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Old 11-11-2017, 03:14 PM   #1
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Alternator charge dash light

Been noticing the alternator charge light coming on ( flickering) on long grades or whatever the rpms are above 1500. My Scangauge is showing the voltage at 13.7 to 13.8 so I知 assuming that the alternator is fine.
The belt was just replaced 6 months ago and the tension seems fine.
Anyone ever had a similar experience? Going to check for a loose wire/ground tomorrow.
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Old 11-11-2017, 04:05 PM   #2
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Were you also running the generator ?

Did it start after the belt change ?
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Old 11-11-2017, 07:14 PM   #3
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Some suggestions.

First, how it works: The alternator has three phase AC output terminals (three of them, effectively all three are the same). Most are stud/nut type connectors. The wire coming off this terminal will have (referenced to ground) voltage only when the alternator is providing power.

This AC terminal (though the magic of a diode in the rectifier bridge) is actually DC in reference to ground, but isolated from the 12 volt battery. So there is a DC voltage there whenever the alternator is producing power, referenced to ground.

That signal is then fed into a relay. This changes the signal from high (or having power) when the alternator is working OK, to low (no power or voltage) when the alternator is working, and high when the alternator is not working. That relay output then feeds the alternator light.

If the light is flickering my first suspect would be the relay. Then associated wiring in the circuit.

I've attached a schematic of a typical system...
Attached Files
File Type: pdf Alternator Circuit.pdf (79.9 KB, 39 views)
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Old 11-12-2017, 08:13 AM   #4
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Quote:
Originally Posted by nehog View Post
Some suggestions.

First, how it works: The alternator has three phase AC output terminals (three of them, effectively all three are the same). Most are stud/nut type connectors. The wire coming off this terminal will have (referenced to ground) voltage only when the alternator is providing power.

This AC terminal (though the magic of a diode in the rectifier bridge) is actually DC in reference to ground, but isolated from the 12 volt battery. So there is a DC voltage there whenever the alternator is producing power, referenced to ground.

That signal is then fed into a relay. This changes the signal from high (or having power) when the alternator is working OK, to low (no power or voltage) when the alternator is working, and high when the alternator is not working. That relay output then feeds the alternator light.

If the light is flickering my first suspect would be the relay. Then associated wiring in the circuit.

I've attached a schematic of a typical system...


Thanks for the schematic. A bad relay or loose wires at the relay seem like a good place to start. I値l post my findings as soon as possible.
Thanks again.
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Old 11-12-2017, 09:21 AM   #5
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The reason I asked about the generator running is because the alternator needs a reference voltage to compare to, to trip the light relay.

If the generator Is running and the house battery charging system is charging the chassis battery, thru a bi-directional isolation relay, the alternator will need to output more then that systems voltage to keep the light off.

The alternator light circuit doesn't know a specific voltage, it senses that it's outputting a higher voltage then the voltage of the battery its connected to.

Going with that, if the belt is marginally loose, it may be slipping at high RPMs.

If you have a belt driven fan, that can also add load to the belt at higher RPMs and temperatures.

This is not all theory, as there are members, on this site, that have the alternator light come on, while running the generator. They find that adding a load, like turning on the headlights, lowers the voltage enough to put out the light.

If this issue started after the belt change, I'd be looking there.
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Old 11-12-2017, 09:50 AM   #6
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Alternator is likely a "Duvac"; using isolator diodes.
OP states "alt fail" light flickers on long grades.
The light is controlled by a 6v relay in the low current plate.
The relay ("alternator fail"), is controlled by the reg output.
High heat from the exhaust manifold caused a reg failure on my 200a Leece.
If the reg or alt is ever replaced, it Must be an OEM LN only.
No after market alt or reg will hold up.
If charge voltage is normal, ok.
When the reg fails, charge volts can go high, pegging the VM.
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Old 11-12-2017, 05:37 PM   #7
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Quote:
Originally Posted by twinboat View Post
The reason I asked about the generator running is because the alternator needs a reference voltage to compare to, to trip the light relay. ...
See my earlier reply and diagram. It doesn't need or use a reference voltage.
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Old 11-14-2017, 01:21 PM   #8
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Thanks everyone for your input. Finally got under the rig to gain access to the alternator wires and the ground stud was loose! That explains the flickering alternator fail light at the higher rpms! I also had a problem with the speedometer reading higher speeds than I was actually going. ( the GPS confirmed this) I知 hitting the road at the end of the week and I値l know then if the problems are related. I知 pretty sure they are but I値l know for sure after I get going.
Thanks again everyone.
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