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Vintage Technical Tips & Tricks Technical and maintenance tips and links. DO NOT POST YOUR QUESTIONS HERE!! Please post questions to the general forum.

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Old 12-11-2009, 10:51 AM   #1 (permalink)
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How to test your alternator and rectifier

In order to successfully test your charging system output you need to be able to put a load on it. According to the Triumph manual you should use a 1 ohm resistor. They show how to construct one, but a simpler solution is to buy one from Nebraska surplus. http://www.surplussales.com/resistor...nd/WW05-1.html
The one I bought is made by Milwaukee and costs around 25 dollars. You need one that can handle at least 200 watts, the one I bought is rated at 240. Attach to each terminal a double set of leads. The white set uses female bullet connectors to connect to the alternator leads. Red set uses some sort of terminal to connect your multi-meter. This is a picture of how I rigged mine. Attach the resister to some sort of stand so air can circulate around it.


Disconnect the two alternator leads where they come out of the primary case and plug them into the two white leads. Connect your multi-meter to the other. Set the meter on 10 volts AC (remember its AC current coming out of the alternator). Start the bike and check the reading. A healthy alternator should be putting out 9 volts.

If thats okay, next check the output at the rectifier. Rig up a jumper with a slide on connector. Connect this to the middle terminal on the side of the rectifier. Connect the other end to one of the white wires. Connect the red wire on that terminal to the negative lead of the multi-meter. Connect the other red lead to the positive multi-meter lead. Ground the corresponding white lead on that terminal to the bike frame. Set the multi-meter on 10 volts DC this time (remember the rectifier converts AC out of the alternator to DC going to the battery). Start the bike again and you should be reading 7.5 volts.

Before I made these tests I was ready to replace the alternator on my bike. It checked out fine but the rectifier proved to be duff. Saved myself about 250 bucks.
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Old 12-13-2009, 02:48 PM   #2 (permalink)
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This rig can also be used to load test your battery. Make some jumpers so you can attach the white leads to the battery posts. Connect red leads to voltmeter same polarity as battery terminals. Connect to battery for 15 seconds. Voltage will drop but should stay above 9.5 volts if battery is healthy.
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Old 07-03-2010, 12:24 PM   #3 (permalink)
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A quicker way to test your stator is firstly to measure it's resistance, it should be about 0.5 ohm.

Measuring low resistance is tricky with a digital multimeter. First measure the resistance of your meter leads connected together (say a reading of 2.4 ohm) then measure your stator (should now read around 2.9 ohms.) Also measure the resistance between a lead and ground. It should be infinite resistance.

If the resistance is OK then with the stator wires disconnected measure their output at 3000 rpm (connect your meter between any two leads in turn if a three wire three-phase stator.) It should be around 32v AC.

If your stator meets these tests it is OK and charging problems lay elsewhere.

Derry.
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Old 07-21-2010, 07:06 PM   #4 (permalink)
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Quote Derry:
"If your stator meets these tests it is OK and charging problems lay elsewhere."

I know I am a pain Derry but No, not really. Some faults are only detectable with a load test. If it passed all of your tests you wouldn't be testing it to begin with.
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Old 12-27-2011, 05:00 PM   #5 (permalink)
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Here's a link to some testing I did earlier

https://docs.google.com/viewer?a=v&p...1ZmQ4&hl=en_GB
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Old 04-05-2012, 07:43 PM   #6 (permalink)
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Quote:
Originally Posted by johntioc View Post
Quote:
Originally Posted by DerryUK View Post

If your stator meets these tests it is OK and charging problems lay elsewhere.
I know I am a pain Derry but No, not really. Some faults are only detectable with a load test. If it passed all of your tests you wouldn't be testing it to begin with.
John
Most of the time people suspect charging system issues is because their battery is dead or keeps going dead, and no other symptoms are visible. In that case a full Lucas test is not really called for, adds a lot of extra cost for something you need to do once every 5 years, and is simply beyond the comprehension of the average rider/mechanic.

I find a faster, cheaper test to be replacing the system fuse with a simple 10-0-10 analog DC ammeter to tell me 95% of what I need to know. The ammeter must have wires long enough to make it hand held so the needle is steady and easy to read.



Above is a photo of the "professional diagnostic tool" I've used as a professional Triumph dealership mechanic since 1971. I remember paying $7 for this. It's obviously just a Britax ammeter from a nacelle model, but it does the job.

All you need is 3 readings:
Idle lights OFF
3000 RPM lights OFF
3000 RPM lights ON

The differences between those readings allows me to test the entire charging system in a single step. Is it fool-proof? No way. But it is fast and easy... and usually points to some silly move on the part of the owner not even related to the alternator.... like installing a 100W headlamp bulb and a hairdryer.
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Old 04-15-2012, 05:43 AM   #7 (permalink)
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Looks good Gamba.

What sort of readings would you expect on your 3 tests? Thanks.
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Old 04-25-2012, 10:16 AM   #8 (permalink)
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Not sure why Gabma hasn't answered Doug but on my bike I would expect:

at idle lights off the meter would read negative
at 3000 lights off the meter would read positive
at 3000 lights on the meter would read zero or slightly positive

If you get negative readings at 3000 rpm in either case it's time to look for problems. The reason for this is that with a negative reading the battery is providing some or all the power for your electrics rather than your alternator providing all of it. In the long term that will probably mean a flat battery.
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Old 04-26-2012, 10:46 PM   #9 (permalink)
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Very close DerryUK. Think about it. You ALWAYS want positive readings (lights ON or OFF) at road speeds. Positive readings indicate the battery is being "charged". Zero or negative readings mean you are "discharging" the battery as you ride down the road. 3 guesses how far you'll get with a Brit bike and a dead battery.

All electrical systems vary from each other. There is NO one single hard and fast answer. Rotor magnetism drops with age, thus affecting the alternator output. Any number of loose or corroded electrical connections might decrease charge rates. Then of course the condition of the battery itself plays a part.

But no matter what...
Idle lights OFF will be negative (a "discharge") This simply checks the connection of the ammeter!
3000 RPM lights OFF should be in the positive region. If there is not a modern regulator (Podtronics and the like), you might see currents as high as a battery-cooking 7A. This is why you never drive on the expressway without the lights turned ON.
3000 RPM lights ON Should also be positive. By how much will depend upon the condition of your system. Dropping back into negative readings is usually too much wattage in the HL bulb. The system was designed for a 35W bulb. For open country riding most riders can use 45W. You really never want your battery to be continuously charged by more than 2A. If you know how and where the bike is ridden, then you can "tune" the charging system with the wattage of the HL bulb.


• An interesting experiment is to see exactly what RPM the meter goes into positive ("charging") territory with lights ON. That is the minimum RPM you can ride if you expect to charge your battery.

Hope this helps!
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Last edited by GABMA; 04-26-2012 at 10:55 PM.
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Old 04-28-2012, 11:35 AM   #10 (permalink)
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RF, good explanation of how to use the ammeter. Thanks!
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