CAN A BAD VOLTAGE REGULATOR DAMAGE THE ALTERNATOR?

If a voltage regulator is damaged, can it in turn damage a field coil alternator? I've read that the opposite is true: that a bad alternator can take down a good voltage regulator. Also, if the voltage regulator is damaged by a bad alternator, what part of the regulator normally does get damaged?

 

regulator fail is unlikely to damage an alternator.

the regulator is a bundle of diodes that turn ac voltage to dc voltage.

diodes fail and connectors melt

you can ohm out the alt. parts and test the regulator with an ohm meter.
also have brushes in alt cover that need to be replaced from time to time

what bike do you have and what problem are you having?

you can find the test specs for the alt. at this link near bottom of page a good write up on testing system

The Ultimate Relay, Switch, Sensor, and Diodes Guide

specs

Checking Alternator Stators:

Measure the resistance across each pair (white1 to white2, white1 to white3, and white2 to white3) of the three white wires (white1, white2, and white3) at the connector; the specifications should be:

0.50 ohms +/- 10% for all XJ550 models.

0.46 ohms +/- 10% for all XJ650, all XJ700, all XJ750, and XJ900RK models.

0.37 ohms +/- 10% for all XJ1100 models.

0.40 ohms +/- 10% for all XS1100 models.

Note that the resistance specifications for the stators are extremely small; they should barely have any resistance at all (compared to most other electrical devices).

Checking Alternator Rotors:

Measure the resistance across the two lead wires (usually brown and green) at the connector; the specifications should be:.

4.5 ohms +/- 10% for all XJ550 models.

4.0 ohms +/- 10% for all XJ650, all XJ700, all XJ750, XJ900RK, and XJ1100 models.

3.5 ohms +/- 10% for all XS1100 models (field coil).


Note that worn, dirty, or damaged alternator brushes can affect these readings, as can "dirty" copper commutator rings on the rotor face (where the brushes contact the rotor):

 

Sir, thank you for replying and all the good info.

Let me provide some history on my yet unresolved charging system problem.
My bike is a 1981 Yamaha XJ650.
Several years ago I was out riding for well over an hour when stopped to get a coffee.
The bike was off for about fifteen minutes. When I went to restart it the starting motor was behaving as if the battery was weak. When I got home I promptly put the battery on a charger and it took about fifteen minutes to recover. This was a fairly new battery from Advance Auto Parts so I did not suspect it as the problem. When I wasn't riding I would keep the battery on a maintainer. So the next time I took a ride she fired right up.
But after stopping for gas the same thing happened: slow cranking as if the battery was not fully charged.
My first inclination was a problem with the charging system so I began with testing the alternator output.
At 2000 rpm the alternator was putting out 14.44 vdc.
The resistance between all three stator coil phases (white wires) is .5 ohms.
The rotor field coil measured 4.5 ohms right at the slip rings.
I then checked the battery electrolyte specific gravity and discovered it had two cells that were already going bad and another that was not far behind them.
I scrapped that battery and began using one from my car that was only replaced due to its age otherwise it was still good.
I cobbled together a set very short jumpers out of heavy copper wire to hook my bike up to this battery as it sat on a bench next to it.
In the meantime, I replaced the regulator/rectifier with a new aftermarket one.
I repeated the same output voltage test; except now the alternator is only putting out 12 vdc at 2000 rpm.
Note: Battery voltage before testing is 13v.
The voltage between the stator phases are each 12 vac at 2000 rpm and the voltage does not increase with rpm.
The resistance between all three stator coil phases (white wires) is .5 ohms.
The rotor field coil measured 15 ohms at the harness connector.
Measured temperature of the regulator/rectifier heat sink at 126 deg F with the engine running.
Rectifier diodes all test out good.
Alternator slip rings are very clean and smooth (no grooves worn in them).
The brushes are brand new.
I also checked for shorts between the alternator coils to ground and there were no shorts.
I haven't done anything more since October so I am going to get back into this so I can get back riding.
Any advice is greatly appreciated.

 

you may have a broken connection to a wire that is in the connector to harness.
are your ground connections clean battery motor and frame?


do you still have the old voltage rectifier?
plug it back in and test voltage

At 2000 rpm the alternator was putting out 14.44 vdc. is good

 

I went over the harness pretty well and it all checked out pretty good.
I believe I still have the OEM rectifier. I'll swap it out with the new one and check the charging voltage.
Also, is it possible that my testing with a car battery was throwing things off?

 

a car battery is ok for jumping a bike but the charging system on the bike will not like charging a car battery depends on the car batterys current charge.
this may be why your voltage is low with the car battery and new regulator