Craftsman PGT9000 Briggs 28hp not running correctly

oakdale81 · Aug 6, 2019

Start of the issue, received a call from my grandpa that his lawn mower would not turn over. They replaced the battery and still nothing. I found a blown fuse and replaced it, turns over but no start. Found no spark and/or poor spark issue.

Intermittent/poor spark, Runs very poorly. Even if I unplug the coils until I turn the key off then it smooths out until it runs out of fuel. With coils plugged in it shuts off like it should of course. I have replaced the plugs.

Ideas?

I was going to flush the fuel tank but seems pointless with it running fine in the off position and it looking like a spark issue.

What all does the little fuel management box up by the valve cover control?

Model: 917289740
Engine family: 9BSXS.8102VS 278786
EU: e11*97/68SA*2004/26*1016*00

RandyT · Aug 7, 2019

The Electronic Fuel Management system consist of the little computer box on the valve cover, a stepper motor, and a thermistor to control the choke. Since this started out with a blown fuel could indicate a short in the EFM box itself, but could also be other issues in the mix also first step is to disconnect the wires to the fuel solenoid, and with the key on see if you have 12 volts when connecting a volt meter between the wires to the fuel solenoid. If 12 volts then that part is good. Next step is when you turn the key on what does the choke do. does it close and stay there, does it close and then reopen, or does nothing.

Let me know what you find, and I will try to lead you down a diagnosis. I am thinking it may be a choke issue related to a problem with the EFM box

oakdale81 · Aug 7, 2019

RANDYT said:
The Electronic Fuel Management system consist of the little computer box on the valve cover, a stepper motor, and a thermistor to control the choke. Since this started out with a blown fuel could indicate a short in the EFM box itself, but could also be other issues in the mix also first step is to disconnect the wires to the fuel solenoid, and with the key on see if you have 12 volts when connecting a volt meter between the wires to the fuel solenoid. If 12 volts then that part is good. Next step is when you turn the key on what does the choke do. does it close and stay there, does it close and then reopen, or does nothing.

Let me know what you find, and I will try to lead you down a diagnosis. I am thinking it may be a choke issue related to a problem with the EFM box

There is 12 volts at the fuel solenoid wires with the key on.

The choke was also on full choke which I already knew with the key on. What I didnt try was pushing it back with my finger with it running, doing that it runs fine with the coil kill wires unplugged. However, with those plugged in it runs very rough all the time.

RandyT · Aug 8, 2019

The wiring harness to the ignition modules has two diodes in the wire. Unplug the wires at the modules and where it connects to the tractor harness and put the red lead of a meter on the tractor end of that wire and the black lead to each of the module kill wires. If you have a setting for diode that is even better, but standard ohm test will also work. continuity between the two diode will cause the modules to cross talk allowing misfires.

If those diodes are shorted in combination with the initial blown fuse I would be concerned with a short and possible battery power reaching the modules which will fry them instantly.

oakdale81 · Aug 8, 2019

RANDYT said:
The wiring harness to the ignition modules has two diodes in the wire. Unplug the wires at the modules and where it connects to the tractor harness and put the red lead of a meter on the tractor end of that wire and the black lead to each of the module kill wires. If you have a setting for diode that is even better, but standard ohm test will also work. continuity between the two diode will cause the modules to cross talk allowing misfires.

If those diodes are shorted in combination with the initial blown fuse I would be concerned with a short and possible battery power reaching the modules which will fry them instantly.

Do you know what color wire I am looking for? Also, what ignition modules are you referring to? I got in late tonight but hope to look at it tomorrow night or this weekend.

Thanks for the help.

RandyT · Aug 9, 2019

The kill wire (normally a black wire) that goes to the coils(ignition modules) contains two diodes. If those diodes short out they can cross talk the modules causing a misfire. If those diodes test good meaning allow flow in one direction only, then the problem is in the mower not the engine.

oakdale81 · Aug 12, 2019

Does not beep but one shows .417 while the other is 1.

RandyT · Aug 13, 2019

The first reading looks OK in the forward direction for that diode, should read 0L or whatever your meter shows for open circuit, and reading of 1 makes me think that you have a bad diode and will need to change the diode harness that goes between the two ignition modules. I would also connect a test light between the kill wire and a good ground and run the key switch through several cycles to make sure we are not getting even a momentary 12 volt power feed to the kill circuit.

oakdale81 · Aug 26, 2019

Sorry, I was on vacation last week a few days. If one diode was bad why would that make spark intermittent to both cylinders? I will pull that harness and check the diodes again. It is easy to remove. Seems to me there is something else going on also?

RandyT · Aug 26, 2019

If one diode fails closed it will allow the spark pulses to interfere with the spark on the other ignition module by causing a ground between the two ignition modules. We call it cross talk between the ignition modules which can cause misfire and no fire issues.