27 volts AC CURRENT across Battery Terminals = Shot Diodes?

[gone2long]

Here we go again,

 

Truck is a 4x4, 1976.5,  720 Datsun with a Z18 engine in it. Alternator light has been dim and recently worse ever since I had it rebuilt about 1 1/2 years ago - new diodes, some wiring, but no rewind.  Asked the shop why the dim red light was still on after they supposedly  "fixed" the alternator, but they just shrugged.

 

Never paid much attention to it because the truck ran ok and the battery never failed.  Had some other wiring issues and got them fixed about 6 months ago with a complete rewiring, but the generator light seemed to get "redder" over time.  Did some checks recently and got contrary results.  But today, the battery still showed 12.65 volts cold.  It dropped to 12.43 at idle, but at a fast rev (guessing about 2,500-3000 rpms) she stayed consistently between 14.1 - 14.35.

 

Still wondered about the red light so I did a "diode test" that i found on the internet.  I kept the engine on idle and set my multimeter to AC current.  Then I probed the pos and neg terminals of the battery with my pos and neg multimeter probes.  Low and behold, the meter showed 27 VOLTS OF AC CURRENT coming through the battery.  Apparently, this means the diodes are shot, no?

 

Interestingly, just out of curiosity, I shut the engine and did the same test, getting the exact same result.  Should that have happened?  If so, why, and is it indicative of any deeper problem or just some kind of residual charge?

 

Thanks, as always, for your comments and suggestions,

 

G2L

 

 

 

 

 

 

Edited May 15, 2019 by gone2long
wording

[datzenmike]

As long as 14.something across the battery terminals

12 minutes ago, gone2long said:

  But today, the battery still showed 12.65 volts cold.  It dropped to 12.43 at idle, but at a fast rev (guessing about 2,500-3000 rpms) she stayed consistently between 14.1 - 14.35.

 

Still wondered about the red light so I did a "diode test" that i found on the internet.  I kept the engine on idle and set my multimeter to AC current.  Then I probed the pos and neg terminals of the battery with my pos and neg multimeter probes.  Low and behold, the meter showed 27 VOLTS OF AC CURRENT coming through the battery.  Apparently, this means the diodes are shot, no?

 

 

 

  The diodes should 'rectify' the AC signal much closer to DC. The AC component it likely what's causing the light to glow.

[gone2long]

21 hours ago, datzenmike said:

As long as 14.something across the battery terminals

 

  The diodes should 'rectify' the AC signal much closer to DC. The AC component it likely what's causing the light to glow.

Thanks Mike,

 

Today, I brought the alternator to a shop and mentioned the AC current of 27 volts, but they didn't know what to make of it.  We opened up the alternator and one of the diodes was burnt.  We replaced the entire rectifier bridge and that seemed to fix the charging problem as I was able to get 14.33 + readings at low idle with no utilities on.  Also, thankfully, the warning light went out.

 

I did an AC test on the battery just for the heck of it, and to my surprise I was now getting 30V of AC current at the battery.  Really have no idea what to make of that.  The electrician's comment was, "it doesn't matter".  Whaddaya think ?

 

G2L 

[datzenmike]

Try this with a different vehicle. I bet there is a small AC component on all alternator cars. As long as 14+ DC I wouldn't worry.

 

BTW 14.33 DC is almost half of 30 AC

[gone2long]

1 minute ago, datzenmike said:

Try this with a different vehicle. I bet there is a small AC component on all alternator cars. As long as 14+ DC I wouldn't worry.

 

BTW 14.33 DC is almost half of 30 AC

Hah! Yeah, you know the math,  did occur to me.  : )  The highest rate at low ideal was 14.5, even closer to half our AC number.  Guess I won't worry about it as long as she keeps running strong.

Thanks again,

G2L

[datzenmike]

Alternators produce AC. DC is from 0 down to 14.5 or negative. Above 0 is positive 14.5. The diodes should block the + component from the battery.