Charging system troubleshooting question

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Arlen Johnson

Arlen Johnson

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1975 Commando MK III:

First test:
- Leads from the alternator disconnected
- Engine running at 3000 RPM
- Voltage across leads =~ 25 VAC

Second test:
- Almost new battery
- Alternator leads plugged in
- Engine stopped
- Voltage at power outlet =~ 12.8 V

Third test:
- Engine at 3000 RPM
- Voltage at power outlet =~ 12.5 V

Conclusion:
The battery isn't getting charging voltage (above 14, or so). In fact, the voltage at the power outlet goes down with the engine running. I assume the voltage at the battery is also about the same, 12.5 V.

Can anyone guess what's going on or have suggestions for other tests?
 
Load test the alternator directly without anything else hooked up to it. Might be as simple as a broken/loose wire. But usually in these old bikes it's some high resistance point that has a bad connection that won't show up with a voltage test. You can test voltages across connections to see it there is voltage, there shouldn't be. It's a pain in the bum. I'm betting on a loose connection.
 
I am rather weak on electrical stuff, so could you tell me exactly how to test the alternator with a load? Also, I don’t understand what you mean “You can test voltages across connections to see it there is voltage, there shouldn't be.” Can you please clarify? I apologize for my ignorance in these matters, but I am learning.
 
A simple test using an Ohm meter.

Engine off.
Select most sensitive scale on meter.
Disconnect both alternator leads.
Connect one lead from the Ohm meter to one of the alternator leads. Connect the other meter lead to frame ground.

Any reading on the meter indicates an internal short in the alternator coils.

For good measure, repeat the test connecting the meter lead to the other alternator lead.

The stator is bad if an internal short is present.

If the stator passes this test, next step is to test regulator/rectifier.

Slick
 
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Well, there's a procedure, I may not be the best one to recommend, but maybe you can look up 'alternator testing'. Or add lucas to it, it may get you to a better procedure. As far as voltages, even if there is a high resistance in a circuit (bullet connector) the voltages can check just fine, but add a load (resistance) to the circuit with juice flowing (the circuit is live) you can measure a voltage across a high resistance joint (bullet connector) where you most likely could not measure the resistance or a voltage if it's not live. It has to do with 'voltage drop' across a resistance. If there is no current flowing, there is no voltage dropping. If there is current flowing, there will be a voltage drop because of the resistance. Sometimes it only takes a few volts drop to make things not work right, and as the current tries to flow more, the voltage drops more, making it even worse.

Hope that helps. But bottom line is use the voltmeter across connections when you have the circuit alive. If you see anything more than a volt or more, it's too much. Remember you only have 12 volts to work with and 1 is almost 10%, too much.
 
Rather than checking at the power outlet, connect your vom leads directly to the battery to see what it actually is getting.
Jaydee
 
Probably a bad earth or loose lead to your rectifier.

Before you start digging too deep and spending money, it is worth taking off your rectifier and both zener diodes and giving the terminals a good clean up - make sure the threads are clean on the zeners and there is no corrosion on the z-plates.

Reassemble with a smear of dielectric grease (or some copaslip if you don’t have any)

Then redo your tests.


As mentioned already, put your multimeter across the battery terminals rather than using the power socket.
It will rule some others variables out of the equation.
 
Hey guys, I really appreciate all your suggestions. I’m going to try them tomorrow and I’ll post after I get some results.
 
I second a good cleanup on all the connectors you can see. Then try it again.
 
Since no volt increase at 3k rpm from non running batt volt, I Suspect the rectifier is not making DC from the AC coming out alt. Check connections. There is a rec test procedure in the shop manual thats pretty straightforward.
 
Another check is the alternator AC voltage while it's connected. Measure at the rectifier terminals, that would include all the wires and connections.
 
Good day to all,

The first test today is to see if both coils in the alternator stator are NOT grounded. With both alternator wires disconnected and my Fluke VOM on the continuity position, I get open line (OL) on both when I check each wire to ground. With the meter on the ohms position, I get somewhere in the neighborhood of 8-12 megohms. When I check both wires to each other I get total continuity and no resistance. I assume this all means the alternator stator is OK. Is there any possibility the rotor could be weak or bad? Remember, in my original post I said the alternator was putting out approx. 25 VAC at 3000 RPM.

Next, I intend to remove both Zener diodes and the rectifier, thoroughly clean all their associated contacts and bench test them.
 
Your alternator sounds normal. Does a headlight bulb across the AC wires light brightly at reasonably low rpm?

My next check would be DC voltage between rectifier output and return or frame, with engine rising up towards 3,000 rpm.
 
Arlen Johnson said:
I'd like to post some photos, but don't know how. HELP!
Click to expand...
Since you're not a VIP member you must post photos on other photo sharing sites then paste a link here to show them.
If you become a VIP you can upload images directly in the messages. A button appears below the comments box "upload a file".
 
I figured out the photo posting by using flickr.

The right side Zener has stripped threads and the ground terminal is quite corroded. The corrosion is an obvious problem, and the stripped threads preclude enough tightening torque. (I hope I wasn't the one that over-torqued and stripped the threads!)



 
Next, I will do Triton Thrasher's suggestion and see if the alternator lights up a 12 V light bulb at low RPM.
 
If the zener diodes were the problem would it not show an overcharge at 3000 rpm, i.e. not shunting excess current?
 
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