Small signal diodes such as switching diodes can be tested in this manner, but the large, hi-amperage power diodes used in auto alternators can have faults not detectable with reverse polarity continuity testing. These need to be tested under high-amperage loading. Also, your DVM in Diode or Resistance Mode doesn't measure voltage --- it measures resistance in ohms. Still, thanks for taking time to make the video. Your testing still has value, because ANY diode which fails reverse polarity testing is bad.
I wish I had seen this months ago, it would have saved me a lot of time fruitlessly searching for the source of a large parasitic draw. I needed a voltmeter with a horizontal display to plug into my accessory socket which is located close to the floor of my Olds Silo van. I purchased a $6.99 battery/alternator tester from Harbor Freight that displayed 3 lights for a fully charged battery and only 2 lights for a good functioning alternator. At first I considered it a toy compared to a real voltmeter. The 3rd light (Red) would be an indication that the alternator was overcharging the battery. One day I noticed that one of the alternator LEDs (yellow) stayed lit when I shut off the engine. That was a sign of current flowing from the battery back to the alternator. I bought a brand new alternator from Amazon and solved the problem. I didn't need that new battery after all. However, my 1992 motor-home did so I bought another new battery for it and plugged in my H/F tester with the engine running. All 3 lights lit on the alternator test indicating that the alternator was overcharging and to make matters worse, the yellow light stayed on when I shut off the engine. Double trouble. I'll check the diodes but am confident that I need to buy another new alternator. UPDATE: I tried your test on the bad alternator that I replaced and it did not show any sign of problems that I do know exist. Perhaps the rectifier board has to be removed from the suspect alternator before checking the diodes. Running the test on a new alternator will not prove anything unless there is a factory defect.
Ally your bad alternator may have failed on thousands of things, most common is voltage regulator, this test only checks the diodes. if your failed alternator has a bad regulator, still may test ok on a diode check.
There are positive diodes and negative diodes constituting what is called the rectifier ... you checked half the diodes, you must remove the upper case to access the rectifier in order to check both positive and negative diodes as either one could go bad ...
Like a ton of other car owners, i hate dealing with electrical issues. for whatever reason my battery refuses to charge. Replaced alternator, replaced the battery, can't seem to get it to hold a charge. a bunch of people told me that i probably just got a bad alternator from a certain irish auto parts store. but the alternator on the 3.4L ford engine is on the bottom, (go ford!) so i really would hate to replace the alternator again.
So is this range of milli-volts the same for all automotive alternators? I just bought a 120 amp for my truck and it reads 1250 milli-volts right out of the box.
I did this on my 2002 suburban, which was having intermittent over-night drain. So, I got the same results as here, which would mean the diodes are good. Yet, when I take the battery feed cable off the back of the alternator, switch my meter to volts, and place the positive lead to the cable end, and the negative to the alternator post, I get 12.6 volts, which would indicate a ground through the post. I thought only a bad diode would let it do that.
I think I have this problem with my 2001 Ford Taurus. With everything turned off and the battery ground disconnected from the battery if I touch the battery ground connection to the ground post of the battery I can see a little spark. If the alternator diode is bad and I disconnect the alternator connections then I should not see the little spark if I again touch the battery ground connection to the ground post of the battery? Thanks!
Your basic understanding is ok but fundamentally flawed while describing it. The alternator produces AC current. The "diode rectifier" is a series of multiple diodes and how many there are depends on manufacturer. A diode is a check valve sure but a rectifier is what is converts AC to DC. Depending on exactly which diode blows in your diode rectifier will depend on how faulty the alternator and/or voltage regulator will behave. It is possible for a completely blown rectifier to produce AC voltage to the battery and that would be very bad. Alternator = AC, Rectifier = AC to DC converter.
HI. I checked mine today as it has ben having problems. Red on case and black on v post got a reading of 1.860. turned the leads round with red now on v post got a reading of .481. I take it that means the diode had had it. When the Alternator was spinning i was only getting 5 volts out of the v post.
Can you say something about the smart charging alternator...would have the same principal, I am trying to find a fault kn a ford transit ( dual battery), old alternator was showing 3v output, replaced it with a second hand one and got the same riding, I'll do some twsting when I'll have a minute but I would like some input. Thanks, good video.
Multimeter can check diodes. It all depends on wiring diagram. This is not going to work for all brands of alternators. Rectifier must be removed put on bench to test each diode. There are 6 to 8 diodes in a three phase alternator. Minimum 4 diodes in case of 1 phase alternator. All diodes must be checked. See service manual. Oscilloscope can show diode failures.
For every one have Saab 9.3 or 9.5 !! the problem with alternator and battery ... the problem is from negativ contact cable (( cause its weak and very dirty )) ..even if you change a new alternator or battery problem it well comes again !! so first check negative cable and clean it .. cause if its dirty than Diode inside alternator well burn after maybe one week ! than the battery also ! . Saab do mistake with cable contact .(speciality the negative cable) ... if you clean it good no need change battery or alternator ... any one have change alternator and battery stil have problem with charging he must clean or put strong negative cable ((from battery to body )) and also (( from battery to engine ))both cable !! (( if body cable weak you well see it becomes worm the head of contact negative cable )) (( if the negative contact cable from the body to alternator weak than alternator Diode well burn .than the electric comes back from battery to alternator the battery becomes very weak )). just do that well fix your problem .... thanks .
the more grounds is never a bad thing, nonetheless consider that european cars don't have grounds from alt. body to chassis, battery to chassis, they have a single chain ground: battery cabled to engine (a big one) then alternator contact grounded to engine annd engine contact grounded to chassis. personally i had no improvement by adding more grounds, but as i said, it surely doesn't make it worse.
Learn how to check for ripple. Ripple happens when you have onbor more open diodes and you get AC on top of your DC. The ripple will drive the computer nuts because things like the cam shaft position sensor pulses get confused with a bad alternator diode AC voltage. This means the computer throws a bunch of bad codes for the wrong thing, the diodes are open. I was hoping for a way to check for ripple with a multimeter but I was disappointed. All the other videos show an oscilloscope method.
Never assume any new part is good without testing. DOA's do happen because not every company has quality assurance checkers especially if it's from China.
Nice I have possible good alternator from the that read 0.548 diods. I have the other that came with the car but the diod test read 0.539 or 0.529 diod test but I thought I saw go the other way which should nt be there way around because it flows One way. Im nt sure if the mechanic checked the connection when said the alternator was bad that came with car.
I tried this because my truck is dying over night but my numbers when doing this are no where near 500 mine are 25 and then slowing start climbing when I swap them I get no reading
thanks, a one way valve is called a non-return valve or check valve, i know i know, it's just a metaphor, this isn't plumbing but it sounds better. (^^,)
Check valves are used in the fuel evap system. To say they are only used in plumbing is incorrect. Car guys actually associate check valves with car parts which is why it’s an easy analogy for car guys to understand.😉 Great video. I have a video on a faulty alternator causing a parasitic drain where I use the same analogy. I didn’t do a diode test though. Wish I did after seeing your video but my diagnosis was still correct. Good video. I think your video will help a lot of people. 👍
I have a red light charging system alarm,I did replaced the alternator the battery ....and the problem persists.....worst some time I didn't have electricity to run the car while riding ...it's driving me crazy
Could be a short in the wiring going from positive battery post to positive alternator post or a grounding issue to the car chassis, starter or fuse box. Try doing a load lest with the volt meter on each section to determine where the drain is coming from if you’ve already eliminated the battery and alternator.
hi i m having very strange problem i applied wd40 on injector and starter relay but did not dry it and start the car resultantly battery warning light came on that alternator stopped charging after that its on problem with its own battery when i start it with other cars battery with leads its ok batteyr light goes off and sometimes on its own battery also but most of the time with its own battery start battery warning light came on in dashboard and if i start the car with push of some peoples help alternator doesnt start charging can you pls guide me ?
Sometimes the simplest information can mean the most useful to someone. Thanks!
It may of taken you several minutes to get to the point, But better than others, At least I understand how to test it now. Thanks
Explanation was exhausted but informative. Thanks.
Small signal diodes such as switching diodes can be tested in this manner, but the large, hi-amperage power diodes used in auto alternators can have faults not detectable with reverse polarity continuity testing. These need to be tested under high-amperage loading. Also, your DVM in Diode or Resistance Mode doesn't measure voltage --- it measures resistance in ohms. Still, thanks for taking time to make the video. Your testing still has value, because ANY diode which fails reverse polarity testing is bad.
Thanks for the input.
my meter is reading 18 when swapped connectors i knew it was bad good video
Thanks for the video on how to test the diodes,Sir .
Great video. Thank you for posting.
Thank you for sharing and explaining everything .
Simple and to the point "love it" well done
Thanks.
First video i understand no bs. Now i can check my alternator
Nice
That is a very nice workbench you have
lol
Great video my Island brother. Thank you.
good video. helped me check my alternator. thanks, bro.
I needed that man..Thank you..easy and to the point.awesome
No problem.
Good video. I am going to try this tomorrow.
I think I may have a weak better, but then again it might be a bad alternator.👍👍
Okay
I wish I had seen this months ago, it would have saved me a lot of time fruitlessly searching for the source of a large parasitic draw. I needed a voltmeter with a horizontal display to plug into my accessory socket which is located close to the floor of my Olds Silo van. I purchased a $6.99 battery/alternator tester from Harbor Freight that displayed 3 lights for a fully charged battery and only 2 lights for a good functioning alternator. At first I considered it a toy compared to a real voltmeter. The 3rd light (Red) would be an indication that the alternator was overcharging the battery. One day I noticed that one of the alternator LEDs (yellow) stayed lit when I shut off the engine. That was a sign of current flowing from the battery back to the alternator. I bought a brand new alternator from Amazon and solved the problem. I didn't need that new battery after all. However, my 1992 motor-home did so I bought another new battery for it and plugged in my H/F tester with the engine running. All 3 lights lit on the alternator test indicating that the alternator was overcharging and to make matters worse, the yellow light stayed on when I shut off the engine. Double trouble. I'll check the diodes but am confident that I need to buy another new alternator.
UPDATE: I tried your test on the bad alternator that I replaced and it did not show any sign of problems that I do know exist. Perhaps the rectifier board has to be removed from the suspect alternator before checking the diodes. Running the test on a new alternator will not prove anything unless there is a factory defect.
Okay
Ally your bad alternator may have failed on thousands of things, most common is voltage regulator, this test only checks the diodes. if your failed alternator has a bad regulator, still may test ok on a diode check.
Thank you! 🙏🏾
There are positive diodes and negative diodes constituting what is called the rectifier ... you checked half the diodes, you must remove the upper case to access the rectifier in order to check both positive and negative diodes as either one could go bad ...
To the point! Thank you.
Cool
Great job, thanks
NP
simple and clarifying
Yep
Good info thank you
Excellent video 👍, thank you. What if my alternator is reading 0.003v in both testing? Means alternator is bad?
Thank u very much
U have explained very well
Cool
Like a ton of other car owners, i hate dealing with electrical issues. for whatever reason my battery refuses to charge. Replaced alternator, replaced the battery, can't seem to get it to hold a charge. a bunch of people told me that i probably just got a bad alternator from a certain irish auto parts store. but the alternator on the 3.4L ford engine is on the bottom, (go ford!) so i really would hate to replace the alternator again.
You need a parasitic battery drain test done
Thanks, very helpful 👍
Good job
So is this range of milli-volts the same for all automotive alternators? I just bought a 120 amp for my truck and it reads 1250 milli-volts right out of the box.
Very helpful thank you
Tnx for the demo
Great tutorial , Einstein robo cop 🚓 you are definitely talented and intelligent probably too smart
Easy test nicely explained.
Thank you
thank for good tips...
Cool
Nice and easy 👍
HI, I'm getting a 14v DC as well as 30v AC at my battery posts, can you advise ?
excellent sir
Thanks
I did this on my 2002 suburban, which was having intermittent over-night drain. So, I got the same results as here, which would mean the diodes are good. Yet, when I take the battery feed cable off the back of the alternator, switch my meter to volts, and place the positive lead to the cable end, and the negative to the alternator post, I get 12.6 volts, which would indicate a ground through the post. I thought only a bad diode would let it do that.
This doesn't rule out a bad Diode
today, i learned my stereo alternator problem is a diode problem.
Where do we buy hi heat capable diodes?
Thanks a million.
Very nice
I think I have this problem with my 2001 Ford Taurus. With everything turned off and the battery ground disconnected from the battery if I touch the battery ground connection to the ground post of the battery I can see a little spark. If the alternator diode is bad and I disconnect the alternator connections then I should not see the little spark if I again touch the battery ground connection to the ground post of the battery? Thanks!
Your basic understanding is ok but fundamentally flawed while describing it. The alternator produces AC current. The "diode rectifier" is a series of multiple diodes and how many there are depends on manufacturer. A diode is a check valve sure but a rectifier is what is converts AC to DC. Depending on exactly which diode blows in your diode rectifier will depend on how faulty the alternator and/or voltage regulator will behave. It is possible for a completely blown rectifier to produce AC voltage to the battery and that would be very bad. Alternator = AC, Rectifier = AC to DC converter.
Yes, thanks for the indebt info.
Is 442 alternator diode good or bad?
How many diodes are in the alternator? Is this a rectifier test?
Thanks for the vedio
NP
Okay to do with the help of my favorite
HI. I checked mine today as it has ben having problems. Red on case and black on v post got a reading of 1.860. turned the leads round with red now on v post got a reading of .481. I take it that means the diode had had it. When the Alternator was spinning i was only getting 5 volts out of the v post.
Might be
Can you say something about the smart charging alternator...would have the same principal, I am trying to find a fault kn a ford transit ( dual battery), old alternator was showing 3v output, replaced it with a second hand one and got the same riding, I'll do some twsting when I'll have a minute but I would like some input.
Thanks, good video.
Okay
Multimeter can check diodes. It all depends on wiring diagram. This is not going to work for all brands of alternators. Rectifier must be removed put on bench to test each diode. There are 6 to 8 diodes in a three phase alternator. Minimum 4 diodes in case of 1 phase alternator. All diodes must be checked. See service manual. Oscilloscope can show diode failures.
Okay
Mahadev, how would you use an oscilloscope to check the alternator diodes?
Thanks!
If I got 1064, what’s that mean? Thanks
I'm get 68 with both ways i try it, I'm guessing my alternator is literally toast!! 🤣
I got the exact same reading when I switched terminals. So is it a bad diode?
Yes, maybe.
No reading means infinite resistance. Silicon diode gives a reading of 0.7 Ohm, reverse is infinite.
Cool
Are all alternators should have about the same reading?
Depends.
For every one have Saab 9.3 or 9.5 !! the problem with alternator and battery ... the problem is from negativ contact cable (( cause its weak and very dirty )) ..even if you change a new alternator or battery problem it well comes again !! so first check negative cable and clean it .. cause if its dirty than Diode inside alternator well burn after maybe one week ! than the battery also ! . Saab do mistake with cable contact .(speciality the negative cable) ... if you clean it good no need change battery or alternator ... any one have change alternator and battery stil have problem with charging he must clean or put strong negative cable ((from battery to body )) and also (( from battery to engine ))both cable !! (( if body cable weak you well see it becomes worm the head of contact negative cable )) (( if the negative contact cable from the body to alternator weak than alternator Diode well burn .than the electric comes back from battery to alternator the battery becomes very weak )). just do that well fix your problem .... thanks .
Thanks
the more grounds is never a bad thing, nonetheless consider that european cars don't have grounds from alt. body to chassis, battery to chassis, they have a single chain ground: battery cabled to engine (a big one) then alternator contact grounded to engine annd engine contact grounded to chassis. personally i had no improvement by adding more grounds, but as i said, it surely doesn't make it worse.
Learn how to check for ripple. Ripple happens when you have onbor more open diodes and you get AC on top of your DC. The ripple will drive the computer nuts because things like the cam shaft position sensor pulses get confused with a bad alternator diode AC voltage. This means the computer throws a bunch of bad codes for the wrong thing, the diodes are open.
I was hoping for a way to check for ripple with a multimeter but I was disappointed. All the other videos show an oscilloscope method.
Thanks !!!
If it's a New Alternator, shouldn't everything/diode be good??
Never assume any new part is good without testing. DOA's do happen because not every company has quality assurance checkers especially if it's from China.
New parts aren’t always good..
Nice I have possible good alternator from the that read 0.548 diods. I have the other that came with the car but the diod test read 0.539 or 0.529 diod test but I thought I saw go the other way which should nt be there way around because it flows One way. Im nt sure if the mechanic checked the connection when said the alternator was bad that came with car.
I tried this because my truck is dying over night but my numbers when doing this are no where near 500 mine are 25 and then slowing start climbing when I swap them I get no reading
Okay, did you found out the problem?
Yeah I did turn out to be a stuck relay
U can check it on the car?
Yes
My alternator puts out good current for 5 to 10 minutes and then drains current from the battery until the car stops running. Is that a diode problem?
Might be
Might be loose connection, like broken solder, in any of the connections in the alternator components of coil, rectifier, regulator or slip rings.
- Is that a GM 4-pin alt?
Yes, i think so
thanks, a one way valve is called a non-return valve or check valve, i know i know, it's just a metaphor, this isn't plumbing but it sounds better. (^^,)
K
Check valves are used in the fuel evap system. To say they are only used in plumbing is incorrect. Car guys actually associate check valves with car parts which is why it’s an easy analogy for car guys to understand.😉 Great video. I have a video on a faulty alternator causing a parasitic drain where I use the same analogy. I didn’t do a diode test though. Wish I did after seeing your video but my diagnosis was still correct. Good video. I think your video will help a lot of people. 👍
Thanks man, i watch your videos too. Keep up the good work.
so what if i get a reading of .845 ?
This isn't 100% accurate
I have a red light charging system alarm,I did replaced the alternator the battery ....and the problem persists.....worst some time I didn't have electricity to run the car while riding ...it's driving me crazy
Take it to a shop
Could be a short in the wiring going from positive battery post to positive alternator post or a grounding issue to the car chassis, starter or fuse box. Try doing a load lest with the volt meter on each section to determine where the drain is coming from if you’ve already eliminated the battery and alternator.
Yes
I lost the will to live about 2 minutes in
K
😂😂
hi i m having very strange problem i applied wd40 on injector and starter relay but did not dry it and start the car resultantly battery warning light came on that alternator stopped charging after that its on problem with its own battery when i start it with other cars battery with leads its ok batteyr light goes off and sometimes on its own battery also but most of the time with its own battery start battery warning light came on in dashboard and if i start the car with push of some peoples help alternator doesnt start charging can you pls guide me ?
Maybe it's the battery or Alternator.
If I rotate such alternator with 200 rpm, will I get Power? If yes, how much?
idk
Hello my friend do you want to explain how to wiring wiper motor with schematic diagram
Don't have one
Dude, if someone asks you a question and you give vague, two-word responses...why should we subscribe?
Dude go figure, help yourself too smh.
you cannot decide the condition of all diodes with this test
Okay
But is it a good test for knowing if there is a problem or not with at least one of them?
It is enought if you have one burnt diode, so what you whant to say?
killjoy Murali. he didnt say that you could decide diode conditons for them all.
Yes
I got .359 diode pack fucked?
I guess
great video, tiny bit slow with your words. perk up a bit. life is good.
Okay
this video is a disaster!
K
@@newyorkerinct2821
"J"
Very nice