1973 GMC 6500 Wiring question pertaining to the alternator.

[Rally]

Hello; So the problem is the alternator doesn't charge. (The battery terminal can be connected to the generator terminal on the firewall to force it to charge only because the reference wire is connected to the battery wire on the alternator and the reference wire is hooked to the field terminal on the alternator to make it charge for now.) (It's not recommended since it drains the battery when the key is off and leaves the reference connection not hooked to anything or ground it your wire is grounded; which would tell your alternator to charge all the time and cause issues.)

Investigating the problem the two issues I found are that the brown field wire doesn't connect to key on power like ignition or any other terminal or ground. The battery wire off of the alternator doesn't connect to key on power or the battery firewall terminal, or battery but it is connected to the generator terminal.

This GMC 6500 has an ammeter in the dash. There's nothing obvious for wire damage. Where should I look next?

It has a 10SI alternator for future reference.

 

[exp500]

Voltage regulator still on firewall? You mentioned Gen terminal. I suggest you look at a wiring diagram or diagram what you are describing. I dimly remember my 74 1 ton conversion to 10SI and a HEI dist. I used keyed power to dist to trigger alt. The Amp gage is a shunt circuit from charge wire. (you need to find it) 10SI has the diodes inside so Batt term can go direct to Batt. If it was originally a generator or split charging alt with external voltage regulator then you have 5 wires. Where/what is original field wire?
Original alt i believe was 10DN a 4 wire.10SI a 3 wire.
another option - https://store.alternatorparts.com/10si-series-12-volt-negative-ground-alternator-conversion-kit.aspx
Hope this helps.

 

[SpareParts]

Try looking up" how to make a GM alternator self exiting" Not sure at all but it might help you out.

 

[LtTahoe]

they make 1 wire alternators

 

[Rally]

My progress so far is that the 10si alternator this 73 gmc 6500 is using is a one wire model currently. (If the field wire is hooked up to key on power it won’t shut off so I don’t need to hook up the brown wire anymore.). I’m getting 14.7V to the firewall gen terminal but not the battery terminal; if a jumper wire is used to connect the gen to the battery firewall terminal I get 14.7V at the battery as well. I found some wiring diagrams for v8 pickups but they appear to use one terminal off the alternator instead of this two row unit on this “medium duty truck”. I tried a wire tracker (touch less unit that beeps when close to the wire I hook the signal generator to). It gets very quiet about a foot away from the alternator but is very faint all the way to the battery but a diode test (beeper on the multimeter) shows no connection from the alternator to the battery wire. (The wire tracker gets quiet about a foot from the signal generator when not connected to a test wire but goes silent not faint after two feet for a base line for comparison.). (This truck came with points and has an hei distributor now, and for the if it was a 10dn or 10si alternator please forgive me since it has what appears to be 78 on the inside of the alternator but it’s faint so it could be 73 and I didn’t notice a voltage regulator. I put a new internal regulator, rectifier, and brushes in the alternator that must have converted it to a one wire setup. I’d prefer a regulator closer to 13V instead of 14.7V but my primary objective is to charge the battery safely then I’ll go for a different regulator of some kind if available. Maybe I’d go back to something like a 10dn with the old mechanical regulators. (The alternator is warm on a 50F day after idling for 5minutes without running any electrical components other than ignition and charging the battery and of course gauges.). Thank you for your time and input!

 

[Doubeleive]

My progress so far is that the 10si alternator this 73 gmc 6500 is using is a one wire model currently. (If the field wire is hooked up to key on power it won’t shut off so I don’t need to hook up the brown wire anymore.). I’m getting 14.7V to the firewall gen terminal but not the battery terminal; if a jumper wire is used to connect the gen to the battery firewall terminal I get 14.7V at the battery as well. I found some wiring diagrams for v8 pickups but they appear to use one terminal off the alternator instead of this two row unit on this “medium duty truck”. I tried a wire tracker (touch less unit that beeps when close to the wire I hook the signal generator to). It gets very quiet about a foot away from the alternator but is very faint all the way to the battery but a diode test (beeper on the multimeter) shows no connection from the alternator to the battery wire. (The wire tracker gets quiet about a foot from the signal generator when not connected to a test wire but goes silent not faint after two feet for a base line for comparison.). (This truck came with points and has an hei distributor now, and for the if it was a 10dn or 10si alternator please forgive me since it has what appears to be 78 on the inside of the alternator but it’s faint so it could be 73 and I didn’t notice a voltage regulator. I put a new internal regulator, rectifier, and brushes in the alternator that must have converted it to a one wire setup. I’d prefer a regulator closer to 13V instead of 14.7V but my primary objective is to charge the battery safely then I’ll go for a different regulator of some kind if available. Maybe I’d go back to something like a 10dn with the old mechanical regulators. (The alternator is warm on a 50F day after idling for 5minutes without running any electrical components other than ignition and charging the battery and of course gauges.). Thank you for your time and input!

not really sure what your issue is, but if it needs a signal to operate, then hook a relay to key on as a trigger, that engages the relay for the signal to work, then when the key is turned off the trigger signal is gone and the relay kicks off (disengages) and then there is no battery drain.
if used to charge the battery you will probably want a heavy duty relay (high amperage) or isolator relay, just make sure it is more amps than the alternator

 

[exp500]

The one wire is your savior! Voltage at 14.7 is great as long as it's not higher. It sounds as if your alternator is Full Fielding, in other words full output because of voltage at field wire.
Fix that and switch from Amp guage to voltmeter or repair amp circuit as described in post #2 to remove traces of external volt regulator. The 10 SI is your friend so do a little research on how it works the the light bulb should light. See link I posted, they used to have excellent Q&A's on each alt and parts kits.
Since your Dist is HEI now, check that exact battery voltage there all the time.
Post the wiring diagrams you are using and we can help there also.

First indication of using wrong wire is truck won't shut off. Alternator powering Distributor.
A one wire properly converted should go from output terminal direct to battery, with no other connections. If it is running battery down, your alternator diode is not working or you are trying to re-use the wrong wire that doesn't shut off with key(backfeeding).

 

[exp500]

Maybe this will help-