Small Engines (Lawn Mowers, etc.): MTD 16 HP tractor - electrical issue, hp tractor, battery cables

Question
Hi there,
I can email specs if you like; I think the problem is fairly straightforward.

I found that the machine wasn't starting regularly, wasn't sure if it was battery or not. Purchased new battery to ensure that was not the issue.

I took the cover off between the seat and controls to expose belts, electrical, etc, and saw that the black (thick) wire from battery was shorting against the set of small red wires (I think these are for the kill switch on seat) because a small piece of conductive material was in the red wire's plastic sleeve. The frame is also threaded, so the black wire from battery was corroded and no longer screwed into the frame. I put a new bolt with nut through the black battery terminal (like an eyelet) and fixed it to the frame.

I expected the problem to be solved, but now the unit doesn't turn off, not even sparingly.

Since I think it was shorting for a while, I am inclined to discount the idea that the short wrecked something.

I also found that if I use a metal tool to bypass the solenoid (connect the red wire on the solenoid to the back wire/bolt against the frame) that the motor turns over. This means the battery is ok.

So would it be more likely to be a faulty solenoid, faulty seat switch or ignition wire from the controls/panel at the front?

Do you have any idea on best way to troubleshoot?

Sorry for being long winded!

I can snap a digital photo, markup with text, and email if you'd like as well.
Many thanks,
Trevor

Answer
Check out this link:

http://www.tulsaenginewarehouse.com/drawings/allwiring.pdf

It covers basic wiring diagrams without safety switches.  Most MTD safety switches are pretty straight forward in that they basically open the circuit to prevent power from reaching the small exciter terminal on the solenoid.  The deck and seat switch usually just ground the ignition which kills the engine by shorting the spark to ground.

Let start with the basics.  Dis-connect the battery cables.  Clean the battery cable ends and battery terminals with a wire brush.  The cable connected to the frame you described is the ground or negative cable.  Dis-connect it and clean the frame and connector with a wirebrush.

When you turn the key to the OFF position, the primary side of the coil is grounded which kills the spark and the engine stops.  Have you checked your key switch to see if it is plugged in?  If so, dis-connect the key switch, remove it and see if matches any of the key switched on the website.  If so, use an ohm meter to test the switch.  Often times I will draw the key switch diagram on a piece of paper so I know what pins are what after I re-install the switch.

Now, trace each wire from the key switch to the termination of other end of the wire.  You should find one wire that leads to the side of the engine block.  This is the primary side of the coil which should be grounded when you shut off the key.  You can dis-connect the wire at the engine block and with the key OFF you can check for continunity...there should be continunity between the wire and ground (metal frame) when the key is off.

Can you send me the mower model number off the sticker on the rear of the mower.  Also, send the engine model number located on a sticker or stamped into the engine metal.

Let me know so I can look up some information.
Eric