Alternator measured voltage question - in AC and DC mode...
09-03-13, 05:30 PM
#1
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Alternator measured voltage question - in AC and DC mode...
So, I replaced the alternator on my LS400 with a rebuild Denso, hi-amp one, a few weeks ago, and being paranoid that I am, I decided today to check the voltage on it.
In DC mode, my multimeter reports 13.6V, quite steady (goes sometimes to 13.59 but mostly it is at 13.6V). The battery is full, as it is new, so nothing to worry about there either I think.
Now, thinking that those diodes may be changed when the alternator was refurbished with el-cheapo kind, I measured the voltage using AC setting on my multimeter hoping to see pretty much - nothing. Well, that was not the case.
Instead of seeing something close to zero in AC mode, what I got was three sets of numbers quickly changing, taking about a second to cycle through these values:
36V
8V
0.2V
36V
8V
0.2V
36
8V
0.2V
and so on. Like I said, it takes about a second to cycle through those values.
Methinks, this points to failing diodes on the alternator.
But, I am far from an expert on these things, so if anyone wants to chip in (Yamae?) please do, enlighten me... Perhaps I am just being paranoid (again - just ask my wife
)?
ps. In case you are wondering what type of multimeter I am using, it is "Aidetek", which seems to be quite good for the money (paid around $100 for it)... easily compares to those costing hundreds of dollars, so I don't think that the meter is lying to me.
In DC mode, my multimeter reports 13.6V, quite steady (goes sometimes to 13.59 but mostly it is at 13.6V). The battery is full, as it is new, so nothing to worry about there either I think.
Now, thinking that those diodes may be changed when the alternator was refurbished with el-cheapo kind, I measured the voltage using AC setting on my multimeter hoping to see pretty much - nothing. Well, that was not the case.
Instead of seeing something close to zero in AC mode, what I got was three sets of numbers quickly changing, taking about a second to cycle through these values:
36V
8V
0.2V
36V
8V
0.2V
36
8V
0.2V
and so on. Like I said, it takes about a second to cycle through those values.
Methinks, this points to failing diodes on the alternator.
But, I am far from an expert on these things, so if anyone wants to chip in (Yamae?) please do, enlighten me... Perhaps I am just being paranoid (again - just ask my wife
)?ps. In case you are wondering what type of multimeter I am using, it is "Aidetek", which seems to be quite good for the money (paid around $100 for it)... easily compares to those costing hundreds of dollars, so I don't think that the meter is lying to me.
09-03-13, 05:47 PM
#2
Lexus Champion
yes! - you are!
those readings you got are fine
Aidetek is one of the better made Chinese knockoffs of a Fluke meter, most models are down below $50 these days
those readings you got are fine
ps. In case you are wondering what type of multimeter I am using, it is "Aidetek", which seems to be quite good for the money (paid around $100 for it)... easily compares to those costing hundreds of dollars, so I don't think that the meter is lying to me.
Last edited by LScowboyLS; 09-03-13 at 05:52 PM.
09-03-13, 06:01 PM
#3
Moderator
LOL. The value change is just caused by your wrong measuring method.
Your multimeter is surprised to get a uneven voltage source and it shows different values each time the sampling is done.
Don't forget to use a capacitor in series with the lead wire when you use the AC mode to measure a DC+ ripples.
You have to eliminate the DC component when you measure the ripples using the AC mode of your multimeter.
Your multimeter is surprised to get a uneven voltage source and it shows different values each time the sampling is done.
Don't forget to use a capacitor in series with the lead wire when you use the AC mode to measure a DC+ ripples.
You have to eliminate the DC component when you measure the ripples using the AC mode of your multimeter.
09-03-13, 07:32 PM
#4
Advanced
Thread Starter
Thanks guys, it will make for better sleep tonight
Yamae (or LScowboy or anyone else who knows for sure), what would the correct procedure be to check for the state of diodes if there is a way to do it with a DMM?
Yamae (or LScowboy or anyone else who knows for sure), what would the correct procedure be to check for the state of diodes if there is a way to do it with a DMM?
09-03-13, 09:22 PM
#5
Moderator
You need an oscilloscope to check each diodes viewing the rectified wave form. But using a DMM, you can mostly check the condition of diodes. The method is to measure the ripples. You need a capacitor like this as is indicated at this post.
Above method is to check the ECU condition measuring the ripples at the 5V DC line generated for processors in the ECU. The 5V DC line is coming to the DIAG connector under the hood and without pulling out and opening the ECU box, you almost can check it. As you see the photo, there is an orange capacitor in series with the lead to the VMM. All you need is to measure the 12V DC line like this way.
The capacitor should be around 0.1-1uF depending on the internal impedance of the DMM. But the value is not very strict and you don't need to worry it too much. Roughly speaking, if the ripple is less than 400mV rms AC, then those diodes in your alternator are good.
09-03-13, 11:22 PM
#6
Lexus Champion
You need an oscilloscope to check each diodes viewing the rectified wave form. But using a DMM, you can mostly check the condition of diodes. The method is to measure the ripples. You need a capacitor like this as is indicated at this post.
Last edited by LScowboyLS; 09-05-13 at 01:52 AM.
09-04-13, 10:17 PM
#7
Advanced
Thread Starter
A DMM can't check everything, but you can consider it is OK as far as you have higher than 13.0V DC at idle and 14.2-14.4V DC at 1500 rpm with headlights ON.
... All you need is to measure the 12V DC line like this way.
The capacitor should be around 0.1-1uF depending on the internal impedance of the DMM. But the value is not very strict and you don't need to worry it too much. Roughly speaking, if the ripple is less than 400mV rms AC, then those diodes in your alternator are good.
... All you need is to measure the 12V DC line like this way.
The capacitor should be around 0.1-1uF depending on the internal impedance of the DMM. But the value is not very strict and you don't need to worry it too much. Roughly speaking, if the ripple is less than 400mV rms AC, then those diodes in your alternator are good.
Where would I tap into the the 12V DC Line - at the battery terminals, or is there 12V available somewhere in the diagnostic terminal inside the cabin, the one on the driver's side?
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09-04-13, 11:22 PM
#8
Moderator
You just can simply measure the voltage at battery terminals. The best point is the +B terminal of the alternator and the housing of it but this point is not easy to access.
You also can measure the voltage at the cigarette plug. This point shows slightly lower voltage compared with +B terminal though.
09-05-13, 08:44 PM
#9
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Thread Starter
You are welcome.
You just can simply measure the voltage at battery terminals. The best point is the +B terminal of the alternator and the housing of it but this point is not easy to access.
You also can measure the voltage at the cigarette plug. This point shows slightly lower voltage compared with +B terminal though.
You just can simply measure the voltage at battery terminals. The best point is the +B terminal of the alternator and the housing of it but this point is not easy to access.
You also can measure the voltage at the cigarette plug. This point shows slightly lower voltage compared with +B terminal though.
09-06-13, 06:48 AM
#10
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