These shunts have provided a lot of confusion on the part of owners for a long time.
I just came across two simple wiring diagrams in the recent BoatUS magazine that should help explain how they work.
Ammeters HAVE to be installed in SERIES with the load because all the power has to run through them for them to work, whereas voltmeters are installed in PARALLEL, and can be installed anywhere in the system to read voltage across the (+) and (-) wiring (like a digital voltmeter).
The Critical Upgrades
) discusses the need to upgrade your cockpit panel if you still have an ammeter (as well as checking the rotten wiring harness connectors original installed at the factory). This is discussed in detail on the C34 Tech wiki, Engine Wiring harness Upgrade (http://www.c34.org/wiki/index.php?title=Engine_Harness_Upgrade
Here's why the OEM cockpit ammeters were bad news:
We need to differentiate between ammeters with internal shunts and external shunts in discussing the appropriate use of ammeters in the cockpit. Most, if not all, production boats were built with ammeters with internal shunts, for cost reasons, and the fact that they only read 0-25A or maybe 0-50A (also see the next post below). For instance, the only Blue Seas ammeter with an internal shunt, model 8005, goes only to 25A, and is true for most ammeters, 'cuz for larger loads the shunts are too big to fit in the small ammeter housing. These internal shunt ammeters are adequate for most on board uses, since while if you turned EVERYTHING electrical ON (all at ONE TIME) you'd most likely exceed that amperage of 25A, we rarely turn our fridge, running lights, ALL interior lights, steaming light, deck light, macerator and bilge pump, and whatever else we have on all at the same time. In the 13 years we've owned our current boat, the 0-25A internal shunt ammeter works just fine. This is NOT in the cockpit, but is down below, different from the factory installed cockpit ammeter. It is inline (series) with the power from the C post of the 1-2-B switch wiring TO the DC distribution panel: switch C post to ammeter (internal shunt) to DP. The only thing that doesn't run through there is our bilge pump.
Those internally shunted ammeters are what most folks have (had!) in their cockpit panels, because that's what Universal provided with their cockpit panels, for our Catalinas and many other brands of boats with Universal and other engines. They were also almost always provided with incredibly undersized small #10 wiring, a real voltage drop issue when the current is need for charging your banks from the alternator, because as the photos show, the charging current had to go a looong way around before it got to the batteries.
The photo on the LEFT below shows the internal shunt ammeter as installed at the factory. This is a KEY picture which may make understanding shunts and ammeters somewhat easier for you.
The downside of this type of installation is that ALL of the alternator output has to run all the way to the ammeter in the cockpit and then way back to the batteries to charge them. In a small #10 wire!!! That's why it's not such a good idea.
However, there are externally shunted ammeters, shown on the right. If the shunt is down below and the ONLY wires to the cockpit ammeter are the small wires from the shunt, then voltage drop is NOT an issue, since the wires to the cockpit ammeter are only carrying a small amount of voltage with little current, only enough to move the meter. The shunt is installed down below, near the batteries from the alternator output wiring. Thing is, these weren't the ones normally installed in our boats.
The Catalina fix linked above is from the "factory" and/or Universal, who would never admit to making a mistake, blaming the boat owners for putting too much electrical stuff on their boats where batteries would need a recharge! How dare those pesky boat owners!!! Oh well, at least we know about it.
An ammeter in the cockpit panel, properly installed with an external shunt is truly helpful, but not if it is an internally shunted ammeter, because it messes up the charging system big time.
In our case, the cockpit panel is limited to the number of meters it can accommodate, so that's why the "fix" is to substitute the ammeter with the voltmeter. In a perfect world, both meters would be a great idea, with an externally mounted shunt down below and the ammeter in the panel.
When discussing this many years ago, Ron Hill suggested that the voltmeter can do the same thing: If the starter solenoid keeps working, or the glow plugs, the voltmeter will drop, providing the same functional indication that an ammeter would. I have found this to be true.