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  #1  
Old 01-04-2015, 12:39 PM
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190e Alternator Test Results

With headlights and all accessories on, I get 13.6V and 56A. Increasing throttle doesn't impact the numbers much. I have a new voltage regulator w/ brushes on the way. My question is about the diodes. An A/C voltage test produces this:

38V
20V
11V
6V
0V

The numbers repeat as a cycle. My book mentions a number above 0.5V indicates bad diodes. But it does not mention cycling numbers. Does 0.5V apply to the highest number?

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  #2  
Old 01-04-2015, 03:38 PM
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if it's 38v and not .38v, you need an alternator. good luck. chuck.
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  #3  
Old 01-04-2015, 03:43 PM
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Yes it was 38. I actually typoed 6V, that number was 0.6V. The others had decimals like 38.5V, etc.
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  #4  
Old 01-04-2015, 06:47 PM
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I rechecked before buying a new alternator. It is in autoranging mode and shows the V symbol. Curiously, when I change to manual mode it only moves the decimal place. So if I hit the button three times, it shows .0038V. Bit when I get it to the mV range it just reads OL as expected.
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  #5  
Old 01-05-2015, 07:42 AM
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Which diodes are you checking? The rectifier bridge? I usually just check these for go-no go directional resistance check.
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  #6  
Old 01-05-2015, 12:07 PM
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Gentlemen,
not all is clear to me, so I'd like to explain the function of a diode first.
A diode may be looked at as a one way valve. There are markers such as ANODE and CATHODE.
The anode side will let a current flow towards the cathode side, if the potential is positive at the Anode side. In the course it will drop approximately 0.6 Volts across the P-N junction, with silicon diodes. If polarity is reversed, no current will flow.
In other terms: A DVM that can measure resistance, can be used to do a quick check on a single diode e.g. Positive lead to the Anode and Negative lead to Cathode. It should indicate some low resistance. Reverse the leads and you should see an OL or a very high resistance beyond several Megohms.

With a fully loaded electrical system (56Amp) and a readout of 13.6 Volt, it seems to be correct to me. If I am not mistaken the 190E had either a 60 Amp or 72 Amp Alternator installed. Even if the alternator is capable of delivering more current, it doesn't mean necessarrily it will do so, this would depend on the load condition.
I take that the current was measured with a high current meter in series with the battery lead?
The fact that the numbers didn't change much would indicate that the alternator is working fine.

One can get easily confused with the measure/test points. There are a total of 6 diodes built into the rectifier and depends on alternator design. It is easy to measure not just an individual diode, but more than one.
Depending on how many diodes are measured, the voltage can peak to the 38V, you have measured. This would occur with the black lead to cassis and the red lead attached to the Cathode(s).
NOTE: The Voltage reading, depends on where the test probes are connected to.....

Your book is probably talking about individual diode measurements, using a diode tester.
If a diode is measured with a diode tester, the number in display is a voltage (not resistance). A single diode has typically a voltage drop of 0.6V.
On the other side I have seen posts, where a voltage test is performed while the vehicle is running or the alternator is spinning. In this test the forward voltage, of a diode, is guesstimated and someone set a pass / no pass number. Again a silicon diode in forward direction can drop a voltage between 0.3 - 0.8 Volts so a 0.5 V is only a reference value.

The Alternator is a energy generator, the diodes will convert the otherwise Alternating Current into a Direct Current.

The 36 Volt are only a peak and with a meter that is fast enough e.g. peak reading, can show these high numbers. This is only the peak of a sine wave and very short in time.

I guess it is enough technical mumbo jumbo
I believe your alternator is doing just what it is supposed to do......

I didn't read your previous post, so I don't know what you are trying to troubleshoot/
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Last edited by oldtrucker; 01-05-2015 at 12:41 PM.
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  #7  
Old 01-10-2015, 02:36 PM
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I didn't measure in series, but put the leads on the positive and negative terminals of the battery. I changed the alternator out this morning, and you are correct, I see the same cycle of numbers with about a 38V peak. I'm not sure how there are so many you tube videos showing the same test with completely different results, unless it is, as you said due to the accuracy of the autoranging feature.

So it appears the diodes in the old one were just fine. The bearings were shot though, and the new one is much quieter. The DC voltage is now 14.5V which is much higher than before, so the brushes were worn out as well.

I'm still scratching my head on the amperage reading. The first thing is I have to clamp it to the positive and negative wires to get any reading at all. I thought the clamp was only supposed to go over the positive wire. I have it in 400A DC mode. But either way, it still read in the mid 50s so that must be the max amps the car uses at full load. The textbook made the assumption that all cars must pull as many amps as the alternator is rated for.
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  #8  
Old 01-10-2015, 05:50 PM
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Maximum alternator output usually occurs at several thousand (alternator rotor) RPM, and with typical drive ratios alternator RPM is usually two to three times engine RPM, so the alternator will not deliver maximum load at idle, but they are usually sized and set up with drive ratios to handle maximum vehicle system electrical load at idle.

I don't have any data for Bosch alternators, but seventies vintage Delco alternators have a specified maximum rotor speed of 18,000 RPM, and I think they delivered maximum rated amperage at about 5000 RPM, but this was usually a good deal higher than maximum vehicle electrical load in order that the alternator support maximum vehicle electrical load at idle.

The voltage regulator opens and closes the circuit to the rotor windings, which act as the field in order to maintain system voltage in the 13.5-14.5 volt range. The raw output is three-phase AC, which is then converted by the rectifier bridge and diode trio to a DC voltage, and a typical voltmeter will measure the average.

If you look at output on a fast scope it will have a sawtooth appearance.

Duke
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  #9  
Old 01-12-2015, 12:17 PM
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Quote:
The voltage regulator opens and closes the circuit to the rotor windings, which act as the field in order to maintain system voltage in the 13.5-14.5 volt range. The raw output is three-phase AC, which is then converted by the rectifier bridge and diode trio to a DC voltage, and a typical voltmeter will measure the average.

If you look at output on a fast scope it will have a sawtooth appearance.
While the test can be performed on a bench, one will have to be careful with the interpretation of the results.
Due to the (no load) shape of the output voltage, the measured value may be much less, if a RMS voltmeter is used. This is because the RMS (Root Mean Square) meter will disect the wave and tries to put it back together in order to get an average value. With a triangular wave it is fairly easy to estimate the average, but one needs to have some electical / electronics background to visualize it.

Are you sure that your Current Clamp can read DC?

Most clamps in the lesser price range are AC clamp meter only. There are DC clamps, but get more expensive, since the way the current is picked up is more difficult. They measure the magnetic field around the wire and run it through a certain algorithm to finally display it. The result may not be very accurate, because of movements, angle and possibly other absorbing obstructions can influence the reading, during measurement.

The best and easiest way to guestimate / test it, would be to connect a battery and spin the alternators axle shaft at (I believe double the engine RPM, depends on ratio) and measure the battery voltage.
If the Battery is fully charged, there will be very little current flowing into the battery and the voltage is close to the alternator (correction "Regulator") output e.g. 13.8 VDC. If it is higher (with battery) careful here, your battery may get a shortened lifespan.
The alternator has to be able to apply a higher than nominal voltage across the battery, otherwise there will be no current flow into the battery.

In general a Starter Motor can pull about 280-350 Amps during cranking. This number depends on the mechanical load e.g. engine compression and anything else that is mechanically attached to the crankshaft.

Right after the engine started, the alternator is now giving whatever it has to give. For example: if the alternator is a 72 Amp alternator, you should see a number not far below. Again, this number largely depends on condition of the battery and how long the starter motor was cranking. The current will very quickly be reduced , because the battery's potential increased e.g. the voltage rose.
A good battery has a larger capacity e.g. can handle a large load for a longer time. Therefore the discharge is not as muich as it would be with a smaller capacity battery. The alternator has to re-charge the battery as fast as possible, therefore will deliver as much current as possible.
As soon as the battery reaches its fully charged state, the voltage regulator will open and the current will stop. Since there are consumer attached at all times, the battery will discharge again, but at a lower rate. The regulator sees the lower voltage and puts the alternator back to the battery to charge it again. This goes on as long as the car is driven / engine is running.

I better stop here!!!!

A word of caution!
With fairly large currents flowing, always make sure that the wiring can handle the current.
If the wire gauge is too small, it can cause a fire or at least the wire will vaporize.
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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

Last edited by oldtrucker; 01-13-2015 at 01:21 PM.
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  #10  
Old 01-12-2015, 01:33 PM
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Quote:
I didn't measure in series, but put the leads on the positive and negative terminals of the battery. I changed the alternator out this morning, and you are correct, I see the same cycle of numbers with about a 38V peak. I'm not sure how there are so many you tube videos showing the same test with completely different results, unless it is, as you said due to the accuracy of the autoranging feature.
The difference is merely where the tesprobes are attached to. I am willing to bet that some of the U-Tube videos are tested across the diode(s). In this case, you'll only measure the voltage drop across the diode e.g. anywhere between 0.3-0.8 Volts.
If you referenced to battery negative post, you'll measure the rectified voltage, which will increase, depending on where the positive test probe is attached to.

Quote:
So it appears the diodes in the old one were just fine. The bearings were shot though, and the new one is much quieter. The DC voltage is now 14.5V which is much higher than before, so the brushes were worn out as well.
Yes, I think you are right. They were just fine.
The higher voltage, however is probably related to the condition of the electrical system and / or the different voltage regulator. You could measure several times under different conditions (Lights on, Light Off etc.) and every time you would get a slightly different reading. The 14.5 Volts are possibly the maximum output voltage that the regulator allows. This is fine as long as it is only for a relatively short time.
The batteries nominal voltage is 12VDC, realistically you'll measure most of the time (engine off) about 12.6 - 12.8 Volts. During the starter cranking this voltage will probably drop close to 11.8 Volts but quickly will rise again after the engine started. Thats when the maximum alternator voltage should occur. This also means maximum current!

Quote:
I'm still scratching my head on the amperage reading. The first thing is I have to clamp it to the positive and negative wires to get any reading at all. I thought the clamp was only supposed to go over the positive wire. I have it in 400A DC mode. But either way, it still read in the mid 50s so that must be the max amps the car uses at full load. The textbook made the assumption that all cars must pull as many amps as the alternator is rated for.
When using the clamp in an AC measurement application, the current is going in one direction and changes into the other direction, depending on wave form, e.g. is alternating at the frequency of the AC.

Not knowing the design of your clamp meter, it is possible that it needs some supply voltage to function in DC mode at all.
In a DC measurement, the current flow is steady in one direction, there is no reference for it, so you'll have to provide a reference, since every electrical energy has to come back to the source. Some of it is dissipated and converted into heat, so it won't make it back as such.
A general rule of thumb when measuring electricity is to set the measuring device to the approximate and expected value. (That's easy to say for me) Measure devices will measure with a larger error if the range is to wide. For example: if I expect a 60 Amp current, I would select a 100 Amp range, if available. In this way the error is smaller than at 400 Amp range. In some case (autorange) the meter will select the range and it is out of your hands.
The text book is right and wrong at the same time.
If I would design a vehicles electrical system, I would start out, what is called a worst case scenario. The worst condition that can happen, that I have to consider. This doesn't mean that the system is always running at this condition.
So, I take all the consumer and I mean all of them!
Add all of them together and get an idea of how much current (Power) I have to provide for it to function under the worst condition. For example: if all consumer require a current of 50 Amps, I will give some room for short term higher consumer etc. and say, well a 72 Amp alternator should suffice. Of course, I would have to built in some protection, perhaps a 150 Amp fuse (the size of the fuse depends on the maximum rating of the alternator). The alternators absolute rating could be capable of supplying more current, but would destroy itself in the course.
So in your test you see the current that is currently required, but not the maximum.

Sorry for those long write ups, but this is a subject that can't be explained in a one liner!
__________________
Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert

Last edited by oldtrucker; 01-13-2015 at 01:33 PM.
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  #11  
Old 01-13-2015, 02:00 AM
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Now that you've got the DMM

This is the Book for Electrical matters (Land or Sea).
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190e Alternator Test Results-screenhunter_170-jan.-13-02.03.jpg  
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  #12  
Old 01-15-2015, 12:52 PM
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Your meter sounds suspect. I usually use a Fluke, but on the occasions that I don't, strange things often occur...

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