Topic: '72 CB750 Problems

[smccloud]

Ok,

I put a new headlight in my '72 CB750 a week ago (Friday the 30th) and tonight the low beam filiment went out, same reason I put a new bulb in to begin with.  I check the voltage when I got home (after finding out the seat latch is now FUBAR) and got the same voltage on both the black and white terminals of the voltage regulator.  Around 12.5-13.0 at 2500RPM.  Is this approximately the right voltage for the RPM?  If not, does anyone know if a stator from a DOHC 750 will fit (K engine if I remember right)?  I'm also guessing I should be able to take a voltage regulator from any bike that uses a three wire regulator and use it, correct?  I only ask the last one because there is a "parts bike" sitting at my dad's place but its a Yamaha 650 Special 2.

--smccloud aka runt aka Shaun

[scondon]

The voltage regulator recharges the battery, your bike runs off the battery not the voltage regulator. Signs of a improperly set regulator are usually either battery losing charge after riding(not enough voltage) or electrolite levels dropping due to battery overheating(too much voltage). Your readings sound about right. If you're not having problems keeping a charge on the battery then I'd leave the regulator alone and look for other cause for your headlight failing.

[smccloud]

The voltage regulator recharges the battery, your bike runs off the battery not the voltage regulator. Signs of a improperly set regulator are usually either battery losing charge after riding(not enough voltage) or electrolite levels dropping due to battery overheating(too much voltage). Your readings sound about right. If you're not having problems keeping a charge on the battery then I'd leave the regulator alone and look for other cause for your headlight failing.

i've only done two things recently concerning the headlight (other than replacing it).  i put a set of controls off a '75 cb750 on my bike because no matter what i did i could not fix the left turn signal side of the switch (wouldn't work most of the time), and i wired another circuit (with relay) to run the headlight and gauge lights.  the first headlight failed after i put the '75 controls on my bike (i chalked this up to the light being old), the second was 6 days after i added the second circuit.  could it be overheating because i run a halogen bulb when its not designed for one?

[TwoTired]

There are three terminals on the headlight, High Beam, Low Beam, and 12V return.  Since you said you rewired the headlight, I have to ask if you power both High and Low beam at the same time. This would be very bright but, make any lamp overheat and fail prematurely.

12-13 volts on the regulator is a normal voltage reading.  However, you need to set conditions that would allow the voltage to be higher, such as a fully charged battery, and the alternator spinning at 4-5000 RPM.  Then read how high the voltage goes.  If higher than 14.5 V then the vreg or some part of the charging circuit is suspect.


Cheers,

[smccloud]

There are three terminals on the headlight, High Beam, Low Beam, and 12V return.  Since you said you rewired the headlight, I have to ask if you power both High and Low beam at the same time. This would be very bright but, make any lamp overheat and fail prematurely.

12-13 volts on the regulator is a normal voltage reading.  However, you need to set conditions that would allow the voltage to be higher, such as a fully charged battery, and the alternator spinning at 4-5000 RPM.  Then read how high the voltage goes.  If higher than 14.5 V then the vreg or some part of the charging circuit is suspect.


Cheers,

nope, not powering both the high and low beam at the same time.  i will check the voltage at the battery when i can get the seat unlatched (and hopefully figure out what caused the problem).

[smccloud]

voltage is swinging from 14.5 to 15v at 4000, i'm sure my neighbors love me

i am thinking voltage regulator.

[TwoTired]

While 15V is a bit on the high side, the fact that it is swinging between 14.5 and 15 indicates it is working.  Perhaps some adjustment is beneficial.  But, it is not damning evidence that the VREG is the headlight killer.

The Vreg has contact points inside that can pit with time and become sticky.  I would look at their condition and smooth then down if necessary (no silica abrasives).  Then set the gaps per shop manual values, and make the adjustment for 14.5 V max battery charge.

IF (and this is a big IF) the Vreg contact points are intermittantly sticking, then the system voltage may go very high during cruise, boil the battery (electrolyte loss) and then be enough voltage to overpower the headlight.

The test you've done, doesn't confirm this IF, however.

What about headlight vibration?  Have you changed anything that might give it more than it would get from a stock mounting?

The last idea I could share would be that of a faulty or intermittant Green wire connection to battery NEG that might allow current to pass through both Headlight filaments at the same time.  Thus overheating the headlight and poof.

Other wise, I'd carefully review your relay insertion connections and method for errors.

Cheers,

[smccloud]

While 15V is a bit on the high side, the fact that it is swinging between 14.5 and 15 indicates it is working.  Perhaps some adjustment is beneficial.  But, it is not damning evidence that the VREG is the headlight killer.

The Vreg has contact points inside that can pit with time and become sticky.  I would look at their condition and smooth then down if necessary (no silica abrasives).  Then set the gaps per shop manual values, and make the adjustment for 14.5 V max battery charge.

IF (and this is a big IF) the Vreg contact points are intermittantly sticking, then the system voltage may go very high during cruise, boil the battery (electrolyte loss) and then be enough voltage to overpower the headlight.

The test you've done, doesn't confirm this IF, however.

What about headlight vibration?  Have you changed anything that might give it more than it would get from a stock mounting?

The last idea I could share would be that of a faulty or intermittant Green wire connection to battery NEG that might allow current to pass through both Headlight filaments at the same time.  Thus overheating the headlight and poof.

Other wise, I'd carefully review your relay insertion connections and method for errors.

Cheers,

i did find a slightly loose connection on the fuse i put in after the relay (insulated female spade connector and not an actual fuse holder yet).  i have double and triple checked the relay, on the day i installed it and since then.  i am not gonna get a whole lot done today since i'm in the bathroom every 30 minutes or so with the runs

plus i forgot to get my dad's points file when i was at his place yesterday.  i did get my old manual though (still says to put the used oil in a jug, cap it, and throw it in the trash).