Alternator testing

Joined
Oct 12, 2005
Messages
235
Location
KC, MO
I have reason to believe my daughter's '07 Santa Fe may be having alternator problems. I replaced the battery just last month. I have been researching online about symptoms and such. Is using a multimeter the best course of action to test an alternator--the reading should ideally be around 12.6? I also read about running the car and disconnecting the negative battery terminal (if the car stops, you likely have a problem with the alternator). Is this second method still a recommended test, or is it outdated?

Thanks!
Ryan
 
Joined
Jul 16, 2005
Messages
584
Location
Montague, NJ
12.6 is the resting charge on a healthy fully charged battery.
Do not disconnect the battery with the engine running. That was OK in 1960 but not now with modern electronics controlling the alternator.
With engine running look for at least 13.2 volts with a meter at the battery to see if alternator is charging.
Turning on lights and blower on high the voltage should be maintained on a good alternator is the shade tree test.
 
Joined
Sep 6, 2007
Messages
2,179
Location
TX, USA
This is a good suggestion to turn on all electrical load components, like AC, all lights even high beam, if the voltage start dimming, then alternator is weak.

Another way is to get a tester.
There are battery analyzer tester that can test alternator also, or you can use load tester with all loads on also.
Or you can bring to a part store, they will probably test it for free with the hope that if it is bad, you buy alternator from them.
 
Joined
Jan 29, 2012
Messages
681
Location
Canada
Voltage with the engine running should be 14-14.5V. it would also be a good idea to test for the AC ripple. Should be no more than 50 mV. I had a really flaky intermittent electrical problem that turned out to be the alternator. I think the diodes were starting to go bad and I was getting 150 mV but those battery testers didn't flag it as a problem. Swapped it out and my ripple was back down to 40 mV.
 
Joined
Sep 20, 2014
Messages
2,811
Location
Seattle-ish, WA
Voltage with the engine running should be 14-14.5V. it would also be a good idea to test for the AC ripple. Should be no more than 50 mV. I had a really flaky intermittent electrical problem that turned out to be the alternator. I think the diodes were starting to go bad and I was getting 150 mV but those battery testers didn't flag it as a problem. Swapped it out and my ripple was back down to 40 mV.

If it is not strictly a not-charging issue, then +1 on checking the a/c ripple. It can cause weird things when diodes are the issue. It varies greatly car-to-car.
 

reemoeMO

Thread starter
Joined
Oct 12, 2005
Messages
235
Location
KC, MO
Just tested it. Everything looks great! I'm thinking she ran the gas tank too low, as the light was on. Could explain why it was difficult to start on her last night.
 
Joined
Jun 8, 2022
Messages
1,316
Voltage with the engine running should be 14-14.5V. it would also be a good idea to test for the AC ripple. Should be no more than 50 mV. I had a really flaky intermittent electrical problem that turned out to be the alternator. I think the diodes were starting to go bad and I was getting 150 mV but those battery testers didn't flag it as a problem. Swapped it out and my ripple was back down to 40 mV.
I can't access the back of my alternator while running. Is it OK to use a jumper wire from the alternator to my meter to check the ripple? I assume so, but everything I read says I need to read directly off the back of the alternator?
 
Joined
Apr 15, 2017
Messages
2,970
Location
Napa, CA.
I can't access the back of my alternator while running. Is it OK to use a jumper wire from the alternator to my meter to check the ripple? I assume so, but everything I read says I need to read directly off the back of the alternator?
You can read it across the battery :)
 
Joined
Jun 8, 2022
Messages
1,316
You can read it across the battery :)
When I do it this way I get 28 AC Volts - with two different meters its about the same - so I assume I am doing it wrong as that seems ridiculous.

I have yet a third meter. When I use it - it cylces too fast to read so when I put it on max hold - it says about 8.6V. I know at least that one reads mv, because with nothing on it it reads 0.000 AC-V
 
Last edited:
Joined
Aug 25, 2022
Messages
186
Location
West Central Indiana
Voltage with the engine running should be 14-14.5V. it would also be a good idea to test for the AC ripple. Should be no more than 50 mV. I had a really flaky intermittent electrical problem that turned out to be the alternator. I think the diodes were starting to go bad and I was getting 150 mV but those battery testers didn't flag it as a problem. Swapped it out and my ripple was back down to 40 mV.
This can be read with the multimeter on ac voltage
 
Joined
Aug 25, 2022
Messages
186
Location
West Central Indiana
When I do it this way I get 28 AC Volts - with two different meters its about the same - so I assume I am doing it wrong as that seems ridiculous.

I have yet a third meter. When I use it - it cylces too fast to read so when I put it on max hold - it says about 8.6V. I know at least that one reads mv, because with nothing on it it reads 0.000 AC-V
No, the alternator will output up to 110v ac with a bad rectifier.
Usually there's a whine/moan coming from the alternator and/or an rpm proportional sound in the radio.
 
Joined
Jun 8, 2022
Messages
1,316
This can be read with the multimeter on ac voltage
No, the alternator will output up to 110v ac with a bad rectifier.
Usually there's a whine/moan coming from the alternator and/or an rpm proportional sound in the radio.
If it's the V6, is the valve cover gasket leaking oil onto the alternator?
My alternator works perfectly - I am simply trying to learn how to measure AC ripple - in case I ever need to know for real someday.

I have tried 3 meters - I know all 3 work fine on AC in my house. At minimum 1 of them for sure measures AC mv. So must be doing something wrong no?
 
Joined
Aug 25, 2022
Messages
186
Location
West Central Indiana
My alternator works perfectly - I am simply trying to learn how to measure AC ripple - in case I ever need to know for real someday.

I have tried 3 meters - I know all 3 work fine on AC in my house. At minimum 1 of them for sure measures AC mv. So must be doing something wrong no?
Try the Volt scale instead of the mv scale.
 
Top