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catmastertech, Technician
Category: Construction and Road Equipment
Satisfied Customers: 2334
Experience:  Field service technician for CAT 19 years.
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I have a Cat 939c I've been having problems with it

Customer Question

I have a Cat 939c I've been having problems with it starting and sometime turning off. I bought a new battery, replaced the ignition. The first time I cranked it, it almost started I forgot to prime it and it had been sitting for 2 or 3 weeks. The next
time I tried to start it I just got a clicking noise coming from behind the dash panel the sound was like the battery needed to be jumped and its brand new and fully charged. The 3rd time it acted if it was dead gauges wouldn't even come on. I disconnected
battery and reconnected it 15 minutes later, the gauges lite up but all I heard was the clicking noise all is good with the alternator and the starter is fine. This has been getting worse, its been off and on the last 3 or 4 times I used the machine. Its only
used at the Ranch on weekends. Thanks for your help
Submitted: 1 year ago.
Category: Construction and Road Equipment
Expert:  catmastertech replied 1 year ago.

Check for power from green wire at key switch when in cranking position. This wire will go to neutral "safety lever" switch in the left console. With the safety lever down, the power will leave the neutral/brake switch and run back to the start relay in the dash on the pink wire.
This closes the neutral relay and then power will travel from the white wire of the start relay to the starting motor solenoid.
If power is going the the white wire on the starter motor solenoid, the start maybe the problem.
Verify you have a good battery cable for both positive and ground then have the starter tested.
If you need further help, please post the full machine serial number with your reply.

Customer: replied 1 year ago.
6DS00922 Serial number, the starter hasn't been the issue, how could the starter impact the gauges not working? Could it be the solenoid in the dash. That is where the clicking noise is coming from. Where is the starter motor solenoid?
I replaced the ignition, something is draining the battery. Cables and connection are good and new.
Could the reset button underneath the dash be a factor?
Thanks for your help
Expert:  catmastertech replied 1 year ago.

The circuit breaker under the dash is for the alternator. If it is tripped out, the alternator will not charge.

Have you checked the main relay power in and out?

This is normally the large relay and the start is smaller. The 6 gage white wire is on the start relay.

The main relay will have red and/or purple wires.

I will verify this on the schematic and post back shortly.

Expert:  catmastertech replied 1 year ago.

The gauges receive power on a White wire marked 123. This is directly from the fuse panel.

Is the fuse good?

Does the fuse panel have power when the key is turned on?

Expert:  catmastertech replied 1 year ago.

The fuse panel receives its power from the main relay, this is next to the start relay.

Battery power to the main relay is on the red wire and goes to the fuse panel on the purple wires.

The main relay is controlled by the key switch on the Yellow wire marked 308.

Verify the yellow wire is powered when the key switch is turned on.

Expert:  catmastertech replied 1 year ago.

I need to correct my previous comment about the breaker. It is not for the alternator, it is the main power breaker for the machine. I was thinking of earlier models.

If this breaker fails, you will not have power to either fuse panel nor the start relay.

Also you should have a glow plug relay between the main relay and the start relay.

Customer: replied 1 year ago.
what do I need to replace, the fuses are fine, it's losing power after the first time a crank it, what about the solenoid or whatever that clinking noise is. I can replace relay switches if need be. It gets power then loses it quickly.
I am not a mechanic thanks again, what would you do first?
Expert:  catmastertech replied 1 year ago.

You should not need to replace anything with this type of problem. It is most likely a loose connection.

I would test the batteries firsts. A load test will show if one is weak. It is possible to have a weak battery and still see twelve volts on a meter.

Then check all the battery cable connections. Look for loose or corroded ends. A bad connection will have voltage but, it will not carry the load for cranking.

That is why you can try the key on and have power but, as soon as you try to start, everything goes dead.

If the cables look swollen, it could be damaged internally. Any cable that has damage to the insulation or connectors can allow corrosion to build up internally and the cable cannot carry a cranking load.

If you have automotive repair ends on the cables, these often cause poor connections and cranking problems. I replace these cable with factory cables when I find them.

If your cable and batteries are good, check in the dash for connections at the breaker and main relay. Focus on the large red and purple wires first, then look for other loose or corroded connections.

I will be away later today but, can help you more this evening. Just let me know what you find and I will be glad to help you with this problem.

Customer: replied 1 year ago.
Thanks, ***** ***** get back after I go back out there. Battery, and terminals are new, cables replaced 6 months ago by a real mechanic. It has to be coming from something behind the dash.
I'll get back to you, thanks I have other issues with equipment also
Expert:  catmastertech replied 1 year ago.

Ok thanks, ***** ***** posted on your progress.

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