Yup. It would be very unusual to not connect the negative battery cable directly to the engine. In theory it doesn't matter but in actual practice it can make a big difference.
Think of the plumbing in your house. If you have a well and water pump and the water comes in at the south end of your house, a pipe will carry it all the way to the north end to the kitchen. From there you tap off and go all the way back to the bathroom at the south end. Will the toilet fill with water? Sure it will but it will take a long time due to the loss in pressure caused by the resistance to water flow presented by the pipe. It would make more sense and be more effective to tap off the pipe right where it comes into the house and go to the bathroom right away.
You're doing the same thing with your ground cables. Besides the loss of power due to the resistance all cables and wires have, every mechanical connection in the circuit presents additional resistance to current flow. That isn't a big deal in low-current circuits like for light bulbs and radio speakers, but it becomes a real big deal in high-current circuits like for starters and generators. You can use an ohm meter to measure the resistance in smaller wires. A typical value for lets say a rear tail light wire might be two or three ohms which won't have much affect on that circuit. In a starter circuit.03 ohms would be catastrophic and would prevent the starter from performing properly, but that value is WAY too small to measure. The ohm meter's leads have much more resistance than that. What we can do is measure the RESULTS of that resistance. We do that with what's called "voltage drop" tests.
Think again of that water pipe in your house. Lets say you have 50 pounds of pressure where the pipe enters the house. No matter where you measure, anywhere in the house, you'll find 50 psi, as long as nothing is turned on. Once you flush the toilet and water flows to refill the tank, it takes a minute for it to fill up. Why is that? It's because the pipe has resistance to the flow of water. If you had a larger pipe it would have less resistance and the water would flow faster. With the little pipe you'd still have 50 psi at the well but perhaps 30 psi at the toilet, but only when the water is flowing.
You can see the same thing with a garden hose. If the nozzle is turned off you'll have the full 50 psi there, even if there's a kink in the hose. It's not until you open the nozzle that the restriction, (resistance), becomes a factor and the pressure drops so much that the little water that does make it through has so little pressure that it dribbles on your shoe. Boy, I do paint a picture!
One final thing to consider with water is the size of the pipe required. A tiny pipe is more than large enough to run a drinking fountain but it wouldn't be much use for feeding a fire hydrant or filling a swimming pool. The size of the pipe has to match the application.
Now equate that to electrical current flow in a wire. Since all wires have some resistance we need to reduce that to the point that it doesn't hinder the operation of the circuit. Starter circuits for small four cylinder engines can draw up to around 150 amps. Just like you would get lower pressure at the toilet, only when it's filling up, voltage, which is electrical pressure, will be lower at the starter due to the resistance in the cables.
The industry standard is a starter motor needs a minimum of 9.6 volts to operate properly. A fully-charged battery starts with 12.6 volts. There is a lot of resistance inside the battery that we can't do much about. It's because of that resistance that you see the RESULTS when you crank the engine or perform a load-test on it. In this story lets say 2.0 volts is dropped inside the battery when high current is being drawn. If you measured the voltage right on the two posts, that 12.6 volts would drop to 10.6 volts during cranking. That is still above the 9.6 volts needed to run the starter.
Now is when we have to factor in the resistance in the cables. Remember that resistance is too small to measure directly with an ohm meter but just as water flow through a pipe causes there to be lower pressure at the toilet, current flow through the resistance in the wire causes a lower voltage to be seen at the starter. That drop in voltage is much easier to measure directly.
In the water pipe you would measure the pressure at the well compared to atmospheric pressure, then you would measure at the toilet compared to atmospheric pressure, then calculate the difference. Wouldn't it be easier to just measure the pressure at the well compared to the pressure at the toilet and get a direct reading? That's what we do with voltage drop readings in a circuit. Instead of measuring from ground to the positive battery post, then ground to the starter terminal, and calculating the difference, we put one meter probe on the positive battery post and the other one on the starter terminal. You WILL find 0.0 volts at first because both probes are at the same point in the circuit. It's when a helper cranks the engine that the results of the resistance in the cable will show up as a drop in electrical pressure which is a "voltage drop".
I hope that made sense. We actually do a voltage drop test in the positive half of the circuit and again in the negative half, meaning the ground cables. There are a couple of other reasons this test works better than taking individual readings but that would require typing another novel.
Now for the important part. The industry standard is you can not have more than 0.2 volts dropped across any one mechanical connection and no more than 0.4 volts total in the positive half or the negative half of the circuit during engine cranking. Mechanical connections include where the cable clamp is bolted to the battery negative post, and where the other end is bolted to the engine block. In this case there are only two mechanical connections. We don't include where the cable is attached to the clamp or where it's crimped to the terminal on the block. We WOULD look at those if testing showed more than 0.4 volts total voltage drop.
The positive half of the circuit can get a little more involved because it has more mechanical connections, and that is the circuit that usually causes the most problems. You still have the cable clamp bolted to the battery positive post and the other end bolted to the starter solenoid which is a big switch. There are two contacts inside the solenoid that are mechanical connections. On some starter designs we can't get at them and all we can do is deduce they are bad from all our other measurements and calculations. Older Ford solenoids sit right up on the fender where it is easy to take voltage readings. Most other starters have the solenoids right on top of the starter where we can get to the terminals to take readings but it can be hard to reach some of them.
To take the readings in the positive half of the circuit it is fastest to take the first reading between the battery positive post, (not on the cable clamp), and the braided starter motor cable where it bolts to the solenoid, if it's accessible. In this instance that second probe is in effect on the ground side until the solenoid is turned on and the starter is cranking so you will measure full battery voltage, not 0.0 volts. It's when the helper cranks the engine that this reading becomes valid. If you find 0.2 volts or less, you're done with that half of the circuit. There's no way any individual connection can be dropping more than that. If you find less than 0.4 volts, that is acceptable for the entire half of that circuit but you still would want to test each individual connection. Each one can have no more than 0.2 volts dropped. When you have numerous connections one could have 0.3 volts which is not acceptable, and all the others combined could have only 0.1 volt. If you find more than 0.4 volts, there are connections that must be inspected and cleaned. You may find every one drops less than 0.2 volts, which is good, but if all together they drop more than 0.4 volts, that's not good.
Now, ... Common sense must come into play too. If you do this test and find 0.5 or 0.6 volts dropped in one half of the circuit and the starter is working fine, don't mess with it or try to fix something that doesn't need fixing. It's when you have a problem and need to find the cause of slow or no cranking that this voltage drop test becomes valuable.
Nothing in any car draws more current than the starter so that is the one that must have the fewest possible mechanical connections and voltage drops. By running your ground wire to the frame or body, then to the engine, you added another mechanical connection and another potential cause of slow cranking. This is similar to tapping off at the kitchen and running back to the bathroom in hopes of taking a shower. You might get wet, but think of how much better it would be if you tapped off right where the pipe comes into the house. The shower would be much more effective.
There is one exception that really isn't exactly an exception. On some cars the negative battery cable has a spot in the middle where a terminal is crimped around it and is bolted to the body. That is not a mechanical connection that is involved with the starter. The cable still goes directly to the engine block. That terminal is simply the point at which the circuit taps off and goes to the rest of the car. In the water analogy the main pipe would go directly to the bathroom, and a smaller one would tap off and go to the kitchen.
Friday, August 2nd, 2013 AT 11:04 AM