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The Trend Setter

Part 3                ONE-WIRE

                                             compared to

                                                              THREE-WIRE        ALTERNATORS




What Happens when the ONE-WIRE cannot do Remote Voltage-Sensing

All voltage regulators for alternator systems will have a “VOLTAGE SENSING” terminal.  The voltage regulator for the “ONE-WIRE” system gets the voltage reading from within the alternator, and it will maintain the alternator out-put terminal at about 14.2 volts.  Lack of compensation for voltage drop that will occur with a long wire routed to a “MAIN POWER DISTRIBUTION JUNCTION” is the problem with being able to read voltage only at the alternator.  In many factory-original wiring systems, the entire electrical system will draw power from the main junction, and the battery will charge from the main junction too.  If we have a 2.5 volt drop in the long length of wire between the alternator and the “main junction,” and start out at the alternator with 14.2, then we only have 11.7 volts at the main junction.  Expect dim lights, weak ignition, and slow electric radiator fans with this system being powered by a “ONE-WIRE” alternator.

Please note that simply disconnecting the original alternator wires, and then installing a heavy cable from the alternator directly to the battery, will only make the alternator effective as a “battery charger.”  (That happens when running a ONE-WIRE alternator with many factory layouts.)  Of course we have to charge the battery, but what about routing power from the alternator to the electrical system?  (ignition, lights, and accessories)  Power would have to flow from the battery to the junction via the old “charging wire.”  And often in a factory-original type harness the “charging wire” is even longer than the wire from the alternator to the junction.  And so the result of this ONE-WIRE method would be dimmer lights and overall weak electrical system performance; in fact often worse performance than with the original, correctly wired, small alternator that was standard equipment on the old cars. 


Okay, all these modern accessories that come with instructions to connect a wire directly to the battery POS, and the instruction to connect a cable from the ONE-WIRE directly to the battery, has resulted with a clutter at the battery area.  And thanks to the 100 amp ONE-WIRE alternator (a high rate battery charger indeed), the wires are a corroded, unreliable, mess! 


The part#CN-1, insulated terminal block, from M.A.D.’s catalog, is a sensible method of connecting a few wires together.  It has a re-enforced shield above it to shed water, and separators between wires.  It’s a great little organizer in general.  And it will be mounted remote from the battery, where it will stay free of corrosion. 

Value of Remote Voltage-Sensing with THREE-WIRE

The well laid-out factory system will have the voltage regulator taking the “VOLTAGE SENSING” sample directly from the “MAIN JUNCTION,” through a wire dedicated to this function.  And that was true with both the external and the internal voltage regulator systems.  And it’s the reason that the old external voltage regulator stayed at the driver’s side when the alternator got moved to the opposite side, beginning with ’69 models (Chevy V-8 engines).

As lighting or any accessories are switched ON, more power is drawn from the main junction, which would lower voltage at the junction.  But the voltage regulator will increase alternator output as needed to maintain the 14 volt level at the junction.  If we have a 2.5 volt drop in the wire between the alternator and the junction, then the voltage regulator will make the alternator produce 16.5 volts to compensate for the voltage drop with routing of power to the junction.

It’s a somewhat “spongy” system, but it does work well and the alternator doesn’t know the difference.  Expect bright lights, strong ignition, powerful accessories, and a properly charged battery when this system is wired effectively.  When running with the factory-original type wire harness, clearly it is an advantage to maintain the “MAIN POWER DISTRIBUTION JUNCTION” at 14 volts.


When voltage available to electrical parts drops about 10% below optimum level, the performance of the parts will typically drop by about 30%.  If voltage delivered to parts is only a little low, performance can be very weak.  (Lights will be dim, electric fans will not move as much air, fuel pumps will be low with fuel pressure delivery, and so on, when these parts operate at low voltage.)  Most automotive electrical parts are rated at about 14 volts.  And the parts will deliver best performance and last the longest when operating at 14 volts. 

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.


For our calculations, we will use a 0.05 ohm resistance, which is a middle-of-the-road number, for an average Chevy factory harness.  The battery will be charged from the junction in the wire harness, and voltage at the junction will be maintained at 14.2 volts.  With the battery in a fully charged condition, it will accept less than 1 amp, from a 14.2 volt source.  We will use 1.5 amp for the charge rate, which is a battery approaching fully charged condition.  The math formula (from Ohm’s Law) can be used to easily calculate the amount of voltage drop that will occur with the small amount of resistance at the “charging wire” between the junction and the battery.  The math formula gives us that Voltage (drop) = current flow (amps) X resistance (ohms).  With the numbers used for this example, 1.50 amps X 0.05 ohms = 0.075 volt(drop).  Rounded off to the nearest 1/100th of a volt, that is a 0.08 volt drop.  And so, with 14.20 volts at the junction, we have 14.12 volts at the battery, which is fine.  (Individual voltage regulator settings will vary more than 0.08 volts.)  The small amount of resistance in this wiring system does not have significant effect on charging the battery, when the battery becomes fully charged.  Voltage drop is a very small amount, when the current flow is a small amount (through the small amount of resistance at the charging wire).


When charging current is delivered to a discharged battery at a full 14.2 volt level, the battery will accept a large amount of current (amps).  And when charging a large capacity battery, if the battery charger or alternator is sufficiently powerful, the amount of current can be a large amount.  In that case we can expect the battery to get warm and produce excessive gas (which results with short battery life and a corroded mess at the battery area).  The battery charger or alternator will produce a significant amount of heat too, so it will need a good cooling capacity.  (With both the battery and the alternator, it’s back to that efficiency loss situation where heat is the by-product.

The low battery being charged by the alternator is a situation when the small amount of resistance in the charging wire will help us.  Power from the junction will reach the battery at a reduced voltage level; which automatically reduces the charge rate.  We could do calculations for this situation, like we did above with demonstration of system performance when the battery is fully charged.  However with the discharged battery situation we don’t have constants.

Resistance in the charging wire causes voltage drop with current flow to the battery, and the voltage drop reduces current flow.  Current flow is also reduced as the battery becomes charged.  We begin with a discharged battery that cannot produce full voltage on its own.  Then as the battery accepts current it becomes more charged and produces more voltage, and then the battery accepts less current flow.  In this battery being re-charged situation, the system is not “static,” it is a changing and self-adjusting system.  The small amount of resistance at the “battery charging wire” makes it is a very gentle and “forgiving system.”

The system is conservative as it helps to prevent battery gassing, battery overheating, and alternator overheating. The small amount of resistance in the “charging wire” does not prevent the battery from reaching a fully charged condition; with a small resistance it will just take a little longer to top off the battery with charge.  (And when using a 100 amp alternator, it can be a very good idea to slow down the charging rate!)

And…while all the above is happening, ignition, lighting, and accessories are being powered-up from the main junction, and at a 14 volt level–and 14v regardless of battery condition! (The exact voltage level will be the setting of the voltage regulator, and we assume that the alternator is capable of operating all the accessories plus charge the battery).  All aspects, considered the factory layout using a “main junction” and “remote voltage sensing” is an incredibly effective system.  Although when the alternator is replaced with a more powerful model, and added accessories draw more power from the system, then the wiring for the system must also be up-graded.

It has never been a good idea to re-charge a low battery with the alternator.  But there are those times when we are caught by surprise… It’s easy enough to accidentally leave the lights on when far away from home.  Or an electric fan run-on function can run the fans longer than the battery can stand.  Eventually the time can arise when we need a jump-start, and then we will drive to our destination.  And with a high powered alternator in place, these are the times when a system like the factory lay-out with a properly wired THREE-WIRE alternator system could really help us out, by slowing the charge rate.  (A low battery should be re-charged with a workshop type charger.  The alternator is intended to keep the battery topped off and operate the electrical system.) 

A POSSIBLE PROBLEM with installing a more powerful alternator and using the existing factory wiring is that when those older cars were built, the wire gauge sizes were calibrated for much less powerful alternators than we are using today.  And back when those cars were built, we were not operating so many powerful accessories.  Nowadays electric radiator fans are a common example of an added accessory that will strain the existing system.

Voltage drop resulting from a small amount of resistance spread out over a long length of wire will behave quite differently than a small amount of resistance at a “bottleneck.”  With voltage drop, the electrical energy lost is converted to heat.  (Again, energy is never lost or destroyed–only converted.)  In the case of a short length, under-capacity wire, or in the case of a poor connection, we have resistance at a small area, and this bottleneck effect can result with destruction of the part through “thermal run-away.”  When we spread the small amount of resistance out over a long length of wire, the voltage drop and heat generated is also spread out over the length of the long wire; and then the heat can be dissipated without overheating the wire.  But as with stress limitations of any machinery part, there are limitations with this wiring system too.  (A thorough discussion and explanation of “thermal run-away” effect is contained in the “tech is made simple” book, available from M.A.D.

A typical ’65 Chevy was often shipped with only a 37 amp alternator, and the wiring was adequate for that much output.  The wire from the output terminal of the alternator was often only 12 gauge, with early alternator systems.  The wiring will have to be up-graded on those early cars, when we use more powerful alternators and accessories.

The current capacity of factory systems in various cars from the Muscle Car period will differ considerably.  But Chevy is more popular than ever before; we will use Chevy for an example.  The ’69 –’71 Chevy with V-8 engine uses a system like the diagram in this feature.  The passenger side mounted alternator has a very long wire from the alternator to the main junction.  The junction is a “welded splice,” in the loom, at the driver’s side.  The wire routing is a very long semi-circle between the alternator and the junction.  And the “battery charging wire” is also very long, to get from the junction back to the battery at the passenger side.  At least in these years of Chevy cars, the alternator output wire and the battery charging wire are both 10 gauge wires.  A 63 amp model 10SI works very well with existing wiring in that system, and the factory built 78 amp 12SI will work well with this system too.  But remote voltage sensing from the junction should always be used with up-grades to SI alternators.

When adding electrical accessories with significant current draw, and the SI alternators to this existing Chevy system, it is very important that the accessories are powered from the wiring at the junction–rather than connected directly to the alternator or to the battery.  But the factory “welded splice,” hidden in the wire harness is not friendly to work with. 

The photo above shows a close-up view of an actual “junction” from a Chevy wire harness.  Four red, 10 gauge wires were all connected together, at what we have labeled a “junction” in our system diagram.  (The GM engineering department refers to the junction as a “welded splice.”  “Buss-bar” would also be correct terminology with this discussion.)  This type of junction, hidden in the wire harness, is very reliable, and it was economical to manufacture; but it is not friendly for up-grade work.  The “junction” does not lend itself to system modification or adding wires for accessories. The insulated terminal block (part#CN-1, from M.A.D.) seen in the photo below is a very friendly to work with “junction,” and it should be used when up-grading this system. 


On the “flip side” of our systems comparison, expect that with a 100amp ONE-WIRE alternator, wired directly to the battery, and the same ’69-’71 Chevy wiring system, performance of the electrical system parts will be significantly reduced.  Wiring from the alternator directly to the battery will charge the battery, but that could be done with a 37 amp alternator too.  The performance problem will come from “back-feeding” power through the old charging wire to the junction.  Driving with lights and factory accessories switched ON, we can have a 30 to 35 amp current load through that 0.05 ohm resistance at the old charging wire.  And then we will have a 1.5 to 1.75 volt drop in the old circuit, which is now feeding power to the junction.  This situation would result with the junction delivering power to various parts of the electrical system at only 12.45 to 12.7 volts, which is no better than just running off the battery!

Also, if we ever have the need to recharge a discharged battery while driving, the ONE-WIRE alternator with a heavy cable connected directly to the battery makes for a high rate battery charger system.  And as previously discussed, fast charging batteries can be destructive.


We often need more powerful alternators than the old cars came with, because we are often installing items that use more electrical power.  But we must do a good job of distributing the electrical power from the alternator to the various parts of the electrical system.  And while installing a ONE-WIRE with a heavy cable directly to the battery will keep the battery alive, it does not always do a good job of supporting the electrical system.  A powerful ONE-WIRE can also be abusive when re-charging a low battery.

It’s obvious that the properly wired THREE-WIRE ALTERNATOR can do a lot more for us than the ONE-WIRE.  The comparison is in parallel with thoughts of engine tune-up mechanics.  There are those tune-up mechanics that are perfectly contented to work on slow, no fun or frills, four door models–And never do more than change spark plugs and other parts, and then verify that the engine has no misfires.  (Although the engine may still be sluggish, or run a little on the warm side when the spark advance and air-fuel ratio is out of focus.)  And then other tune-up mechanics are most happy when tuning an engine for maximum performance.  And they do very well at getting the most from an engine.  The distributor spark advance system gets custom re-curved, the carburetor gets custom calibrated, the valves get properly adjusted, and so on.  The ONE-WIRE is like the tune-up that keeps the car running, but does not optimize performance.  The THREE-WIRE provides the option of getting the best performance from the alternator.

The only disadvantage of the THREE-WIRE alternator compared to the ONE-WIRE alternator is that a little more knowledge will be required to properly wire the THREE-WIRE into a particular system.

At M.A.D., we have that only disadvantage covered.  We do have extensive experience with wiring work and electrical service on most of the popular models of the older cars and trucks.  And M.A.D. prefers to sell alternator wiring kits directly to the customer (and also many other of our popular kits).  It gives us the opportunity to learn a little about how the car is equipped.  And then if needed, we can customize the kits for best performance and provide the most simple of installation methods.  The factory wiring systems vary among the many makes, years, and models.  And we have batteries at different capacity, and alternators at different capacity, and various accessories that can use only a little power or a lot of power.  Therefore, of course there will be times when we need to customize the alternator wiring system.  (As with choosing a camshaft, there is not one grind optimized for all applications.)

  • Alternator wiring kits from M.A.D. provide hook-up for original ALT and GEN warning lights at the dash.  We provide option for adding a Warning Light at custom dashes too.
  •  And when a system will benefit, we always provide the correct hook-up for “remote voltage sensing” function.
  • M.A.D. also provides the correctly calibrated Fusible Link wire kits for short circuit protection.  (Fusible Links are the ultimate in reliability for short circuit protection in High Amp systems.)  Fusible links should be installed in the original dash area “main power-up” wire, high current draw accessories, and at the alternator system.


  •  M.A.D.’s part #ALT-1 alternator wiring kit or M.A.D.’s CS-130 alternator wiring kit.  (All the best of wiring parts, and great installation & troubleshooting manuals are included in the kits.  They are in M.A.D.’s catalog)
  • Part # TB-1, “tech is made simple” book available from M.A.D. catalog.  It teaches craftsmanship techniques and has other information needed to install custom wiring.  (especially important with High Amp systems, as with alternator wiring)
  • TOOLS  A good wire terminal crimp tool is of critical importance with alternator wiring.
  • PART #  CN-1, insulated terminal block from the M.A.D. catalog is the best part available to serve as the “main junction,” when installing a custom version of the system we have discussed with this feature.


We are working on a parts kit for a new “POWER DISTRIBUTION SYSTEM.”  The new system will perform equally well with ONE-WIRE and THREE-WIRE alternators.  The system will work well with 63 amp model 10SI alternators, or equally well with a 140 amp custom alternator.  The new system will do a very good job of delivering power from the alternator to the various parts of the electrical system.  And, something that the industry has never attended to, the new system will be gentle to the battery and the alternator when working with these modern high-powered alternators and accessories.

The new system is simple to work with, and it has been proven to be very effective.  (We have already been doing it for many years, with custom wiring and High Output alternators.  Many years ago it was the only way stock-type alternators could last in abusive motor home applications.)  The new system can be adapted to the factory-original type wire harness, or it can be used with custom wiring systems.

It delivers full power to all parts of the electrical system.  And it will reduce the chance of alternator overheating and “boiling” batteries, should a low battery be recharged from a High Output alternator, while driving.  The new system is an improved and more adaptable version than the factory wiring discussed in this feature.



  • See information with M.A.D.’s part # ALT-1 kit, it’s in the M.A.D. ELECTRICAL catalog at this web site
  • VOLT gauge compared to AMP gauge, in the electrical tech section of M.A.D.’s web site (you’re in the M.A.D. site now)
  • Visual identification and features of the 10SI and 12SI alternators, and “CLOCK” positioning of the wire connections.  (It’s also in this tech section of M.A.D.’s web site.)
  • AutoMeter for many models of quality VOLT gauges, and discussion of AMP vs VOLT gauges in the TECH TIPS/FAQ section, at www.autometer.com 
  • POWERMASTER for ALTERNATORS at www.powermastermotorsports.com

   ONE-WIRE move over!



    The THREE-WIRE system is the winner here!



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