troubleshooting alternator problems - tachometer / charging

[sgtPluck]

On both my engines (Yanmar 39hp, catamaran) the tachometer will jump up to idle RPM level when engines start, but when I rev up they do not go past that level and the alternators are not outputting current. They were both working until fairly recently, although sometimes would exhibit this issue sometimes not. Now they have the issue every time I run the engines.
The belts are fine and seawater is circulating properly.

Any idea why I'm getting a reading on the tach for just idle RPM and nothing more? If the alternators have failed, it seems suspect that they did so around the same time.

[GordMay]

ALTERNATOR VOLTAGE TESTS:

1. Set Voltmeter to DC Volts (20V scale), and connect the DC Voltmeter test leads to the battery post [Red lead to Positive Post, and Black(or Yellow) to Negative]. With engine “off”, read and note battery voltage (see “Battery Testing”).

2. Start the engine, and at “Idle” RPM, check the voltmeter reading. At very low speeds the alternator output may be lower than the regulator setting; hence you will expect to read the battery voltage.

3. Increase engine RPM, and note increasing voltage reading to a maximum of about 14.2 volts (12V Nominal System - expect ± 29.5V in 24V system).

If no voltage increase noted, you likely have a bad voltage regulator, or poor wiring
(Alt’ to Batt.). Go on to test 3B.

If Voltage increases go on to test 4.

3B. With engine “off”, connect Positive test lead to Alternator Output (“B+”, or “A”), and Negative Lead to the Negative Ground Post (“Neg”, or “E”) of Alternator. Start engine. If Voltage increases with increased RPM, your regulator is OK, and the problem is in the wiring. Go on to test 3C.

3C. Discharge the Battery to less than 12.5 Volts. Check for Voltage drop between the Alternator and Battery, as follows..
Connect the Positive Test Lead to the Alternator Positive Output Terminal (B+) and the Negative Lead to the Positive Battery Post. Crank the engine, and increase RPM to a fast idle. Observe Voltage. If the voltage climbs to as much as 0.02 Volts (or more), the positive cable has a high resistance. Repair or replace cable.

Check for voltage drop in the negative cable, as above - but: connect the Positive Lead to the Alternator’s Negative Output Terminal, and the Negative Lead to the Negative Battery Post. Analyze as above.

[roryboy]

Turn off any solar panels, wind generators and shore power charging. Turn on a bunch of high amp equipment like blowers and bilge pumps and give it a rev.

[tkeithlu]

There are two approaches. One is testing that while complicated, will isolate a problem. Gord has laid that out for you. The other is to go to the most likely source and see what you find. It might not be the problem, but it was the most likely, so you might solve it with little effort. For that:

Your description sounds like a wiring problem, and since it occured for both engines, somewhere in the central wiring. Wiring in a marine environment corrodes and fails with daunting frequency. So, trace both setups, cleaning connections with some fine sandpaper and looking for loose or broken wires. See if that solves the problem.