warning light for alternator

I wired a delco 10si alternator to my m farmall and followed an article I found on this website that says there has to be a warning light wired into it. Is this necessary? What is its purpose if it is?

I use the same alternator on my MF35 and I don't use a warning light... (I do have a voltmeter so I can tell if the ALT goes south)...
8)

Without a warning light, appropriate resistor, or diode in the "excite" circuit, the alternator can charge OUT of the excite terminal, causing the engine to continue to run after the ignition switch is shut off, and/or possibly damaging the diode trio or internal voltage regulator.

A #194 lamp, a 10 Ohm, 10 Watt resistor or a 3 Amp 1000 PIV diode (1N5408) will each serve the same purpose... limit or stop reverse current flow OUT of the "excite terminal".

Also, in the case of a poor or open connection in the charging circuit the alternator will try to pump it's entire output out of the "excite terminal" letting ALL the smoke out of the alternator.

P.S. Forgot to mention that mine is diesel, so engine shut-off is fuel related, not electrical related...
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well it is and isnt.problem is if you dont have either a diode OR a light voltage CAN backfeed to the ignition and tractor wont shut off.some folks i think use the accy poition on an aftermarket key switch to break that line and keep it from backfeeding from alternator.a diode of course works sort of like a check valve that only lets voltage flow one way,while a light bulb actually raises the resistance up to where current wont flow.the indicator light on the dash of your car or truck does the same function.it may or may not be neccessary depending on how your tractor is wired.ive changed all my fords over to 12v and personally i use both simply because i work them a lot at night in the heat of summer,and the guages are not lighted.overkill of course.

I cut the previous post short, as I had to run an errand. Here's a little more information:

Here's an explanation of what can happen if a lamp, resistor or diode is NOT used...

<img src = "http://www.gondtc.com/~blweltin/Bob/DelcoSIA1A.gif">

NORMAL charging system operation.

<img src = "http://www.gondtc.com/~blweltin/Bob/DelcoSIB1A.gif">

What happens when there's a defect in charging circuit wiring and no lamp, resistor or diode in the "excite circuit"..

A diode is the easiest/cheapest way to go and will cost you between $0.05 and $2.00, depending upon where it's purchased. Or, as I said, a lamp or resistor. It's up to you!

Here's how to connect a diode...

<img src = "http://www.gondtc.com/~blweltin/Bob/AlternatorHarness3.jpg">

"some folks i think use the accy poition on an aftermarket key switch to break that line and keep it from backfeeding from alternator"

NOT a good idea! See my other post for details on what can happen if a lamp, resistor or diode is not used.

(quoted from post at 08:51:57 10/27/11) P.S. Forgot to mention that mine is diesel, so engine shut-off is fuel related, not electrical related...
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Click to expand...

Ok Mr. Smart Guy...

Without the light or resistor or diode, the alternator will simply drain your battery dry when the engine is not running.

(quoted from post at 14:00:18 10/27/11)

(quoted from post at 08:51:57 10/27/11) P.S. Forgot to mention that mine is diesel, so engine shut-off is fuel related, not electrical related...
8)

Click to expand...

Ok Mr. Smart Guy...

Without the light or resistor or diode, the alternator will simply drain your battery dry when the engine is not running.

Click to expand...

Yep... Sure would... except for the fact that the relay that controls that circuit cuts off when I turn the ignition to OFF...
8)

"Without the light or resistor or diode, the alternator will simply drain your battery dry when the engine is not running."

NO, it will not. Once the engine is shut down the lamp, diode or resistor in the "excite circuit" have NOTHING to do with draining the battery.

There's just TOO much mis-information on this stuff floating around!

Bob, alternator repair guy.

IH used a 25 ohm wirewound resistor on their earliest tractors with alternators. Probably the most durable device for the purpose. And resistances up to at least 55 ohms will work.

Many different ways to wire it with no indicator light. Back when the 10SI was "modern", GM used exciter relays for some hookups (instead of an indicator light). Some were hooked to engine oil-pressure switches.

The alternator (unlike a brushed generator), does not have enough residual magnetism to "excite" itself and charge at normal RPMs although some will after being spun around 2000 RPM. So, a small amount of current must be fed to it, to create magnetism, to get it started. The problem is, once the alternator DOES charge, current can backfeed through that exciter wire to the IGN circuit and prevent you from shutting off the engine. This is especially true with diesels that use low-current shut-off solenoids in the injection pumps.

So - you can use a light bulb as a resistor (and charge indicator), or a diode, or a relay, or just install a $15 "self-excite" regulator and hook with just one wire.

$2 spent at your local Radio Shack is the easiest. j$2 will get you a four-pack of 6 amp/50 volt diodes at Radio Shack. Part # 275-1661. The 3 amp units e.g. 271=1144 and 276-1143 also work fine. If you use the smaller 3 amp units, hook two in parallel to be safe.
A 276-1661 diode is all you need.

Bad memory. Think I got a pushbutton I hooked up. Start tractor, push button, keeps charging until shut down. Diode also. Dave

I disagree. Bob is ABSOLUTELY RIGHT. I have used aftermarket switches with both the accessory position and ignition positions being seperate while using the 10SI alternators, and had no problems with it at all. If hooked to (a) seperate accessory position it kills power to the alternator while also seperating the power feedback to the switch (which would feed the coil circuit) to keep the engine from not shutting off on a gas tractor. It has worked fine for many years on tractors that I have owned. You also need to know that not all switches work the same. Some switches the accessory and ignition switch are "hooked together" in the switch and may not really "disconnect" from themselves once off. In this case they will feed back. I use a switch from NAPA which uses 2 accessory positions with the ignition position. You should test them before using them. You DO NOT need a light to run this alternator. My 2 cents Leon in Mn.