Craftsman 27HP Blowing PTO Switch

GoldHunter331July 21, 2012

I have a Craftsman Model 917276340 with a 27HP Briggs. I bought it used and it worked fine for a little while and then it started blowing the PTO switches. I have replaced the darn things twice and it just did it again. Any idea what could be going on? I have checked the wired for burning or shorting and don't see any thing. I adjusted the electric clutch because it was dragging a little and I thought that that might be put to much load on things. When I install a new switch it works great for several cycles of off and on and the it just stops working. I checked the fuses and it's ok and just can't figure this cotton pickin thing out. Any help would be really appreciated.

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mownie(7)

On this model, it could be a couple of things.
The clutch might be drawing too much current, and that is overloading the PTO switch.
And the other thing is that your clutch features what is called a "snubber diode".
This diode minimizes arcing flash over the contacts inside the PTO switch that would occur whet the PTO switch is turned to the OFF position (which collapses the electromagnet field that the clutch operates on).
You need to test the amp draw and air gap of the clutch.
The snubber diode might be built into the clutch.
If it is an external snubber diode it can be serviced with a diode from Radio Shack or other electro store.
There have been a few old threads on here where members replaced the snubber diode if though it was not available through Sears.
Check your e-mail.

    Bookmark   July 21, 2012 at 10:06PM
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bill_kapaun

If you look at your schematic, you'll see the clutch will draw current from the alternator before it ever gets to the ignition switch and on to the fuse.
Since the alternator is rated for 16 AMPs (about 200 WATTs), that in itself would probably "toast" the PTO, before it ever started drawing current from the battery which WOULD go through the 30 AMP fuse.

IF you have an OHM meter, check the resistance of the PTO windings.
It should be around 3-4 OHMS.

What mownie said about the diode is true, and I THINK that is the culprit.

All the windings in the PTO act like an automotive ignition coil when the switch is opened. Basically it feeds back a large voltage spike as if it was trying to fire a spark plug.
The diode acts like a check valve, "snubbing" a good part of the AC voltage "spike".

Your schematic shows the diode to be part of the clutch assembly.

You can still add an external diode across the circuit with a bit of electrical surgery.
The orientation of the diode is important, so buy 2.
If the 1st one blows, install the other pointing in the opposite direction.

    Bookmark   July 21, 2012 at 11:41PM
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justalurker

We've had this diode conversation before and EVERY diode I've ever seen on a Sears, AYP, Husqvarna was in the pigtail EXTERNAL to the PTO on the tractor side of the harness connector even though the specific wiring diagrams show the diode IN the PTO. Look for a lump in the pigtail.

    Bookmark   July 22, 2012 at 12:37AM
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    Bookmark   July 22, 2012 at 12:51AM
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mownie(7)

Thanks jal, that was the thread I was trying to remember. Attaboy to ya!

    Bookmark   July 22, 2012 at 9:17AM
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justalurker

It was easier to find that thread than it may be to find a Radio Shack... they still sell the 276-1143 diodes.

    Bookmark   July 22, 2012 at 12:08PM
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mownie(7)

I know whatcha mean. RS is on the endangered list 'round here.

    Bookmark   July 22, 2012 at 12:22PM
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justalurker

Long gone are the days of Allied Radio and Lafayette Radio...

    Bookmark   July 22, 2012 at 12:32PM
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GoldHunter331

Thanks Mowie and Everyone for your responses! I found the diode location and it's right at the clutch harness connector. We have a RS right here in town so I'll pick up a couple of diodes and replace them and see how that goes. I'll followup once I get that done. Thanks Again!!!

    Bookmark   July 23, 2012 at 11:49AM
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justalurker

In case you're electronically challenged... diodes are directional so it matters how they are installed in the circuit.

You still might need a new PTO switch and while you're there check the clutch itself. You'll find a clutch troubleshooting guide at the link I posted.

    Bookmark   July 23, 2012 at 11:58AM
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mownie(7)

That's nice but you still need to check the amp draw of the PTO clutch as well.
If you have located the diode in the harness you should check it BEFORE you run off to start replacing things.
If the diode is bad and has shorted, it WILL show continuity on an Ohmmeter no matter which way the Ohmmeter leads are connected.
If the diode is bad and open, it WILL NOT show continuity on an Ohmmeter no matter which way the Ohmmeter leads are connected.
If the diode is good it WILL show continuity WITH THE LEADS CONNECTED ONE WAY, but not when the leads are reversed.

    Bookmark   July 23, 2012 at 1:50PM
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