~ 1991 STX38: Starter engages, engine cranks but won't start, when key is turned off, engine continues to crank. Disconnected battery to stop engine from cranking. Upon attempting to reconnnect battery engine cranked again without seat safety switch being made up. Also engaged PTO and tried to reconnect battery but it cranked again. Safety switches seem to be bypassed by this problem. Is this an ignition switch the likely problem?

Won't start how ? no crank - no start , which means when you turn the key to the start position nothing happens , starter doesn't engage. Or the starter turns the engine an it doesn't start an run. a crank - no start ?

604 is not a complete model number for MTD. However, it looks like you may have the following machine which was manufactured in 1991. MTD 131-604-000 (1991) Lawn Tractor

Found Manual for Model U604H
This manual also for: M3800s-bn.
Download manual

Do check the oil or see if you can manually rotate the engine. Be sure you did not overheat the engine and cause it to freeze up. If this has happened none of the rest of this post is going to help.

Your problem sounds like a Key Switch failure. The Key switch is the one component that the starting circuit and the run circuit have in common.

Note: If you have a good electrical diagram and an ohms meter you can test the Key Switch directly. However these tests let you test the switch without a schematic. They should work on any riding mower.

You can test this theory by jumping the starter. Hold a wrench or fat screwdriver across the two large posts of the starter solenoid. This bypasses the key switch and directly drives the starter. Your starter should crank. Don't crank it for more that a couple seconds. You just want to verify the battery and the starter are both in good shape.

If the starter cranks try one more test. Run a small jumper wire from the positive post of the battery to the little terminal on the solenoid. You may have to remove the small wire off the terminal on the starter solenoid in order to do this. When you apply this current from the battery to the terminal it should make the starter crank. This test bypasses the key switch and shows you the entire starting system is working. You could do this test first and if it works you do not need to do the first test.

If these tests cause the engine to crank, odds are high the key switch is bad. If you want to do a further test to verify this do the following.

Consider the engine kill system. Modern day safety systems include a kill wire. This will be a single wire going to a connecting terminal on the coil (magneto). The wire is small and can be disconnected from the magneto. Disconnecting this wire isolates the engine from all the possible bad switches, relays, diodes, and wiring setup designed to kill the engine if safety procedures are not followed.

With the kill wire disconnected the Coil functions entirely independent of the rest of the wiring systems on the machine. If you disconnect the kill wire and the coil is good you will get spark to your engine regardless of the condition of the rest of the entire wiring system. Disconnected the Coil works all by itself; it does not need the battery or the key switched on.

You may need to remove the fan shroud in order to perform this test. You must find and disconnect the kill wire. You do not need to remove the shroud if you can find the wire that comes from the magneto and disconnect it from the rest of the system. Otherwise remove the shroud and disconnect it directly at the magneto.

NOTE: Many systems have a fuel shut off solenoid. This is a small black cylinder on the bottom of the carburetor. It has two wires running to it. This fuel solenoid will likely not work if your Key Switch is bad. If your carburetor has a shut off solenoid you will have to poor some fuel into the carb before performing the rest of the test. After poring in some fuel set the throttle to fast run, not choke.

Once you have disconnected the kill wire, then jumper the starter solenoid like before. Run a small jumper wire from the positive post of the battery to the little terminal on the solenoid. This is going to crank the engine. Unlike the first test we want to continue to crank the engine until the engine starts. Please do take the solenoid wire off when the engine fires or you will damage your starter. With the kill switch disconnected from the magneto and the starter solenoid jumped to crank the engine, if the engine is in good shape and has fuel, IT WILL START. This combination test definitively identifies a bad Key Switch. Replace the Key Switch.

Note: it is likely, with a disconnected kill wire the engine may start, but when you turn the key off, or otherwise try to shut the machine off, the engine will keep on running. Just slip the kill wire terminal back on and the engine will die, or wait till it runs out of fuel, or short the kill terminal to ground.

do you hear a click if you do its probably your starter cylinoyed if it works when you jump the wire you are bypassing it hope this helps pat

Find the starter relay under the hood. It may be bad and stuck so power continues going to the starter. Try a new relay Or try pulling off the small signal wire on the relay (it is from the switch), and hook battery up. If starter continues to run, the relay is bad. If starter is stopped, put the signal wire back on relay. If starter now starts operating, and key is still off, then switch may be bad .Location of relay may be at left front fender apron, in front of shock tower (like on earlier taurus models).

It sounds like you have a bad actuator. This is attached to the starter and has a solonoid that pushes the starter motor gears into the engine gear. If the solonoid is faulty this may happen. Also check alignment of the same.

Define - It attempts to turn over but won't catch and run ???? When you turn the key to start does the starter engage an spin the engine ? This would be a crank no start condition . If the starter doesn't engage an spin the engine this would be a no crank no start condition . Sorry to ask but i have only been an auto tech for thirty years an have heard a lot of different discriptions for these simple actions . crank no start or no crank no start . Very different diagnostic procedures . So when you turn the key to start , does the starter engage an spin the engine ,but won't start an run ?

did you check out the battery cables,some times they can get corrosion inside where it cant be seen and cause a short, and lack of output and sonetimes will mess with the charging system.it happend on my 87.

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What does engage mean to you.
To me it means a loud click sound followed by the engine cranking.
Actually engage is just the click sound.
Cranking is the small electric starting motor forcing the engine to move as if it were actually running. Once it moves just a bit a spark plugs fire and the resulting explosions makes the engine keep going until one of the following happens.
Loss of air/fuel mixture (i.e. run out of gas or air intake blocked)
Loss of spark (i.e. battery low or dead, bad coil, bad plugs)
The two above do not come together at correct time (timing)
Something physically stops the engine from continuing to operate (engine seizes, too much load)

If battery is low starter will not engage or crank.
If electrical connections are bad (usually loose or dirty) current will not flow properly and starter will not engage or crank.
Both can be checked with a multi meter.
Measure battery voltage when attempting to start.
It should be 12-13.2 volts and should not drop below around 10 volts
If it is not dropping but still not engaging/cranking, continue to take voltage readings move from battery toward the starter. When you find a sudden voltage drop, investigate wiring just before that point upstream toward the battery.

From what u describe, I believe u have a starter and possibly a flywheel problem. When u turn the key to start the car, the starter gear engages w/ the flywheel, turning the flywheel, which turns the engine over. It sounds like the teeth of the flywheel and starter are worn, (they should mesh together smoothly) which would make the grinding noise u describe. If this is the case, the flywheel and starter would need to be replaced, and its a labor intensive job, as engine and trans. have to be separated to remove/install the flywheel. The rrrrrrr sound could be the starter gear spinning but not engaging. The starter gear extends on a shaft, then engages the flywheel. It is possible the gear isn't moving forward on the shaft, just spinning w/out engaging, making the rrr sound. So, I would recommend u remove the starter and have it tested (many parts stores will do this for free) and check the teeth on the gear to see if they are worn or broken. U will also be able to see a portion of the flywheel after the starter is removed, and u should check the teeth on it also. Let me know what u find, hopefully it's just a starter problem, and the flywheel is ok.

the starter should be replaced. The solenoid is kikcing the pinion out but there is no contact being made to turn the starter motor. I have the exact same truck and before i changed the starter used the old starter tap trick. Tap the starter casing with a hammer and it should start turning again. hope this was helpful for you. Good luck.