Monday, Jan 20, 2014 at 20:07
It has to do with the voltage sensing requirements for the ECU.
The ECU needs to know load levels on the system in order to control alternator output (contrary to what has been said elsewhere, it is not simply temperature control).
It does this by a variety of sensors and gets its info. via the CAN Bus in most cases.
If you bypass the main earth wire from the chassis to the battery, then you will, in many instances, bypass the current sensing facility which will leave the ECU with a lack of info. on real load conditions.
A case in point is the later model Ford Rangers which have the sensor on the negative cable adjacent to the battery terminal.
For that sensor to give accurate load indications, all currents in both directions need to be accounted for, and that won't happen if your extra neg. cable goes straight to the battery.
The list of vehicles with ECU controlled variable voltage alternators is extensive.
Holden Commodore range from 2006 VE up, Mazdas - 6, BT50, Nissans - Patrol V8, Navara, Pathfinder, Hyundai Santa fe, Range rover, Landy Disco, Subaru Forester, BMW X5, Pajero, Ford Ranger, etc.
Voltage outputs from these can vary from 15v down to 12.3 and sometimes a complete shut off.
As an aside, provided a chassis connection is made correctly (operative words, clean, tight, and sealed against moisture ingress) you will get far less circuit voltage drop using the chassis as the return path than you will using a dedicated cable.
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