Testing Ignition and Safety Switches for MTD mower

strong(z5MO)May 22, 2010

I have an MTD model 673 riding mower with a Briggs 2 cylinder engine (model 42A707 type 2238E1 code 9902055A). It won't turn over when the ignition switch is turned. However, I jumped the starter solenoid and the engine turns over OK, so the starter and battery are good. This was a sudden problem - ran the engine a week ago.

Using a test light(and with both the blade, clutch and seat safety switches closed) I get no light when I turn the key on and touch the test light to the solenoid terminal going to the starter, and no light when the key is turned on and I touch the single spade terminal from the ignition switch. The solenoid ground is good. It would seem there is a problem with either the ignition switch or one of the safety switches since I'm not getting current to the solenoid with the key on.

The seat switch is a simple "ground out" - when you sit on the seat it breaks the metal contact (easily visible, not a button switch.) I manually pushed in the buttons on both the blade and clutch switches, so either there is an internal lack of contact within these switches or else the ignition switch is bad.

I removed the ignition switch (5 terminals marked G, L, M, S and "blank" - I assume "blank" is the Bat terminal?) and tested for continuity. With the key in the off position I get continuity between L and "blank". With the key in the on position I get continuity between S and blank. If S is starter and blank is battery, then it would seem the ignition switch itself is OK because I'm getting continuity between S and blank (battery?) When I use the test light at the wire connectors for the ignition switch terminal (with clutch depressed and blade lever up), I get no light on any of them (shouldn't one of those five wires at the ignition switch be a hot at all times???) I can't see any damage to the wiring.

If either the clutch safety or blade safety switches aren't working, would this prevent current from reaching the ignition switch? I used the test light probe on all the safety switch wires (clutch safety= yellow, yellow, white, orange, orange; blade safety = orange/white, orange/black, yellow, yellow, yellow) and get no light.

The wiring diagram in the MTD parts book is virtually useless - no clear idea where the wires connect, wire colors don't totally match what I have, no indication where fuses might be).

Any help in figuring out which component is responsible and how to further test the ignition, clutch and blade switches or discover where the fuse links are would be appreciated!

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***"If either the clutch safety or blade safety switches aren't working, would this prevent current from reaching the ignition switch?"***
Well, you have that backward, the ignition key switch is the source of the current for the starter control circuit, not the recipient of any current.
What you say about the test light not burning is more significant here than the continuity tests.
Your test light should burn when you touch the BATTERY terminal on the key switch. If it does not burn at this point, you have no battery power reaching the key switch. Check for a blown fuse.
As for the tests you have done already on the seat switch, brake switch, and PTO switch...........if you have NOT been holding the key switch in the START position while you have been doing your checks, that is why you are not detecting any voltage at those switches.
Check this, Do you have 12V power to the BATTERY terminal of the key switch? If no, check for blown fuse or broken wire. If yes, turn the key to start and check for 12V power at the S terminal of key switch. If no power is present, replace key switch. If no, you need to recheck all the switches you say you have been checking, and make sure to hold the key in the START position while you do your checks. Beware that if you wiggle a wire and re-establish continuity in the circuit, the starter could engage without warning. Keep your hands and other body parts away from the engine while testing the wires and switches.

    Bookmark   May 22, 2010 at 11:19PM
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Thanks Mownie! It was in fact a broken 16 gauge stranded wire leading from the positive post of the solenoid to the ignition switch. How that wire could be broken inside the intact insulation where the wire was running straight and in a protected location I don't know. Looked fine on the outside and suddenly the test probe stopped lighting. I suspect the wire was defective from the get go because this mower has always had an intermittent tendency for a long time to hesitate when turning on the ignition key. Now it starts like a charm.

    Bookmark   May 23, 2010 at 9:43PM
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Man, I'm tickled to hear of you getting the problem nailed down solid. This type of circuit failure is one of the rarest, and often hardest to find. As to "how" these actually "get caused" is impossible to say, but it could have happened where the wire itself was manufactured when the new wiring was spooled the first time. Could have happened when the wire was assembled (woven into a harness). Could have happened during assembly of the tractor. And on and on and........
Glad I could help (you just needed to be pointed in the right direction)

    Bookmark   May 23, 2010 at 9:54PM
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i sure hope u guys are still around. i have a yardman mv20s with a kolher engine. i too put a new switch because the old one melted on two wires, no i dont remember which two. i dont get any power to the switch either and as far as i know theres only one fuse which is good. after reading here i have to still check the safety switches. and the wiring does not match to the book. the wires i have coming to the switch are 2 yellows together, 2 reds together, 1 orange and 2 greens together.. im guessing the greens are the grounds, and the reds the power, i hope, dont laugh,. any way on the solonoid i have one cable goes to he battery and the other goes to the starter and on the side of these are two small orange and white wires which arent on the book, great. so im going to check the safty switches tomorrow. when i turn the key i should get power to the B terminal right.. and as to the letters on the switch what do the others mean,M,L,S,B,G..thanks

    Bookmark   February 24, 2011 at 11:30PM
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***"when i turn the key i should get power to the B terminal right.."***
No, you should be able to detect 12V battery power at the B terminal without turning the key (or even if the key is in your pocket).
The B terminal is where power arrives at the switch after leaving the battery. The fuse is between the battery and the B terminal.
***"theres only one fuse which is good."***
And how do you "know" it is good? Have you tested it with an Ohmeter? or just by looking at it? Have you tried a new fuse?
That you have 2 "melt wires" can be an indication of a shorted wire somewhere........or it might mean that there was a bad connection that was putting up too much resistance and overheated the wires that way. Sometimes the switch itself is the "bad connection" and just kills itself (often taking out some wiring with it).
B is for Battery, this is power into the switch. The switch then distributes that power to various other terminals to send power to the proper destination according to the key position.
M is for Magneto, this is the terminal that grounds the magneto circuit to stop ignition sparking when the key is in the OFF position to kill the engine.
L is for Load, this terminal sends power out to any component that requires a steady supply of power from the battery in order for the component to work. Some components might be a fuel solenoid. If the tractor has an electric PTO the power to the PTO switch usually comes from this terminal. Likewise if the tractor has battery powered headlights, the light switch will be fed from the L terminal.
S is for Starter, this terminal sends power to operate the starter solenoid when the key is turned to the start position. There may be some "safety interlock" switches or devices between the S terminal and the starter solenoid that inhibit the flow of power unless certain "safety requirements" have been fulfilled.
G is for ground, this terminal is there so that a ground wire can be connected to the switch if the switch is mounted in a nonconducting material such as plastic or fiberglass. If the switch is mounted in a metal dash which is grounded, there is no need for this terminal to be connected to any wiring.

    Bookmark   February 25, 2011 at 1:23AM
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thanks for getting back to me..I will start from the top as soon as the rain stops. you have given me lots of good info. will get back soon thanks again

    Bookmark   February 26, 2011 at 5:41PM
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I have an enhanced wiring schematic for the MTD-600 mower. I took it from the Operator Manual, imported it into Windows Paint, and cleaned it up with color codes on the wires, and components symbols, which make it very much easier to read and understand.
I could send it to anyone who wants a copy -- can we post email addresses here? Alternately I think there's some way to put documents and pictures into the Journal here, but either I'm ignorant or the instuction pages here just aren't making sense. Has anyone here done a Journal page on GardenWeb, can give me a rundown on how it's done?

    Bookmark   March 10, 2011 at 10:24PM
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Anyone wishing to e-mail James, click on his name in the "posted by" line of his post, then click "send me an e-mail" when his member's profile page opens.
After both parties have made e-mail contact, the forum platform is no longer needed for e-mail communication between the two parties.
This only works if the member enabled the e-mail function when establishing his/her member profile.

    Bookmark   March 11, 2011 at 12:29AM
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Hello!! I also have (I think) a safety switch problem. the tractor is a 14hp mod 13am660f062. The blade and seat appear ok There is a single wire (yellow) attached under the shift selector (forward reverse and neutral) I believe it prevents blade rotation in reverse, but I'm not sure how it works. I have found I can start the tractor by giggling the shift selector. What I'm not sure of is if there is an actual neutral switch elseware causing the trouble. Can you tell me if there is? If there isn't, how does the single wire terminal next to the direction shifter work? (The term is insulated from the chassis)

    Bookmark   July 10, 2011 at 4:02PM
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Thank you so much goes out to james garfield for the terrific wiring diagram of the MTD 600. He did a fantastic job making it easier to comprehend.

    Bookmark   September 23, 2011 at 8:44PM
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i have old land mark 12 hp 38 in. cut. (mtd) i have seat safety and reverse safety switches unhook. is it possible to disconnect deck safety switch?

    Bookmark   May 22, 2013 at 2:34PM
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I doubt there is a deck safety switch. Seat safety switches save lives btw.

    Bookmark   May 22, 2013 at 11:26PM
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I'd like to add my story to this thread.

I had the same problem. Turned the key, nothing worked. Check out all wires - multiple times - and everything checked out. Finally, after disassembling most of the engine compartment, I noticed that the button in the safety switch on the blade engagement arm didn't seem to be pushed in all the way. After playing with the arm - forcing it all the way back toward the seat - I got current and was able to start the mower.

Turns out, there is a spring underneath the mower, above the mower deck, whose sole purpose in life it to keep tension on the arm so it will properly depress the button when disengaged. In this case, that spring had broken. I saw it handing underneath and figured it had nothing to do with the safety system because it didn't seem to serve a purpose. Wrong assumption, obviously.

    Bookmark   May 25, 2013 at 8:32PM
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