Hey Chris
read this and make a copy for your truck's electrical binder
http://www.madelectrical.com/electricaltech/onewire-threewire2.shtml How the THREE-WIRE Voltage Regulator Works
(the Turn ON & Warning Light circuit)
With the “THREE-WIRE” alternator model built for cars and trucks, one of the voltage regulator terminals is wired to an ignition switched OFF/ON source. And the OFF/ON circuit may also be used to operate a warning light at the dash. (The THREE-WIRE type can be wired with or without a warning light at the dash. The warning light is an option–not a requirement.)
REMOTE VOLTAGE SENSING, a THREE-WIRE advantage
The other of the two plug-in wires at the SI series alternator is the “voltage-sensing” terminal for the voltage regulator. Through the sensing-wire, the voltage regulator monitors electrical system voltage and makes adjustments to alternator output. The regulator will adjust alternator output as needed to maintain the place where the sensing-wire is routed (at about 14.2 volts). And the sensing-wire can be routed to a place remote from the alternator (downstream).
This “remote voltage-sensing” feature is often a big advantage when running with a factory type wire harness from the Muscle car period (60’s and early 70’s period cars and trucks). Then the alternator output will be adjusted according to what goes on far downstream from the alternator–at the main power distribution junction, which operates the whole car plus charges the battery!
A couple of very important considerations
regarding this factory-original type system, which uses a main power distribution junction, and remote voltage sensing.
(And of course M.A.D. did not invent the system, we are just observing operation of a system that was successfully used for many years. And it is a system that we understand well enough to up-grade, make it friendly for addition of accessories, and make it stronger to accommodate a more powerful alternator operating more powerful accessories.)
(1) The system only works properly when the “battery charging wire” connects from the junction to the battery. There will never be a wire connected from the alternator directly to the battery.
By now the thought may have occurred, that a possible installation could include a heavy gauge charging wire from the alternator directly to the battery. And then leave in place the original wire from the alternator to the factory main junction. Route the voltage sensing wire to the junction, in effort to keep the lights and ignition happy. It’s true that then voltage regulator would maintain the junction at the 14.2 level. However, the fault in the plan is with the voltage drop that will occur between the alternator and the junction. As was previously discussed, the system will compensate by raising the voltage level at the alternator. The problem would be overcharging the battery through the separate charging wire, as voltage at the back of the alternator could be over 16 volts.
(2)The second important consideration of the typical Muscle Car period factory system is the effect of the long battery charging wire (which routes from the junction to the battery). It’s a rare occasion when a little resistance in wiring can actually help us, and this is one of those rare occasions. A small amount of resistance in the “charging wire” leg of this system will serve as a cushion. The amount of resistance will be much too small to be measured with an ordinary ohmmeter, as accuracy of the ohmmeter will only be good to about a plus or minus 0.2 ohm. But the amount of resistance can be calculated after measuring current flow and voltage drop.
To understand how this “cushion” effect with a small resistance at the charging wire works, we must first have knowledge of how batteries behave when being charged. When recharging a low battery, battery voltage will be low, and current flow (amps) to the battery will be a large amount. As the battery becomes charged, battery voltage increases, and it will accept less charge, so charging current (amps) will decrease. Eventually during charging, battery voltage reaches the level of the voltage setting at the charger, and the battery stops accepting all but a very small amount of current.
(See more about how discharged batteries behave when being charged, in photos and captions, at our “RE-CHARGING LOW BATTERIES” page, in this tech section. And see more about special maintenance battery chargers at our BATTERY CHARGING for MAINTENANCE and STORAGE page in this tech section.)
When working with more than a 60 amp alternator, the actual amount of resistance at the “charging wire,” and also the wire from the alternator to the junction, should always calculate to less than 0.1 ohm. The actual amount of resistance will vary with wire length and wire gauge sized used in various models, and the amount of resistance will also change with temperature. (A warm wire conductor will have more resistance than a cool wire conductor.) A new or good condition, factory built harness on a Chevy from the Muscle Car period will have about 0.02 to 0.05 ohm resistance at the wire from the alternator to the junction, and about the same at the charging wire from the junction to the battery.
there is more to read @ mark's MAD Electric site.
above link.
this should clear it up for everyone
