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not have enough voltage to switch on the diode and you may think it’s open. Switch the
meter leads and you should see it conduct one way and not the other. A-B1 should
conduct with A more positive and B1. A-B2 should behave the same way. E-A should
conduct when E is more positive than A. B1-B2 should not conduct either way. You can
use a battery operated test light to check the diodes as well.
Connecting a jumper between the alternator connection and the vehicle battery
connection on the isolator essentially returns the vehicle to the typical vehicle
configuration and “bypasses” the diodes in the isolator. Now the voltage at the battery
and the alternator should be about the same.
It’s been reported that with a discharged house battery, the battery light may come on
when running the engine. In this case, the alternator can’t deliver the high current
demanded by all of the loads including the vehicle loads and the batteries. As a result the
voltage falls and the light comes on. This is abnormal and the alternator is likely
delivering all the current it can. The current follows the voltage, the higher the voltage,
the more current the alternator delivers. Effectively it’s “browning out” because it can’t
produce the power demanded at the normal voltage. The maximum current is limited by
the design of the alternator.
Operating the alternator at maximum output is hard on the equipment and can lead to
early failure. Under this condition, I would expect the alternator output to still be about 1
volt greater than the battery voltage indicating that it is producing power and delivering
current to the loads.
Not all alternators are created equal and some produce more output than others. You
could have an alternator, which appears to work, but is damaged and cannot produce full
output. Roadtreks are normally equipped with a high output alternators to handle the
demands. When a battery is very low, it will demand a lot of charging current, but this
usually falls off fairly quickly as the battery charges.
Running batteries very low is a recipe for early failure of the batteries and perhaps other
components as well.
A DC clamp on ammeter which is very useful in troubleshooting such problems. Normally
to measure current you have to “break” the circuit, but with this type of meter you clamp it
around the wire to measure the current. Be sure and get one which measures DC
current, since they are less common than AC only clamp on meters which won’t help in
DC circuits. Knowing the voltage is only half the story, the current and the voltage will
help you understand the situation in much better detail.
Isolators are solid state devices and can have variable life spans. They can easily last
the life of the vehicle. If they are operated very near their capacity they will get hot and
heat kills electronics. Momentary short circuits can instantly open the diode in an isolator
and it becomes an expensive fuse.
