Using a DMM to test an alt.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 967
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Using a DMM to test an alt.
Hi guys. Thanks for the warm welcome in my other threads. This place is great for helping me with all my fun projects.
I was getting some testing done on my electrical system this last weekend, and found I have no clue how to use my DMM. I want to test my alt. I know how to use it as a volt meter, but what setting do I put it on to test amps? I have an older style alt, but I don't know how many amps it puts out, and I can't find the instructions to this Multi Meter.
Thanks for the help everyone. Sorry for the dumb question.
I was getting some testing done on my electrical system this last weekend, and found I have no clue how to use my DMM. I want to test my alt. I know how to use it as a volt meter, but what setting do I put it on to test amps? I have an older style alt, but I don't know how many amps it puts out, and I can't find the instructions to this Multi Meter.
Thanks for the help everyone. Sorry for the dumb question.
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 967
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Originally Posted by Kalway
Well if you don't have a rear window defroster (I don't know what year you have) it should put out like 60 some odd amps.
It's a 1977, but I don't think it's the original alt. It has a reman sticker on it but doesn't say what size. I just want to know what setting to put the DMM on to test amperage.
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Be careful, unless it is a very expensive dmm, it will only handle about 10 amps although that is usually fused.
For quick tests, I use an old Snapopn induction ammeter that you just lay over the wire in a channel on the back of the meter and it tells you the amperage and direction of flow.
http://buy1.snapon.com/catalog/item....re&dir=catalog
For quick tests, I use an old Snapopn induction ammeter that you just lay over the wire in a channel on the back of the meter and it tells you the amperage and direction of flow.
http://buy1.snapon.com/catalog/item....re&dir=catalog
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 967
Likes: 0
Originally Posted by noonie
Be careful, unless it is a very expensive dmm, it will only handle about 10 amps although that is usually fused.
For quick tests, I use an old Snapopn induction ammeter that you just lay over the wire in a channel on the back of the meter and it tells you the amperage and direction of flow.
http://buy1.snapon.com/catalog/item....re&dir=catalog
For quick tests, I use an old Snapopn induction ammeter that you just lay over the wire in a channel on the back of the meter and it tells you the amperage and direction of flow.
http://buy1.snapon.com/catalog/item....re&dir=catalog
Cool tool but that's a lot to spend for just a test that I'll only do once.
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Instructor
Joined: Jun 2006
Posts: 125
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From: East Windsor CT
Originally Posted by noonie
Be careful, unless it is a very expensive dmm, it will only handle about 10 amps although that is usually fused.
For quick tests, I use an old Snapopn induction ammeter that you just lay over the wire in a channel on the back of the meter and it tells you the amperage and direction of flow.
http://buy1.snapon.com/catalog/item....re&dir=catalog
For quick tests, I use an old Snapopn induction ammeter that you just lay over the wire in a channel on the back of the meter and it tells you the amperage and direction of flow.
http://buy1.snapon.com/catalog/item....re&dir=catalog
I agree, even most fluke meters (200+ bucks) will only do 10 amps. You need to find one with a ring terminal (wire goes through or over terminal) compared to an inline time with like your dmm. You could put your dmm on Vdc and measure the voltage at the battery terminals with the car on it should read anywhere from 13.5 - 14.4 volts. If its reading 12.5 then its dead. Plus your alt. will only produce current when its needed. So trying to measure it unless you have your electrical under full load is kind of a waste.
Don
Thread Starter
Burning Brakes
Joined: Sep 2006
Posts: 967
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Originally Posted by gorrie16
Plus your alt. will only produce current when its needed. So trying to measure it unless you have your electrical under full load is kind of a waste.
Don
Don
Yea, I was gonna turn on everything. Heater, lights, stereo, and anything else with a switch. Do you think anyplace locally has a tool for rent?
Instructor
Joined: Dec 2005
Posts: 196
Likes: 0
From: Burlington WI
Alternators are usually tested for voltage output with a load. That what a voltage reg. does. You can bench test and see whats going
on in the inside of the alternator but you have to have it off the car
and than the ground the reg. so it full output. You can test it on the
car but if its a delco SI alternator you still have to ground the regulator
to get full amperate at so may rpms. You can take a amp meter like
I have had for years and put in line with your hot wire. In other words
from alternator thru amp meter than wire. My has clips with #10 wire
. If you hook it up wrong, ( reverse wires) it will read neg. amps. Use
a 60 amp gauge meter.
on in the inside of the alternator but you have to have it off the car
and than the ground the reg. so it full output. You can test it on the
car but if its a delco SI alternator you still have to ground the regulator
to get full amperate at so may rpms. You can take a amp meter like
I have had for years and put in line with your hot wire. In other words
from alternator thru amp meter than wire. My has clips with #10 wire
. If you hook it up wrong, ( reverse wires) it will read neg. amps. Use
a 60 amp gauge meter.
Instructor
Joined: Jun 2006
Posts: 125
Likes: 0
From: East Windsor CT
Originally Posted by Block_Head
Yea, I was gonna turn on everything. Heater, lights, stereo, and anything else with a switch. Do you think anyplace locally has a tool for rent?
Autozone or Advanced Auto Parts or what have may have them to rent but I dont know since they have the testers there. Try calling them, its worth a shot. Even if you turn everything on, I doubt you'll see the 60 amps, car altnator sizing was chosen not to be close to power power for safety and realibility issues.
Instructor
Joined: Dec 2005
Posts: 196
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From: Burlington WI
I don't believe that. 15 amps. I work at a alternator, starter rebuilding
business and that is all we do. Most likely reg was not loaded and thats
all the system was calling for. Like I said. Test the with a volt meter
across the battery. should be in the 13's. 13.8 is good but can go into
the 14's. 15 is to high and anything 12.5 or lower is not enough. Turn
on heater fan on high and headlights and wipers. The rear defogger
does suck up volts.
business and that is all we do. Most likely reg was not loaded and thats
all the system was calling for. Like I said. Test the with a volt meter
across the battery. should be in the 13's. 13.8 is good but can go into
the 14's. 15 is to high and anything 12.5 or lower is not enough. Turn
on heater fan on high and headlights and wipers. The rear defogger
does suck up volts.
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Originally Posted by jayr
Turn
on heater fan on high and headlights and wipers. The rear defogger
does suck up volts.
on heater fan on high and headlights and wipers. The rear defogger
does suck up volts.
Wouldn't this be pretty much a full load for the alternator?
Race Director
Joined: Mar 2006
Posts: 14,112
Likes: 28
From: Florida
Here is a link to the machine one of the local Advance Auto stores uses. It is not a Snapon but a Vat and they will test for FREE.
http://www.snapondiag.com/vat45-batt...ing-system.asp
It will put a full load draw on the alternator to see what it's max output is. It can also test your normal output at any rpm etc.
BTW, I have had 63amp units test out at 20amps or less before. It means they are bad.
http://www.snapondiag.com/vat45-batt...ing-system.asp
It will put a full load draw on the alternator to see what it's max output is. It can also test your normal output at any rpm etc.
BTW, I have had 63amp units test out at 20amps or less before. It means they are bad.
Race Director
Joined: Feb 1999
Posts: 19,659
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From: Lenoir City Tennessee
Ci 6, 8 & 10 Veteran
St. Jude Donor '05-'06-'07-'08-'09-'10-'11-'12-'13-'14-'15
Your digital multi meter will not read amps that high. Do not try to do this. It will blow the fuse in the meter if you are lucky. If you do not have the meter on the correct setting, it could burn the meter up or possibly harm the user. To use the meter as an ammeter, you have to put it in series with the circuit. When you measure voltage, you are putting the meter in parallel with the circuit.
Instructor
Joined: Dec 2005
Posts: 196
Likes: 0
From: Burlington WI
If your want to drain your battery stone dead and jump it and start
the engine you will get a full amp charge at the very beginning but will
taper. If you run the alternator too long with a dead battery you cook
the alternator. They are not a battery charger. They are design to maintain the battery charge will running the car. That is why the volt
age regulator tapers off. Also discharging the batteries now day is not
the smartest thing to do as they will ruin the battery . I was told recently that the batteries today will only take 3 complete discharges
and the life of the battery is greatly shorten.
the engine you will get a full amp charge at the very beginning but will
taper. If you run the alternator too long with a dead battery you cook
the alternator. They are not a battery charger. They are design to maintain the battery charge will running the car. That is why the volt
age regulator tapers off. Also discharging the batteries now day is not
the smartest thing to do as they will ruin the battery . I was told recently that the batteries today will only take 3 complete discharges
and the life of the battery is greatly shorten.
Race Director
Joined: Sep 2005
Posts: 18,640
Likes: 121
From: Farragut,TN
Thats what i like about my '69 with an amp meter in the instrument cluster it shows what the alt. is putting out at any given time. don't know why they switched over to a volt meter in the later cars. my alitnator puts out about 40 amps on start up and falls to just above 0 after running. I have another car that has the voltmeter. it will show 14.5 volts on start up and after running for a while it will show about 13+ volts when the battery is charged and still running.