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rudolphs 13-03-2015 19:38

Exciter Wire needed on Diesel Alternators?
I left off the Exciter wire when replacing my 1984 Universal Catalina 30 alternator with a new 51 Amp Sierra 18 6263 “Internal Regulator” alternator. The new belt is extremely tight; two brand new 105 Ah Walmart EverStart batteries. The prior and very suspect alternator/wiring had no exciter wire, so I left the new Sierra-supplied Exciter wire in the box. Now I’m seeing intermittent “no charging”. Some days I’ll go out and run the engine in idle for most of a 4-hour trip and find that my house battery has discharged during the entire trip. Need to switch batteries to start. Also, I seem to need to rev the engine once to “wake up” the new alternator. For load I run a small GPS and VHF Radio, which I’ve measured with a current clamp to be drawing 1.3A combined.

When the alternator is producing, I’ve measured it outputting 14.67V at full throttle between the positive terminal of the starter to ground. FYI, I have added a beefy shortcut lead from the alternator output to the positive battery terminal to bypass the long run to the console and back. Without the shortcut lead I only see 13.8V at the same test location.

Was I dumb to leave off the Exciter wire? Would the purple fuel pump power wire be a good candidate for T-connecting into the EXC. terminal of the alternator. I’ve attached a picture as currently wired as well as the Mando wiring diagram that came in the Sierra box. And a digression: if you happen to know, which terminal should the tachometer be connected too, again I could have wired it wrong, currently tach wire is connected to “indicator light” but I also tried it on “A/C TAP Tachometer” to no avail - I think my tach guage is caput.

Main problem is intermittent charging. Should I wire fuel pump wire to EXC.? Thanks in advance for all help, help, help.

As currently wired:

Mando wiring diagram that came in the Sierra box:

btrayfors 14-03-2015 09:31

Re: Exciter Wire needed on Diesel Alternators?
OK, I'll try again....damned computer/Board lost most of my answer.

I see at least three problems:

1. yes, you need the exciter (field) wire connection;

2. the battery sense lug requires a connection directly to the battery positive post, not the little jumper wire shown which only measures the alternator's output voltage; and

3. you need a ground wire, at least the same size as the positive output wire, connected directly to the battery negative lug. Grounding thru the alternator case/engine isn't enough.

A "beefy" positive wire directly to the battery is a good idea...alternator should NOT go thru the switch. However, you need an appropriate fuse in this wire near the should be rated at least 50% greater than the maximum alternator output amperage.

Others more expert with this alternator may have additional comments.


cal40john 14-03-2015 09:53

Re: Exciter Wire needed on Diesel Alternators?
Is the indicator light you're referring to the charge/alternator light? Then yes it is likely where the exciter wire goes.

The exciter needs to be on a switched connection, it will drain the battery if not. Typically it is connected to the charge indicator light. When the ignition switch is on it supplies power to the indicator light. The alternator starts out producing no power, and the field coil is low enough resistance that the light comes on. Since the light is supplying electricity to the coil, the alternator starts producing electricity. The output of the alternator, through the regulator, is also connected here, so once the alternator is producing electricity the field is powered by the alternator output and so also there is no voltage difference across the light, so it goes out.

What kind of tach do you have? Mine is mag sensor on the flywheel that has nothing to do with the alternator. Others connect to the AC (alternating current) post on the alternator. This is before the diodes so the current is reversing, the tach counts how fast the current is reversing.

thomasow 17-03-2015 13:32

Re: Exciter Wire needed on Diesel Alternators?
Hello, the direct answer is: yes - you will need to reconnect the exciter wire.

Some internally regulated alternators are able to 'self excite', but even those which are not often are able to get themselves going providing there is sufficient residual magnetism left in the core. This is why you are seeing intermittent functioning (as well as reving the engine can at times help 'wake it up'). I had a like situation (caused by a wire failure in the engine wiring loom). Alternator would work, and then stop. Took it in for testing, and it tested fine (as they connected the exciter) and it still worked after I reinstalled it due to the residual magnetism. For a while [IMG]file:///C:\Users\Win7\AppData\Local\Temp\msohtmlclip1\01\c lip_image001.gif[/IMG]

Connect it to some power source that comes on with your engine, a fuel pump would be fine. You might connect it through a diode and/or light bulb - not sure the specs on your specific alternator, but some need protection on their exciter wire to prevent the alternator from trying to pass high current back through the exciter wire. In any case, adding in a diode or light bulb will not hurt even if it is not needed. (Diode - a standard 2-5A / 25v or higher one will work just fine. Put it so that the 'arrow' points towards your alternators exciter connection)

Related tach: IF it is indeed a tach designed to be driven from the alternator, you would connect it to the TACH terminal. But know that this only works when the alternator is actually charging! So, given the thing with the Excite terminal, if the alternator has not been 'excited' into working, there is no chance for the Tach to work either!

Best of luck, hope this has helped some.


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