Komatsu PC50UU-1 Sn1825 - No electrical power

[JayBee59]

Hello all,
I have taken delivery of a reconditioned Komatsu PC50UU-1 Sn1825 (see the one I purchased here

)
I have only driven the thing for 12-15 hours since delivery a few weeks ago.
On 04-02-23, I shut the unit off as normal. The next morning, I initiated the glow plug procedure, but no response from the key switch in any direction. I checked the fuses. All good. I tried jumping the battery with my F250,. The jumper leads arced as if I had accidentally reversed the polarity. I then used my Dewalt 30AMP Automotive Portable Car Battery Charger with 80amp engine start and alternator check (Home Depot). The unit's processor couldn't display anything. I then disconnected the battery and reconnected the Dewalt and it began charging the battery without any problem. The battery showed only 9.2V and the Dewalt went into "recondition" mode immediately.
I then removed the control/starter panel in the cab. All connections were intact with no apparent issues. In the engine bay, the battery is well grounded to the motor mount and is a clean metal to metal contact.
This excavator has run perfectly for 12-15 hours with no hesitations or power issues. The machine is slow to start in +/- 35F temps (using the glow plug procedure).
I have very limited electro mechanical knowledge. Might anyone explain the "reverse polarity" effect I experienced when attempting a standard jump with HD cables? Did I lose a ground further up the chain of contacts? I wish I would have paid more attention in "electrical for dummy's" class!
Thanks in advance for any help you may lend!

 

[Simon C]

Not sure but most of the Komatsu's I worked on had a large relay in the battery compartment. There should be some decent sized cables hooked up to it to. Need to see if there is power to the relay , fuse blown, or what ever else. Maybe someone can get you a schematic.
Simon C

 

[John C.]

I don't have a book for a 50 but do have one for a 45. I'll try to post a diagram for that. The system is pretty simple. You likely have a problem with the battery disconnect or the wiring from the disconnect to and from the key switch.
View attachment 1680647968695.png

 

[JayBee59]

Hi,
There is no relay visible and the battery is plainly accessible on the starboard side of the engine compartment. I have resized the original 5"x8" to 8.5"x11" and attached the electrical diagram covering serials 1001-3400 (SN1825 is mine).
I will be attempting a direct jump from my Ford F250 (completely bypassing the disconnected PC50UU's battery) to the battery cables of the PC50. If is get a reverse polarity spark upon contacting the final connection of the positive from the Ford cable to the corresponding batt cable of the machine, the result should mean that the short is on the machine. If there is no short, the key in the PC should function and it should crank. This should mean that the battery has malfunctioned. I am a caveman when dealing with electrical, but a tech has assured me that as long as I do not ignore the reverse polarity sparks for the quick contact, there should be no problem. Fingers crossed.
Thanks for your responses!

 

[John C.]

In looking at your wiring diagram, I have to question if the machine is actually a Komatsu. If you machine conforms to that wiring diagram, you have no disconnect from the battery when the key is turned off. The alternator has diodes that can fail and allow current to flow to ground continuously.

A spark happening when you touch the jumper cable to the battery cable only indicates current flow in the system and not a reversed polarity. You can check this by installing a test light between the jumper cable and the battery cable lug. If the light comes on, there is current flowing indicating a short. You can disconnect wires at the key or on the alternator to see if the test light goes out.

 

[JayBee59]

Hi john. Thank you for your response. This is a komatsu. Also, when attempting to hook the positive, it was not a spark but an actual welding attempt by the lead to the post. Something in the machine has shorted. I brought another 12 volt battery and when touching the positive cable to the battery another weld attempt occurs. Thank you for any help you may lend.

 

[John C.]

You have a big cable worn through and touching the frame somewhere or something like the alternator has a massive short.

 

[JayBee59]

Diagnosed - massive starter failure with scorch marks and all! the techs believe the short also damaged the battery beyond recovery. Any suggestions for a solid battery replacement?
Thanks for your help!

 

[Vetech63]

Any suggestions for a solid battery replacement

Don't buy a battery from an auto parts store.