Alternator bad? Verify my troubleshooting please.

[undy]

What manufacturer and model alternator are you currently running? A lot of Cobras run the GM alternators, especially the 1 wire ones. Your alternator sounds like it's definately bad. My GM 100 amp 1 wire puts out 13.8 VDC @ 65 amps, at an idle. Mine's also convertable to a 3 wire setup. Doing so will give me the full 14.5 volt alternator output. Mine's a Powermaster. I plan on doing the mod in the near future in conjunction with my new cooling fan install.

Also, make sure your charging circuit/wiring is up to the task of a 100 amp alternator. Painless wiring and others typically are not. I run a #4 copper wire from my alternator to my fuse block/starter/battery and #8 copper to my engine cooling fan.

[Juggernaut]

Originally Posted by undy

What manufacturer and model alternator are you currently running? A lot of Cobras run the GM alternators, especially the 1 wire ones. Your alternator sounds like it's definately bad. My GM 100 amp 1 wire puts out 13.8 VDC @ 65 amps, at an idle. Mine's also convertable to a 3 wire setup. Doing so will give me the full 14.5 volt alternator output. Mine's a Powermaster. I plan on doing the mod in the near future in conjunction with my new cooling fan install.

Also, make sure your charging circuit/wiring is up to the task of a 100 amp alternator. Painless wiring and others typically are not. I run a #4 copper wire from my alternator to my fuse block/starter/battery and #8 copper to my engine cooling fan.

Undy, it's a Summit alternator, SUM 810304 for a 289, their name brand. Not sure of the wire size from the alternator although 4 sounds right, I can check on that.

Also, yours runs 13.8 VDC @ 65 amps, where does the advertised 100 amps come in, see I need a little education on this electrical stuff.

Is there an advantage to running a GM 1 wire alternator over a Ford one? Where would I get the Powermaster for my 289?

Thanks,

[undy]

I don't know if a GM alternator is better than a Ford one. I wouldn't imagine so. That was what mine was originally equipped with. Yours is definately a Ford unit.

That's what the idle amp and volt rating was @ 1000 rpm, 13.8 @ a 65 amp draw. The 100 amp rating is "max amps". I place more value on the idle amp rating than the max amps. It's at idle where we experiance most of our under-voltage conditions anyway.

Dave

[white rose]

Take the alternator to any auto parts store and they can check the output, under load your alternator should put our 100 amps. Also when you charge a battery with a battery charger it should read about 13 volts when fully charged. Any auto parts store can also load test a battery.

[Juggernaut]

Originally Posted by white rose

Take the alternator to any auto parts store and they can check the output, under load your alternator should put our 100 amps. Also when you charge a battery with a battery charger it should read about 13 volts when fully charged. Any auto parts store can also load test a battery.

Sounds good, man you guys are great!

Thanks to all for the help!

Randy

[Juggernaut]

Originally Posted by undy

I don't know if a GM alternator is better than a Ford one. I wouldn't imagine so. That was what mine was originally equipped with. Yours is definately a Ford unit.

That's what the idle amp and volt rating was @ 1000 rpm, 13.8 @ a 65 amp draw. The 100 amp rating is "max amps". I place more value on the idle amp rating than the max amps. It's at idle where we experiance most of our under-voltage conditions anyway.

Dave

Dave,

Thanks for explaining that to me, now I understand about the 100 amp rating. And, it also makes sense that under voltage conditions happen mainly at idle.

Looks like I'll be getting a new alternator, I'll let you know the outcome!

Thanks again for the help,
Randy

[Argess]

You are correct with your test....12 VDC across battery with engine off, 13.8 (or thereabouts)with engine running.

Just to be certain, try checking right at the output terminal of the alternator for the 13.8 volts. Try your voltemeter ground probe on both the alternator housing and the engine (block, intake, whatever is a good ground) when you do this test.

I know you said you checked grounds, but is the alternator case itself grounded? Painted adjustment straps etc could be a problem.

The AMP rating of an alternator is the max current that can be drawn safely from an alternator. It is usually defined by the designer based on the max current the diodes can handle without burning up. The alternator is actually a device that produces 3-phase AC and has a 6 diode bridge rectifier circuit in it.

I'm not familiar with your particular alternator, but I don't suppose there is mention of an output voltage adj in the literature that came with it?

All in all, your tests so far indicate a bad alternator.

[Juggernaut]

Originally Posted by Argess

You are correct with your test....12 VDC across battery with engine off, 13.8 (or thereabouts)with engine running.

Just to be certain, try checking right at the output terminal of the alternator for the 13.8 volts. Try your voltemeter ground probe on both the alternator housing and the engine (block, intake, whatever is a good ground) when you do this test.

I know you said you checked grounds, but is the alternator case itself grounded? Painted adjustment straps etc could be a problem.

The AMP rating of an alternator is the max current that can be drawn safely from an alternator. It is usually defined by the designer based on the max current the diodes can handle without burning up. The alternator is actually a device that produces 3-phase AC and has a 6 diode bridge rectifier circuit in it.

I'm not familiar with your particular alternator, but I don't suppose there is mention of an output voltage adj in the literature that came with it?

All in all, your tests so far indicate a bad alternator.

Argess, funny that you mentioned to check the mounting of the alternator case, I just checked that a little while ago & it's good. Wow, maybe I'm starting to understand this electrical stuff??

I'll check the alternator at the lug on the alternater as you suggested just to confirm everything, heck, it can't hurt!

I don't have the paperwork anymore that came with the alternator, all I know is that it as listed as a 100 amps.

Thanks for the alternator lesson, too, man I just learning more!

Randy