How-To: Wiring a GM 3-Wire Alternator into your Patrol
Posted: Sun Jun 22, 2014 2:05 pmMy green 66 Patrol came with a GM 3-wire SI series alternator installed in it, and whoever converted it did a really shabby job (but no more shabby than any of the other previous modifications done on this Patrol, 
)
So, I tore out everything they did and re-wired it, and I thought I'd do a "how to" for anyone who wants to scrap their generator in favor of an alternator -
This post assumes that you have already removed your generator, and done all the hardware and bracket related changes to get the Alternator mounted to the engine- We're only covering the wiring here.
Also, this wiring procedure assumes that your Patrol's factory ammeter is operational, and that you want to continue using it (along with re-using as much of the factory wiring as possible) with the alternator...
So, Once your generator and voltage regulator have been removed, you should be left with one factory wiring pig-tail that has 3 wires wrapped in it - the white/red generator "A" lead, a ground, and the yellow wire for the temp sensor:
Since so many Patrols have been converted to negative ground, let's also do a couple double-checks to make sure the circuits are set up correctly for the alternator. Pull the multi-gauge from the dash, and flip it around and make sure the Ammeter polarity is correct: Black wire to positive, white wire to negative:
Also, check the fuse block and make sure the polarities there are correct as well: Black wires to the BAT terminal, and White wires to the center Load (or accessory) terminal:
Now, my alternator was wired in using the "It works, but it is not ideal" method- someone had just jumped the red load sensing terminal (terminal 2) directly to the Bat terminal in the alternator - This is a very common method that a lot of guys use when they wire in a GM 3-wire alternator, and like I said- it works, and it saves a few feet of new wire; but it also presents one key disadvantage: When wired like this, the load sensing wire does not pick up the vehicle's total electrical load when the vehicle is running, until the battery does, which means that the Alternator does not "Switch on" as early as it could/should when the vehicle is running (anyone who has ever been in an older, alternator-converted vehicle where the headlights come on dimly, then get brighter after a second has probably seen this result first hand)
The way I am doing the wiring here allows the alternator to fully power the vehicle's load while it is running, as well as providing whatever current the battery needs to remain fully charged. This will be evident in your Patrol as the Ammeter will swing to the negative side while the starter is cranking, but once the engine is running, it should gradually swing back to the positive side, and as long as the vehicle is running, the ammeter should not go negative when you hit the headlights or high beams, or when you fire up that thumping 7000 watt stereo system...
So, the white/red generator 'A' wire from the factory harness goes to the red terminal 2 wire on the alternator's 2 wire plug, and the black ground wire from the harness goes to the ground point on the bottom of the alternator case - Now it's time to run the 2 new wires that need to be added to allow your Patrol's new alternator to work correctly. First is the actual output wire from the Alternator - This wire needs to be a minimum 10 gauge wire, and should run from the Alternator's BAT terminal, to one of two points - Either the BAT terminal on the starter solenoid, or directly to the battery positive terminal: I chose to connect mine to the starter's battery connection-
The 2nd new wire we need to run is the little white wire from the number 1 or R terminal on the alternator's 2 wire connector - This is the lead for the charge indicator lamp. It does not need to be a thick wire (14-16 guage is adequate) and you will need to add a new, 12v two lead lamp inline between this lead and a switched ignition source. This light should come on whenever the ignition is hot, but when the alternator is not charging (or not turning) . It should go out as soon as the alternator is turning/charging- and if the alternator goes bad, then the light will come on and stay on, even when the alternator is turning.
Testing the circuit is easy- switch your key into the ON position, and confirm that the indicator lamp lights up, then fire up the Patrol and confirm that the light goes out. Then, while the engine is running, take a voltage reading between the BAT and ground terminals on the Alternator, and then take a voltage reading across the battery- they should show exactly the same voltage.
)So, I tore out everything they did and re-wired it, and I thought I'd do a "how to" for anyone who wants to scrap their generator in favor of an alternator -
This post assumes that you have already removed your generator, and done all the hardware and bracket related changes to get the Alternator mounted to the engine- We're only covering the wiring here.
Also, this wiring procedure assumes that your Patrol's factory ammeter is operational, and that you want to continue using it (along with re-using as much of the factory wiring as possible) with the alternator...
So, Once your generator and voltage regulator have been removed, you should be left with one factory wiring pig-tail that has 3 wires wrapped in it - the white/red generator "A" lead, a ground, and the yellow wire for the temp sensor:
Since so many Patrols have been converted to negative ground, let's also do a couple double-checks to make sure the circuits are set up correctly for the alternator. Pull the multi-gauge from the dash, and flip it around and make sure the Ammeter polarity is correct: Black wire to positive, white wire to negative:
Also, check the fuse block and make sure the polarities there are correct as well: Black wires to the BAT terminal, and White wires to the center Load (or accessory) terminal:
Now, my alternator was wired in using the "It works, but it is not ideal" method- someone had just jumped the red load sensing terminal (terminal 2) directly to the Bat terminal in the alternator - This is a very common method that a lot of guys use when they wire in a GM 3-wire alternator, and like I said- it works, and it saves a few feet of new wire; but it also presents one key disadvantage: When wired like this, the load sensing wire does not pick up the vehicle's total electrical load when the vehicle is running, until the battery does, which means that the Alternator does not "Switch on" as early as it could/should when the vehicle is running (anyone who has ever been in an older, alternator-converted vehicle where the headlights come on dimly, then get brighter after a second has probably seen this result first hand)
The way I am doing the wiring here allows the alternator to fully power the vehicle's load while it is running, as well as providing whatever current the battery needs to remain fully charged. This will be evident in your Patrol as the Ammeter will swing to the negative side while the starter is cranking, but once the engine is running, it should gradually swing back to the positive side, and as long as the vehicle is running, the ammeter should not go negative when you hit the headlights or high beams, or when you fire up that thumping 7000 watt stereo system...
So, the white/red generator 'A' wire from the factory harness goes to the red terminal 2 wire on the alternator's 2 wire plug, and the black ground wire from the harness goes to the ground point on the bottom of the alternator case - Now it's time to run the 2 new wires that need to be added to allow your Patrol's new alternator to work correctly. First is the actual output wire from the Alternator - This wire needs to be a minimum 10 gauge wire, and should run from the Alternator's BAT terminal, to one of two points - Either the BAT terminal on the starter solenoid, or directly to the battery positive terminal: I chose to connect mine to the starter's battery connection-
The 2nd new wire we need to run is the little white wire from the number 1 or R terminal on the alternator's 2 wire connector - This is the lead for the charge indicator lamp. It does not need to be a thick wire (14-16 guage is adequate) and you will need to add a new, 12v two lead lamp inline between this lead and a switched ignition source. This light should come on whenever the ignition is hot, but when the alternator is not charging (or not turning) . It should go out as soon as the alternator is turning/charging- and if the alternator goes bad, then the light will come on and stay on, even when the alternator is turning.
Testing the circuit is easy- switch your key into the ON position, and confirm that the indicator lamp lights up, then fire up the Patrol and confirm that the light goes out. Then, while the engine is running, take a voltage reading between the BAT and ground terminals on the Alternator, and then take a voltage reading across the battery- they should show exactly the same voltage.
