I've searched diligently in the forums for this problem, and found no entries. So here goes. Symptoms: (1) Fusible link between battery and ignition switch (Kubota 67111-55190, 20 AWG, ~6 amp) heats rapidly (less than 3 sec) and blows during preheating or cranking. (2) Insufficient voltage (only 6.8v) available at starter motor solenoid activation terminal during cranking (battery shows 12.7v), so solenoid clicks but will not activate starter motor). (3) Glow plug indicator heats to bright yellow (not red) in under 3 sec regardless of ambient conditions -- if held any longer, it would melt. (4) Several crimp connectors in the glow plug indicator wiring and the starter motor wiring show signs of high heat (melted insulation, stiff wires, blackening, etc.). (5) Very hard to start, as if glow plugs aren't working.
So, referring to the wiring diagram in my Owner's Manual, I've made the following discoveries. (1) When the starter switch is rotated CCW (preheat position), both the glow plug indicator and the glow plugs themselves are activated. (2) When the starter switch is rotated CW (start position), the glow plugs are still activated but the glow plug indicator is not. So whether preheating or cranking, the glow plugs are activated -- they are inactive only when the starter switch is in the center position. (3) The glow plugs are wired in parallel (from one to the next to the next to the next by a single wire feed wire, and then are all grounded to the head by their base. At 7 ohms apiece, their total resistance together should be about 2.2 ohms.
My troubleshooting process has been to open the dashboard (a real pain on the L355S) and remove the starter switch, then one by one reconnect circuits to it, checking all wiring harnesses for open or shorted wires first. I've also pulled the starter motor, polished the motor commutator and brushes, pulled the solenoid and smoothed/polished the solenoid contacts, and cleaned and re-greased the gearbox (at the service bench the starter motor unit, including solenoid, performs to spec). Disconnecting the wire from the starter switch to the first glow plug, the remaining glow plugs together measure about 2 ohms with a digital VOM -- right on target. Together they should draw about 6 amps, which matches the fusible link capacity. The starter switch unit was pulled and put on the test bench to see if any of the connections were damaged or introduced resistance that would cause the voltage drop to 6.8v. It performed to spec. So I started reconnecting things, first the wire to the starter motor solenoid, leaving the glow plugs and glow plug indicator disconnected. The starter motor performed correctly, spinning the motor at a rapid speed against normal compression. The fusible link did not get hot to the touch after several seconds of cranking. Conclusion: The problem must be in the glow plug circuit side of things. I reconnected the glow plug wire to the first glow plug. When I connected only the glow plugs to the preheat connection (NOT final wiring, just for testing, leaving the glow plug indicator out of the circuit), and turned the starter switch to the preheat side, the fusible link got warm after several seconds, but not too hot to touch. An inductive ammeter showed an initial surge current of 6 amps when the switch was initially turned to preheat, as expected. So finally I put all of the glow plug circuit wires back where they belonged on the starter switch. And the problem reappeared. I pulled the glow plug indicator out of the dashboard panel and discovered that the hot-side terminal was rusty and blackened (signs of high heat) with the lead wire stiff and insulation melted. But the other terminal was nice and shiny, no signs of heat, and the lead wire was fine.
My conclusion is that the glow plug indicator is the culprit, although I haven't verified that by replacing it yet (had to order one, won't be here for a week). My thinking is that the (probably factory original) glow plug indicator's resistance changed with age and heating. The cruddy nature of that one terminal would have been highly resistive when cold, causing about half of the battery's 12v to be consumed in the indicator itself, which in turn made the indicator's coil heat to yellow-hot within a couple of seconds and somehow causing insufficient voltage to appear at the solenoid activation terminal at the starter motor. This would also cause most of the voltage to be applied to the indicator and very little to the glow plugs during preheating, essentially making the preheating position of the switch a "heat the indicator a lot but the glow plugs very little" position. Then when turning the switch to the starting position, the glow plugs are initially cold and to get them warm enough to do any good you'd have to crank for so long that the fusible link gets overheated.
So in the end, I'm out my one-and-only tractor for a week. I'll replace all wires with signs of heat stress at connection terminals, and I'll get a fistfull of new little rubber connection caps. And when it gets here, I'll install the new glow plug indicator, and update this post with my findings.
I'd sure be interested to know if anyone else out there has experienced this issue (primary symptoms hard starting, low starter solenoid voltage and a glow plug indicator that gets way too hot way too fast), or sees other ways that these symptoms could be caused. Since the glow plugs resistance "in bank" was right, I've put off replacing them, especially since replacing the front one requires dismantling the fuel injection system and intake manifold. We'll try replacing the glow plug indicator first and see what happens...
So, referring to the wiring diagram in my Owner's Manual, I've made the following discoveries. (1) When the starter switch is rotated CCW (preheat position), both the glow plug indicator and the glow plugs themselves are activated. (2) When the starter switch is rotated CW (start position), the glow plugs are still activated but the glow plug indicator is not. So whether preheating or cranking, the glow plugs are activated -- they are inactive only when the starter switch is in the center position. (3) The glow plugs are wired in parallel (from one to the next to the next to the next by a single wire feed wire, and then are all grounded to the head by their base. At 7 ohms apiece, their total resistance together should be about 2.2 ohms.
My troubleshooting process has been to open the dashboard (a real pain on the L355S) and remove the starter switch, then one by one reconnect circuits to it, checking all wiring harnesses for open or shorted wires first. I've also pulled the starter motor, polished the motor commutator and brushes, pulled the solenoid and smoothed/polished the solenoid contacts, and cleaned and re-greased the gearbox (at the service bench the starter motor unit, including solenoid, performs to spec). Disconnecting the wire from the starter switch to the first glow plug, the remaining glow plugs together measure about 2 ohms with a digital VOM -- right on target. Together they should draw about 6 amps, which matches the fusible link capacity. The starter switch unit was pulled and put on the test bench to see if any of the connections were damaged or introduced resistance that would cause the voltage drop to 6.8v. It performed to spec. So I started reconnecting things, first the wire to the starter motor solenoid, leaving the glow plugs and glow plug indicator disconnected. The starter motor performed correctly, spinning the motor at a rapid speed against normal compression. The fusible link did not get hot to the touch after several seconds of cranking. Conclusion: The problem must be in the glow plug circuit side of things. I reconnected the glow plug wire to the first glow plug. When I connected only the glow plugs to the preheat connection (NOT final wiring, just for testing, leaving the glow plug indicator out of the circuit), and turned the starter switch to the preheat side, the fusible link got warm after several seconds, but not too hot to touch. An inductive ammeter showed an initial surge current of 6 amps when the switch was initially turned to preheat, as expected. So finally I put all of the glow plug circuit wires back where they belonged on the starter switch. And the problem reappeared. I pulled the glow plug indicator out of the dashboard panel and discovered that the hot-side terminal was rusty and blackened (signs of high heat) with the lead wire stiff and insulation melted. But the other terminal was nice and shiny, no signs of heat, and the lead wire was fine.
My conclusion is that the glow plug indicator is the culprit, although I haven't verified that by replacing it yet (had to order one, won't be here for a week). My thinking is that the (probably factory original) glow plug indicator's resistance changed with age and heating. The cruddy nature of that one terminal would have been highly resistive when cold, causing about half of the battery's 12v to be consumed in the indicator itself, which in turn made the indicator's coil heat to yellow-hot within a couple of seconds and somehow causing insufficient voltage to appear at the solenoid activation terminal at the starter motor. This would also cause most of the voltage to be applied to the indicator and very little to the glow plugs during preheating, essentially making the preheating position of the switch a "heat the indicator a lot but the glow plugs very little" position. Then when turning the switch to the starting position, the glow plugs are initially cold and to get them warm enough to do any good you'd have to crank for so long that the fusible link gets overheated.
So in the end, I'm out my one-and-only tractor for a week. I'll replace all wires with signs of heat stress at connection terminals, and I'll get a fistfull of new little rubber connection caps. And when it gets here, I'll install the new glow plug indicator, and update this post with my findings.
I'd sure be interested to know if anyone else out there has experienced this issue (primary symptoms hard starting, low starter solenoid voltage and a glow plug indicator that gets way too hot way too fast), or sees other ways that these symptoms could be caused. Since the glow plugs resistance "in bank" was right, I've put off replacing them, especially since replacing the front one requires dismantling the fuel injection system and intake manifold. We'll try replacing the glow plug indicator first and see what happens...
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