| Alternator exciter wire power source [message #362415] |
Mon, 15 February 2021 17:28  |
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RF_Burns
Messages: 2277
Registered: June 2008
Location: S. Ontario, Canada
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Senior Member |
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So I have had a power resistor hanging around the coach forever that I planned to wire-in in place of the dreaded nichrome resistor wire. I checked out the wiring diagram and for all years it shows the source of the exciter power as being from the ACCY power from the key.
I thought that's crazy because that means there is current flow in that resistor wire if someone leaves the key in the Accessory position for any period of time. What's even more crazy is the GEN light is fed from the IGN contact. So that means there is current flow in the resistor wire (alternator exciter wire), but the GEN light is not powered ON.
Is this the reason the nichrome wire has overheated in some coaches??? The key was left in the ACCY for long periods of time...? In which case the APC cable would not prevent this current flow.
I could see these connections being reversed, then in the ACCY position the GEN light will be ON to remind you that you are discharging the battery, but there would be no current flowing in the exciter wire.
So I planned on wiring the exciter current through my resistor from the IGN feed rather than the ACCY feed. The only time there should be current flow in the exciter wire is when the Ignition key is ON and the engine is NOT running. In other words, the few seconds it takes to start the engine. The only other time current will flow in the exciter wire is of course during an alternator failure.
So as part of my preparations to install my exciter power resistor, I proceeded to measure the voltage at Pin 1 of the alternator (exciter input) I turned the key to ACCY and read 0 (zero) volts. So then I turned the key to IGN and read about 2 volts. So, my exciter wire only gets power with the IGN ON (as it should be). I have tried to find the power source in the wiring harness, but that seems near impossible.
So are the wiring diagrams wrong and the exciter power is fed from the IGN feed only from the factory?
Or has my coach been modified to supply power from IGN as it should be?
Has anyone noted alternator exciter power in the ACCY position?
Can someone (preferably more than one person) test the exciter power at pin 1 of their alternator. This is on the 2 pin connector on the alternator. Pin 1 is the exciter current and should show something between 2 and 4 volts with the engine not running. Try it on ACCY and IGN positions. The second wire in this connector is the voltage feedback wire which should have battery voltage on it at all times.
Hopefully the wiring diagram is wrong and the exciter only gets power in the IGN position. If others have power in the ACCY position, well then I don't know what the Engineers were thinking, but it could explain the overheated wiring harnesses.
Let me know what you find!
Thanks.
Bruce Hislop
ON Canada
77PB, 455 Dick P. rebuilt, DynamicEFI EBL EFI & ESC.
1 ton front end
http://www.gmcmhphotos.com/photos/showphoto.php?photo=29001
My Staff says I never listen to them, or something like that
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