Alternator or wiring issue on ls swap 74 c10

[Malicious]

Hi all,

I have installed a 2003 ls1 from a local Australian commodore. This should be similar to the first year gto and potentially the early Camaro. It has a two pin alternator.

I am having issues with the alternator or the wiring for it.

I have wired the top white wire to the new constant power bus bar I’ve installed ( I’ve removed the old two post unit on the fire wall). The bottom brown wore I have hooked up to the existing brown alternator wire that goes through the firewall to the ignition switch. I have cut and wired in a 3v t10 globe into this wire between the firewall and ignition switch for an indicator light and added resistance as this is what I have read the alternator needs to avoid burning out.

Everything seems to be running great, engine and the alternator is charging the system, but the alternator light will not illuminate.

That’s my first issue.

Second issue: while trying everything to work out wtf is going on I noticed with the battery connected the alternator got warm. After talking to a couple of mates I decided to replace the alternator as the seems to be a lot of people saying that indicates the diodes are stuffed.

After replacing the alternator with a new one, the new alternator is also getting warm(I don’t think as warm however) and the light is still not illuminating.

Can someone help me work out wtf is going on?

Cheers!

[VetteVet]

Based on your limited description of the alternator, and the wiring, I would guess that your problem lies in your using an LED bulb for an indicator. This type bulb only allows current to flow one direction. The indicator bulb uses positive voltage from the key switch and grounds through the alternator diodes until it begins to charge. Once it starts charging it provides positive voltage to backfeed the light and it goes out.

The indicator needs to be a regular incandescent bulb. It's duty is to excite the alternator through the internal regulator to charge the alternator fields to begin charging.
I suspect that the current is going to the alternator full time causing it to overcharge which would account for the warm condition. You can check this with a voltmeter on the output wire of the alternator.

The wire off the ignition switch is a resistance wire equivalent to 10 ohms so it was used as a backup for the bulb in the charging light dashes and when the factory went to all gauges they kept the wire in place as the resistor they needed for the alternator. It must have some resistance to the back current generated by the alternator or it might cause the engine to continue running when the key is shut off.

If you are using a newer CS style alternator, then 10 ohms is not sufficient to protect the alternator diodes and more resistance is needed. The range varies depending on who you listen to but 35 ohms 5 watts to 300 ohms and 1 watt is what I see the most.
Perhaps you can post some pictures of your project and give us the year to help us help you.

[Malicious]

Or sure photos will help much but see attached.

The light used is a standard 3w incandecent globe, I know LEDs are no good for resistance.

I’ve worked out the warmth is coming from the voltage sensing wire being hooked up to the batt not ign power. So I’ve swapped that over. Happy days.

Now I need to get the globe working.

You can see the stock brown wire I’ve used in the photo listed as 16-Brn-25. The globe is spliced in between the firewall plug and the ing switch where it meets the orange 12-or-300 and taking power from that I am assuming.

One other thing to note, for whatever reason the heater fan is on as soon as the key gets into the on position, no matter what the fan is set too. Is this normal? Could this be effecting the alternator light as it is linked on the same circuit?

[VetteVet]

I think you might have one of the newer style alternators that are regulated by the PCM/ECM and the voltage sensing is controlled internally in the alternator. Are you using the computer to control the ignition and the fuel system?
When I googled the 3v T 10 bulb it came up as an Led unit thus my response.

There is a forum here for LS conversions and up to 1997 trucks that some one my be more able to help you. I assume your truck is a 73 to 76 model from the diagram you posted. It may become necessary for you to get a CS or SI model alternator with a plug designed for that year wiring with the terminals labeled P L I S or F S L P, or in case of the SI there are 2 terminals labeled NO.1 and No.2.


Do you have a model year for the alternator that you have? In 1974 GM converted to the 10 SI alternator which was internally regulated and which used the brown wire from the ignition switch as the exciter wire.

[Malicious]

Yeah not a computer controlled alternator. I’ve looked into it

The alternator is from a 2003 model. Not sure what was originally in the C10 the engine is long gone. But yeah it is a 74 build date.

I’ll check out the ls forum. See what I can find.

[Malicious]

Actually where is the LS section? I can only see an LS classified section?

[starterman99]

Your alternator is a Mitsubishi 12 volt 140 amp and uses an S L plug configuration. The S= Battery sensing circuit and should have full voltage all the time. The L = Light circuit and should go off and on with ignition switch. If the L terminal is not going off with Ignition it will keep the regulator turned on and the unit will be warm to the touch after sitting. check you don't have the two wired in reverse as the light would not work and the regulator would stay on making it warm to the touch when it should be cool.In the picture above the terminal on the right is L it is closest to the output battery terminal on the back.

[Malicious]

Originally Posted by starterman99

Your alternator is a Mitsubishi 12 volt 140 amp and uses an S L plug configuration. The S= Battery sensing circuit and should have full voltage all the time. The L = Light circuit and should go off and on with ignition switch. If the L terminal is not going off with Ignition it will keep the regulator turned on and the unit will be warm to the touch after sitting. check you don't have the two wired in reverse as the light would not work and the regulator would stay on making it warm to the touch when it should be cool.In the picture above the terminal on the right is L it is closest to the output battery terminal on the back.

Thanks you were exactly right. Yesterday after posting I went out and swapped the wires on a whim and it worked.

I had tried both ways on the old alternator and it didn’t make a difference so I put them in the way I thought was right but didn’t try the other way with the new alternator. Stupid, but turns out the old alternator was bad and the wiring was back to front.

I am a very happy man now. Thanks for the help!