3 wire to 1 wire alternator

[Swampyankee74]

I hope this pic is better. I labeled a wire "A" I was planning on tying "A" to the red wire that went to the alternator which goes back to the firewall junction box, splits and goes to the bulkhead. Please correct me if that's wrong.

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[Swampyankee74]

I just spoke to a friend who mentioned to run the 6 guage wire from the alternator with a 200 amp midi fuse in the line to battery positive. Then go from positive terminal on your battery down to your starter like normal. And coil your 3 old alternator wires up and dont use them. Said everything should function just fine. That sounds right and super simple. That would feed power from the starter up to the junction block on firewall. Does this sound safe and unlike a fire waiting  to happen?

[bd]

Wiring it that way would work if you had a voltmeter but will defeat a factory ammeter.  Sometime this evening or tomorrow I'll create and post a diagram (or modify the existing diagram) that illustrates how you can integrate the alternator into the factory wiring harness so that the original ammeter continues to function.

[Swampyankee74]

That sounds excellent bd! Are you saying it would fry the ammeter or just wouldn't function? Yea, it makes my head is spinning a little.

[bd]

The ammeter would indicate zero and battery discharge only.  It would never indicate the battery charge current.

[VileZambonie]

IIRC there are two 4 amp inline glass fuses for the ammeter on each wire. One wire comes off the fusible link at the starter solenoid and the other off the junction block on the firewall. The other stud on the junction block should have a fusible link and go to the main stud on the alternator. As long as these wires are connected the ammeter should function. Let BD do his analysis and wizardry and we can post it in the tech section. I'm working on my 74 as well, trying to get it done! Good luck

[Swampyankee74]

Vz I'm not by the truck now. But that sounds how mine is wired. I found those 2 fuses 4 amp fuse. We'll see what bd drafts up! I get a little nervous,  there is a million different ways people wire these things up. Don't want an inferno as an end result! Haha!

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[bd]

Patients, young Padawan!


                     

[Swampyankee74]

Bd, that is a thing of beauty! Now that I can wrap my head around! That is very much appreciated,  thank you for your time!

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[bd]

Where to begin?

The factory wiring that connected to the original alternator can either be snipped and removed completely or taped up inside the harness (in case you wish to revert to a three-wire alternator sometime later).

During your initial installation, all of the original wiring between the firewall junction block and the starter, including the 16-gauge black wires and dual 4-amp inline fuse holders that connect to the junction block and run to the ammeter can remain unmolested (all of the items contained within the green box in the preceding image, except as noted*).  The preexisting 12-gauge red wire that connects the dual stud firewall junction block to the firewall bulkhead connector MUST BE INDEPENDENTLY PROTECTED by a 16-gauge fusible link!  To accommodate the significant increase in alternator output, the alternator B+ charge lead size (wire gauge) should be upgraded in accordance with the alternator manufacturer's recommendations.  For reference, up to 175 amperes of alternator output can be accommodated safely by a 6-gauge SXL-rated charge wire (which will exhibit a 3% voltage loss at 175 amps of current flow - well within industry-accepted guidelines).  Note that the alternator and its charge lead should be isolated from the battery through a suitably rated slow-blow fuse, such as the MIDI, MEGA, or ANL series fuses or an appropriately gauged fusible link.  The fuse rating must be at least 120% of the maximum current output of the alternator.  For convenience, the charge lead fuse holder can be mounted on the firewall adjacent to the junction block.

Introducing a high-amperage alternator (>100 amps) into a 1973 - 1975 C/K truck with a factory ammeter and original 37 - 55-amp charging system can significantly impact the ammeter's function and sensitivity, resulting in dramatic needle deflection during periods of high charge.  The periods of high charge and concomitant extreme needle deflection are generally of short duration as the vehicle battery quickly recovers from cranking the engine during startup and the charge current decreases.  Still, needle deflection might appear excessive to the vehicle operator. 

The factory ammeter is a shunt type that connects across a specified section of the charge lead, or primary buss, that runs between the battery and the rest of the electrical system in a parallel configuration.  Although the gross amount of current moving between the battery and electrical system is carried by the heavy gauge primary buss, a trickle of current bypasses the primary buss through the light gauge parallel configuration routing through the ammeter.  Ammeter needle sensitivity to the net flow of charge current is dependent on the proportion of the current passing through the primary buss relative to the current passing through the ammeter.  Thus, the wire gauge of the primary buss has a direct effect on ammeter sensitivity.  This begs the question, should the factory primary buss be upgraded all the way through from the alternator B+ output stud to the battery cable connection at the starter solenoid B+ stud during the initial alternator installation?

Circuit Analysis:

The factory 10-gauge wire that connects the firewall junction block to the starter solenoid B+ stud will consume (waste as heat) ~7% of the charging voltage while the alternator is generating at its maximum capacity of 140 amperes.  This equates to ~1 volt of loss across the 4-foot length of 10-gauge wire.  Generally, such a loss isn't particularly significant since the battery will recover fairly rapidly and the charging current will decrease accordingly.  As the charging current decreases, the voltage loss across the primary buss will also decrease in an exact relationship according to Ohm's Law (Voltage = Current x Resistance).  But a factory ammeter introduces unique circumstances.

If the resulting ammeter needle deflection is wildly excessive or the ammeter's inline 4-amp fuses are overburdened (melt), you can decrease the current flow through and desensitize the ammeter's response by decreasing the voltage loss across the primary buss.  This can be accomplished by simply increasing the size (decreasing the gauge) of the original 10-gauge section of the primary buss that connects the junction block to the starter solenoid.  Of course, a new appropriately gauged fusible link would need to be installed as well to safely protect the new wire.  For example, increasing the size of the primary buss between the firewall junction block and the starter solenoid B+ stud from 10 gauge to 8 gauge would decrease the associated voltage loss during the alternator's maximum output from ~7% to ~3%.  Whereas, reconfiguring the primary buss to 6 gauge would decrease the voltage loss to just under 2%.

Based on percentages and the fact that current and voltage are directly proportional, we can estimate the correct wire gauge for the primary buss.  The original 10-gauge primary buss, by factory design, normally exhibits <2% voltage loss while carrying the maximum charge current from the original 37 - 55-amp alternator.  That means that <2% of the charge current flows through the factory ammeter at maximum charge, which equates to ~1 ampere at a 50-ampere charge rate (2% of 50 amperes).  Well, it so happens that only ~2 amperes of current flow through the factory ammeter results in full-scale needle deflection.

The ~7% voltage drop across the primary buss during periods of maximum alternator charging current output after upgrading to a 140-amp alternator will force ~7% of the charge current through the factory ammeter, the ammeter's wiring, and the inline fuse pair — 7% of 140 amps is 9.8 amperes or nearly 2.5 times the current rating of the ammeter's 4-amp inline fuses and ~5 times the current that results in full-scale needle deflection.  Therefore, increasing the size (decreasing the gauge) of the primary buss is necessary.  Increasing the buss size to 8 gauge produces a maximum ~3% voltage loss with ~3% of the maximum charge current flowing through the factory ammeter — 3% of 140 amperes is ~4 amperes, which is still greater than desired or needed.  Increasing the primary buss to 6 gauge produces a maximum voltage loss of <2%, forcing <2% of the maximum charge current through the factory ammeter — 2% of 140 amperes is 2.8 amperes.  Hence, by estimation, the factory 10-gauge primary buss connecting the firewall junction block to the starter solenoid B+ battery cable 3/8" stud needs to be upgraded to a 6-gauge SXL wire protected by a 6" long, 10-gauge fusible link installed at the solenoid B+ stud in order for the factory ammeter to function within design parameters.

[Swampyankee74]

Wow BD you are on top of your shoot! Very nicely explained with a nice schematic to match!

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[bd]

After crunching some numbers I changed my initial thinking regarding modifications to the size of the primary buss and edited my preceeding post and the circuit diagram, accordingly.  Assuming my calculations are correct, the 10-gauge primary buss between the firewall junction block and the starter solenoid B+ post needs to be upgraded or the ammeter fuses will blow.

[Swampyankee74]

I will change that wire bd. Thank you again!

[VileZambonie]

Nicely explained BD 

[bd]

Thanks, VZ.  Hopefully, at least some of it will prove to be true.   


Swampyankee, keep us apprised so we can incorporate any corrections before it migrates to the Tech Forum...  Thanks.