96f350psd

I am running a serpentine.

CN24

mine does the same thing, but after a rev it's fine. Even back at idle.

formula31

Id be more inclined to think your battery is junk.

mopower

Check the battery voltage AT THE BATTERY. Don't always trust the dash volt meter. I've seen them be incorrect...many times. Not so much newer boats but eight and ten year old boats are notorious for this. Corrosion on connections causes this...even stuff you can bearly see

CN24

Both my batteries were brand new.

Lmarth

Just a few brief comments on this thread. My Merc manual shows two manufacturers of alternators-Mando and Prestolite (Motorola) which may explain why Reed is seeing what he is reporting. Accurately measuring battery voltage with the engines off, at idle and at elevated rpm's is necessary to diagnose these problems. Having the alternator/voltage regulator checked out at Autozone,etc can also simplify things. Assuming the voltmeter is accurate,Post #7 tells me the the alternator is not producing output at idle (the 10 volts is battery voltage under load) but is at elevated rpm where you're reading the 13 volts. Assuming the regulator is good,alternators typically work or are dead-nothing in between (diodes and semi-conductors either work or they don't). I'd look at the regulator and for worn/sticking brushes in the alternator together with excessive runout on the alternator shaft. We're not told how many hours are on the alternators. For post #13, I'd definitely look at the brushes,etc. There's not a lot of spring pressure on the brushes when the brushes are worn. A little runout on the armature shaft and one or both brushes can bounce off the shaft and stick. Blip the throttle and they can reseat. And you thought HP500 valve springs were a pain...

tcelano

I agree with Lmarth. Either it's working or it's not. I installed a Gaffrig voltmeter which seems to track exactly with a good multimeter. I installed an all-new electrical system in an older boat, so I know all the connections are good. With ignition off, with batteries not charged for a while, I see about 12.8 volts. With ignition, EFI and fuel pump running, I see about 12.5. As soon as it starts, even at idle, there is a dramatic increase to about 13.8 V or better. If you are not getting a positive behavior from your alternator, check the connections to the excitation terminals. Depending on whether you have a single-wire, three wire or four wire connection, you will be looking for different things. Mine is a three-wire Mando 65A, and you should see battery voltage at the "S" terminal and the alternator output regardless of ignition switch position. When you turn on the ignition, you should see ignition voltage at the "EXC", and it should go away immediately when you turn off your ignition switch. I'm not personally familiar with what the fourth wire does on those applications. If you are seeing behavior as described above, and it still won't charge, than either your alternator is junk, or there is a bad connection somewhere. It is common to get corrosion at the terminals which allows you to see voltage, but you won't get any current flow. Try retorquing the mainline terminal nuts. Another thing to check might be your battery switch. Some boats are set up with auxiliary terminals on the switch to save the alternator should the battery switch get turned "off" with the engines running. A bad connection or switch failure there could cause the alternator not to charge.

Hope that helps some and Merry Christmas!

Reed Jensen

I rarely rely on what the dash gauges say about charging systems. I have a Snap-on AVR... alternator,voltage regulator and battery tester. This isn't the little chrome toaster version.. This has battery leads the size of large jumper cables and an inductive lead to put around the alternator or battery cables to see how much amperage is being produced and how much voltage. I can also short the battery with up to 500 amps to test it's output too. What is nice is while the engine is running... I can put a load on the charging system and see if the regulator is upping the amperage. This thing was about a thousand bucks when new... but it's paid for itself many times.

PPlay33

aljolyn, you probably have a marinized Delco marinized alternator. It has screens in it to act as a flame arrestor and a specil self-exciting regulator that does not charge until it exceeds about 1200rpm. IT will stay charging until tou stop the engine. This regulator allows the alternator to operate with just a single wire and not have a battery drain.

Lmarth

Tcelano and Reed are providing good info. The numbers tcelano is reporting are virtually identical to what I usually see. I have a scribbled matrix on the garage wall for our three cars that shows static voltage (engine off), after startup and after the charge rate has stabilized.Anytime I sense something's strange, I compare to my baseline. As for boats, I rarely go to the marina without my multi-meter. They should come in the glovebox with every new boat. After uncovering the boat, I check the batteries with the onboard volt meter which is quite accurate. Whether I turn on the shorepower is based on what the voltmeter says. I'm kinda paranoid about not overcharging or cooking my batteries. Just tryin to help... Lou