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engine swap... question about diode...

Posted by dieseln none (My Page) on
Mon, Aug 4, 14 at 21:58

Hi, I just swapped a older Briggs12hp into a newer (09?)Murray Select that had a 17.5 briggs. I got the Murray for 100 bucks w/o the engine. The rest of the tractor appears to be new. I spliced into the newer wiring that came with the Murray.

It runs fine but it blows the ignition fuse either after it starts or when I shut it off. (i don't know which). I put in a new fuse and as long as i don't shut it off i can cut the lawn. However I'm using a lot of 15 amp fuses.

I did some thinking on it and found that i had inadvertently cut off the diode on the red wire coming out of the stator on the 12hp while splicing in the wiring.

What does this diode do and could it cause the ign fuse to blow?

Also the Murray has a grey wire which used to run to a 'anti-backfire' device. I just left it dangling - it isn't touching anything.

Thanks for any advice!
PS - I'm impressed how well the 12hp performs in the newer tractor.


Follow-Up Postings:

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RE: engine swap... question about diode...

The diode converts AC from the alternator to DC for battery charging etc.
It basically acts as an electrical "check valve", only allowing the positive part of the AC sin wave through and blocking the negative part. The result is a pulsating DC current.

IF you no longer have the diode, this is the cheap solution.

www.radioshack.com/product/index.jsp?productId=2062591

This post was edited by bill_kapaun on Mon, Aug 4, 14 at 23:55


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RE: engine swap... question about diode...

Thanks Bill. ..I'll be at radio shack this morning.


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RE: engine swap... question about diode...

Thanks Bill. ..I'll be at radio shack this morning.


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RE: engine swap... question about diode...

Thinking about this a bit more RE blowing the fuse.
My first thought was no, but then I considered-
Some tractors have a direct connection between the diode and the positive post of the battery via the small red wire connected to the + solenoid terminal. Others have this switched out via the key switch.

I don't have a schematic, but if it's the first style, you would have 12V feeding directly back through the stator coil from the battery and no diode to block it when the engine wasn't running.

This may blow the fuse, but even worse, it may have "fried" the stator.
IF it didn't, trying to charge the battery with AC tends to kill batteries.

This post was edited by bill_kapaun on Tue, Aug 5, 14 at 11:22


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RE: engine swap... question about diode...

I'll finally get a chance to install the diode tonight - the consequence of a fried stator would mean 'no battery charging'?
I'll let you know...
thanks again


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