Starter verses Alternator

ambianceack · #1 (**permalink**) 10-10-2006

Attempting to winterize the boat and she wouldn't start. It only started twice after some hesitation. Turn the key for the Yanmar diesel and voltage meter drops from 10 volts to 0 and the engine does not turn over. The starter was hot to touch. The starter selinoid began to smoke.

We charged the batteries and purchased a new No.1 starting battery. We are not sure why the voltage meter is registering 10 volts when it should be 12? A few tests and sounds we are aware of indicate the starter selinoid or starter have failed and need replacement. But how can we be certain it is not the alternator? Any thoughts?

capn_dave · #2 (**permalink**) 10-10-2006

Where are you reading the battery voltage? On your electrical panel or on the instrument panel? Dose the key have to be on to read the voltage? Did the new battery read 10 volts? Did you run the engine and check the voltage while running?
While cranking the voltage will drop, as the alternator is not charging at this point and the starter is drawing allot of current. Most alternators will not start charging until they get to a certain rpm. For sake of argument about 1000 engine rpm then once excited will continue charging even at idle. When the alternator is charging and the engine reved up a bit you should be seeing around 14 volts.

Fair Winds

Cap'n Dave

dave.verry · #3 (**permalink**) 10-10-2006

The fact that the starter solenoid was smoking is a clue!

Batteries have the ability to produce thousands of amps when shorted. The alternator does not produce any current until the engine is up to speed, definitely not occurring while starting. This makes the alternator nothing but a drag until the engine has started, all the power is coming from the battery.

The 10 volt reading simply meant that you had an excessive load on the battery. The voltage drop could have been in the battery or the battery cables, I “volt” for the battery cables (sorry, I couldn’t help myself).

The alternator is probably fine. Check the battery voltage once the engine is started, if it is up to 14 volts + then it’s OK.

Rockter · #4 (**permalink**) 10-10-2006

It does sound like the alternator is down. It really should be raising the voltage to about 14 V.