QUOTE(oakdalecurtis @ Jun 16 2023, 01:09 AM)
I always wondered why the didn’t just run a ground directly to the starter, instead of a convoluted trunk to tranny approach, but it was probably simpler and cheaper…
The reason this isn’t done is because it will basically double the voltage drop due to cabling. You want the voltage drop in cables minimized and as close to zero as you can by keeping cables as short as possible.
Here’s the math:
4 gauge battery cable is about 0.2533 ohms per 1000 feet at 70 degrees F.
0.2533 / 1000 = 0.0002533 ohms per foot
0.0002533 ohms per foot x 4’ cable = 0.00010132 ohms for the 4’ negative cable not counting resistance at the connections to sheet metal.
Average 4 cylinder starter draw is about 150 amps - can be higher when cold and trying to crank a motor using thick oil.
Voltage drop = amps x resistance
150 amps x 0.00010132 ohms= .152 volts dropped in that single ground cable when everything is perfectly new.
It is far better to use only a 6 - 8” negative ground strap between the body and engine/trans to cut that voltage drop down to almost zero.
A 0.152 volt drop in the negative cable doesn’t sound like much but remember you also have about the same cable length on the positive side for a total of 0.3 volts lost in cabling. This is not insignificant. Likewise this voltage drop will only get worse over time as the cable and the end connections corrode. Temperatures in and around the engine will increase resistance of the cable over ambient. And a starter working hard and pulling higher amperage will drastically increase it.
That is why OEMs don’t run negative cables all the way to the starter. Nothing to do with cost. Has everything to do with ensuring the system has minimal voltage drop and starts reliably.