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I have been trying to fix my neighbor's Husqvarna riding mower. The mower did not start last summer. He changed 3 safety switches, the starter and the solenoid, but the mower will not start. The front light will come and the hours are displayed, but that is it. I pulled on the belt and the blade is not frozen, however the engine will not start. When I turn the key to the run position, I hear a 'thump' from the solenoid, but the engine will not start.

Today I replaced the battery and the mower did not start. I decided to jump the solenoid with two screw drivers (key on run position) to see if the engine would turn over. After jumping the solenoid the engine would spin and spin, but the engine would not run. I also pulled a spark plug and I could see a spark. I believe the engine was getting gas, but still no start.

I think that he has a wire in the wrong position, but I don't know. Any ideas?

The solenoid is not polarity sensitive so either cable on any terminal is fine
If you have all the controls in the correct position & the yellow wire shows better than 10 V and the black/white wire is ground but the starter does not spin then either the starter or the solenoud is deceased.
The next test is to jump from the battery + directly to the starter
IF the starter does not spin then add a second jumper from the battery - to the engine
Starter spins = bad ground
Try using the key with the ground jumper still in place
Starter spins confirms bad ground connection
Starter does not spin = bad solenoid .

If the starter iself is bad then it can draw too much current & drop the voltage so low that the fuel shut off solenoid on the carb will not open.

Thank you for the fast response. I jumped the red battery wire directly to the red starter wire and the engine started spinning but did not start. I checked for spark and the spark plug was sparking.

In the picture below, the vacuums line is draw rotting. I taped it over just to see if that was the problem. The mower engine will not turnover with the key. The engine will spin if it touch both red wires.

The solenoid makes a thud sound when I turn the ignition to run, but will not start.

So if the starter does not spin then the solenoid is shot.
The terminals burn out .
Next step is to put some fuel down the plug hole and crank the engine.
If the engine fires & runs a second or so then the ignition timing is OK

Should that be the case remove the solenoid from the carburettor
If it has 2 wires then turn the key on & off
The plunger should retract quickly every time you turn it on and snap out when you turn it off
If ithas a single wire you will need to ground it to test it.

Ok I will try in the morning. I will keep everyone updated. Thank you all!

I cleaned the big ground wire and it did not start. The mower will not start with the key, however, when I attempt to start the engine, the solenoid makes a thump sound.

After jumping the starter wire, the mower started and ran for 2 seconds when I sprayed starter fluid in the plug holes. I was happy to hear the engine run again, but the mower will not start with the key.

Using the volt meter I found the following...
The mower will not start with the key. I noticed the yellow wire (left lug/looking towards the rear) has 12 volts with the key on.
The black and white wire (ground - left lug) when used as a ground on the volt meter, shows 12 volts with the key off.

When I jump the yellow wire to the red starter wire, the fuse blows every time.
When I ground the black/white wire, the engine will not start with the key.

When I unplug the sensor on the big steering handle, the solenoid does not make the thump sound.
When I unplug the sensor under the seat, the solenoid will make the thump sound.

The two wire part hooked into the bottom of the carb shows 12 volts with the key on.

I think I am following what you are doing.
So pull the yellow wire & the black /white wire off the solenoid.
Turn the key on
Jump 12V to one of the small terminals on the solenoid and ground the other terminal .
Does not matter which way they are connected .
If the starter cranks the engine then the solenoid is good.
If itjst goes thump and the starter does not spin then the solenoid is bad
From what you have posted it appears that the mowers wiring is OK.

Ok. Thank you very much. I will follow you directions when the sun comes up. Thank you again!

Ok. The ground is good and the solenoid is new. I am sure the original problem I was having was due to a bad adjustment on safety switch (tall steering handle.)

The testing went horribly wrong and I jumped 12 volts to the wrong wire and now the mower will not make the thump sound when I turn the key. I assume that I burnt up the PTO switch. Does the PTO have an internal fuse? Will the PTO stop the mower from starting?

I followed the thin yellow wire from the solenoid to the PTO. The yellow solenoid wire goes into the PTO and comes out the PTO into a yellow wire that goes into the ignition. If I touch both of the yellow wires together, the mower starts with the key.

Again, the mower starts with key when I touch the yellow wires together. Simply, I have power going into the PTO, but no power coming from the PTO to the ignition.

Is the PTO bad? Why is the power not coming out of the PTO?

If it cranks with the key when the two yellow wires are bridged and does not work when the switch is connected ( pto off ) then the you are correct & the switch is not bridging the wires as it should.
Thus the PTO switch looks like being the guilty party all along.
When doing a diagnosis on mowers, I pull all of the plugs off and jump the plugs as the switches would have done.
I no longer try to instruct owners how to do this after one managed to cross the plug diagonally and burned the magneto out then PMed me endlessly demanding I stump up for the new coils .
The terminals on the outside of the PTO switch extend exactly as they are on the outside inside the switch.
Inside there is a slide with some brass strips that either connect or disconnect the opposing flats of the terminals.
They all connect left right, there is no diagonal or complex bridging,
The brass strips are quite thin and easily burned, worn through or as one poster here found can have ants nesting in there.

Because you posted the photo I did not bother to download the wiring schematic .
There was no need till we confirmed weather you had both power & ground to the solenoid & that the solenoid was in fact working .
The wiring schematic for most Husqvarnas is at the end of the owners manuals .
However most are wired basically the same .
The cranking circuit is a daisy chain from the key switch through the PTO & Brake switches to the solenoid.
On ZTR's it may also go through both lap bars, or the lap bars might control the ground if it has a 4 pole solenoid like in your photo.
The key is the wiring colours
A plain black wire is generally a plain ground
A black wire with a coloured trace is usually a switched ground which is why I asked you to test for a ground on the Black/white wire.

Thank you Bertsmobile1, Lawn World and anyone who helped with the thread. I really appreciate that you would take the time out of your schedule to help us. I sleep well knowing that there are good people like you in the world.

I don’t know much about mowers, but I know cars and I am notorious for assisting people on the car forums. Thank you for all that you do! It means a lot. I hope you are blessed for all that you do. I will keep you posted about the progress. As soon as the PTO arrives, I will let you know. Thank you very much.

The PTO fixed the mower. Thank you very much!!!

And thank you for hanging in there and even more so for the follow up to close the thread.
For every one who posts a problem, a lot lurk in the background seeking answers in private and forthem long threads like this where we have worked through a problem methodically and come to a conclusion that worked will be of great use to them.