Lights alternate between dim and bright

[dp.guzzi]

Re: 2002 Lemans. 27k miles.

When I ride at night, I notice that the lights (headlight and instrument lights) get dim, then get very bright, then dim, then bright , etc. Not a regular pattern, rather seems random.

As an experiment last night, I noted that when I honked the horn, the lights dimmed.

No other trouble. The battery is relatively new and is charged.

What could be the problem?

[GuzziYang]

I too have the same exact problem while riding at night the instrument light will go bright then dim for no reasons. My bike is also a 02 V11 Le Mans Champaign color so maybe its a special feature of this particular model

 

My guess is the voltage regulator not doing its job of regulating the voltage but I like to heard from more electrical minded owners as to its cause and solutions.

[Kiwi_Roy]

It could be several things, I'll list a few obvious ones

 

Bad ground on the regulator.

All of the charging current has to get back to the alternator via the regulator case.

On a new bike the regulator is grounded by a good accidental connection to the horn bracket, from horn bracket to chassis however as the bike ages the contact points rust and corrode and all you are left with is the tiny wire Luigi ran from the case back to the battery. Any Voltage drop in this wire is subtracted from the battery Voltage.

You should have a nice fat wire running from the regulator case to an engine bolt.

 

Loose 30 Amp fuse

The alternator doesn't put out a steady DC current like a generator, it puts out half wave pulses that are quite high, perhaps over 40 Amps, this can cause the fuse holder contacts to heat up eventually melting the plastic of the fuse or welding it in place.

After a run touch each of the fuses and see if any are warm.

The fuse holder should have a nice firm grip on the fuse

Check the headlight fuse also, look for any discouloration of the plastic or blade contact area.

 

Bad contact in the relay base

Relay 2 (headlight relay) also provides the reference Voltage to the regulator, horn, idiot lights, 12V to the tacho and a few other things.

The base contacts can spread apart or even if tight they corrode a little causing some extra resistance.

Pull the relay and check the contacts, in the process you will give the contacts a scrape and create a better contact.

On some bikes they wired the headlight relay in series with the starter relay so check that as well.

 

Dirty ignition switch

After a few years the grease in the ignition switch goes hard which takes some tension off the contacts building up resistance.

Somewhere on the forum I have a post showing how you can check the resistance without pulling the switch.

I don't think this is your case because the V11 Sport has one of the better Guzzi wiring schemes but if you haven't cleaned the contacts in a few years it might help.

 

Dirty Dimmer switch contacts

As above, but I don't think this is your problem, do you have after market headlight relays?

 

Bad contact on the headlamp base connector.

These are known to overheat and melt, easily fixed because they are a standard item in any auto parts store.

I don't think this is your problem since the panel lights are effected also

 

Bad battery connection

Your battery terminals should be scraped and have Vaseline applied to prevent oxidation.

 

Bad Regulator

Sometimes one of the diodes lets go

Hopefully not the case but I can show you how to check.

 

Take note, do the panel lights go bright and dim with the headlight or opposite (headlight dim, panel lights bright)

 

Some bikes have the horn on the same fuse as the headlight, you could check that by pulling the headlight fuse.

If this is the case I would suspect the relay bases of headlight fuse holder

 

I'm sure there are others, PM sent.

 

Moto Guzzi, making electricians out of riders for 90 years.

[mznyc]

Moderators could you move this to "Technical Topis"?

Thanks