Whose switch logic is in Top form this evening?

I have spent the last 7 hours in and out of a sweltering hot storage building trying to pinpoint why my riding mower will not crank and am officially too tired to think. 

If anyone can tell me if, and how, the now removed ignition switch I have in my hands now can be tested via a simple continuity check I would be eternally grateful.  What I'm trying to do is determine whether the switch itself has an internal problem before going out and buying a new one.  In other words, what pins should or should not be making contact with each other in OFF vs START?

The key has 4 positions:  1) OFF, 2) RUN/LIGHTS, 3) RUN, 4) START

So far I know that:

1)  pin 6 (ground) and pin 4 (Magneto) appear to be making contact in OFF - connection is clearly dodgy and meter will swing from a few ohms to 5M and is sensitive to key jostling.  So does a bad motor OFF connection mean NO START situation?

2) Measuring from pin 2 (Hot lead from Battery) I get continuity ONLY to pin 5 (safety switch to solenoid lead) ONLY in the 2nd position (RUN/LIGHTS). So NO continuity from the Hot lead to any of the other non ground pins at ANY switch position

3) NO continuity from any of the other pins to one another at ANY switch position

So, only 2 continuities:  1) The dodgy one from pin 4 (Magneto) to ground at OFF 

                                      2) Solid one from pin 2 (Hot) to pin 5 (Solenoid) ONLY at the RUN/LIGHTS                                                                position



Does this sound 100% conclusive of a bad switch?
                                     

lassoharp said:

I have spent the last 7 hours in and out of a sweltering hot storage building trying to pinpoint why my riding mower will not crank and am officially too tired to think. 

If anyone can tell me if, and how, the now removed ignition switch I have in my hands now can be tested via a simple continuity check I would be eternally grateful.  What I'm trying to do is determine whether the switch itself has an internal problem before going out and buying a new one.  In other words, what pins should or should not be making contact with each other in OFF vs START?

The key has 4 positions:  1) OFF, 2) RUN/LIGHTS, 3) RUN, 4) START

So far I know that:

1)  pin 6 (ground) and pin 4 (Magneto) appear to be making contact in OFF - connection is clearly dodgy and meter will swing from a few ohms to 5M and is sensitive to key jostling.  So does a bad motor OFF connection mean NO START situation?

Click to expand...

No... short there properly kills the spark when off. Dodgy intermittent open means motor could keep running when turned off. If other kill switches not working.

2) Measuring from pin 2 (Hot lead from Battery) I get continuity ONLY to pin 5 (safety switch to solenoid lead) ONLY in the 2nd position (RUN/LIGHTS). So NO continuity from the Hot lead to any of the other non ground pins at ANY switch position

Click to expand...

AFAIK Pin 2 to 5 should only have continuity in start position, in run mode, pin 5 should be open so starter motor releases, and pin 4 should not be grounded in run position so magneto will spark.

3) NO continuity from any of the other pins to one another at ANY switch position

Click to expand...

not sure what pin 3 is doing. 

So, only 2 continuities:  1) The dodgy one from pin 4 (Magneto) to ground at OFF 

                                      2) Solid one from pin 2 (Hot) to pin 5 (Solenoid) ONLY at the RUN/LIGHTS                                                                position

Click to expand...

2 to 5 seems to me like it should be intermittent contact, only while cranking. .

Does this sound 100% conclusive of a bad switch?
                                   

Click to expand...

I don't know... The actual circuit is pretty simple, but all the safety cut outs make it look complex.

To start all you need to do is un short the magneto kill winding, and crank the motor.

But the starter solenoid being engaged when on sounds wrong.

and lights wont work if no connection through switch.

JR

> mower will not crank

Disconnect spark plug.

Jump battery + to small terminal on solenoid. Starter-motor should crank.

If not, disconnect big wire to starter motor, listen close while jumping batt + to solenoid small terminal. Solenoid should Klunk.

DANGEROUS trick: use BIG jumper (like car jump-start cable) to jump the two big solenoid terminals to each other. (Big sparks.) Starter should spin no matter what's-up with solenoid or switch.

It is UNWISE and DANGEROUS to defeat or bypass all those safety switches. However they do go bad. And it can be hard to get replacements for older machines. While you can in theory logic-out how to jump these safeties open/closed, in fact the drawing is confusing and the shed is too hot for clear thinking. My 1974 Sears now has _no_ safety switches, which makes debugging simpler, but I risk my feet every time I go near it, and count my toes a lot.

Your "continuity #2" seems wrong. You should have 2-5 only in the spring-loaded "Start" position of the switch.

If you are willing to live dangerously, just run the "essential" wires. Battery (through fuse!) to switch (pushbutton if keyswitch is sick, 1A rated) to solenoid small.

You do NOT need the MAGneto to get cranking action. You will need it to Run, of course. As John says, the usual rule is GROUND the "mag" wire to STOP. If left floating, the magneto runs just fine without battery power (it makes its own power when turned). So my neighbor leaves the MAG wire loose, and holds it to frame to kill the engine. (Beware: the back-EMF can sting.) A more timid man might buy a toggle switch.

FWIW, on this machine the lights only run on "stator" power, i.e. only when engine is turning.

FWIW: the "key" does not keep your neighbor from using your tractor. There is a near-universal key available at Tractor Supply. Apparently most of the mid-market machines use the same key. So if button and toggle gets you cranking and stopping, fine.

2 to 5 seems to me like it should be intermittent contact, only while cranking. .

Click to expand...

That makes most sense to me - at least it did with the older 5 pin ( 4 + ground) switches.  For sure on those it was Battery to Solenoid at Start position.  And you would get Battery to Lights in the  2nd position, like in a car.  That would have been pin 2 to pin 1 on the new switch and I get nothing there.


I'll pick up a new switch tomorrow and check the continuity before buying.  The safety switches are another possibility of failure and I haven't checked those yet.

Jump battery + to small terminal on solenoid. Starter-motor should crank.

Click to expand...

It cranks fine when doing this, and when jumpering the the bigger solenoid leads together.


One other detail - when I opened up the panel to check the solenoid I found that the ground wire that runs to the key switch was dangling loose.  I thought this was surely the problem so I secured it and still no start.  What was kind of odd was that there seemed to be no obvious signs the ground had recently become unsecured.  I just mowed problem free last week and without a ground connection it would seem that if the ground had come loose during mowing that the engine would not have turned off (grounding magneto).  I suppose it could have coincided with engine shutdown.  There were no loose bolts or nuts that I could see.  My first reaction upon seeing the ground wire so "neatly loose" was that the previous owner (just bought this a month ago) had installed a new solenoid ( looks too shiny for 10 year old mower) and forgot to connect the ground wire to the solenoid mounting bolt.  But if so, could it have still started and stopped?  This ground wire is the same one shown on the schematic going from pin 6 to the solenoid housing.

I reattached the ground to one of the frame holes that stick out on either side.  You can see one in the pic.

.

The main thing the ground wire to the switch does is kill the magneto when turned off... but this is in parallel with other kill switches so you might not notice if missing.

JR

> the ground wire that runs to the key switch was dangling

It is possible the ground is also through the switch body, the wire is both redundant and handy for servicing with the switch out of its hole.

The false connection in Run and the lack of connection for Light suggests a bit of metal has come off the Light contact and for now has lodged near the Run contact. Maybe you could live without lights (mine stay in the shed with the annoying hood) but loose metal bits will find some way to make trouble.

Yes, if that switch is available, blow the $15 for another.

-------------------------------------------------
I suppose it is sick tractor season.

When I bought this land there was a lot of junk on (and in) it. There was an ancient Sears lawn tractor. I jumped the battery and it ran, but only at 5/8th choke. Cleaned the carb, which led to about a pint of pine needle peat blocking the bottom of the fuel tank. The tank is 6 inches lower than the carb, so there was a fuel pump working on crankcase suction. This leaked vacuum, I bashed it, it ran a couple years but was dying. A new replacement is $68!! (As much as the one in my plow truck!) I got an electric pump on eBay, made a good tunk-tunk-tunk noise at first, but burned-up in an hour.

Hell. Gravity sucks, reliablly. All I need is a tank with a fitting in the bottom. Looks like I've converted the gasoline engine to an alternate fuel: insecticide.

Wow!  I'd call that kind of a  Rube Goldberg device as interpreted by Stephen King!  Kind of hypnotically cool.

That's like a mini version of the thing that chased Dennis Weaver across the desert in "Duel"

I suppose it is sick tractor season.

Click to expand...

Indeed.  The reason I have the new mower.  Back in May I went out one Sunday afternoon to mow.  Went to crank and as the engine turned over I heard that distinct whipping POOF! and watched in horror as a thin stream of flame shot out of the side of the engine and the lower front of the mower went up in flames!  I leapt off and grabbed the gas can and ran hard, praying the flames did not go back into the engine and hit the tank.  After calling the Fire Dept I managed to grab a hose and put out the fire that had spread beneath the mower. The front tire and bumper were still burning when the Firemen pulled up.  What had happened was the muffler/manifold had lost the bolts and fell off the engine. It apparently had happened at the very end of the previous weeks mowing.  I completely missed it when fueling up and checking the oil.  The choke had injected fuel hard enough to squirt several feet out the open hole and the spark timed it just right into the open air.  This was an MTD and over the years I had to constantly tighten and replace the block bolts.  Apparently they have issues with other bolts too.

This is the reason why we have no grass in our yard.
Best,
Bruno2000

PRR said:

> the ground wire that runs to the key switch was dangling

It is possible the ground is also through the switch body, the wire is both redundant and handy for servicing with the switch out of its hole.

The false connection in Run and the lack of connection for Light suggests a bit of metal has come off the Light contact and for now has lodged near the Run contact. Maybe you could live without lights (mine stay in the shed with the annoying hood) but loose metal bits will find some way to make trouble.

Yes, if that switch is available, blow the $15 for another.

Click to expand...

+1

-------------------------------------------------
I suppose it is sick tractor season.

When I bought this land there was a lot of junk on (and in) it. There was an ancient Sears lawn tractor. I jumped the battery and it ran, but only at 5/8th choke. Cleaned the carb, which led to about a pint of pine needle peat blocking the bottom of the fuel tank. The tank is 6 inches lower than the carb, so there was a fuel pump working on crankcase suction. This leaked vacuum, I bashed it, it ran a couple years but was dying. A new replacement is $68!! (As much as the one in my plow truck!) I got an electric pump on eBay, made a good tunk-tunk-tunk noise at first, but burned-up in an hour.

Hell. Gravity sucks, reliablly. All I need is a tank with a fitting in the bottom. Looks like I've converted the gasoline engine to an alternate fuel: insecticide.

Click to expand...

You need to find a mower/tractor junkyard and find a proper gas tank... yes, gravity feed is reliable... as long as carb float valve doesn't screw up...
---------
My riding mower would drip fuel when parked and one time when mowing the float valve stuck full open, with raw gas pouring out of the air filter... I respect the energy content of raw gas pouring out by the hot muffler...    I replaced that carb last summer (damn ethanol).

At the moment I'm replacing the magneto on mine (got new part already and will swap it out this weekend), because it has trouble re-starting when hot (no spark)...  I have double and triple checked all the safety switches and even with them disconnected it still screws up, so my diagnosis is a subtle insulation breakdown when hot inside the magneto coil, which reduces the spark just enough that it still fires when running,  but won't start when hand cranking hot.

===

PRR: Not Rube Goldberg at all ... to direct and effective, Maybe you could buy one of those red plastic gas cans... but that company was put out of business by class action lawyers.  : A square metal gas can might work.

JR

Well, another twist via replacement part compatibility . . .

The listed OEM switch part for my model is listed as 92377 MA.  The new switch you see in the pic is, AFAIK, this part - according to local  Murray Lawnmower parts guy.  The twist is:

Re physcial position, On the new switch,  B ( Battery, pin 2) is swapped with S (solenoid, pin 5),  and, X (aux, pin 3) is swapped with L ( Lights, pin 1)

I can see that the basic START connection of pin 2 to pin 5 will be preserved, as will pin 3 (aux) to pin 1 (lights).

Also, from testing the new switch connections, the hot lead from the battery will not make an internal connection to the magneto in any position.  This worries the most as it would likely burn up the coil pretty quickly.  I also am not sure what could happen via the external circuit connections with the pins in swapped positions.

You would think that the switch designers wouldn't leave such a potentially costly mistake hanging out there in replacement parts but who knows. 

Anyone see potential problems?


For reference:

L = pin 1

B = pin 2

X = pin 3

M = pin 4

G = pin 6

lassoharp said:

Well, another twist via replacement part compatibility . . .

The listed OEM switch part for my model is listed as 92377 MA.  The new switch you see in the pic is, AFAIK, this part - according to local  Murray Lawnmower parts guy.  The twist is:

Re physcial position, On the new switch,  B ( Battery, pin 2) is swapped with S (solenoid, pin 5),  and, X (aux, pin 3) is swapped with L ( Lights, pin 1)

Click to expand...

Do what works.

measure twice cut once.

I can see that the basic START connection of pin 2 to pin 5 will be preserved, as will pin 3 (aux) to pin 1 (lights).

Click to expand...

momentary start is connecting B to S ,,, good

On or run position connects  X to L,  with M open ,,,, good

Also, from testing the new switch connections, the hot lead from the battery will not make an internal connection to the magneto in any position.  This worries the most as it would likely burn up the coil pretty quickly.  I also am not sure what could happen via the external circuit connections with the pins in swapped positions.

Click to expand...

Don't worry too much about it, just don't connect it wrong.

You would think that the switch designers wouldn't leave such a potentially costly mistake hanging out there in replacement parts but who knows. 

Click to expand...

There are limits in engineering to how idiot proof you can make a design. There are obvious benefits from making it hard to manufacture wrong, diminished benefit from making it hard to repair wrong.  In fact recent trends discourage adding significant cost to a product that buyer doesn't appreciate at point of sale (like repairability). Since you can lose the race before the starting gun if customer buys cheaper competition.

Clearly labeling the terminals by function is good, IMO.

Anyone see potential problems?


For reference:

L = pin 1

B = pin 2

X = pin 3

M = pin 4

G = pin 6

Click to expand...

and
S= pin 5.

If concerned you can connect up wires in sequence to confirm each works correctly, before doing next.

1)  connect B and S first... Mower should actually start but without M and G connected switch will not turn motor off. Grounding the magneto wire will stop it. 

2) connect M and G next to confirm it kills motor after running

3) then connect X and L for Lights, action and camera....


JR

Thanks John, I just tested the all the safety switch paths and it appears that regardless of switch being dodgy that one of the connectors has an internal short.

I'm getting to that overheated and tired point so I'm going to post results and rest for a bit before hacking into the harness.

What I found was that measuring  from the Magneto lead (Yel, SW pin 4)) on the key switch wire harness gave continuity to ground in both seat switch positions.  If I'm understanding the schematic correctly this switch should open the ground connection when the seat is occupied so that the Magneto isn't grounded out.


The switch itself tested ok but the harness connector leads showed short.  An easy enough fix with a set of spade connectors but I would like one more opinion on what I've proposed in case I've overlooked something.  Easy for me to do under normal conditions and even easier after several hours in the sweltering heat.   

lassoharp said:

Thanks John, I just tested the all the safety switch paths and it appears that regardless of switch being dodgy that one of the connectors has an internal short.

I'm getting to that overheated and tired point so I'm going to post results and rest for a bit before hacking into the harness.

Click to expand...

Time for a cold beer and some time in the air conditioning.  8)

What I found was that measuring  from the Magneto lead (Yel, SW pin 4)) on the key switch wire harness gave continuity to ground in both seat switch positions.  If I'm understanding the schematic correctly this switch should open the ground connection when the seat is occupied so that the Magneto isn't grounded out.

Click to expand...

be careful, don't over think this.  I happen to have a new magneto sitting here, that I plan to replace this weekend, and the magneto kill winding, when not shorted by an external switch contact, measures about 2-3 ohms to ground.  So a simple continuity tester will not discern any difference between a couple ohms and a dead short.

A low ohms scale VOM measurement might, if you can read the difference between a few ohms and probably less than 1 ohm for shorted, while tester leads and probed connections could be a major fraction of 1 ohm.

The switch itself tested ok but the harness connector leads showed short.  An easy enough fix with a set of spade connectors but I would like one more opinion on what I've proposed in case I've overlooked something.  Easy for me to do under normal conditions and even easier after several hours in the sweltering heat.   

Click to expand...

The proposal to connect up in stages should help if you are still concerned...  If the wiring harness really does have a short to ground, on the magneto kill terminal, the motor would never ever run.

I suspect the switch is probably a single point failure, if the mower operated OK before.

JR

> the hot lead from the battery will not make an internal connection to the magneto in any position.  This worries

I may be mis-reading, but: the battery should NOT connect to the Magneto. The best feature of magneto ignition is you do not need a happy battery. All you need is a way to kill the magneto when done. You hold the points closed (via a shorting switch, typically to-ground to save a wire) and it stops sparking.

> wire harness gave continuity to ground in both seat switch positions.

There is often an and/or logic. On the Murray I could take my butt off the seat and not kill the engine IF the parking-brake was mostly engaged. The Sears has another path through the PTO (mine is electric) but that may have been so I could not Start with blades engaged. (That safety is long-gone now.)

The proposal to connect up in stages should help if you are still concerned...  If the wiring harness really does have a short to ground, on the magneto kill terminal, the motor would never ever run.

I suspect the switch is probably a single point failure, if the mower operated OK before.

Click to expand...

So I attached the new connectors to seat safety switch and still get continuity to ground in both positions.  I pulled the wires loose and tested.  There is a another path to ground from the incoming safety switch line and pin 4 (Mag) on the wiring harness.  Also, I do not currently have the ignition switch connected to the harness, where pin 4 would normally be grounded in the OFF position.  So unless I'm missing something on the schematic there's another short to ground somewhere.  For the Yellow Wire path starting at pin 4, I'm seeing an NC path through all 3 safety switches and a ground at a single point coming out of the seat safety switch.

I also checked the seat connector I cut loose and it does still show an internal  short.  Thinking maybe too much current (but not enough to open the fuse) or arcing caused another internal failure in one of the other switches.  Off to test that . . .

I am repeating myself.

Is the wiring harness attached to the magneto? 

Are you measuring Ohms or just continuity (buzzer)?

I measure about 2.5 Ohms to ground from my brand new magneto kill terminal. So even with all the safety switches open, with the magneto attached, a continuity tester will see continuity to ground.

Have you tried starting your lawnmower?

JR

PS: Life is a test, often with trick question...

JohnRoberts said:

I am repeating myself.

Is the wiring harness attached to the magneto? 

Are you measuring Ohms or just continuity (buzzer)?

I measure about 2.5 Ohms to ground from my brand new magneto kill terminal. So even with all the safety switches open, with the magneto attached, a continuity tester will see continuity to ground.

Have you tried starting your lawnmower?

JR

PS: Life is a test, often with trick question...

Click to expand...

Yes, I see this now - the harness is connected to the Magneto which is grounded at the armature . . .
Am measuring Ohms.  Usually get 1.0 or less will pay attention to Magneto ground path reading.

I haven't tried starting mower with new switch.

Will double check ground path with Magneto plug out.

> magneto attached, a continuity tester will see continuity

Good point.

> with Magneto plug out.

If you get as far as cranking: be sure you have some way to kill the engine. If mag-short harness is dubious or unplugged, tie a nylon rope to the spark plug boot so a good yank (from several feet out on the rope) will be sure to kill all spark. (Fuel shut-off works but very slow: I forgot to turn-on today and got 50 feet before it sputtered.)