LM2 not charging

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   1979, LM2 - on the last couple of rides, I have noticed that the charging voltage was initially low, basically just battery voltage so not charging. On both occasions after a few minutes it 'corrected' itself and reverted to charging correctly so I wondered if it was just a loose connection to the voltmeter?
Yesterday, I took the bike out and the voltage remained low, it didn't correct itself so after about 10 miles, I came home so as not to run out of battery power and need recovery.

As far as I can tell, assuming the voltmeter and (Gammatronix) charge monitoring light are correct then there is basically no charge coming from the alternator/regular/rectifier with only around 12v showing constantly (fluctuates between 11.8 and 12.2) and it actually drops as revs increase - I presume due to more firing pulses taking more power which is then not being replenished?

So I need to find the cause, I presume the likely culprits are alternator and/or voltage regulator/rectifier.

To assist in this, does anyone know what sort of voltage I should see from the alternator when running?

I assume it will be a fairly high AC voltage that is then regulated/rectified to the correct DC for charging.

Any advice/tips would be greatly appreciated. 

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Dukedesmo Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 17 Apr 2015 Location: Leics Status: Offline Points: 926	Post Options Post Reply Quote Dukedesmo Report Post Thanks(0) Quote Reply Topic: LM2 not charging Posted: 26 Aug 2025 at 15:11
	1979, LM2 - on the last couple of rides, I have noticed that the charging voltage was initially low, basically just battery voltage so not charging. On both occasions after a few minutes it 'corrected' itself and reverted to charging correctly so I wondered if it was just a loose connection to the voltmeter? Yesterday, I took the bike out and the voltage remained low, it didn't correct itself so after about 10 miles, I came home so as not to run out of battery power and need recovery. As far as I can tell, assuming the voltmeter and (Gammatronix) charge monitoring light are correct then there is basically no charge coming from the alternator/regular/rectifier with only around 12v showing constantly (fluctuates between 11.8 and 12.2) and it actually drops as revs increase - I presume due to more firing pulses taking more power which is then not being replenished? So I need to find the cause, I presume the likely culprits are alternator and/or voltage regulator/rectifier. To assist in this, does anyone know what sort of voltage I should see from the alternator when running? I assume it will be a fairly high AC voltage that is then regulated/rectified to the correct DC for charging. Any advice/tips would be greatly appreciated.
	Le Mans 2 Ducati 916 Ducati M900

theone&onlymin Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 14 May 2014 Location: United Kingdom Status: Offline Points: 2427	Post Options Post Reply Quote theone&onlymin Report Post Thanks(0) Quote Reply Posted: 26 Aug 2025 at 15:25
	I'd look at the brushes and slip rings first so I would. 10 Min job. Then head down the test route as described elsewhere on this forum. Cheers Min

Brian UK Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 13 May 2014 Location: Surrey Status: Offline Points: 18226	Post Options Post Reply Quote Brian UK Report Post Thanks(0) Quote Reply Posted: 26 Aug 2025 at 16:04
	I think you need to clarify exactly what charging system you have. Is it original Bosch, or has it been modified? Original Bosch, then slip rings and brushes as Min said, but the charge warning light would also stay on. However, you mention Gammatrinix.
	Brian. Better 5 minutes late in this world than years early in the next.

Stevex Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 24 Nov 2020 Location: North Yorkshire Status: Offline Points: 486	Post Options Post Reply Quote Stevex Report Post Thanks(0) Quote Reply Posted: 26 Aug 2025 at 17:19
	Do the easy stuff first. Put a voltmeter across the battery terminals and see what you're getting. Check brushes. Check stator for continuity between windings and earth, windings resistance and if they're OK check the AC output of each yellow wire with the engine running. If output is good check all 3 yellow wires for continuity up to the rec reg. Fault diagnosis is everything, be systematic. This is obviously if you still have the original components fitted. My LM2 still all oem, so if you need to compare readings let me know. Edited by Stevex - 26 Aug 2025 at 17:22
	Steve

#1moto-uno Members Profile Send Private Message Find Members Posts Add to Buddy List Falcone Joined: 11 Jan 2022 Location: burnaby b.c Status: Offline Points: 86	Post Options Post Reply Quote #1moto-uno Report Post Thanks(0) Quote Reply Posted: 26 Aug 2025 at 20:26
	FWIW , my LeMans 2 (with 1/4 million kilometers) is on it's 4 alternator rotor ! Slip some paper under the brushes and check for the rotor resistance , around 3.5 to 4.2 ohms is okay . Mine have all shown infinite resistance . (broken wire) . I actually took to taking the rotor apart and rewinding it and reassembling , BUY NEW .

Dukedesmo Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 17 Apr 2015 Location: Leics Status: Offline Points: 926	Post Options Post Reply Quote Dukedesmo Report Post Thanks(0) Quote Reply Posted: 27 Aug 2025 at 07:22
	Thanks for the advice, I've not looked at it yet but as far as I know it's all original, other than the voltmeter and charging light which I fitted when I replaced the LM2 dash with LM1 style instruments and I'm confident that the voltage readings are correct. Bike is low mileage (less than 20k) so, hopefully not worn out, I'll get on it in the next few days and update with findings.
	Le Mans 2 Ducati 916 Ducati M900

Brian UK Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 13 May 2014 Location: Surrey Status: Offline Points: 18226	Post Options Post Reply Quote Brian UK Report Post Thanks(0) Quote Reply Posted: 27 Aug 2025 at 17:51
	What is the charging light doing?
	Brian. Better 5 minutes late in this world than years early in the next.

Dukedesmo Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 17 Apr 2015 Location: Leics Status: Offline Points: 926	Post Options Post Reply Quote Dukedesmo Report Post Thanks(0) Quote Reply Posted: 27 Aug 2025 at 18:23
	The Gammatronix charging light; https://gammatronixltd.com/epages/bae94c71-c5b6-4572-89a1-e89006e78fbe.sf/en_GB/?ObjectPath=/Shops/bae94c71-c5b6-4572-89a1-e89006e78fbe/Products/E Is flashing yellow, meaning voltage is around 11.8v i.e. not charging - It should show steady green (13.2v) and was doing until a couple of weeks ago. This tallies with the (digital) voltmeter which fluctuates between 11 and 12v.
	Le Mans 2 Ducati 916 Ducati M900

Brian UK Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 13 May 2014 Location: Surrey Status: Offline Points: 18226	Post Options Post Reply Quote Brian UK Report Post Thanks(0) Quote Reply Posted: 27 Aug 2025 at 20:25
	So how does the rotor winding get it's supply? The Gammatronix is just a monitor, but the original charge light is a vital part of the circuit.
	Brian. Better 5 minutes late in this world than years early in the next.

Dukedesmo Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 17 Apr 2015 Location: Leics Status: Offline Points: 926	Post Options Post Reply Quote Dukedesmo Report Post Thanks(0) Quote Reply Posted: 28 Aug 2025 at 08:17
	Good question, I had overlooked the 'Gen' light at as it appeared to out when running but, I've just started it up in the garage where it's not so bright and in fact the charge light is lit but only very dimly when the engine is running. In fact it only lights dimly even when the engine is stopped and doesn't change whilst running.
	Le Mans 2 Ducati 916 Ducati M900

Brian UK Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 13 May 2014 Location: Surrey Status: Offline Points: 18226	Post Options Post Reply Quote Brian UK Report Post Thanks(1) Quote Reply Posted: 28 Aug 2025 at 10:39
	That points at a poor connection between the slip rings and brushes.Though it could be a bad connection elsewhere. If you have a multimeter, disconnect the two wires to the brushes and measure the resistance between those brushes, should be in the region of 4 ohms. If higher, check the reistance between the two slip rings, again should be around 4 ohms. Edited by Brian UK - 28 Aug 2025 at 10:42
	Brian. Better 5 minutes late in this world than years early in the next.

Dukedesmo Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 17 Apr 2015 Location: Leics Status: Offline Points: 926	Post Options Post Reply Quote Dukedesmo Report Post Thanks(0) Quote Reply Posted: 28 Aug 2025 at 11:48
	Firstly, it's definitely Bosch; I checked the connections, I get no reading at all between the slip rings even after cleaning them, the brushes look good. Plus, to add; no voltage output at all from the alternator. Edited by Dukedesmo - 28 Aug 2025 at 12:03
	Le Mans 2 Ducati 916 Ducati M900

Brian UK Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Joined: 13 May 2014 Location: Surrey Status: Offline Points: 18226	Post Options Post Reply Quote Brian UK Report Post Thanks(1) Quote Reply Posted: 28 Aug 2025 at 12:14
	Sounds like the rotor has gone open circuit. No magnet, no voltage output. A place called West Country Windings can rewind it.
	Brian. Better 5 minutes late in this world than years early in the next.

Dukedesmo Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: 17 Apr 2015 Location: Leics Status: Offline Points: 926	Post Options Post Reply Quote Dukedesmo Report Post Thanks(0) Quote Reply Posted: 28 Aug 2025 at 14:26
	I called West Country Windings, seemed very knowledgeable and confirmed the diagnosis, so I have sent it off for rewinding. Thanks for the help.
	Le Mans 2 Ducati 916 Ducati M900

Don-Spada Members Profile Send Private Message Find Members Posts Add to Buddy List Falcone Joined: 14 May 2014 Location: Market Harboro' Status: Offline Points: 93	Post Options Post Reply Quote Don-Spada Report Post Thanks(0) Quote Reply Posted: 12 Sep 2025 at 20:12
	The standard charge light is an important part of the charging circuit and needs to be a standard filament bulb (not an LED) as it supplied the voltage to the slip rings to energise the rotor windings. Have aread of these 2 articles https://www.gutsibits.co.uk/pr/TheShed/Downloads/BoschChargingSystemFaultFinder.pdf ; https://www.thisoldtractor.com/guzzitech.dk/pdf/charging-around-the-circuit-john-noble.pdf ;
	1978 Guzzi Spada 1965 Guzzi Stornello