Aftermarket Alternator Question

[Drnobeam]

Tom's got it. To verify the accuracy, you can compare the voltage drop readings with an inexpensive multimeter set to DC amps. These usually won't check anything over 5 to 10 amps (anything more takes heavier circuitry = $$). If you operate the brake lights, and get a 7 amp reading, the voltage drop readings should indicate the same number.

Like Tom said, you're simply picking two points on the ground cable about 12" apart. If you're ground strap is less than 12", make a longer length of ground cable. Might be a little more or less depending on the size of the ground cable. What you're actually checking is voltage drop, but the reading will be identical to DC amps, without the expensive electronics.

With the engine off, you can check the amperage load (draw) of various circuits by turning them on one at a time. If one is excessive, it may show a direct short. With the engine running, the alternator should overcome the load and charge the battery.

 

[Nickodell]

Tom; I thought that was the theory you were referring to - you measure voltage at the start and at the end and from the resistance calculate the amps by the formula I = V/Ohms. I doubt that this is practical for Eric's purpose, as not only do you need to know the length of cable run and its exact linear resistance (which varies by manufacturer), you also have to factor in unknown resistances of other components like fuse(s), relays, switches or connectors.

IMHO, it's more of a lab practical test in electrical theory than real-life. Instead of fiddling about with that, for $125 he can find out how much Lucas smoke his alternator is putting out against increased loads, which is what he needs to know. To do that he needs either an in-line ammeter (which his car doesn't have) or a clamp-on.

AS Dennis Miller says: that's just my opinion, of course; I may be wrong.

 

[Drnobeam]

Nick, this is a simple build for the cost of a length of ground wire. I demonstrated this to dozens of technicians in more than 30 dealerships in New England when I worked back east as a manufacturers rep.

There's no calculations either. You set the meter to millivolts, and the readings/scale reads the same as DC amps.

You do not need to factor in fuses, relays, connectors, etc. You are simply measuring voltage drop. The reading is identical to DC amps. This isn't bench racing theory. It's actually a very practical way to check the alternator output.

Really. We're just talking about a 12" piece of wire.