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Old 09-05-2005, 11:10 AM
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One-wire to 3-wire Alternator change

I realize this topic has been beaten to death, but I still have issues with my setup. I went to a GM 100 amp alternator on my Mustang for the purpose of having more juice at lower RPM to run the electric fan. I tried the one-wire - bad choice. So I rewired to use the 3-wire capability of the alternator. Much better, but it still won't charge at idle (600 - 700 rpm). Just off idle it works just fine. I guess my question is - do I need a new internal regulator, or is the alternator simply a dud? It is newly rebuilt item. Thanks.

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Old 09-05-2005, 12:33 PM
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Are you using the stock pulley on the Alt??
It may be to big... You probably just need
to find a pulley with a smaller diameter...

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Old 09-05-2005, 07:18 PM
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Doc here,

A properly operating Alternator will charge at ANY speed faster than 300 rpms...If you are losing charge on idle X 2 alternators, you have a load or wire problem that should be addressed Now.

Place a DMM across the battery at idle after about 5 minutes run time after start (which is a large load drain) IT Should read 14.4 volts...If not and you rev it up and you read 14.4 than the regulator is shot.

To insure the alternator is properly working your system Should be wired this way...

Ground System..Main ground cable to block bolt AT or VERY near The starter..

From the same bolt, run a battery type cable with ring terminals on BOTH ends..(auto parts store about 6 bucks..)

Attach and run the OTHER end to the FRAME anywhere easy for you..

Next from the block bolt, run 2 ~ 10 gauge Black wires...The First of which will run and terminate at the Firewall, the second will run and terminate at the alternator mount, ground lug, or mount bolt.

Get some Ground Braid (Rat shack..Cheap) Run a braid from the hood, to the firewall, from the doors to all door posts, the trunk to the body, Gas flap to Body.

At EVERY Ground point, Burnish ALL paint and grease from the area, use a proper star washer, and if required, Tech screw or Sheet metal screw or proper sized bolt, and Locks.

Do this ground system, and you'll have a system that will serve you well for many years, trouble free, good charge rates, and no heat-soak problems..


If your Alternator bracket is powered white..(as well as the alternator case..) Pull them and sand or water blast them back to bare semi shiny metal as well as their hardware..

Most mounts on SBC's as well as the Alternator case (What the hell were they thinking...??) ARE aluminum..Meaning everywhere they come into contact with STEEL you get electrolysis..(the white junk..) and poor electrical bonding and it eats metal as fast as a weak acid..

IN FACT..If you DO have this as you pull it down, you might want to consider the replacement of the water the impeller usually goes first..get it all in one shot.

When you Reinstall the new hardware, Get some ALUMINUM wire past and coat all the hardware..and everywhere the brackets come into contact with steel. This will give you great bonding AND never corrode again!


Be sure your Red main wire from the alternator goes from the alternator directly to the main terminal bolt on the starter, and at the Starter a proper Fuse link installed..make sure all connections are tight (not only on the posts but on the ring terminals) and a proper inner star washer is used.

Pull and Check your positive cable, inspect for corrosion, and tightness in the clamps, this includes any secondary wires attached to the cable..If you have corrosion, lots of it..slice the insulation back about 3 inches If not and inspect..If it's up there, get new cables, If not heat shrink or tape, and replace. Same applies to the ground side..

Charge your battery for a good 6 to 8 hours on a 10 amp charger if needed, an alternator is not designed to bring a previously dead battery back to snuff..they are only designed to support the vehicle system while replacing any surface charges (Starter loads, lights while not running, Etc..)


Do you have a shunt type AMP gauge in operation? If so did you replace it with a higher current gauge? at 100 amps now the prior was shunted for 60 amps..Should you (rare but happens) place a 100 amp load on the total system, that gauge will smoke on you right in the panel..Most folks forget about it until it happens, then they don't have a clue..hummm..60 amp gauge..100 load...don't know why it burned up...


Are you running relays on those fan (s) ? You must run relays..for safety AND current delivery to the fan motors..they should be wired, Relay coil and ground Via the Fan Control Circuit, and the N.O, C.W. contacts direct via proper fuse link to the battery and fan..most overlooked, will cause the problems you are having..AND in the case of a rotor lock..(plastic bag caught in the fan..) can draw EVERY BIT of that 100 amps, which is why the relays and a proper link is important..without them it will burn up the next weakest link.. The DON"T want that to happen.


You can just change out the regulator , the diode pack is about $7 bucks (maybe more for 100 amp) is very simple to do and takes about 1/2 day to do it right..

However I would get the full rebuild kit, about $20 bucks, gives you New brush/holders/springs, bearings/bushes, and rectifier diode pack, as well as the can burnish the slip rings to shiny metal, and follow the above instructions for electrolysis while your in there.

That should get you running better than new..

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