Disconnecting fuse stalls engine

[freddy3]

Stock 1972 XS2 (owned since '75)...
Question: What would cause the engine to immediately stop when the fuse running from the battery (located just below the left side of the seat) is disconnected?

Background: 30 minutes into trying to adjust the voltage regulator for 14.5+ VDC (lights dim at idle, but increase with higher RPMs), I shut the engine off to clean the regulator contacts. Then, I reconnected the fuse and re-started the engine to continue testing the regulator. A few minutes into testing, I noticed that I'd forgotten to disconnect the fuse again before rechecking the system voltage. So I disconnected the fuse and the engine immediately stopped.

Thinking it was a fluke (because I'd been running the bike for the past 30 minutes w/o the fuse connected without issue!), I reconnected, restarted and disconnected the fuse again. The engine stopped. Rinse, repeat.

What did I do to cause the engine to stall when disconnecting the fuse all of a sudden?

 

[5twins]

I think that's normal. I think the question you need to be asking is how did it run before when the fuse was disconnected. I don't think it should. I'm pretty sure all you have is that one fuse, a main fuse for all the power. It's in the power line running from the battery to the key switch, then that switch sends it to the whole rest of the bike, including the ignition. Take the fuse out and you disconnect the power to the whole bike. It shouldn't run. Nothing should work, no lights, horn, starter, nothing.

 

[freddy3]

I think that's normal. I think the question you need to be asking is how did it run before when the fuse was disconnected. I don't think it should. I'm pretty sure all you have is that one fuse, a main fuse for all the power. It's in the power line running from the battery to the key switch, then that switch sends it to the whole rest of the bike, including the ignition. Take the fuse out and you disconnect the power to the whole bike. It shouldn't run. Nothing should work, no lights, horn, starter, nothing.

I've been troubleshooting the ignition system this way for nearly half a century and that's the way it worked until I cleaned the regulator contacts.

Both the OEM Service Manual and Clymer book both detail similar troubleshooting processes...

 

[jpdevol]

I've been troubleshooting the ignition system this way for nearly half a century and that's the way it worked until I cleaned the regulator contacts.

Both the OEM Service Manual and Clymer book both detail similar troubleshooting processes...

So the main circuit (Red wire) actually starts at the rectifier and goes to key switch, the fuse is on a leg in parallel that goes to the battery. So, the bike, once running, is powered via the rectifier once the fuse is pulled. If there is no output from the rectifier (via charging system) the bike will quit if the fuse is pulled.

 

[Jim]

So the main circuit (Red wire) actually starts at the rectifier and goes to key switch, the fuse is on a leg in parallel that goes to the battery. So, the bike, once running, is powered via the rectifier once the fuse is pulled. If there is no output from the rectifier (via charging system) the bike will quit if the fuse is pulled.

Before the age of computerized engines, that was a common "side of the road" troubleshooting method.
Get a jump to start your dead battery car or bike.
Once running, disconnect the battery and see if it kept running.
If it did you had a bad battery.
If it died you had a bad alternator.

 

[freddy3]

Before the age of computerized engines, that was a common "side of the road" troubleshooting method.
Get a jump to start your dead battery car or bike.
Once running, disconnect the battery and see if it kept running.
If it did you had a bad battery.
If it died you had a bad alternator.

So you think I fried the stator and/or rotor (i.e., alternator)?

 

[Raymond]

So you think I fried the stator and/or rotor (i.e., alternator)?

Not necessarily - perhaps a bad connection in the alternator-regulator wiring. Take a look, starting with the wiring you were working on, maybe pulled a connection loose?

 

[freddy3]

Not necessarily - perhaps a bad connection in the alternator-regulator wiring. Take a look, starting with the wiring you were working on, maybe pulled a connection loose?

I suppose that's not impossible, but I'd just completed a thorough check/cleaning of every associated wire/connector condition before this happened, and I didn't see anything that looked even remotely worn, loose or dirty. Literally, all I did between the time the bike ran w/o the fuse and, then, it didn't run w/o the fuse is to slide a small piece of paper in between the regulator contacts (it came out clean) and then reconnect that wire & restart the bike. A minute into testing I noticed I'd forgotten to disconnect the fuse and that's when the engine began quitting when the fuse was disconnected.

Where to go from here?

 

[jpdevol]

As you describe the bike charging prior to working on the regulator, perhaps the problem is there

 

[freddy3]

As you describe the bike charging prior to working on the regulator, perhaps the problem is there

Sorry, you lost me? The original problem was that the headlight dimmed at idle (brightened when the RPMs increased) and that's why I was attempting to adjust the voltage regulator. But, now, I can't really perform any of the other tests because the engine stops when I pull the fuse. Or, are you suggesting something else?

 

[jpdevol]

The light getting brighter indicates it was charging. With fuse in place, should charge 14.5VDC @2500rpm = system good. If voltage is slightly off it's regulator or voltage drop (poor connection). Charge voltage around idle speed will be close to 12VDC. IIRC most per manual checks can be done with fuse in place

 

[xjwmx]

The light getting brighter when you rev is normal. The system voltage rises from about 12.5 at idle to about 14.5 at 3000, so lights get brighter. That's a function of the output of the generator. Your regulator adjustment will control the top but not the bottom...

(only earlier bikes have adjustable regulators, in case late a late model reader is scratching his head now over where the screw is)

 

[freddy3]

The light getting brighter indicates it was charging. With fuse in place, should charge 14.5VDC @2500rpm = system good. If voltage is slightly off it's regulator or voltage drop (poor connection). Charge voltage around idle speed will be close to 12VDC. IIRC most per manual checks can be done with fuse in place

Before the fusing issue, I was getting ~12.9~13.9VDC @ 2500RPM. Whilst mucking with that regulator screw, once or twice it hit ~14VDC on the Fluke meter, but it was just for a split second. After checking for loose/damaged wires or poor grounds (none found), I cleaned the contacts. That's as far as I was able to get before the engine starting stalling when I pulled the fuse. The Yamaha OEM Service Manual describes the same fuse disconnection process as the Clymer book.

Since the engine ran w/o the fuse and now it doesn't, I need to find out why and fix whatever's wrong. Trouble is that I no longer have a guide to follow since I can't run the engine without the fuse. So???

 

[freddy3]

The light getting brighter when you rev is normal. The system voltage rises from about 12.5 at idle to about 14.5 at 3000, so lights get brighter. That's a function of the output of the generator. Your regulator adjustment will control the top but not the bottom...

I think you may be right about the brightening headlight, but I don't recall that happening early on in the bike's life (I've had it since the mid-70s) and I know that generally indicates a charging issue. Either way, now I've gotta figure out what I damaged that's causing the bike not to run w/o the fuse. That definitely indicates something's changed.....and is wrong somewhere.

 

[xjwmx]

^Absence of the brightening is what would indicate a charging issue. I habitually look for the green neutral light to fluctuate when I start up to see if I have charging.

 

[jpdevol]

Had to review: yeah, so the only reason the book instructs fuse removal is for fine adjustment on the regulator - it's simulating a dead/low battery so the system produces full voltage. If the battery is reading above 12.5VDC, the regulator cuts power to brushes and there is no output.

Anyway, I'd clean and set the core gap and point gap on the regulator with power off, then start and fiddle the adjusting screw to get 14.5+VDC @2500.

I don't think anything is damaged. See if that will work.

 

[freddy3]

^Absence of the brightening is what would indicate a charging issue. I habitually look for the green neutral light fluctuate when I start up to see if I have charging.

Me, too. And that's definitely not happening (i.e., nice, bright green neutral light).

 

[Jan_P]

Lets keep this simple .
Perhaps already been said here
On my 80 it works like this
The system can get power from 2 sources the charging circuit A or the battery B
If the charging A is broken It drains the battery B and eventually stalls the engine
If it charges via A at the same time it is providing power to the system it also charges the battery B
As an over winter battery tender.

It is fork in the wiring ( logically ) so if the wire going to battery B when running engine goes open circuit
the regulator adjusts It senses that and ooooopss now something fex B is disconnected and adjust to the spec around 14 V

So fex If one is running at idle the battery B can drain a little and If the charging is low or nonexistent with time the stalling on battery can happen even for a working A since the rpm is low.
Which did not happen at the beginning of the day. So charging the battery is a first. CSF Rule Cheapest Simplest First

It can of course be a partial short somewhere but that can be a long an difficult fault finding process
I looks as a well kept machine and I believe you dont want to change anything :but a Better regulator can be
A thought.
Alternator Brushes Inspected ?

 

[freddy3]

Had to review: yeah, so the only reason the book instructs fuse removal is for fine adjustment on the regulator - it's simulating a dead/low battery so the system produces full voltage. If the battery is reading above 12.5VDC, the regulator cuts power to brushes and there is no output.
Anyway, I'd clean and set the core gap and point gap on the regulator with power off, then start and fiddle the adjusting screw to get 14.5+VDC @2500.
See if that will work.

Didn't work. I'd already done that. The best I got was ~14V, but just for a second or so. At best, it's running at ~13.9VDC @ 2500~2700RPM.
That and the brightening headlight makes me think there's still an electrical issue....besides the new fuse issue.

 

[freddy3]

Alternator Brushes Inspected ?

Before I even began testing, I thought it was the brushes (they were a few years old, but within spec), but I just replaced 'em anyway. So they're new.
And I cleaned all of the rotor/stator contact surfaces.