Friday, September 17, 2010

Journey of discovery: Following the traces

Here's the manufacturer's wiring schematic for the alternator.  Looks pretty simple, doesn't it?  The field connection (terminal 1 on the alternator) goes thru a current limiting resistor, thru an oil pressure switch, and then direct to the positive battery terminal on the starter.  Makes sense.  You don't want the field drawing current all the time - only when the engine is running.  And when the engine is running, there will be oil pressure (well, hopefully).

Now looking at the actual alternator, we discover something:  The brown/white wire and the green wire are no where to be seen.  Stuff has been rewired.  From my previous forensics, I know that the Previous Owner installed a 60 amp or greater alternator but didn't upgrade the wiring to handle the extra current, causing a major melt-down in the engine wiring bundle.  There have been a lot of splices. 

Our actual alternator terminal wire (past the short section of white wire at the plug) is pink.  The wire bundle goes from the alternator (lower right in the picture) to the back of the engine (upper left in the picture) before it joins any other wires.
At that junction, we find additional evidence of the wiring melt-down:  three of the four wires coming from the alternator have been spliced, again.  And the colors have changed, again.  Instead of a pink wire, we are now interested in a lavender wire, which enters the larger engine wiring bundle and heads to the left...

Under the floor, towards the cabinet behind the pantry...

Up the inside of the cabinet in a large bundle of wires...

Finally arriving at back of the cockpit engine control panel.  Here we discover that it is connected directly into the wiring coming from the ignition switch, specifically, the circuit that is energized when the switch is turned on.

This is nothing at all like the manufacturer's schematic.  But it should still work to deliver current.  There is a risk of burning up the field windings if the ignition switch is left on for long periods when the engine is not running.

This is not good.  But still, it should work.

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2 comments:

Erick said...

Loving these posts. Keep up the good work, I'm going to have to start doing electrical work sooner or later and the alternator scares me a bit.

bob said...

Erick: I'm not there yet.

But don't let alternators put you off - they are really very simple machines. Those with internal regulators (Delco...) will have 3 wires:

o A big fat one, to carry the output
o (Delco) Terminal 1: the field supply that I've been talking about in this series of posts
o (Delco) Terminal 2 - the voltage sense connection - the regulator tries to make the voltage it sees on Terminal 2 match its setpoint. You could just connect this to the heavy output terminal, but then you'd be getting the desired voltage at the alternator, not at the battery. Instead, this is usually connected directly or close to the battery, so the voltage supplied to the battery can be sensed without the interference of the voltage drop in the heavy current carrying wire.

If you stick with automotive-type alternators, that's pretty much all you need to know...

Oh... note that the last point above means that if the sense wire is not connected to anything, the regulator will run the voltage as high as it can (and destroy itself in the process), trying to make 13V appear at the sense terminal.

OK, one more thing... automotive regulators are not the fancy 3-stage regulators you want to have when your batteries sit on the charger for months on end.

OK, one last thing :^)... Diesel engine tachometers typically feed on the pulses produced by the alternator (being that there is no ignition system). If you use an automotive alternator, you'll have to bring a wire out of the case which is connected directly to one of the stator terminals for this. You can see mine in one of the pictures.

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