I recently added an ammeter and a real oil pressure gauge to my 2000 Ranger. I am aware of the arguments for and against amp meters versus volt meters, but I opted to install an ammeter. I didn't trust myself to work with a hundred amps (more or less), so I had it installed by a real live ASE certified mechanic. I have the smaller, 95 amp, alternator, and had him add a 100 amp ammeter. The meter reads about 65 amps just after a cold start, and about 35 amps after a mile or two. With the engine off it shows zero amps (duh). So far, so good.

What I can't understand is why the reading never drops below about 25 amps. I guess I can accept that the current is going somewhere, but turning on the lights or tapping the horn doesn't result in any change in the reading. Even stranger is the fact that with the engine off and the meter showing zero, turning on the lights and the horn doesn't show a negative current flow. This seems counter-intuitive to me. Even if the lights and horn draw little current compared to the 100 amp full deflection, I think I should see some deflection when I turn them on.

The meter is in series with the 125 amp fuse. It is between the rear fuse connector and the black/white wire, which was connected to that terminal before. That looks right when I consult the wiring diagram. The battery stays charged, and the instrument cluster voltmeter looks like it's where it should be (with no voltages shown it's hard to tell). It's the way I have installed lower amp ammeters in previous cars I owned.

Does this make sense to you guys? Can you explain what is going on?

 

Your battery of course supplies the power to start the truck.After it starts the alternator recharges the battery then drops down to 25 amps to run any and all electric devices. Sounds like it is working just fine to me. The reason you do not see a power drop when you turn on the lights with truck not running may be the way it was wired in,,,Have you tried just turning the key on without starting truck and then turn headlights on to see if you get a power drain ??? Depending on the gauge and how it was wired in you may have to start the truck for the gauge to work,,,,that explains why you do not see a power drain when you turn lights on without starting the truck. Sounds like everything is just fine to me .

 

Well, it's wired directly in line with the battery. All the juice coming out of the alternator goes through that wire, as far as I can see, so all the current coming out of the battery should too, unless I'm missing something. There is a second (red) wire connected to the battery, with a 30 amp fuse, but the diagram shows that going to the voltage regulator. I just noticed the 30 amp fuse on the red wire is labeled "head" on the diagram. They wouldn't have two fuses, one for input from the alternator and one for output from the battery, would they? If so, why would the 30 amp circuit show as being terminated at the voltage regulator?

 

I'd have to look on my truck (99 Ranger) but I'm pretty sure that 125A fuse is JUST for the alternator. If that's the cable you're measuring current in then it won't show current leaving the battery, only what leaves the alternator.

 

In light of Dixie460's response, I thought I would bring closure to this by reporting the results of my playing around. I finally decided to take the bull by the horns and measure the current at the battery terminal. I have a clamp-on ammeter, and once I had extended the cable at issue by about 6 inches (so I could get the clamp around the cable) I confirmed that at that point current was flowing both directions. Zero with the engine off, about 65 amps in upon starting, and about 15 amps out with the engine off and the lights on. So somewhere inside the wiring harness the cable splits with one side going to the alternator and the other going to the power distribution circuits.

Now I have to go back to the mechanic and get them to admit they did it wrong and fix it. If they won't I guess I'll do it myself.

Thanks for the responses and especially to Dixie460 for confirming my suspicians!