Topic: 1975 cb750k5 Electrical problem: dash lights dim

[splurgurnurk]

Hello all—

I'm a new poster here, though I've already gleaned a lot of great stuff from this community.

Problem started during a pit stop mid-ride, so bike had just been running fine. Getting ready to go again, I hit the electric start and the power cut — nothing on, not even dash lights. After a few seconds, dash lights slowly brightened again. Kick started fine, but when I switched on the headlamp all power cut once again. After minutes of waiting and turning the key off and on, jiggling what connections I could reach, still nothing.

Back in the garage: battery holds charge at 12.5v. Fuses look and test good. I see dim dash lights, but nothing else. Won't kick start.

I'm suspecting a faulty or corroded connection somewhere, as it seems the problem was caused when there was a draw from the electric start, then the headlamp?

I have a multimeter to continue testing — any suggestions where to look from here?

Thanks,
Matthew

[TwoTired]

Look at battery voltage with a load on it.   Cause it sounds like your battery has depleted.  If so, any kind of a load will have the voltage drop like a rock.  Batteries can do a limited self recovery.  But, not under load.

Next would be charging system diagnostics.

Cheers,

[bryanj]

12.5 is nowhere near charged

[MauiK3]

It’s the battery or I’m a Harley rider (not a Harley fan)

[Kevin D]

I had a similar occurrence a couple weeks back:

http://forums.sohc4.net/index.php/topic,187283.0/all.html

A new battery put things back to normal for me.
What kind of battery do you have?
How old?
How well do you treat it?
What’s your off season like for your battery and your charger?

…..and, Hi Matthew

[splurgurnurk]

Hi all,

Thanks or the replies. Battery is an AGM Scorpion, 3-5 years old? Every winter it's been off the bike and on a trickle charger in a heated space - I've been meticulous with it because I've learned the hard way in the past. I charged it up, and same thing happened. Just to make sure I swapped it out for a new battery, fresh off the charger reading 12.7v. Same symptoms as with the old battery.

To clarify symptoms:
When I turn the key, battery voltage drops to 4.4. Dash lights are very dim and flicker. Won't kick start. This all started the moment I hit the electric start after a mid-ride pit stop.

Any ideas where to look next?

Thanks everyone.

[Kevnz]

Sounds like something is shorting out when you turn on the key. Possibly not a straight short, as that would result in smoke leaking out of the wires , but like a circiut with high drain; possibly the starter solenoid stuck on and trying to spin the starter. Try removing the headlight to gain access to the wiring and disconnect various circuits one at a time while monitoring the battery voltage. If the voltage returns to normal as you unplug a wire, that's where your problem lies. I would start unplugging the wires from the RH switchblock first, there's more circuitry in that one. Or try pulling fuses one at a time.

[TwoTired]

To clarify symptoms:
When I turn the key, battery voltage drops to 4.4. Dash lights are very dim and flicker. Won't kick start. This all started the moment I hit the electric start after a mid-ride pit stop.

Any ideas where to look next?

A battery fresh off the charger should be at least 13.8V or up to 14.5V, until the battery has rested for an hour, then 12.7 v is a good read.

So, the CCA rating of a battery is where a load can be placed upon it and the voltage won't drop below 10 V.  If a known good and load tested battery actually drops to 4.4V then something like twice the CCA rating of the battery is being placed upon it.  That's something like 400 Amps, which is enough to weld metal bits or make them molten.  If your fuses haven't been replaced with metal shunts, and you aren't seeing smoke from melted bits, about the only place that could draw that much amps is the starter motor circuit.   But, for this to be cause you would need a double fault, both the solenoid has to be fused closed, and the starter motor is shorted.

To seek cause, disconnect the big cable that routes from solenoid to starter motor, then switch on the ignition.  If your voltage holds above 12.4V, you can probably kick start the bike.  If good so far, then take apart the solenoid and free the fused contacts, or replace the unit.  This would clear one of the faults, but may return if the starter motor is indeed grounding out and activated again.  The next fault to clear would be the starter motor.  Probably the armature bearings are worn which allows the armature to contact the grounded coil housing.  Might even be welded there.  Short of removing and replacing the starter motor, to verify fault you'd need an ammeter capable of reading about 200-300 amps., where you can actually test the current draw of the starter motor.  it usually only draws 25 A, or 150A stalled (unable to turn).  If it draws more than that, it's bad and needs repair or replacement.

Remember this all assumes you actually have a known good and load tested battery, which could be performed where batteries are sold.

Good luck!

[splurgurnurk]

I just want to say thank you everyone for your replies. Just had a baby recently so garage time is limited. I'll post an update when I'm able.

[newday777]

I just want to say thank you everyone for your replies. Just had a baby recently so garage time is limited. I'll post an update when I'm able.

Congrats on the baby

[splurgurnurk]

Success! At least the first step. The fault(s) were indeed in the starter motor circuit. I disconnected the starter solenoid relay and was able to kickstart and ride as normal. Now on to investigating the starter motor per TwoTired's advice. Thanks again.

[Bodi]

This is puzzling. If the solenoid was stuck on, the starter would run all the time key off or on. So how can it load down the battery only with key on, and be OK with the battery to solenoid cable removed? Makes no sense to me.