Gain extra benefits by becoming a Supporting Member Click here find out how!

Classic Performance Products Classic Industries
American Auto Wire
Danchuk Catalog
Hellwig Products IncPerformance Rod & CustomNu-Relics Power Windows



Username Post: altenator diode check        (Topic#364851)
sting199 
Poster
Posts: 57

Reg: 12-12-19
04-07-21 10:36 AM - Post#2817657    

In trying to check my altenator for occasional dead battery, I got some crazy readings doing a diode check. With my meter on AC, I got about 30 volts, but when I switched leads(pos to neg) the voltage went to 0. Tried the same thing on another battery with the engine off and got the same readings. Is my meter fried? On the AC scale, there shouldn't be any difference in the reading when the leads are switched.



 


65_Impala 
Very Senior Member
Posts: 4684

Reg: 12-29-02
04-07-21 01:03 PM - Post#2817667    
    In response to sting199

You should not get an AC reading with the engine off.

Try measuring the DC voltage with the headlights on and running at 1500rpm. Should be around 14V. If not, then you need to dig into the alternator further.





 
beagrizzly 
"12th Year" Gold Supporting Member
Posts: 2231
beagrizzly
Age: 70
Loc: south texas
Reg: 08-04-12
04-08-21 07:05 AM - Post#2817692    
    In response to sting199

You can pull the negative cable from the battery. Then put your voltmeter between the post and the cable. It's got to be on D.C.

If you are showing voltage, you have something drawing voltage.

Unhook the alternator lead. check it again. If you don't have any voltage draw, you have bad diodes in the alternator.

Another thing you can do is disconnect the battery. Then let everything cool down. Hook the battery back up and feel the alternator. If it is pulling voltage, it will get warm to the touch.

Just a poor man's troubleshooting process.

You can also take it to a parts house. Most of them can check it for output, and diode function off the engine.

I am lucky enough to have a starter/alternator rebuilder locally that can do all of that, and rebuild it if it needs it.

Griff

if you're gonna be a bear..................

1960 Biscayne (the 6T)
2005 Yukon XL
2007 GMC Sierra Classic 8.1
2009 Silverado
2011 Escalade ESV


 
bobb 
Super Senior Member
Posts: 5775

Loc: paradise
Reg: 09-05-03
04-10-21 08:28 AM - Post#2817795    
    In response to sting199

poor mans test.engine off, disconect battery and run a light bulb between the cable and battery. a 1056 bulb should not light up. if it does disconect alternator and try again. diodes fail one ata time normaly. engine running you should not have much ac voltage, sorry i dont know the max number at this time. i always test voltage at the battery terminal and at the alternator terminal. not on the connectors.

70 L camino, grampa engine, g-force 5 spd, road rage suspension. Pray first before all else fails.


 
gchemist 
Member #271 Senior Chevytalk Moderator
Posts: 25408
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
04-12-21 12:02 PM - Post#2817913    
    In response to bobb

Would a clamp meter work too? Clamp on the positive side. Then disconnect the alternator? If amps drop to 0, it's a bad alternator? "Asking for a friend".

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles
'08 GMC Acadia 3.6L SLE


 
sting199 
Poster
Posts: 57

Reg: 12-12-19
04-12-21 12:15 PM - Post#2817914    
    In response to 65_Impala

Ran that test,,, and the voltage was 14V. Still have a slow battery failure, and I am suspecting faulty diodes.



 
sting199 
Poster
Posts: 57

Reg: 12-12-19
04-12-21 12:16 PM - Post#2817915    
    In response to gchemist

Not sure. sorry



 
sting199 
Poster
Posts: 57

Reg: 12-12-19
04-12-21 12:19 PM - Post#2817916    
    In response to beagrizzly

That would detect a leaking diode, but not a blown diode, would it?



 
gchemist 
Member #271 Senior Chevytalk Moderator
Posts: 25408
gchemist
Loc: Austin, TX 78748
Reg: 05-09-00
04-13-21 05:42 AM - Post#2817945    
    In response to sting199

Good question. I'm tracking down a parasitic draw. The alternator or starter are my next items to test. I was wondering if using a clamp meter would tell me if one component is bad. A few common parasitic items have been eliminated. Either by unplugging a component, pulling a fuse, or replacement.

'83 Silverardo XST - ZZ4 powered
'95 Jimmy SLT (Bought @131,814)
'96 GMC Jimmy LS Ret. @236651 miles
'08 GMC Acadia 3.6L SLE


 
65_Impala 
Very Senior Member
Posts: 4684

Reg: 12-29-02
04-13-21 09:04 AM - Post#2817956    
    In response to sting199

  • sting199 Said:
Ran that test,,, and the voltage was 14V. Still have a slow battery failure, and I am suspecting faulty diodes.



Disconnect the main charging wire and see if the battery still discharges.

Best bet is putting a light bulb or a meter set to read current between the battery negative post and the cable. Disconnect things until the bulb goes off or the meter reads less current. Current should be below about 20mA on most any car with memory storage type electronics in it (radio with clock for example). Otherwise, it should be zero.



 
85Camaro 
Senior Member
Posts: 1150

Loc: East TN
Reg: 07-14-01
04-14-21 08:06 PM - Post#2818052    
    In response to sting199

Research how to do a parasitic draw test. Acceptable parasitic draw is 30 milliamps, anything more than that is unacceptable. A draw is measured in amperage, Your digital meter should have an amperage setting. Disconnect your negative battery cable and hook a bridge between them, one of those battery shutoffs with the green knob. If you don't have one hook a test lead between the negative cable and the negative post. Be sure everything in the car is turned off and the doors are closed. Leave the test lead in place and hook your amp meter between the negative cable and the negative post and turn it on to the amperage position. Now remove the test lead you had installed so any amperage will be passing through the meter and see what it reads. Give it a little time for everything to go to sleep and it shouldn't be over 30 milliamps. if it is start disconnecting things and pulling fuses to fins the draw.



 


Icon Legend Permissions Topic Options
Report Post

Quote Post

Quick Reply

Print Topic

Email Topic

210 Views
FusionBB
FusionBB™ Version 2.1
©2003-2006 InteractivePHP, Inc.
Execution time: 0.096 seconds.   Total Queries: 16   Zlib Compression is on.
All times are (GMT -0800) Pacific. Current time is 03:28 AM
Top