Main fuse blows when connected to battery

[doig]

Hi everyone,

Recently purchased an 81 Special, rode it around the first day and it ran like a dream. Parked it and accidentally left it in Park mode. Needless to say, I came back in the morning to find it with a dead battery. I recharged the battery and dropped it back in the bike, only now to find out that the main fuse has blown. Replacing the main fuse and reconnecting the battery terminals causes the fuse to blow immediately.

Researching the forums has led me to believe I have a short to ground somewhere. After running around the bike with a VOM I didn't see any shorts. The only thing that I've been able to do in order to prevent the fuse from blowing is unplugging the rectifier/regulator cable.

Could this be where the problem is? Is it common for a rectifier to go bad after a battery has been drained?

Thanks in advance!

 

[retiredgentleman]

Page 158 and 159 of the Haynes manual shows how to test the rec/reg for your bike.

Each red to white and each black to white should be checked for continuity, (6 diodes to test). I'll quote from the book," If any diode shows continuity or no continuity in both directions then the complete unit must be renewed." They say to use the ohms x100 scale.

Welcome to the group.

 

[doig]

Thanks for the help retiredgentleman! I have some updates on my issue...

Checked the rec/reg cable for continuity and there appeared to be a significant amount of resistance there. My multimeter doesn't have an x100 scale, and actually I couldn't get a reading until the knob was turned to 2000k Ohms. Is it correct for the resistance to be several orders of magnitude higher than the scale suggested in the Haynes manual?

Uplugging the rec/reg and turning the ignition to ON gives power to the taillight and pushing the starter button turns the rotor. However, I discovered that attempting to turn on the turn signals blows the signals' fuse. Could this be related to why my main fuse is still blowing?

I'm assuming that I have a short to ground somewhere, what's the best way to go about searching for the short? I'm a complete newbie when it comes to this stuff, so the simpler the explanation, the better!

 

[hmusket]

Since the turn signal fuse blew, did you try putting the reg/rec back in the circuit without the turn signal circuit connected to see what happens?
Another thought, if you take all the fuses out, except the main, start your bike, do a voltage reading at the battery, that should give an indication on whether or not your charging system is working. Then putting fuses back in one at a time, you could see which circuit is faulty.

 

[doig]

Just tried removing all but the main fuse and then connected the rec/reg. The moment the rec/reg was connected the main fuse blew (the only fuse in place). Is it safe to assume its an issue with the rec/reg?

 

[sgallaty]

Here's a link you'll find useful.

http://xs650temp.proboards.com/index.cgi?board=Charging&action=display&thread=3461

If you can't plug in the rectifier without blowing the circuit, try to isolate it. Test your rotor first, that is a very common short.