 | Home > Technical > Alternator: Bypass excite wire question |
 
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| jst Member Since: 14 Jan 2008 Location: Taunton Posts: 8059   
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If the battery is fully charged the alternator will reduce its output.
On one of my setups the twin alts would charge 2 pairs of two batts (2 banks of batts) on a switched circuit between the two banks. When they were full the alt light would glow as the alt output decreased. If you wire as you suggest if you have alt internal failure, there is no indication Presume kill switch has diode to avoid spikes when all power is killed? Cheers James 110 2012 XS Utility 130 2011 M57 bespoke Camper 90 2010 Hardtop 90 M57 1988 Hardtop |
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26th Feb 2020 7:21pm |
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| Marks Landy Member Since: 09 Feb 2015 Location: Cheshire Posts: 299  
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Had the same issue last week on my 200TDI.
The Engine to chassis harness disconnect plug was full of green corrosion. It wasn’t getting enough current to excite the alternator. Fixed with a little wire bush! |
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26th Feb 2020 9:32pm |
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| rdavisinva Member Since: 11 Sep 2020 Location: Chesapeake Posts: 5 |
For many years the AC Delco 10SI was the most widely used alternator.
It came in 2 varieties: self exciting (without a warning light) and with a warning light. The self exciting version had the field directly connected to the (+) lug internally. This went directly or indirectly (through the starter solenoid) to the (+) battery post. Both versions worked fine, but as mentioned in an above post by JST, a self exciting alternator does not provide a means to monitor the healthy or unhealthy alternator state. Add a volt gauge and monitoring problem is solved. |
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12th Sep 2020 1:51am |
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| sako243 Member Since: 08 Jul 2014 Location: Wales Posts: 1225  
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Under normal circumstances DO NOT short the alternator field windings to the battery live. Unless the alternator is spinning then the windings are a dead short to ground. The light does not go out because the terminals are switched when the engine is running. The fact the alternator is spinning means that via the laws of electromagnetism a current is induced in the windings (and as one terminal is a dead short to ground) then that means the excite terminal voltage has to rise.
In your circumstances with an isolator then you could do it, but forget to isolate it after an event and you'll have a flat battery in no time at all. Separately if you think about the circuit you have battery live -> ignition switch -> indicator light -> alternator field windings -> ground. There is no other switch as I mentioned above. You've (most likely) eliminated the alternator being faulty which means that in order for the indicator light to illuminate whilst the alternator is running that means you have a short to ground between the indicator light and the alternator. If you circumvent the problem by jumping the alternator terminals you still have an intermittent short to ground somewhere in the loom which will likely cause issues further down. If you've got one chaffed wire then you almost certainly have more on the way. My recommendation - find the broken wire and fix it. Sorry for the longer post but thought I'd explain why you should fix it not bodge around it. Ed 82 Hotspur Sandringham 6x6 95 Defender 110 300Tdi |
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12th Sep 2020 10:10am |
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| rdavisinva Member Since: 11 Sep 2020 Location: Chesapeake Posts: 5 |
So the way the ignition warning light works is when the key is switched on electrons flow from the (+) white circuit via the IG SW that is attached to one side making the bulb filament glow because the other side is attached to the alternator field (+) brown with yellow
When the engine is started, the alternator starts producing (+) current of equal voltage to the (+) current that the white circuit has and this makes the ignition warning light bulb filament stop glowing. This is how the warning light works with a healthy alternator. With an unhealthy alternator the ignition warning light stays on or will glow dim or intermittently when the engine is running. With a bad alternator field the ignition warning light will never come on also never coming on can indicate a burned out bulb, circuit failure, or burned out alternator diodes. sako243 brings up an interesting point I have never thought about before today, that if the field wire is connected to the battery will it run flat and if that is the case, you could connect a wire from the white IG circuit to the alternator field. Then when the key is switched off, there is no (+) current to the field coils. |
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12th Sep 2020 11:59am |
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| sako243 Member Since: 08 Jul 2014 Location: Wales Posts: 1225
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Electrons are negatively charged and they "flow" from negative to positive... Ed
82 Hotspur Sandringham 6x6 95 Defender 110 300Tdi |
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12th Sep 2020 12:06pm |
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