A few months ago, I swapped a 94 amp GM 12SI 3 wire alternator in my 74 F250. Now, if the truck sits about a week and half, the battery goes dead. I was told I need a diode inline from the alternator wire to the starter solenoid. I searched, and found that they supposedly have an internal diode. Does this do something different? If I do need an in-line diode, can someone recommend a specific one, or tell me how to select one? Thanks in advance

 

You should not need a diode. If you are sure the alternator is causing the drain, then the alternator has a shorted diode inside it.

It has 6 diodes inside that change the 3 phase AC voltage that it generates, into DC. A auto elec shop can change them out easily, or you can get a rebuilt.

The only time a diode is externally added to one of these alternators, is in the line that goes to the keyswitch from the alt. If this wire is tied into the same wire as the ignition coil, then when the key is turned off, the alt will backfeed voltage up the wire to the coil, and the engine will not shut off. You apparently do not have this problem.

 

Thanks. I am just a little perplexed as to what could cause the drain, since this wasn't a probem before the swap. Other than this, I really like the conversion. Any ideas?

 

Do the test above. If the light glows bright, you have a drain like you suspected. Take all the wires off the alternator. If the light goes out, your alternator is bad.

That is assuming you have it hooked up correctly. The "F" terminal should go to a hot in run power source.

The "L" terminal should be run to a dash lamp and then to a hot in run wire.

The large output terminal "S" should be hooked to the battery +.

 

First determine it is your alt that is causing the drain.....

On the SI series there is a Diode "Trio" inside the alt, very cheap easy fix. In this picture it is going from the regulator to the rectifier, the diode is the black sqaure thing


The easiest way to hook up the delco SI's is to run the charge output wire to the bat/relay. The sense wire to the charging stud, then the other plugin on the regulator to a key on power only to turn the alt on.


To me it sounds like your alt is staying on all the time

 

For some reason, I thought he had the newer version of the GM alt, thus the different terminal designations.

Paul you are correct, if he has the earlier internal reg version, the #1 terminal should go to a key on source seperated from the ignition coil, #2 terminal can be jumped to the output terminal, and of course the output goes to the bat +.

 

Guys, I think this just helped a bunch. On the plug-in, I do have the red to the batt+ charge wire, but I ran my white to my guage. I was told that is what to do with it. Now the guage always reads real high, and my battery drains. It sounds like I need to cut the white and route it to keyed power. Thanks.

On a 77 wiring harness, what do you think would be the best place for keyed power under the hood? I don't want to run a wire in the cab if there is already something I can use close by. Also, how do I get my amp guage to work now?

 

Check the old external regulator plugin for the factory ford alt....there might be something there. other than that I can't remem anything else. I have a large continous duty relay under my dash that uses key on power so i ran mine there.

Can't help ya on the Amp gauge i didn't get mine working again, voltmeter would be alot better to use anyway. I think one of the wires that are in the old ford external regulator plug in are for the ammeter. The ammeter has to be in the direct path of charging and draining. Just a PITA to deal with.

 

Paul is correct again. There should be a red/lightgreen wire that went to the old Ford regulator "S" terminal. Hook this to your white wire and you are set.

Your original ammeter is really just a very sensitive voltmeter. It reads the voltage drop across a special piece of wire Ford had in the old alt harness called a "shunt". They never did work very well when new.

If you want to try it, subford gave me an idea the other day that might work. There should have been two small wires running from the old Ford alternator output wiring harness up into the dash to the factory gauge. If you can find these wires, hook one to the large output terminal on your new alternator, and hook the other to the battery + terminal. Turn the headlights on, with the truck off, and see what happens with the gauge. If it moves a little bit, great, it might work. If it moves a llittle bit, but the wrong way(it should move to "D"), then go out and swap the little wire's positions at the alt and the bat +, and it should read the other way.

 

Ok, guys. I finally got a few minutes to look at it, and I do in fact have it hooked up correctly, but it is draining. I have a white w/black wire that connects to the old external regulator wiring harness. Now it is unused. It connected to "s" on the ext regulator. It doesn't ever show keyed power. Just a thought on what is wrong, since it am stumped.

 

Ok, something doesn't jive here. It sounds like your truck was originally wired with a dash "gen" light. That explains the white/black on the "S" terminal of the old reg.

When it's wired with a dash indicator instead of a factory gauge, your hot wire to trigger the alt system from the key is located on the "I" terminal of the old reg. Your truck is older, so I am not sure of the colors, but the later trucks with the dash light had the lightgreen/red wire there.

This keyed hot wire that goes to the alt system, actually comes from the dash light. So that all has to be hooked up or it will not work. If you have installed a custom dash, and the light has been eliminated, or someone swapped the stock dash for one with a factory ammeter, then you probably will have to run your own keyed hot wire to your GM alt.

 

Thanks, I did the swap. I cut the 74 harness behind the guages and the 77 harness behind the guages and spliced them together. I wanted the 77 harness since it was in better shape than the 74 and had duraspark too.

So what you are getting at is that I should now consider splicing what the "I" wire ran to or running my own keyed hot? If I can remember correctly, the green w/red ran to the ammeter or idiot light, so it should be right, but isn't. What did "I" do anyway?

 

All alternator systems have to be turned on or brought "online" when the key is turned on. If they stay "on" all the time, they run the battery down like yours. The aftermarket one-wire types have a special internal reg circuit that turn them on automatically.

That is what the "I" terminal did. It is what told the original system to come online and get ready for charging. With the idiot light hooked to the I wire, it also gives a indication if the alt system is working.

What should you do in your situation? I believe you are sunk as far as using any of the original Ford wiring to turn the GM alt on(or a Ford alt for that matter). You have no place in the dash and the circuit board for the "gen" light.

Do you want a "gen" idiot light? They are pretty good for indicating problems and give you an instant warning when fan belt comes off. What you will have to do is find a spot to mount a small indicator light, and the indicator light socket has to be the plastic type with two wires. You cannot ground the light. You will take one bulb wire and hook it to a spot that has 12 volts with the key in run. The other bulb wire will run all the way out to the alternator and hook to the white wire.

If you do not want a light, you can instead use a diode in place of the light. Put the band of the diode toward the alternator. This will keep the voltage from feeding backward, and not letting the engine turn off. You could then get a voltmeter to monitor the charging, or you could take the dash apart and try to hook the factory ammter up like was discussed before.

 

Thanks. I thought I was ok with electrical, but you guys are MUCH better I know what I have described may sound confusing, but you are really helping. I tried to test the white w/black that comes from the cab for keyed power. What volts should it read? My reasons for trying to find a factory wire is that I am trying to avoid what past owners do- hack and run pointless wires. I don't want to run a wire from the cab to provide keyed hot if there is already one there.

 

Since you changed the dash out, you do not have a key-on hot wire that works under the hood anymore that you could use.