How to test my alternator: SOLUTION

SOLUTION: As a couple of gentlemen have pointed out, it looks like the alternator light coming on isn't a major issue. If I'm understanding it right, when the generator is on, the batteries are already fully charged and so the alternator isn't putting out as much as the inverter/converter charger already is. This causes the alternator light to come on. This would mean that the RV is perfectly safe as far as taking it on trips. Thanks to all for your replies; I appreciate it.

bluebarry1964b wrote:
UPDATE: I did some experimenting. For some reason, as one person mentioned, when I have the generator and engine on, the "alternator" light is, indeed, coming on. When I shut the generator off, but keep the engine on, the "alternator" light is going off.

What I can't figure out is why now? I've had the RV for 8 years and never encountered this problem before. The one change is that I just replaced all of the chassis and house batteries. I did look over the wiring and it looks like the batteries were hooked up properly.

Any ideas as to what could be causing this now?

I've been researching this lately since my coach only has a relay delay that works in one direction.

Holiday Rambler/Monaco used several devices over the years to try to keep both sets of batteries charged. Sounds as if you have the bi directional relay delay device (BIRD) that does not have the lead that inhibits operation of the relay tying the batteries together if ignition is on and the generator is on.

Here's what I think is happening:
The BIRD sees alternator input, see's the chassis battery set is charged, and closes the Big Boy (or other) relay tying the chassis batteries and the house batteries together so that you can get your house batteries charged while driving down the road.

This is all well and good, but now you decide you need the generator running so that you have 110 volts for some reason. Now the inverter/converter sees 110 volts and begins to provide power to the house batteries....but the house batteries and the chassis batteries are still tied together because the relay is closed. The inverter/converter and the alternator are now BOTH trying to charge both sets of batteries.

Someone that understands electronics better than I do can probably explain better why you're seeing the dash light under this circumstance.

One model of the BIRD (INTELLITEC RV/MOTORHOME BI-DIRECTIONAL RELAY DIESEL MODEL: 00-00839-000) has a 12 volt lead from the generator that allows the BIRD to inhibit the operation of the relay. Here's Intelletic's description of the operation:
"When the generator is operating, a 12 volt signal is applied to the generator input. This inhibits the isolator from
operating. This is to prevent the generator/converter combination from supplying power to the chassis functions which might over-load the converter or the generator circuit breaker."

As to your other question Why is it diffent now? Maybe the battery install cleaned up some bad connections, or maybe one set of your batteries were so bad the BIRD never energized the relay (of course I'm guessing here).

I hope this limited info helps.

If you start looking for your BIRD, some of them are circuits mounted on the large fuse/relay board in the electric bay. Some of them are mounted (I think) near the batteries themselves. My one directional isolator relay delay is mounted in the battery bay.

Fred

Not that uncommon.

If the converter, charger or inverter/charger output is at higher voltage than the alternator, the alternator light will (and should) come on.

UPDATE: I did some experimenting. For some reason, as one person mentioned, when I have the generator and engine on, the "alternator" light is, indeed, coming on. When I shut the generator off, but keep the engine on, the "alternator" light is going off.

What I can't figure out is why now? I've had the RV for 8 years and never encountered this problem before. The one change is that I just replaced all of the chassis and house batteries. I did look over the wiring and it looks like the batteries were hooked up properly.

Any ideas as to what could be causing this now?

K Charles wrote:
Maybe that "unhook the battery" trick worked in 1958 but it may not work the same way with electronic stuff you have today. An auto parts store might check it for free.

in 1958 they had generators and generally if the engine idled it would stall if you unhooked the battery.

long about 196x they started putting in alternators and one of the "Brags" (Commercials) they did was start a car and remove the battery and drive it without any battery at all.

That said. I agree.. However I can fix that too.

bluebarry1964b wrote:

rgatijnet1 wrote:
When testing with a voltmeter, what you are looking for is an increase in voltage above what the voltage is with the engine off.
ANY increase usually indicates that the alternator is OK. A lower than usual charging voltage reading can mean a bad or shorted battery cell, or other battery problem that is dragging the voltage down. An alternator cannot overcome a bad battery and a shorted cell will certainly cause the charging voltage to be low.

Now, you've got me thinking . . . I just had all of my batteries replaced 300 miles away at the last place we were staying. I sure hope one of the new chassis batteries doesn't have a bad cell in it, or something like that.

Or they got a wire crossed when installing the new batteries.

rgatijnet1 wrote:
When testing with a voltmeter, what you are looking for is an increase in voltage above what the voltage is with the engine off.
ANY increase usually indicates that the alternator is OK. A lower than usual charging voltage reading can mean a bad or shorted battery cell, or other battery problem that is dragging the voltage down. An alternator cannot overcome a bad battery and a shorted cell will certainly cause the charging voltage to be low.

Now, you've got me thinking . . . I just had all of my batteries replaced 300 miles away at the last place we were staying. I sure hope one of the new chassis batteries doesn't have a bad cell in it, or something like that.

The most simple thing to do would be to buy a digital cigarette lighter volt meter. You plug it into the cigarette lighter socket and watch the meter. Of course, your lighter socket must be powered by the chassis battery. Also, as this is simple, it is not the professional way to check voltage. It is good to keep one of these meters on board for a quick check at any time.

Alternator light on means the alternator is drawing more power than it is generating, THIS may be due to a faulty alternator, slipping drive belt or just flat low engine speed. (However usually the engine will stall if it is going that slow).

It can also be a bad alternator.

Easiest way to check it: Some Auto parts places can hook up a simple (well very complex internally but simple externally) device and you do what they tell you regarding engine and headlights and it says GOOD/BAD. Some can even measure current and such.

There are many ways an alternator can fail. Some are easier to detect, but a good test device, will.

Alternators have three 2 phase outputs, a total of six diodes, inside the alternator, to provide the output.
They are usually in one replaceable unit.
Failure of any one diode of the six will result in a lower than normal charge current, and depending on the battery state, a lower than normal charge voltage.

The "alternator light" is looking at a small difference in charge voltage.
Many things could cause the light to go on, not all meaning something bad.

As an example, running the generator while driving down the road is a sure way to make a Monaco DP light the alternator light.