Ammeter is pegged

[Gord May]

good eyes

George S:I don't doubt your word on the meter's tested accuracy. I agree that it's well worth the cost - but not a lot more. I do expect that you've got better eyes than I. I couldn't differentiate 2A on either scale. Even on the 100A scale, each "tick" represents somerthing like 8 A.I do own, and have used, one, though not often.Gord

 

[Seattle Scott]

Update re meter pegged

Replaced meter, no go still pegged. Resumed search for ammeter shunt. Found a likely candidate (still need to confirm the wire runs back to the meter) with a disconnected bar between the two posts. I also need to trace main wire on the shunt back to the alternator. Anyway, looks like I will have something to do this rainy weekend. I did get a new 200 amp shunt with the meter so pehaps a fix will occure.

 

[patrick]

Measure the voltage at the meter terminals -

it should be exactly the same as the voltage at the shunt. You must use a good digital meter as you will be measuring a very small voltage difference when the shunt is working properly. If the shunt is good, the voltage measured should be on the order of a few millivolts (ie: almost but not quite zero). Pay close attention to the actual numerical value. Normally, with only a few amps of current, the voltage will be very small. If you measure more than 50 mv (0.05 volts) (which, from your discription, you probably will), you probably have a bad shunt. To find out for sure, measure the voltage at the shunt. If the shunt voltage is as it should be, you have to find the source of the voltage at the meter. Don't leave the meter hooked up for too long if it is seriously pegged. It can burn out that way.

 

[Gord May]

NOT a missing shunt

Patrick is exactly right. Most ammeters are actually voltmeters, with full scale (in amps) at 50 millivolts. It uses a known resistance (the shunt) and measured voltage to derive amperes (Ohm’s Law, where Amps = Volts / Ohms).You’re situation does NOT suggest a “missing” shunt.

 

[Seattle Scott]

Yep, it was the shunt

the bridge between the posts was broken and the plastic base had melted in the area where the posts were screwed into the mounting base. Replaced the 200 amp / 50 mv shunt with one with the same rating and hooked up new ammeter and everything appears to work. The shunt was wired into the positive side and before replacment I was not getting any alternator output to the battaries. After replacement the meter was registering and I was getting 14.5 volts at the battaries. As an aside, I did find two more shunts. One is a 400 amp shunt wired into the negative side, which from reading Calder's book, I think services the battary monitor computer thing, that will tell me amps consumed and remaining. There is another shunt mounted on the battary monitor fuse board, on the positive side, which I guess is used to measure volts at the battary terminals, which the monitor display will also show.Nice thing was, that after spending a couple hours crushed into the cockpit locker swapping out the shunt and getting it to work, wife called and invited herself out for a sail, since Sunday turned out to be nice weather.Checked the shunt after motoring back to the slip and it was cool to the touch. That melted plastic was disturbing

 

[patrick]

Another meter somewhere

"There is another shunt mounted on the battary monitor fuse board, on the positive side, which I guess is used to measure volts at the battary terminals, which the monitor display will also show."Scott, a shunt is only used to measure current. There must be another meter or connection somewhere. Why not follow the small wires from the shunt and see where they lead? That will tell you where the current is indicated by that shunt. While you are at it, that information might prove useful sometime.

 

[Seattle Scott]

Patrick, boat does have another meter

which measures amps in/out of house battary bank and also measures volts of both banks. I think those wires are actually labeled. I always have trouble tracing a wire since it seems that within a foot of any connection all wires dissapear down a "rat hole" with about 15 other wires and where a specific wire comes out, is anyone's guess. The way I figured out the ammeter shunt, was I disconnected the small wires at the shunt and clipped on a 12 volt portable automotive battary charger to the disconnected wires, then put my voltmeter on the wires disconnected from the ammeter, showed 12 volts at the meter end of the wires when the all the boat battaries were disconnected.