Alternator bad? Verify my troubleshooting please.

[Juggernaut]

I finished my Cobra March of 2008. I'm running a Summit 100 amp 1 wire alternator (internal regulator) on my 289 and ever since it's been on the road, my volt guage always read 11, sometimes a little lower.

Yesterday while on a cruise & after about four stops, I noticed the volts were down to about 10. Stopped for lunch, went to leave & it wouldn't start but cranked real slow. After a jump start, it started fine but the volts were at 10 & never went up. Never happened before but this is the first time it had that many starts on it in one day and it's the most I've driven it in one day (200 miles). All my other trips were around the local area & didn't stop & restart the engine that many times.

Today I checked all the grounds & they appear good.

So, I charged the battery to 12.04 volts today, started it, took it to 1500 RPM's then to 2000 RPM's and checked the voltage at the started solenoid on the firewall where the alternator hooks up. It read 12.04 at both 1500 & 2000 RPM's. I believe it should be about 13.8 or so? I also checked the volts at the battery while it was at 1500 RPM's & it was at a steady 12.04 also.

Is my next step to have the alternator bench checked? Should I just go ahead & replace it? I'm certainly no brainiac when it comes to electrical issues.

Could I get a 100 amp one wire alternator (internal regualtor) from NAPA that would fit the 289 or would I have to order it through a catalog?

Thanks in advance for any help you guys can give me.

Randy

[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

[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.

[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 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

[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

[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

[cobred]

You may have another problem. The car could not be driving for a year and a half running 11 volts or less. You may have a bad volt meter or connection problems.

[Trueoo7]

You could also disconnect the positive side of the battery with the car running, if it stalls, its most likely the alternator.

[Juggernaut]

Thanks for the help guys, I'm at the point now where I'll pull the alternator & have it bench checked to confirm my suspicions.

I'll let you know what I find out.

See ya,
Randy

[trularin]

I do what true007 does when I suspect the alternator - pull the positive terminal while it is running and see if it dies.

[Argess]

Probably not a good idea to disconnect the battery with the engine running. Alternator output can go very high and not only can they self-destruct, they can take out other electronics, such as your expensive ignition computer.

Here's some info:

http://www.aa1car.com/library/charging_checks.htm

ps: I guess it can't be the problem in your case as you've been driving so long, but sometimes alternators are shipped with the brush hold-back wire still installed. They shouldnt be as all alternators should be bench checked before shipping and the wire would cause the test to fail. It's a little wire that is temporarily inserted to hold the brushes back until the armature is installed. Goes in a hole on the back of the housing. Then the wire is supposed to be pulled out. Also, some manufacurers put a shorting stip from a ground stud to the output terminal to keep static electricity from damaging the diodes. Something that has to be removed before use.

pps: Biggest problem I've had with alternators is dirty contacts. Dismantle and clean usually fixes things up. Second most common problem is fried diodes.

[Juggernaut]

First of all, I want to thank everyone for their help. What a great group of folks here on the forum!

I took the alternator to a highly recommended local alternator / starter repair shop, had them rebuild it and they found a bad voltage regulator. Charged the battery and installed the alternator tonight, started the car and the volt gauge jumped right up to 14 VDC.

I'm on the road again!

Thanks again,
Randy

[undy]

Great news!! That's what it's all about, Cobra guys helping other Cobra guys.

Oh, and Cobra gals too!

Dave