How to Test an Alternator
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Date: 2021-02-01
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Comments and reviews: 10
Mike
Chris, at about 40 secs in you showed us a loose negative battery clamp.
Later, when you checked voltage drop between alternator body and battery negative post, you actually measured drop between alternator body and the negative wire before it went into that loose clamp.
I think you under estimate the effect a small voltage drop has on charge rate. The relationship between voltage and charge rate is astounding, and very easy to illustrate. I also think you underestimate the voltage at which batteries start to take a charge. You will need a variable voltage DC supply, an ammeter, and a battery to check my thoughts. The experiment will make a good video.
Incidentally, the easy way to check for voltage drop between alternator and battery is to
1) Measure the voltage between the battery posts.
2) Measure the voltage between the alternator post and the alternator chassis.
3) Take the first reading from the second reading.
If you have a significant drop, then check between battery post and alternator post. If that doesnt find much of a drop, then you know you have a problem with the earth link. A cable from an alternator mounting bolt to the earth clamp on the battery will solve that. The cable needs to be at least 16mm2.
As a rule of thumb, if extra lights, sound equipment, etc have been added, then bigger, or more wires need fitting between alternator and battery.
Manufacturers cut things to the bone.
reply
Chris, at about 40 secs in you showed us a loose negative battery clamp.
Later, when you checked voltage drop between alternator body and battery negative post, you actually measured drop between alternator body and the negative wire before it went into that loose clamp.
I think you under estimate the effect a small voltage drop has on charge rate. The relationship between voltage and charge rate is astounding, and very easy to illustrate. I also think you underestimate the voltage at which batteries start to take a charge. You will need a variable voltage DC supply, an ammeter, and a battery to check my thoughts. The experiment will make a good video.
Incidentally, the easy way to check for voltage drop between alternator and battery is to
1) Measure the voltage between the battery posts.
2) Measure the voltage between the alternator post and the alternator chassis.
3) Take the first reading from the second reading.
If you have a significant drop, then check between battery post and alternator post. If that doesnt find much of a drop, then you know you have a problem with the earth link. A cable from an alternator mounting bolt to the earth clamp on the battery will solve that. The cable needs to be at least 16mm2.
As a rule of thumb, if extra lights, sound equipment, etc have been added, then bigger, or more wires need fitting between alternator and battery.
Manufacturers cut things to the bone.
reply
autos
chris in 9 minutes you taught me what my shop teacher failed to teach me in a month, especially now during covid i wasnt there all the time so notes where harder to take and he never went over it with us again. after struggling for so long im not sure why hes making me do every test even after turning it on and running the lights dropped it to 11 volts but its good to learn them. This video helped a lot thank you
reply
chris in 9 minutes you taught me what my shop teacher failed to teach me in a month, especially now during covid i wasnt there all the time so notes where harder to take and he never went over it with us again. after struggling for so long im not sure why hes making me do every test even after turning it on and running the lights dropped it to 11 volts but its good to learn them. This video helped a lot thank you
reply
USMAN
Hi very useful video. I have smart charging alternator changed twice and battery is new for ford focus 2011. Electrician checked all connections and wiring is good but still get battery light when use heating and headlights even sometimes without them and goes off when driving on motorway. Could it be PCM or alternator causing problem?
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Hi very useful video. I have smart charging alternator changed twice and battery is new for ford focus 2011. Electrician checked all connections and wiring is good but still get battery light when use heating and headlights even sometimes without them and goes off when driving on motorway. Could it be PCM or alternator causing problem?
reply
Carlito
Chris, I'll be honest, when you create videos with this format in mind, they are so much better than the ones where you're running through everything fast with increased volume in your voice. Here, you are calm and taking your time, just more enjoyable to watch and listen, you don't have to be loud to get my attention.
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Chris, I'll be honest, when you create videos with this format in mind, they are so much better than the ones where you're running through everything fast with increased volume in your voice. Here, you are calm and taking your time, just more enjoyable to watch and listen, you don't have to be loud to get my attention.
reply
PoPo
Hi Chris just had a boiling battery last night use the car for 5-6 hrs by delivering, let it cool down for a night and today I did all the test you had on your videos all pass both alt and batt, no physical damage on batt and its still start and run the engine, what would be the reason for that? Any idea? Thanks
reply
Hi Chris just had a boiling battery last night use the car for 5-6 hrs by delivering, let it cool down for a night and today I did all the test you had on your videos all pass both alt and batt, no physical damage on batt and its still start and run the engine, what would be the reason for that? Any idea? Thanks
reply
Jason
Dude, nice video, but please add a warning about electrical shock. People should know to be careful about grounding out the positive terminal, and they shouldn't be touching battery terminals or alternator studs with their bare hands like you were doing.
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Dude, nice video, but please add a warning about electrical shock. People should know to be careful about grounding out the positive terminal, and they shouldn't be touching battery terminals or alternator studs with their bare hands like you were doing.
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LaShaye
Just did this test today on my 07 Jeep and image passed the test! Thanks Chris! You are freaking awesome!
I went for an oil change and was told I NEEED to replace the alternator within a few days! I decided to take matters into my own hands!
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Just did this test today on my 07 Jeep and image passed the test! Thanks Chris! You are freaking awesome!
I went for an oil change and was told I NEEED to replace the alternator within a few days! I decided to take matters into my own hands!
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Deuxiit
This depends on the battery you have. I have agm battery and its brand new car. It charges 14, 4v and when cold weather or battery is low it charges 14, 8v. Its perfectly normal for the car.
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This depends on the battery you have. I have agm battery and its brand new car. It charges 14, 4v and when cold weather or battery is low it charges 14, 8v. Its perfectly normal for the car.
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KayBee
What about when the multimeter reads zero from negative to alternator case, and 12. 1 volts across the battery while the vehicle is running? Is that just a bad alternator?
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What about when the multimeter reads zero from negative to alternator case, and 12. 1 volts across the battery while the vehicle is running? Is that just a bad alternator?
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No
I just got my alternator tested at auto zone and it reads 13 volts and they told me its good. I have a sound system in my car is the alternator strong enough?
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I just got my alternator tested at auto zone and it reads 13 volts and they told me its good. I have a sound system in my car is the alternator strong enough?
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