Help me with this electrical problem...

[frischtr]

Alright, I'm stumped here...

Started mowing this afternoon, made it about 3 laps around the yard and the PT425 just dies. No lugging or sputtering, just all stop on the engine as if I turned the key off. I turned the PTO switch off, won't turn over, no clicking, nothing.

Checked the voltage to the key switch, 13 volts. Turn the key to run position, hour meter does not begin timing. So I figure it is a bad key switch. I pull it out, it meters out good. Still have 13 volts on the red hot wire in the wiring harness.

Checked PTO switch for correct continuity (to make sure it's not a false PTO cutoff situation), all good there.

I unplug all load from the key switch in the run position, which is only the hour meter, headlight switch, and fuel cutoff solenoid. Recheck voltage at the key switch in off position, 13 volts. Hook up hour meter only. Switch to run position, hour meter begins counting, 13 volts at hour meter.

Add the light switch back in (light switch in off position). Key switch to run position, and the hour meter begins counting. Turn on light switch, no lights, and hour meter stops. Now 0 volts on red hot from wiring harness, but still 13 volts at the battery itself. Turn off light switch, still 0 volts on red wire. Recycle key switch to off, then to run position again, hour meter begins counting again. So I figure bad light switch or lights. Well, the light switch meters good, headlights work when wired directly to the battery.

I left the lights unhooked, and tried messing with the fuel cutoff solenoid. When I add that load to the run position, again 0 volts at the key switch, and it won't pull in.

I have checked grounds everywhere, everything meters and seems good. Looked for shorts or continuity issues in all wiring harnesses, everything seems good. All fuses good. If I add any load in addition the hour meter, voltage at the key switch drops to 0, and hour meter stops, but battery voltage remains good at 13 volts. Never had any issues starting previously, started like a champ 10 minutes before this happened.

What else can I try?

Thanks...

 

[PTWannaHave]

Crap...! I am stumped as well!

 

[BigRol]

It could be any load you add is shorting to the meter ground which would leave you with two hots to the meter? only thing I can think of. good luck.

 

[2bON2b]

What else can I try?

Jumper key switch in same order that you were adding loads to it in your description. Sounds like switch contacts have carbon build up on them. Which also may explain how the machine cut off like it did. Loose contacts in a switching device leads to arcing which then leads to failure.

 

[frischtr]

Jumper key switch in same order that you were adding loads to it in your description. Sounds like switch contacts have carbon build up on them. Which also may explain how the machine cut off like it did. Loose contacts in a switching device leads to arcing which then leads to failure.

Yeah, I hadn't thought to bypass the switch altogether since it ohm'd /metered good, but I'll try that tomorrow.

 

[Leejohn]

I know you said you check the grounds, but you should recheck them.

After thinking about it after I posted a bad ground wouldn't just shut the engine down when running, so like was said before the key switch.

 

[frischtr]

Bypassing the key switch altogether doesn't help, same issues... Battery is strong, when wired directly to the starter motor, it cranks away.

 

[marrt]

This sounds like a bad safety switch. Now, I'm not aware of any safety switches on the PT. However, if the PTO switch is on, the tractor won't start (if I remember correctly). Maybe something to consider. I assume you already checked the fuses?

 

[Baby Grand]

I had a fusible link that was partially fused. If you moved the wire, the circuit would close. Move it a little more and it would open. Drove me crazy trying to diagnose why the tractor would shut down while in use, or start up on one day but not on another. Finally found it while checking resistance of all the components in the starting circuit. Was holding the link in my hand and it checked okay, then accidentally bent one of the wires and the meter read open.

 

[frischtr]

All switches, fuses, and wiring ohm good. Any load wired directly to the battery works, but if I use the factory wiring harness/connectors from the rear to the front, I get nothing (despite the fact that I have excellent continuity).

Every time I try something, it points to one thing, but then that thing will check out good, but points to something else, etc, until I end up back at the beginning.

Could it somehow be the voltage regulator?

 

[frischtr]

I had a fusible link that was partially fused. If you moved the wire, the circuit would close. Move it a little more and it would open. Drove me crazy trying to diagnose why the tractor would shut down while in use, or start up on one day but not on another. Finally found it while checking resistance of all the components in the starting circuit. Was holding the link in my hand and it checked okay, then accidentally bent one of the wires and the meter read open.

I'm afraid that's what it may be... Trying to find something like that now.

 

[marrt]

Hmm. This is going to be a tough one.

Have you even seen an old incandescent light bulb with a broken filament that would still burn a while then go out when the filament heats up? The broken wires touch and make contact at first. Then, when the element heats up, the filament wires expand enough to break contact. It's possible a similar thing is happening here. The very small load of a continuity meter isn't enough to heat up and break the circuit. This is a guess of course. But this scenario fits your findings.

Or, perhaps a wire is abraded somewhere and shorts to ground under full load.

I'd go ahead and replace all fuses since it's easy and would be the most likely culprit for the "broken filament" scenario. After that, I'd make up a fairly long bypass wire the shielded alligator clips and start bypassing each wire, one by one, until I found the culprit.

Or, you could just wait until tomorrow and call Terry.

 

[MikeInEburg]

I think you'll find that's it's a safety switch causing your grief. Seat or PTO active or similar.

 

[frischtr]

I think you'll find that's it's a safety switch causing your grief. Seat or PTO active or similar.

No seat safety, and the PTO safety is not the issue (issue is still present after complete PTO switch bypass)

 

[frischtr]

Alright, got her running.

I went back and was checking continuity on all wiring while jiggling/bending all connectors, fuses, and terminals. I believe the culprit is the inline fuse holder between the starter and the wiring harness that leads to the front of the machine. I'm pretty sure i was having a loose connection there, so I took it apart. Looked good, put it back together and now everything works.

I couldn't replicate the connection issue there after pulling it out to where I could see it better, but I still suspect that's the issue. Going to get a replacement inline fuse holder and swap out the fuse. Fingers crossed, that is the issue.

At least now I can work on it in the garage instead of 100 yards away from my tools...

Thanks for the suggestions!

 

[Baby Grand]

Hmm. This is going to be a tough one.

Have you even seen an old incandescent light bulb with a broken filament that would still burn a while then go out when the filament heats up? The broken wires touch and make contact at first. Then, when the element heats up, the filament wires expand enough to break contact. It's possible a similar thing is happening here. The very small load of a continuity meter isn't enough to heat up and break the circuit. This is a guess of course. But this scenario fits your findings.

Or, perhaps a wire is abraded somewhere and shorts to ground under full load.

I'd go ahead and replace all fuses since it's easy and would be the most likely culprit for the "broken filament" scenario. After that, I'd make up a fairly long bypass wire the shielded alligator clips and start bypassing each wire, one by one, until I found the culprit.

Or, you could just wait until tomorrow and call Terry.

Yes - I've had that same experience with an incandescent bulb! Thought it was possessed.
Also had a 286 computer (remember those?) with a cracked circuit board.
When turned on, it would start up and by the time it was done booting, the board had heated up enough that it expanded & the crack opened, shutting it down. Absolutely, maddeningly, mystifying until I actually saw the crack in the board on the third or fourth tear-down.

frischtr,
Hope you nailed it!
Let us know if your diagnosis holds up after a few uses.

 

[frischtr]

Hopefully! I installed a new sealed, weather-rated inline fuse holder with a new fuse to replace the crappy clamp-on type. I'll be back to mowing shortly, so we'll see if it lasts...

 

[k0ua]

Well, I was going to try and help you, and I see you have it tracked down to a high resistance connection in the fuse holder. Ohm's law at work here. Just because you measure 12.6 volts in one of these circuits, does not mean it will deliver the current necessary to run all the things that need to operate on your tractor. A high resistance connection will still measure with a voltmeter the battery source voltage, but when you place a larger load on the circuit the voltage can drop to 0 due to the high resistance connection.

The hour meter likely only draws a few milliamps of current and was not enough to drop the voltage across the high resistance connection.

As an experiment to prove this to yourself, take a 1000 ohm resistor and put it in series with the battery and measure the voltage. You will find that the voltage measured thru the resistor is the battery voltage. Now take and put a load like a tail lamp in series with the resistor and to battery negative, and it will not light, and the voltage measured on the load side of the resistor will be near zero, and the voltage on the battery side of the resistor will be the battery voltage.

 

[k0ua]

Well, I was going to try and help you, and I see you have it tracked down to a high resistance connection in the fuse holder. Ohm's law at work here. Just because you measure 12.6 volts in one of these circuits, does not mean it will deliver the current necessary to run all the things that need to operate on your tractor. A high resistance connection will still measure with a voltmeter the battery source voltage, but when you place a larger load on the circuit the voltage can drop to 0 due to the high resistance connection.

The hour meter likely only draws a few milliamps of current and was not enough to drop the voltage across the high resistance connection.

As an experiment to prove this to yourself, take a 1000 ohm resistor and put it in series with the battery and measure the voltage. You will find that the voltage measured thru the resistor is the battery voltage. Now take and put a load like a tail lamp in series with the resistor and to battery negative, and it will not light, and the voltage measured on the load side of the resistor will be near zero, and the voltage on the battery side of the resistor will be the battery voltage.

To troubleshoot these types of problems. Place negative lead on battery post. Measure with positive lead along circuit path. You would have found the voltage drop across the fuse holder quickly by measuring one side and then the other side and noting the voltage drop with a load placed on the circuit.

 

[ponytug]

To follow up on K0ua's comments: having the load on is important. Put differently, you want to measure the current flowing, not just the voltage. You can pick up a good clamp meter on Amazon, or an OK one at HF. Woodland farms pointed to a good one awhile back.

Switches can look good by voltage until current flows, and then you discover that it actually had 100Ohms of resistance in it.
You can get copper loaded dielectric grease for coating relay sockets, and fuses. It keeps them from corroding and it keeps the connection at low resistivity. I use it all the time on boats, tractors, and english sports cars...:laughing: A little bit goes a long way.

All the best,

Peter