How do you Check an Ampmeter?

[scallywag]

How do you check an ammeter? I just had one ammeter melt down. I bought an replacement and it started to show moisture(?) on the plastic lens just like the one did before it melted. I turned off the diesel before it melted all the way down. I took off the alternator and regulator and had them both tested. The both tested fine. It was suggested that maybe the new ammeter was bad and replace it with another new meter. The motor is a Westebeke, the alternator is a Motorola, and it is a 12 volt system.

Sounds to me like the meters are just getting water into their guts. It would not take very much water to cause a "meltdown".

Is the meter located in a place where is is exposed to weather, or dew? Is there any way that water can get into it from the back?

There are are not liquids that are part of a ammeters construction, so what you see on the face has come from outside somewhere.

Bill

[Alan Wheeler]

A little more info would be helpful.
What current output is the Alternator?
What current rating is the Ampmeter?
Is the Ampmeter in series, as in, all the current flows through the meter? or
Is the ampmeter a low current device and a current shunt is fitted in line with the main cable?

[scallywag]

Bill and Wheels,
The ammeter is a TeleflexMorse. It is a 40 or 60 amp, direct reading meter and it is in series. A 6 gauge wire comes from the alternator to one side of the meter and on the other side a 6 gauge wire runs to the splitter for the batteries. The meter showed 30+ amps passing through the meter. I am going to start the deisel and see if anything is shorting out the two posts where the 6 guage wire connects.

Thanks for the responses,

John

[Maddog]

The meter is showing 30 amps with the engine off? Bad meter or bad wiring! Look for a short. When you say a splitter, I assume you mean an isolator. Ditch it, run the 6ga wire from the meter to the positive post on your house bank directly and install a charging relay between the postive posts of your house and starting banks. Bluesea has a great system for this, including a e-series battery switch that isolates both banks when they are on. Battery happiness and sunshine have rained upon my boat after I installed this system. Check out the Bluesea web site for the products and diagrams.

[Alan Wheeler]

You would find an improvement going to a "shunt" type system. This makes for a safer system and there is less loss.
I doubt a short in the cable would cause a problem. You would see the meter needle go off scale.
When you say the meter burns up, what exactly do you mean. The gauge melts??

[scallywag]

Maddog and Wheels,
The meter shows 30+ amps when the engine is running. When the engine is cranking it shows a negative reading. The original gauge seemed to show moisture on the lens, then the guage melted. The guts of the guage fell out of the back. The replacement guage started showing moisture(?) last night while the engine was running and the guage showed 30+ amps. I shut everthing down and decided to have the alternator and regulator checked this a.m. The alterator rebuild shop said everything checked out fine. The engine schematic of the boat shows a splitter. I have not seen the device. The s/v ran fine until I had the deisel overheat and then yard replaced all the hoses and freshwater cooling pump replaced. I just can not understand what would cause the original and new guage to melt.

Thanks for your thoughts and responses,
John

[hellosailor]

Scallywag-
An analog ammeter, of any kind, is really just a voltmeter which is measuring a voltage drop across a known resistance. That resistance is usually a piece of special metal bar, called a current shunt. For low power (40-60A) the current shunt can be small enough so it is located in the ammeter body itself, so the ammeter is just wired into the main power cable. For larger currents, the shunt tends to be larger and it needs to stay cool by disippating heat, so the shunt gets installed remotely (in the main power cable) and the meter is installed somewhere else, connected by two thin wires.

Either way...an ammeter generally is really robust. A little moisture in the air, or condensation, would just cook off without much harm. That the ammeter was MELTING means that there's something grossly wrong. Either way too much current is going through it, or there's a physical defect, a loose connection, etc. in it.

Most unusual that two would melt that way--and TM are generally considered a decent quality, so you might want to ask them if they want both units back for examination and replacement. (I'd guess they'd replace it under warranty, and if necessary with a different unit.)

Since most alternators can put out more than 60Amps, it is possible the ammeters have simply been frying because they are not the right capacity for your alternator.

[scallywag]

Hellosailor,
Thanks for your thoughts. The meter only shows 30+ amps going to the batteries. I could understand the meter melting if the needle on the meter was pegged at 60 amps. I was tinking about e-mailing TM about the problem but I wanted to check here first.

John

[Maddog]

"The s/v ran fine until I had the deisel overheat and then yard replaced all the hoses and freshwater cooling pump replaced. I just can not understand what would cause the original and new guage to melt."


I'll take a flyer on this. Any chance the boat yard rerouted an exhaust hose or intercooler/heater hose near the meter and it is melting from a source outside the meter. The engine exhaust warms up next to the meter, a little condensation forms, exhaust gets hotter until the plastic melts. Just a thought. this could happen even if they shared the same enclosed space with no ventalation. Let us know.

[scallywag]

Maddog,
I had to pull the instrument panel, which is above the engine, to replace the guage. From every angle the guage it looks clear. I hope pulling the panel tonight when the engine is running will give an answer to this situation. I will let the forum know when the answer is found.

Thanks for your suggestion,

John

[scallywag]

Just started up the diesel and the ammeter had a moisture looking lens. The meter was showing about 50 amps. Pulled the panel and I did not see any fluid hitting the back of the panel. Turned off the engine and I felt the wires and the posts to the meter. Neither felt hot, just warm. After a few minutes the meter's lens returned to crystal clear.

John

[Alan Wheeler]

I could understand the meter melting if the needle on the meter was pegged at 60 amps.

These meters can become grossly inaccurate the closer they reach full scale. Normaly you would have a meter that is capable of reading a level higher than your alternator can produce, or you will indeed melt the meter. I have not seen one of theses meters with an internal shunt. Normaly the shunt is a coil of wire across the back of the meter. The current is carried across the wire and the meter simply measures the voltage created across the coil due to resistance. When I say coil, it is simly bare wire of say a ft in length twisted into a "spring" shape coil. The same coil can be fixed down in the engine room and the "signal voltage" from across the coil can be sent to the panel. It saves on heavey wire running to the panel and the loss involved along the way and you don't have a coil on the back that is bare and can get hot.

[scallywag]

Wheels,
Thanks for the information. I plan to redo the electrical system when I add a wind gen and solar panels. I am just not at that point. I am completing other simple projects.

John

[donradcliffe]

A poor connection at the terminals of the ampmeter can easily create enough heat to melt the meter--make sure that your wires/connectors are not corroded.