I am currently troubleshooting a start problem on a komatsu pc600 lc. The intitial problem was the main power relay was burnt up. After replacing, I now have power to the cab. Repaired several other problems to get signal voltage to the starter relay mounted on the starter. I have had the starter out of the machine several times and hooked to a bench test. It works fine out of the machine sending battery voltage to the signal wire but when installed the switch will not close to send power to the solenoid. After hours of in and out I realized that the only difference in and out of the machine is battery voltage to the other wire that runs to the start relay. The second wire that runs to the switch is tied in to the "r" terminal on the back of the alternator which should not have 24vdc with the key switch in the on position? I thought maybe there was a short in the alternator but with the wire removed from the "r" terminal I still have 24vdc on that wire. I am getting battery voltage through the harness to the second wire (not the signal wire) as soon as the machine is keyed on. This is the only difference between in and out of the machine. That wire is tied to the following; battery relay, alternator "r" terminal, lamp switch, starter cut-out relay, light relay, swing lock switch, and a couple different dead head plugs. Probably missing something...... any help is greatly appreciated. I need to know why I have voltage on the second wire to the starter relay or if anyone has run into something similar. This is my first time using one of these forums. I apologize for the length but am getting desparate.
update to the above post....I removed that second wire from the circuit to verify that it would start in the machine and it does. With the machine running, I verified that the swing lock, hydraulic lock out, light switches, and other accessories were still operating correctly and they all seem to be. Could that voltage be a safety to keep the machine from starting? I do not want to send it with a safety bypassed. I am kind of hoping that running it like that will burn up a fuse so I at least have somewhere to start. Anyone have any suggestions?
:Banghead
One of the dead plug you're talking about should be actually a diode.It is a diode build in a DT(deutsch) female connector. When the alternator is not running, the ''R'' terminal is grounded, so the starter solenoid will engage.Once the engine is started (alternator running), the ''R'' terminal output 24vdc, the solenoid cannot engage anymore, it prevents accidental starting while the engine is running. Here comes the interesting part, there's a wire that goes from the ''R'' terminal to the ''+'' signal of the main power relay as a back-up.There is a diode somewhere in this circuit that prevents current going fron the ''+'' main relay signal to the ''R'' terminal when the key is turned on.If the diode is burnt, current will go the ground side of the starter solenoid and the starter will not engage. Hope this help
I have the same issue on a slightly small machine, a PC75UU2. I have a switch wired around this to allow starting when the diode acts up. I have no Idea how to find diode in harness or even access the alternator.
The diode is inside the alternator on the bigger machines. The small machines that had the Cummins engines had an external diode and a different wiring to prevent the starter from engaging when the engine was running. If you take the alternator to a rebuilder you will have to tell them about that R wire and the starter cut out function or they will tell you there is nothing wrong.