Ammeters & Shunts 101

[Stu Jackson]

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 page (http://c34.org/bbs/index.php/topic,5078.0.html) 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.

[Stu Jackson]

QUESTION:  For those of you who used to have ammeters in the cockpit panel (raise your hands if you STILL have one!!! ), could you please let me know if these ammeters were simple 0 to, say, 50 A ammeters, or were they 0-50-0 meaning they'd show both positive and negative amperage.  Thanks.

[Stu Jackson]

Wiring & Installing Battery Monitors by Maine Sail

http://forums.catalina.sailboatowners.com/showthread.php?t=125606

See also Electrical Systems 101 here: http://c34.org/bbs/index.php/topic,5977.0.html

[Ron Hill]

For what's worth -- From my limited experience with amp meters here's what I found :
Ususlly analog ammeters that register for 0 - 50 have in internal shunt (inside the gage itself).  If you measure more than 50 amps on the ammeter, you'll need an external (stand alone) shunt.  
Found this for the ammeters that were shunted off the positive (charging).

[Stu Jackson]

http://bluesea.com/category/6/23/productline/140

Ammeters over 25A have external shunts, at least for Blue Seas.

[2ndwish]

Our recently replaced ammeter was -0+ (plus and minus).
T

[Ron Hill]

2nd : If that ammeter that you just replaced was in the engine instrument panel, I surely hope that you have read about the wiring harness upgrade (Critical Upgrades) !!  A thought

[Stu Jackson]

Maine Sail's wiring harness description:

http://www.pbase.com/mainecruising/universal_wiring_harness_upgrade

and a better a more readable version of it is:

http://forums.catalina.sailboatowners.com/showthread.php?t=135558

[Ron Hill]

Guys : All of the 1986 and 1987 C34 production had an engine instrument panels with and ammeter.  Most of the early 1988 C34s also still had an ammeter.  All production from about hull # 700 and later all had voltmeters installed.

All C34 MKI production still have to worry about the 8 pin trailer connectors and the routing of the power back to the engine panel and then finally back to the batteries -- till about the 1992 production year.

[scotty]

It's funny how things pop up.  I was just browsing around, as I often do on this site, and came across this thread.  Yes, I replaced (took the "plug" out and soldered the wires) my wiring harness, and at that time checked out the ammeter circuit.  Being kind of dumb, I couldn't figure out how the wire from the alternator was obviously upgraded (I forget if it goes directly to the starter or the battery?) but I still had a alternator - with a thinn guage wire -  on my pannel???  Now I get it.  I must have an external shunt.  I'll go down when it's not raining and check it out.  Thanks. 
P.S.  I just went down to the boat to check it in the rain.  To cold to check wiring, but (well), don't forget to close all the hatches (Opps)!

[Ron Hill]

Scotty : If you still have an ammeter in your engine instrument panel reread my # 3 reply above. 

The instrument gage in you engine panel (if it's an ammeter) most assuredly has an internal shunt. 

[scotty]

Ron, If the wire going to the ammeter is small guage, can that be an internal shunt.  I have a lot to learn about this!  Thanks for your help.

Scotty

[Stu Jackson]

Nope.  That's one of the basic issues with the old wiring harness:  all the wires were small.  The purpose of this initial topic was to explain how ammeters and shunts work.  Unfortunately, the OEM wiring in our boats was too small and there were no external shunts when it came to ammeters in the cockpit panels.