2nd Gen Non-Engine/Transmission Propperly Testing alternator amps and volts

[firefighter_2000]

I have a digital clamp multimeter and Im trying to test the output (amps and volts) of my alternator. I thought there was a thread on this recently, but I cant seem to find it.

I put my clamp meter around the cable coming from the alternator today, and either idle or 1500 rpm it would show around 40 amps. Im not sure if Im doing this correctly.



Can someone please explain the propper way for testing voltage & amps from the alternator.



Thanks

Craig

 

[GAmes]

The voltage part is easy. Put your leads on the pos and neg terminals of a battery with the engine off to see what the static charge is. Start it up and check voltage again, should be about 14-15 volts. Amperage is draw, so the more items you turn on the higher the amp meter will read. I don't know how to check what the alternator's output is capable of.

 

[fest3er]

40A at idle does not sound right. That's over 500W. Even for a 24V, that's a lot of power. Engine off, you should see no current flowing to or from the alternator; you *will* see current flowing to the alternator if a diode has failed. Engine off, key off, you should see less than 1A draw on the driver's side B+ lead from the battery to the starter, heaters, PDC; EOKO, headlights on, you should see less current than the headlight/running light fuse ratings. Engine running and nothing else turned on (lights, HVAC, etc. ), I might be surprised if the alternator produces more than 5A.

Locate the control leads for your intake heaters (the small posts). Use a voltmeter to determine which of them is B+ and which is ground, then reconnect the B+ leads. Start the engine and use a throttle stick to set the idle at 1500 RPM (so the engine uses enough air to keep the heaters from overheating the intake manifold and such). Put your ammeter clamp on the alternator output wire. Connect one of the relay ground posts to ground. Check alternator output; it should be producing 100A or so. Ground the other relay control post. Check alternator output; it should be producing 136A or so (or more if you have a higher output alternator).

Unground both relay posts and reconnect the leads to them. Turn on headlights and high beams. Current draw should be close to or less than the headlight fuse rating (close because other things may be drawing current, too).

 

[WestTN]

You can't check the amperage accurately with a clamp on meter. These are made to check AC sinusoidal anperage. The alternator output is largely DC with a small ripple component.

 

[B.G. Smith]

I believe the 99 alternator is 140 amp output. Check the voltage across the battery terminals as stated above with the engine running after the heater grids have quit cycling. Then turn all accesories, AC with fan on high, high beam of head lights, etc. With engine idling the alternator should still maintain the voltage you read at battery. If it does it is probably fine. The alternator output is controlled by the PCM so I would be leery of checking max. out put with it on the truck. The output is maxed out when the field of the alternator sees high voltage (12volts). Auto parts stores (most) can test the alternator by running it and putting voltage to the field. bg

 

[DieselDoghouse]

OH BOY!! Somebody stop me (!!!) As noted you can test for a voltage rise with the alternator running, testing the amperage output requires a shunt type current meter: its ohms law in action—a precision resistor (R) is placed in the circuit and the voltage drop across it is measured and…badda bing



voltage drop/ resistor = current output (E/R = I).



But under no/low load a bad alternator can look OK. The next step requires either a 'waveform meter' (oscilloscope) or a laptop or meter that can display waveform.



Your alternator is 3 separate (single phase) generators in one. Six diodes ('in and out' pairs for EACH of the three windings) ensure that all windings contribute equally to the charging function. The scope will show a good alternator as a string of sharp sign waves all in a row. A bad diode(s) will drop out its winding and you'll see a skipped-tooth pattern. This is an alternator that MAY produce charging voltage with no load but will fade off rapidly as load is added, since power generated from the bad winding bleeds into the other 2 windings(generating much heat BTW). Short of this the alternative (IMHO) is to crack the case and test the continuity of each of the 6 diodes forward/backward individually.



Now. I've never understood in a world where CPM's now tell you the % loss of oil capacity in the engine or changed viscosity, why the 'puter can't alert you to an alternator that is no longer 3-phased?? (Oh cwap - did I just let another patent loose??)

 

[kscheffler]

Why would a fluke meter with a clamp on current device set to read DC amperage not work? I never heard that clamp on DC amperage meter would not work on a DC circuit? Technically why does a clamp on not work on a DC circuit?

 

[Road Dog]

You're not measuring the alternator output, you're measuring the load on the alternator. Add more load and the current will go up. To properly look at the alternator output you need an oscilloscope.

 

[kscheffler]

You can't check the amperage accurately with a clamp on meter. These are made to check AC sinusoidal anperage. The alternator output is largely DC with a small ripple component.

__________________

Dennis McCormick





I understand that load will increase amperage. I am questioning this posted statement. A DC clamp on amerage meter with a DC and AC selection swithch should accurately read DC amperage; right?

 

[B.G. Smith]

I have a clamp on amp meter but it won't read DC. I can live with that. I also have a small meter with a metal channel in the back, you place the channel over the wire and it reads DC amps. I think it was made to check starter loads as the scale on the gage is pretty high. I also have a shunt hookup that can read DC amps up to about 50 I think. The only experience I ever had with an oscillascope was about fifty years ago and all it showed on an auto at that time was the ignition system out put. I know it will show a sine wave for AC current but DC? Guess I missed too many days at school. bg

 

[jelag]

Checking full alternator output can be done by loading the battery with a carbon pile at 1500 rpm and using a clamp on style amp meter... . as you drop the battery voltage below 9. 6 volts the circuit will sense low battery voltage and run the alternator to full output based on the temperature control circuit in the ECM... .

In addition a simple test for the diode bridge and diodes in the alternator is to put an AC V meter across the battery... . there will some static AC voltage, maybe up to 1. 0 volts but there should be no more than that... . when you see 3 volts or above you've got a bad diode or set of diodes...

A Sun or Snap-on VAT (volt amp tester) performs the tests I've listed above but some simple tools will do it...

Understand that most clamp on meters are not designed for DC, it requires a powered circuit board and a hall effect senor in the clamp to read DC amperage... the simple AC amp meters that are used on household or industrial applications for single and 3 phase circuits won't work...

We use such an amp meter and digital voltmeter to check starter draw, charging voltage and other related tests when we need to...

Using the AC Voltmeter when the alternator does some work (charging over 30 amps) will look for bad diodes like diesel doghouse has mentioned above... that's how the VAT that's sold does it... they switch the voltmeter to an AC voltmeter with a low scale and put a green good bar and a red bad bar on the meter...

 

[JoeBioDiesel]

I don't recommend dropping the voltage below 12V with a carbon pile to test alternator output. Doing so will cause alt output beyond it's normal range. You'll risk smoking it.

However, as jelag says, the only foolproof way of testing output is with a carbon pile. Then check for AC leakeage.



Joe