Electrical short

Since I finished ny rebuild I've put almost 6,000 miles on my 72 Commando.
I had put a new electrical harness on and a Pazon Sure Fire elec ignition.
Has run without any electrical problems.
After a 45 minute ride I pull up at a view point, put her in neutral and after sitting for a couple minutes she cuts out.
I didn't have any lights or brake on.
After checking the obvious stuff I find my 20 AMP main fuse at the battery has blown.
I put in my spare fuse with the ignition off and as soon as I put it in holder it blows.
Figure I have a wire rubbed through so pull tank and look for obvious source but can't see.
I pulled brown-blue wire from battery by way of rectifier and zenner diode at master switch to see if short in switch or downstream. Still blows fuse.
Figured I'd start at fuse and work my way toward master switch so I disconnected brown blue-double wire at rectifier and when inserting fuse it still blew??

In looking at wiring diagram the rectifier is the first device downstream of main fuse.
Shouldn't this mean when disconneceted the only part of the wiring that is hot is between the battery and the connector for the rectifier.
Other than damaged fuse to rectifier wiring what should I be looking for?

TIA

Bob

If the wiring is stock, with no other component or additional wire added to the neg terminal of the batt, then, as you note, the rectifier is the first item in the circuit. So if the fuse blows when the connector to the rectifier is disconnected, their must be a short in the wire between the battery and the rectifier. Not sure how that would happen within the harness though...

Try isolating the systems one by one until the fuse stops blowing---this will tell you which system wiring is causing the fuse to blow.

Check the leads to your zener diode. Even if you pull the brown/blue from the rectifier it's still connected to the zener and has a chance to ground. If the negetive lead has come off the zener and grounds to anything it will do what you describe.

illf8ed said:

Check the leads to your zener diode. Even if you pull the brown/blue from the rectifier it's still connected to the zener and has a chance to ground. If the negetive lead has come off the zener and grounds to anything it will do what you describe.

Click to expand...

You make a good point. The wire to the rectifier is joined to the one leading to the Zenner so pulling it from the rectifier doesn't break the circuit.

Was the ignition switch turned on or off when the fuse blew after the first time? If it was off then it's probably something to do with the rectifyer/zener circuit. They are wired in paralell before they are connected to the switch, they are always connected with the switch on or off. If it only blows when you turn on the ignition then it's something else.

Edit. By being wired in parallel, I meant the they are wired in parallel with the battery and the fuse.

As others have advised check the zener wire. I had a similar problem years ago. I had tightened the zener with the spade fitting touching the frame. It worked for ages until the insulation on the spade fitting rubbed through which caused the short.
ando

Yep... as has been stated - the zener(s) are connected to the fused wire at the terminal that connects to the Rectifier. I had to go out and look!

I have shorted out the fuse several times while working on the bike down around the zeners (I have one on each side) but I had never really noticed (or if I did, I had forgotten) that the zener wire(s) are directly connected to the fuse wire. So, as everyone else has stated - the problem is most likely related to the zener/wiring since pulling the fuse wire from the rectifier does NOT disconnect the fuse wire from the zener.

With every wire between the battery and master switched connected except at the Zenner there is no problem. Lights work etc.
As soon as I attach the wire at the Zenner the fuse blows.
Except for the Zenner going bad and shorting through it I can't see what else could cause short.
Before I order a new Zenner are there any other ideas?

TIA
Bob

You could probably get a new transistor regulator and rectifier for the price of that zener. Unless you want to keep it original style.

Dave
69S

DogT said:

You could probably get a new transistor regulator and rectifier for the price of that zener. Unless you want to keep it original style.

Dave
69S

Click to expand...

Dave
After reviewing other threads on the Zenner and considering the benefits I'm leaning to getting the Podtronics Rectifier/Regulator that appears to be the unit most people mention as being good.

Thanks for everyone's input on this so I'll find out in a few days if the Zenner is at fault.

Bob

You can test the zener with a simple ohm meter. You should read very high resistance in one direction and low ohms in the other. If the zener is shorted it will show low ohms both ways. Disconnect it to test.

Dave
69S

I've got a zener diode you can substitute for your questionable one if that would be helpful


Tim Kraakevik
kraakevik@voyager.net
'72 Combat, '74 RH10 850

If you get the podtronics, I think Old Britts has a good price on them, get the larger one, that way if you want to upgrade your stator or use a 14AH battery, you'll be good to go.

Dave
69S

DogT said:

You can test the zener with a simple ohm meter. You should read very high resistance in one direction and low ohms in the other. If the zener is shorted it will show low ohms both ways. Disconnect it to test.

Dave
69S

Click to expand...

How do you go about doing this test? On the Zener, it only has one wire attached... is the other terminal the bolt that mounts to the frame? Thanks

If your rectifier is bad, it can cost up to $40 or so to replace from OldBritts and a Zener can be twice as much. I advise against buying used. You already have a used one. :lol:

I am not sure how much of a purist you are, but a high percentage of us remove the zener and rectifier and replace it with a solid state unit for $35. (see link below) This also cleans up some un-needed wiring. This is very simple to do and adds much needed reliability, of which you are experiencing, to the highly questionable stock electrical system. For what it is worth, I also remove the assimilator and starter capacitor. I revamped my headlight shell light to leftturn red, rightturn green and center lamp highbeam blue.

http://www.oldbritts.com/17_17104.html

Feel free to start a thread on this to get more responses. Some may even suggest bumping up to a high output 200 watt system or even a 3 phase system. Much more $'s, probably $150 to $200 respectively.

pvisseriii said:

If your rectifier is bad, it can cost up to $40 or so to replace from OldBritts and a Zener can be twice as much. I advise against buying used. You already have a used one. :lol:

I am not sure how much of a purist you are, but a high percentage of us remove the zener and rectifier and replace it with a solid state unit for $35. (see link below) This also cleans up some un-needed wiring. This is very simple to do and adds much needed reliability, of which you are experiencing, to the highly questionable stock electrical system. For what it is worth, I also remove the assimilator and starter capacitor. I revamped my headlight shell light to leftturn red, rightturn green and center lamp highbeam blue.

http://www.oldbritts.com/17_17104.html

Feel free to start a thread on this to get more responses. Some may even suggest bumping up to a high output 200 watt system or even a 3 phase system. Much more $'s, probably $150 to $200 respectively.

Click to expand...

Thanks,
Definately not a purist when it come to Lucas... I would prefer reliability over short circuits electrical fires. Lol.

I am a newbie and really would not be completely confident to instal the podtronics on my own without some pointers. Do you guys know if anyone has put together a guide specifically for commando owners (with pics)?

Johnnymac, This job is so easy. The 2 wire from the stator go the yellow wires on the new unit , the red wire to the ground (positive) and the black wire output (negative). You can and should put a fuse inline of the black wire to the negative side of the battery. That's It! The instruction in the link I sent show everything you need to know.

I swear, it is that easy. You have already done much more compicated things by checking the output of the stator and changing the battery.

pvisseriii said:

Johnnymac, This job is so easy. The 2 wire from the stator go the yellow wires on the new unit , the red wire to the ground (positive) and the black wire output (negative). You can and should put a fuse inline of the black wire to the negative side of the battery. That's It! The instruction in the link I sent show everything you need to know.

I swear, it is that easy. You have already done much more compicated things by checking the output of the stator and changing the battery.

Click to expand...

I did get the Podtronics unit from OldBritts and the installation was a snap. It came with simple instructions.
Used nylon tie straps to attach the unit under backbone tube above battery.
My problem is now gone and bike runs great.

Bob