The New And Improved Unofficial R65 Forum V2

Technical Discussion => BMW Technical Q&A, Primarily R65 => Topic started by: billygoat on July 01, 2009, 01:08:18 AM

Title: GEN light
Post by: billygoat on July 01, 2009, 01:08:18 AM

 My gen light started comming on and flickering now it does not come on when I switch the key on. I think it's the rotor after multiple checks. So I put my ohm meter across the contacts and got 20 ohms??? [smiley=embarassed.gif] I managed to drop the rotor and re-measured it and got a different reading!  My bike has over 100,000 miles so I guess it's time for a new one.

 Has anyone heard of the 'penny test?

From:http://mysite.verizon.net/reso8sug/id4.html


"Gary Smith, "that's the guy we need to talk to, the expert, the guru, "the man" of BMW airhead charging systems."  After introductions and my responses to three pf Gary’s questions, He diagnosed my problem as a failed rotor. After removing the front cover (disconnect the ground wire first) we reconnected the ground wire and performed the penny test.   With ignition on Gary place the penny between the commentators the generator light illuminated verifying that the gen. bulb was okay and the rotor was shorted. .  When it comes to diagnosing Airhead charging problems, this guy pulled the sword from the stone."

 What do you think they mean by 'commentators'? Where do I put the penny? What ohm reading should a rotor have?

Thanks.

Title: Re: GEN light
Post by: Andy-Gadget on July 01, 2009, 03:26:10 AM

Rotor should be between 4 and 8 ohms, so it sounds like a rotor to me, it is the most likely bit to go.

When you pop it off, check slip rings to body as well.

Make sure you use a cut off allen key or similar to pop off the rotor, or suffer the problems  >:(

Title: Re: GEN light
Post by: Bob_Roller on July 01, 2009, 08:00:42 AM

To check the rotor resistance with it still installed, you need to pull the brushes up and put a piece of paper between the brush and slip ring, otherwise you are not going to get an accurate reading .

Title: Re: GEN light
Post by: Justin B. on July 01, 2009, 09:41:58 AM

The "guru" is using the wrong terminology.  Commutators are what the brushes ride against in a DC motor (or generator) whereas the term "slip rings" is used in an alternator for the copper rings the brushes ride against.  I think what he is recommending is to short the two slip-rings together with a penny and the light should come on.  If the light does not come on you have a brush, bulb, or wiring issue, if the light comes on you have an open rotor.  You have already "ohmed" the rotor and it shows to be defective but this is just one more "sanity check" you can perform before sending Mr. Jones $100 for a new rotor...

Title: Re: GEN light
Post by: billygoat on July 01, 2009, 03:15:44 PM

Quote

To check the rotor resistance with it still installed, you need to pull the brushes up and put a piece of paper between the brush and slip ring, otherwise you are not going to get an accurate reading .

 Why? That doesn't make sense, the paper isolates the rotor, so does removing it.

Title: Re: GEN light
Post by: Bob_Roller on July 01, 2009, 03:58:32 PM

I didn't see in the posting, that the brushes were removed, or that the rotor was removed from the bike, when the resistance test was done .

If you want a good troubleshooting guide for airhead charging systems (Classic Boxer Charging Version 2), Motorrad Elektrik has  it for about $25, and steps like the 'penny' test are in there .

http://www.motoelekt.com/

Title: Re: GEN light
Post by: Justin B. on July 02, 2009, 02:01:56 AM

If removed, or brushes isolated, the penny wouldn't do anything either.  I'm corn-foozed...  :-?

Title: Re: GEN light
Post by: Bob_Roller on July 02, 2009, 07:12:08 AM

I meant to do the resistance check of the rotor with it still installed in the bike, if you don't insulate the slip rings, you can get a false reading, if the rotor is open, then you are measuring the resistance of the regulator circuit .

When it was mentioned that the rotor had 20 ohms, it didn't sound right, the rotor usually checks with around 4 ohms for a good rotor, infinite resistance for an open rotor, and 0 ohms for a shorted rotor, it sounded like an open rotor, and the 20 ohms was the resistance of the regulator circuit .      

Title: Re: GEN light
Post by: billygoat on July 03, 2009, 12:46:35 AM

 I got three readings, 20, 17 and 12.5 then 0 ohms. I mentioned I dropped it. Between each measurement I dropped it 3 inches onto a wood table to see if I could get it to fail and got the funky readings. I compared my readings with a 20 ohm resistor each time so the meter is accurate. I got a used rotor today from a great guy here in Sacto ;D for $45 and it tested at 3 ohms so I should be good to go.

 I ordered an electronic voltage reg last year and my wires and diode board are tested and good. I have a rotor core if someone needs one for cheap.

 I like this guys idea about grounding the diode board.

http://www.evolutioncycles.com/airhead_ideas.htm