1997 Komatsu PC75UU-2E intermittent electrical stating problem help?

[mike69440]

I really could use some help.
The problem is this intermittent electrical problem hat I could never figure.

I own a 1997 Komatsu PC75UU-2E Excavator S.N 13095, that for several years has exhibited an intermittent stating problem.
When it starts it fires right after a ½ seconds of cranking,
This is a farm machine that otherwise would be dependable enough to work commercial. I am considering doing this as I am an job seeking mechanical engineer.

The machine last did 40 hours without this problem.

The problem is when you switch the ignition off the engine kills, but the accessories are still on and he starter will not reengage if switched to start. Panel lights stay on with key in the off position. Remove the key and all panel lights are still on.
The fan, lights, wiper, horn etc run with the key in off position. I disconnect the batteries and when I do, the relay by the battery clicks. If I reconnect the battery the relay clicks again.

What does this relay do? is it the main power relay?

Switching the starter to start, I hear a faint click coming from the starter (I think), but the starter does not crank. I doubt it is a dead spot in the starter as tapping the starter does nothing.

So I disconnect the battery, wait a few hours to a day, and when I try a restart, it to date will restart fine. It will then start normally until the next time it decides to repeat the above.

I can’t afford to have the machine serviced, so I want to correct this problem myself.

I have attached pictures. I have the operator and parts manuals.

I would be grateful for any help.

Regards,

Mike

 

[Directional]

I've got the same machine, as well as all the manuals. Never had those problems myself, and my Shop Manual only calls it a battery relay.

I did search for that relay and found a few places that say they have it. Might be worth the few dollars to change it and see if it helps.

 

[mike69440]

Starting problem understood. Now how much to fix??

I am 95% certain I now understand the problem. The Battery rely is fine. My starting issue appears to be a result of a safety circuit that prevents the starter from being engaged when the motor is running. This is to prevent starter motor and flywheel damage in the event some idiot tries to start the motor while it is already running. There is a diode in the alternator that prevents the starter relay from energizing when the alternator is making charge, i.e. motor is running. When the diode goes bad, the starter relay is deactivated and the battery relay is kept from energized.

I am talking with the dealer about a service visit. I need to find out first, if this requires an alternator replacement and if it can be done in one visit.

In the mean time I have rigged a switch that will allow the motor to start and then once the motor is running I switch back to normal so the alternator charges and all the accessories are powered.

 

[joelmartin]

curious how much an ignition switch is. if 60 bucks I would replace it for entertainment!
Or study the wire colors and diagram, see if those wires that power accessories stay live when you turn key off.

 

[John C.]

You are probably correct on the start circuit being part of your problem. However Komatsu changed the circuit in the newer machines and instead has a diode under the seat that provides start circuit protection in many of the smaller machines.

Look at you alternator to see how many wires there are. The older machines will have three wires, the big battery terminal, a small black wire for ground and another wire for that works the safety relay on the starter. If you have only the battery and ground wires you will have to look for the diode. Last machine I worked on was a couple of years ago so I don't remember exactly where it was but I do recall having to pull the seat to find it.

The issue with the battery relay not dropping out is probably something different. Have you installed another electrical component? Radios and heaters can be a big problem. Have you recently had a component failure like a wiring harness or maybe a fire? The reason I ask is that your relay works on the ground side of the circuit. When you turn the key on it only provides a ground that lets current flow through the coil. A short circuit in any component or the wiring harness will keep the battery relay engaged. You might try pulling fuses and keeping track of what they supply. When the relay drops you might have found your problem.

Good Luck!

 

[bigbob]

Looks to be a gray market machine. Before having the dealer show up, make sure they have an alternator on the truck. They are not to hard to change and you might get a better price through outside sources. Cat wanted $1900 for a starter on one of my excavators and I found a brand new one on line for $500.

 

[Kobe130]

To avoid problems like this on my 1998 Kobe sk130 I removed the original battery relay which was bad and pricey to replace and put a manual HD battery disconnect switch in its place to make sure all power can be cut when I leave the machine. Following that I'm removing the safety relay tossing it and putting a std 24 volt starter relay $30 in to power up the starter. Nuts to the safety relay and the alternator controlling it. Switch out the original ignition switch with a $35 Cole Hersee lever operated and you're in business. There is no good reason to pay $200 or more for these electrical items from OEMs like Kobelco or Komatsu. The simpler you can make these circuits the fewer problems you will have in future with the machine. What are the chances that I'm going to hit the starter when that little 4BT is already running to grind the flywheel gear?

 

[Kobe130]

I should answer my own question and say pretty slim. I would also chase back the now redundant wiring and take it out of the wiring harness for safety. Like the one that used to go from the starter relay to the alternator, etc. Now the "voltage sensing safety relay" will never, ever cause you a problem again. Also instead of using some of the OEM relays for other functions like lights, heater etc you can easily use 24 volt relays (or whatever voltage your machine runs) from Omron as replacements and they cost about $20 a piece.