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The aftermarket ammeters do have to be connected in series. That means all the power for your truck is going through the ammeter wires! A better choice would be to throw the ammeter as far as possible out the back door, and get a voltmeter for it.
The voltmeter is easier to wire, monitors the charging system better, and is much safer.
You can't measure 'reserve' amps, but a voltmeter will tell you when you've reached the alternator output limit.
I absolutely agree with the loneranger. You know why everyone quit installing ammeters? Because they kept catching fire. Not to mention having to run battery cable all the way to the dash and back to the fuses. If you want that much power put an extra battery in the back and chare it off a combiner.
Adam, it's too bad you've already ordered it. I looked in my Summit catalog and couldn't find an ammeter that went above 60 amps, so Franklin2 is correct.
They do show some good looking voltmeters, however. Something like an Autometer would probably give a more precise reading than the stock one.
A battery at rest will read about 12.5 to 12.8v. You can compare the volt reading with the engine off, to the reading when idleing and the loads turned on (lights, amp, wipers, etc). If you have a multimeter, a more accurate check would be directly from the battery posts.
When you get this all set-up with the voltmeter, don't panic if you have a heavy load on the system, and the voltmeter drops below 12v when idling. This is normal since the alt cannot put out it's full potiental at a slow rpm, and there will be some voltage drop in the wiring from the battery to the rest of the wiring in the truck. After you run it awhile, you will be surprised how informative a voltmeter can be in monitoring the system.
I have a few comments on altenators. Remember all altenators are basically the same. The only real difference is in the location of the voltage regulator. An altenator at full output is 14.6 volts. A battery at rest is 12.6 - 12.8. That would be between 1.260 and 1.280 specific gravity. A "dead" battery with no load adn at rest is considered to be 11.2 volts. Hard to swallow that number, but that is according to Battery Council International. The governing body for batteries. The altenator output should fall at idle. The pulley on an altenator is sized to deliver a rate of aprox. 2.5 to 1 on the crankshaft. The most important and over looked factor in altenator swaps is the pulley size. It is crucial that the pulleys are the same diameter. Factory voltmeters are notoriuos pieces of junk. If you are that concerned about your altenator output, be sure to get a quality meter. If you have voltage drop on a wire between the battery and the wiring you are in trouble. A corroded wire will only drop 0.2 volts. A diode will drop 0.9 volts. Anything more than that and you have problems in your system.