Alternator light on with 14.16v at isolator

[timthenomad]

Hi All,

My alternator light has been coming on intermitantly. I checked the voltage at the center of the isolator and it has been a consistent 14.1V. I do have the APC cable on there. The batteries have been staying charged. It was on dimly yesterday and when I turned on the headlights it got brighter. Its a newish alternator, and other than the light the electronics have been behaving fine. The coach starting battery is a low-end Walmart one with a 12/20 date sticker on it but still seems to be doing the job. Should I just swap at the alternator and see if the light goes off or try to dig deeper?

Thanks

--tim

 

[RF_Burns]

The GEN lamp is basically wired between the battery positive (+) and the secondary output. The GEN lamp turns ON whenever there is voltage difference (Higher or Lower) between the battery positive and the alternator output.

Further, since you have an APC cable, the GEN lamp can only turn ON when the alternator voltage is Lower than the battery voltage.

Later coaches (75 and after I believe) have a diode in series with the GEN lamp from factory to prevent a dim GEN lamp due to the voltage drop across the Isolator, so for those coaches the alternator voltage needs to be lower than the battery voltage for the GEN to turn ON.

The "Secondary" alternator output is taken from a smaller rectifier in parallel with the main output. This second rectifier is called a "Diode Triode". So, with that in mind, your fault appears to be in the alternator itself. If you are capable of repairing your own alternator (they are very simple), I would look for loose connections at the Diode Triode leads, and the voltage regulator, or one or both of those could be faulty. Also a fault in the rotor and brushes.

You actually should have more than 14.1V at the center terminal of the Isolator. The isolator has a 0.7-0.8 volt drop internally, so that only leaves about 13.4V at the output to the battery. You should have something around 14.3V to the battery, so about 15.0 volts at the input to the Isolator. The OEM Delco alternator has a Voltage Sense wire coming from the wiring harness. This feeds battery voltage back to the alternator regulator to allow the alternator to compensate for any voltage loss between the alternator output and the battery.

 

[tyler]

I think Bruce's option is more likely, but as a maybe...I had some intermittent gen light, and it turned out to be my ignition key cylinder was not returning fully to run position from the start position. The test function of the light would stay lit.

 

[RF_Burns]

I think Bruce's option is more likely, but as a maybe...I had some intermittent gen light, and it turned out to be my ignition key cylinder was not returning fully to run position from the start position. The test function of the light would stay lit.

Actually there is no "test function" for the Gen light. The alternator is not outputting any voltage before the engine starts, so there is a voltage drop across the Gen lamp and it lights up. The designed operation of the Gen light gives you a test. If the Ignition switch wasn't returning to apply power the the accessory output, the alternator would be outputting voltage but the Ignition switch side of the lamp would be at or near zero volts and the Gen light would light up. You must have a 73 or 74 coach, because 75 and later coaches had a diode in series with the Gen light which would have prevented current flow through the lamp in this case. They should have installed two diodes in parallel in the opposite direction to indicate this failure.

I don't think a faulty Ignition switch could be his problem because regardless of the year of the coach he said he had an APC cable. That would prevent current flow back through the Gen light in that type of failure.

 

[Matt Colie]

Tim,
This is another case where a sigfile could stop a lot of misunderstanding. If we knew more about you coach, that could end a lot of Bruce's required conjecture. Plus, it is nice to know who it is we are trying to help, and it makes it much less awkward if we ever meet and a location makes that a better chance. We are a very supportive and helping community and someone in striking range may offer to assist you.
Matt_C

 

[Rdenney]

I think Bruce's option is more likely, but as a maybe...I had some intermittent gen light, and it turned out to be my ignition key cylinder was not returning fully to run position from the start position. The test function of the light would stay lit.

And if (I said "if") it can be the key switch, it can also be the wiring harness on the steering column, which on my coach has needed cleaning a couple of times. Bad connections in that big inline plug can do all sorts of weird things.

Rick "who has bypassed the nichrome wire with an inline 12-volt bulb that illuminates briefly on startup" Denney

 

[RoadWarrior]

A leaky or failing diode in the alternator can cause the symptoms you describe. Unfortunatly the best way to positivly test for this is with a Oscilliscope. If you have a meter with a high enough impedance you might be able to read AC voltage across the battery. Assuming this is the problem, it can be detrimental to the battery.