Reading my Amp guage

[Shriveguy]

Hi Don -
First of all I wanted to say thanks for helping me breathe life back into my late model A-4 in my recently purchased 1969 Coronado 30. My wife and I have slowly been restoring our boat, and your manual and parts website have helped out greatly. After getting the motor running I ran into a charging problem. I bought a new alternator from a local source. When I installed the alternator initially it squealed (like the regulator was over-exciting the diodes) then the squeal went away. It also charged initially (14.2 volts at the positive post that goes to the amp guage), but eventually failed after maybe twenty minutes of running. During this time the Amp guage was reading between +15 and +30. I am not quite sure what to make of that. The place I purchased the alternator from replaced it, I just want to make sure that I am not missing something. I have checked all of my wiring and it looks correct. I am an old car mechanic (big groan, another car guy), and am used to seeing the Amp guage read in the negative when the alternator is under a load, and zero when everything is hunky dory. What do you think?

[Don Moyer]

I'm not sure why your alternator failed, but 15 to 30 amps continuously would suggest to me that your batteries are quite old and developing a bit of internal resistance, or something else in your electrical system is creating a similar load. But again, most alternators today are rated at 50 or more amps, so you might want to get assurances from whoever you're buying your alternator from that it's capable of supporting the loads you applying to it.

Don

[Shriveguy]

Thanks Don -
I guess I am confused on the Amp guage reading. It has been a while for me, but I always thought the guage reading in the negative meant that the alternator was under a load, I was never quite sure what the positive side is for. Could you please clarify for me what I am seeing when I read the Amp gauge. I have asked everyone I know and no-one seems to be able to give me a straight answer.

Thanks
James - Seattle

[policecentral]

Ammeter

Simple. If your ammeter is wired correctly a needle in the + side indicates that current is flowing into your battery from the alternator: e.g. it is being charged. Needle on the - side, indicates that current is flowing out of your battery: e.g. it is being discharged.

Indicated value either way indicates the number of amps. After starting the engine, the ammeter generally should start out with larger number of amps being charged, and gradually move toward zero, indicating that the battery is charged and that the alternator is supplying the small current required to keep the engine running.

JSM

[Shriveguy]

Thanks for the info - That puts a better perspective on it for me - I did get a new alternator and ran the engine for a couple of hours this weekend with no problems and no irregular amp guage readings - it was charging a solid 14 volts the whole time - I really appreciate the insight into reading my amp guage

[policecentral]

You might note that you may have two meters in your boat. The Voltmeter registers voltage (like the water pressure in a hose) and should read 14+ volts when engine is running. Your battery will measure 12.6 volts when fully charged and the engine off, so the alternator always needs a little more voltage (like pressure) to charge the battery.

Your Ammeter measures amps, like the volume of water pushed through a hose). Amps multiplied by volts equals watts. So a 12v light that is rated 60w draws 5 amps from your electrical system.