Testing the Lucas 49072B rectifier
- Thread starter fireflake
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Simple test on rectifier is to test each diode with a multimeter set to ohms and make sure they pass electricity in one direction and not the other. The Zener is more complicated as it designed to swap excess charge to heat, section J in the workshop manual has a setup but it's not simple, if your rectifier checks out and your alternator is good then you can just monitor the battery voltage as the engine is running, the voltage should increase as the revs rise but then stop climbing at about 14.5 to 15V, if it carries on climbing then its faulty or is not in circuit or the earth connection to the heat sink its screw into is bad. I had a alloy heat sink corrode with that white furry deposit and that stopped the zener working, cleaned it up and zener started working again.
Onder
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I just went through this. The stock system is pretty reliable despite the Lucas Reputation. Agree that you make sure each item should be
checked out as per the factory book. Clean all connections both physical and electrical.
Also agree that if you put in a modern regulator/rectifier you will be better off: cheaper and works reliably. A modern rectifier alone is only
about 10 USD or less : https://andover-norton.co.uk/en/shop-details/16668
If you run an electronic ignition then it probably is a good idea to scrap even a working Lucas rectifier and zener as I have noticed
they dont really keep the volts down to 14.2-4 at all times.
checked out as per the factory book. Clean all connections both physical and electrical.
Also agree that if you put in a modern regulator/rectifier you will be better off: cheaper and works reliably. A modern rectifier alone is only
about 10 USD or less : https://andover-norton.co.uk/en/shop-details/16668
If you run an electronic ignition then it probably is a good idea to scrap even a working Lucas rectifier and zener as I have noticed
they dont really keep the volts down to 14.2-4 at all times.
marshg246
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Even if everything tests OK, that rectifier should not be trusted. It is very important that the center bolt not turn, and it has so it will fail, if not now, soon and it is probably adding resistance to the circuit now - not good.
Also, it appears that you are testing without the battery connected - that's not a valid test. The test is to check the battery voltage with the bike not running and then with it running. If higher when running, the battery is being charged. It may not be higher at idle but when you bring the revs up it should be. With a fully charged battery, the voltage will be only slightly higher.
A fully charged AGM battery, disconnected, should be between 12.7 and 12.85 volts. If less after fully charging, the battery is at least near the end of it's life. Unlike Lead/Acid, an AGM battery at 11 volts is discharged (zero capacity left).
Also, it appears that you are testing without the battery connected - that's not a valid test. The test is to check the battery voltage with the bike not running and then with it running. If higher when running, the battery is being charged. It may not be higher at idle but when you bring the revs up it should be. With a fully charged battery, the voltage will be only slightly higher.
A fully charged AGM battery, disconnected, should be between 12.7 and 12.85 volts. If less after fully charging, the battery is at least near the end of it's life. Unlike Lead/Acid, an AGM battery at 11 volts is discharged (zero capacity left).
concours
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I understand all of the above.
And what I read in the book.
And how a diode works.
Thank you.
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