Testing amp draw with VOM

[Californio]

OK, pardon my ignorance, but I want to figure out how much total draw is on my alternator, and I don't know how. I disconnected the alternator wires and put the VOM leads between the wires and the amp lug, but it didn't give any reading. The + lead was in the "unfused" socket of the VOM but it blew the fuse anyway!

It's a 90 amp Bosch alternator, and it's giving me 14.2 volts at the cigar lighter. This sounds fine but when I turn on all the accessories (high beams, AC, fridge, blower motor) it drops down to around 12.1--not enough to charge the aux batteries correctly on a cross-country mid summer trip. I have three aux batteries and one starter battery interconnected with a 125-amp voltage-sensitive relay.

What I'm ultimately trying to figure out is if I need a high-output alternator to charge the four batteries. Which is why I wanted to figure out the total draw in the first place.

[Dogpilot]

The amp meter needs to be inline as part of the circuit. So you would pull off the output + put on lead of the amp meter to that and the other lead to the line you removed. The big caveat is, most inexpensive and many expensive meters have an 10 amp load limit. So you may have popped the fuse in the meter, which in my Fluke is a big honking buss fuse that cost $7.
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Geology with a Syncro rocks!
86 Syncro Westy AKA "The Bughunter"
98 Disco I
08 Range Rover SC
08 VW Rabbit S
1951 O-1G

[Californio]

That's the way I did it. Maybe the whole VOM is kaput as I had the lead in the unfused socket. I'll replace the fuse and see if it still works.

So what do you do to measure a high-amp draw? There has to be some way--a different type of meter?

[presslab]

Is your meter capable of measuring 100 A? Your test leads too?

The Fluke fuse is $$$ because it's rated to 1000 V; that's because the meter is rated to that as well. Of course in a car you only need 12 V, kinda sucks.

I have a clamp-on meter from Sears that measures 0-200 ADC. You don't have to break the circuit and there is no chance for blowing fuses. It's not very sensitive though, so measuring something like a battery drain won't work. But for your type of measurement it is ideal.


But really, if the voltage right at your alternator droops when everything is running, it doesn't have enought current to meet the demand.
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1986 Vanagon Westfalia EJ25
1988 Subaru GL-10 EJ20G --- 2000 Honda XR650L
2010 Titus El Guapo --- 2011 On-One 456 Ti

[Wildthings]

Are you revving the engine up when you are checking the voltage? It would be normal if the alternator couldn't keep up with all the loads at idle

If a 90 amp alternator isn't putting out enough to charge the batteries when revved and you don't have any unusually high loads installed, like a large inverter, then I would guess that you might have a diode out in the alternator. You might just want to take it in to an automotive electric shop and have them look at it.

You could pickup up a generic ammeter from your wrecking yard or your FLAPS. Not talking about a hand held meter, but one that is for permanent dash mounting.

[crazyvwvanman]

You measured the voltage at the wrong place to start with. If you want to know if the alternator can keep up with the load you MUST measure at the alternator itself. The fact that the voltage at the dash dropped to 12.1 under load means nothing with respect to the alternator. This is basic auto electrics 101. You need to check the voltage at the alternator, then add the dash loads and check again at the alternator. I bet your voltage drop under load is in the wiring between the alternator and the cig socket and if so you could put a 1000 amp alternator in there and it would make no difference at the dash voltage.


You can't measure the full current draw in amps without a special meter so forget about it. Measure the voltage properly and go from there.

Measure voltage at the dash and alternator, then add the loads, then measure both the dash and the alternator again to see what changed and by how much.

Mark

[PDXWesty]

You can buy an adjustable voltage regulator for the Bosch alternator and set the output voltage for whatever you want. There is another post that covers this.

http://www.thesamba.com/vw/forum/viewtopic.php?t=363115

The problem is that if you adjust it high enough to compensate for the voltage drop as the amp draw increases, you are at too high a voltage when you don't have all your accessories on and it will over charge the batteries. I use one and have it set to 14.8 volts measured at the alternator output.

What also helps to minimize the voltage drop is upgrading the wiring from the alternator to the starter and making sure your battery cables and grounds are in top condition. I added a second #8 wire from the alternator to the starter which helped keep the voltage higher when the loads are on.
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89 Westy 2.1 Auto

[Californio]

Yes, I've been measuring at idle. But the alternator pulley is only about 1 inch diameter (Tiico w/serpentine setup) so I thought it'd be going fast enough. Probably better to check at 2500 Rpm or so?

The suggestion to install an ammeter is good. If the ammeter has to be inline, how would you do that?


I've also been looking online for a small unobtrusive digital voltmeter that could mount in the spare knockout on the dash. No luck, anyone have an idea where to find one?

[crazyvwvanman]

The dash mounted voltmeter won't tell you very much unless it connects directly to an important point in the electrical system on it's own wires. Either directly to the battery terminals or directly to the alternator. Most people want to know if their battery is getting a good charging voltage or if their battery is still strong enough for use while parked, or both. The only way to measure this with any precision is to connect the meter wires right at the battery you are concerned about.

Mark