissimilar metals in contact with water, this simple mechanism causes a current to flow - but changing the chemistry of the water (specifically the pH or the compounds in it) can increase the ease at which electrons flow (current).
If you disconnect your battery (engine not-running, ignition off), and you see a voltage between the water in the radiator and the battery negative terminal, then your water and the cooling system components are likely a direct cause - flush your water system, put in a sacrificial anode (zinc), and put in fresh water (or use pre-mixed 50/50 antifreeze). If you mix your own, and your tap water is "hard" or more acidic, you would be better served by using distilled water (not de-ionized water) to fill your cooling system. Flushing is a good idea too since with age and galvanic corrosion and just general vibration, you can have small metal particles in the water which will improve the conductivity of the water, almost as much as the pH being off. Also, you are draining and replacing your coolant every two years... right?
If you connect your battery (again engine not running, but this time ignition on), and you see a voltage, try pulling all of your fuses and check again. If your fuses are out, there should be no systems powered - so it could be a main feeder cable that is shorting somewhere (or about to short). Then one-by-one install your fuses, turn on the load attached to that circuit, and check your water for voltage. If one circuit causes the water to show a voltage, then you should have narrowed it down to that circuit and its loads. Check the wires for cracks, and all of the devices on that circuit (switches, motors, light bulbs, etc) for good clean grounds. If you find one bad circuit, don't stop looking!generating power) so this should show the quality of the grounds for that system. Then with a helper, and with FIRM pressure on the brake pedal, shift the transmission through the ranges and put tension on the drive train (idle or very low throttle is good). This will make the drive train and suspension a good ground path by pressing shafts against bearings, gear teeth against gear teeth, and shifting anything that is held by a rubber isolator towards a metal mounting point (you might see this as a wonky gauge going down the road). If any voltage appears in your water here, that means your engine/transmission-to-chassis-to-battery grounds are spotty (i.e. engine mounts make terrible grounds
In general, you should have a thick cable from the battery positive to the alternator, and a cable from the alternator or battery positive to the starter motor. There should be a cable from the low-battery (if you kept your 24/12 system) negative to the engine block, and another cable either from the low-battery negative or the engine block to the frame/chassis. One last cable should go to the main fuse block from the appropriate battery.
Anything that is not specifically a + voltage input on a device (or the wire to the fuse to the switch to the device) should be grounded either through mounting holes, or even better, with a dedicated ground strap. The direction that the current passes is also very important - if your water can be a path to ground, you need to break that circuit; insulate the heater core from the body, and provide a return strap to the engine block, and do the same for the radiator, insulating it from the body and core support, and only include a good strap to the engine block. This makes it impossible for the current from the body or frame to pass through the water circuits on the way to the alternator, and ensures that all parts of the water system are at the same voltage potential (and if you strapped your engine to the frame to the battery - that should be "ground" or 0-volts).
In an ideal world you would want to be able to map out your electrical system and have it look like a tree - where the truck hits the ground is your battery and alternator, and every branch out from the trunk should have a fuse - and no branch should touch any other branch once it's split. Your positive and negative distribution should be mirrors of each-other. If power goes from a device on the engine to the chassis, there should be a ground from that device to the chassis. If power goes from a device on the chassis to the body, there should be a ground from that device to the body.
All of your painted surfaces should be maintained so that the "presence of water" is prevented at the metal - which will act to limit the electron movement (galvanic corrosion). There are certain metals that when in contact will lose electrons and thus material mass (these are anodes) to the other metal (these are cathodes). Picking fasteners (bolts, nuts, screws) that are compatible with the surfaces that they are attached to is important too. You want a larger anode surface or mass to be in contact with a cathode.
In fact, MIL-STD-889B Notice-3 has some good guidance about material compatibility, and I see it referenced frequently in the private sector. This spec has a handy chart that show how a large list of common materials interact with each other in different environments.
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I both agree and disagree with this. You must be from NNSY or similar, because i received the same training. Galvanic corrosion is an electrochemical process that creates electricity. like a battery, or dissimilar metals of a boat sitting in salt water corroding by creating their own voltage. this is galvanic corrosion.
Well, yes, it's oversimplified.
