12Si Alternator Install Diode with Battery Gauge

[andrewpclayton]

Working on a 69 GMC 1500 and I have been following VetteVet's posts as best I can, but this is absolutely not my strongest area. Hopefully he can chime in

I am swapping in a 350 engine from a 77 4wd (I think) that has the alternator on the passenger side. As of now the engine is out, and nothing is plugged in but I want to have everything ready to plug and go. Will be installing an alternator from a 85ish C10.


I have taken the wiring tape apart and removed the plug for the external harness, so its set for the brown wire to go from firewall to #1 on the alternator

I have a battery gauge, I assume this means there isnt also a light, thus I need the diode?

If that is the case, I cannot figure out what a diode even looks like, nor which to get.


Can one of you, any of you, all of you tell me

A) am I on the right track in terms of from the harness, the brown wire going to #1, the red going to #2 and the junction box, and power from battery via starter

and
B) if the diode is necessary, can someone link me to the correct unit to buy?

thanks!

[andrewpclayton]

Also my apologies, I know there are many similar posts, I just cant find confirmation re the diode

[VetteVet]

If you have the battery gauge you also have a resistance wire on the key switch at the auxillary terminal along with a large brown wire (12 gauge). This resistance wire is there to give the alternator the resistance it needs in case the light fails or if you don't have the light, in your case.
Just go ahead and connect the brown wire to no;.1 and it should work fine.

Be sure you run the red output wire to the same junction as it was and not directly to the battery or the starter post or the battery gauge will not work.

A diode is a small device in a wiring circuit that will only allow current to travel in one direction (like a check valve) It is different than a resistor which will allow current to travel in either direction but applies a resistance to the current to lower the voltage to a load which requires a lower voltage.

The diode will have an arrow showing the direction of flow of the current and a resistor will have a series of bands on the body that are color coded to show its resistance value. Some have the value (ohms) labeled on the body.

https://www.youtube.com/watch?v=I24e7gF6k5o

[andrewpclayton]

Thank you so much for your response you’re the very best. I just finally got around to getting everything going ..

I have the brown wire extended and connected to #1 on the alternator, 2 goes to junction box. I started the engine and it isles but the voltage stays in the 12.3 range which I believe means the alternator isn’t working. Is there anything you can think of that I should check? Or is testing the alternator at adnvace auto the next step?

[VetteVet]

The brown wire usually comes from the resistance wire on the key switch accessory terminal on the gauge dashes and from the light on light dashes. It runs through the firewall to the external voltage regulator or in your case to the no.1 On the SI alternator. You should see 12 volts on the wire with the key on engine off and zero volts with the key off. If you are not getting that then your alternator is not charging.

[andrewpclayton]

Looks like I am getting 12 volts to that wire.. What should I change for the alternator to start charging?

[andrewpclayton]

That image shows a connection from the nut on the alternator to the junction box in addition to the #2... am I seeing that correctly? That might could explain it!

[VetteVet]

Originally Posted by andrewpclayton

That image shows a connection from the nut on the alternator to the junction box in addition to the #2... am I seeing that correctly? That might could explain it!

The no. 2 wire has to supply the current to the alternator fields as the regulator reads the system. The brown wire just turns on the alternator and it reads the voltage carried by the no. 2 wire. So yes if you don't have the no. connected to a full time 12 volt supply (junction block) the alternator fields will not become more magnetic which increases and decreases the alternator output. The battery voltage must be applied to the no.2 wire to allow the alternator to maintain the 14.5 volts it needs to power the system and charge the battery.

Think of the regulator as a water valve and the no. 2 wire as the supply hose. If you need more water from the supply hose the regulator opens more and lets the alternator pump more juice to the system.

[andrewpclayton]

It’s working! Thank you so much you’re the best

[Warrens69GMC]

Good job VetteVet!!