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Dan, Technician
Category: Construction and Road Equipment
Satisfied Customers: 5132
Experience:  17+ yrs. shop and field experience with all major brands of heavy equipment and trucks - 1995 WyoTech graduate.
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Please don’t try to answer with general info. I’m fairly

Customer Question

Please don’t try to answer with general info. I’m fairly sure the problem is somehow connected to a komatsu way of not allowing the started to engage when engine is running. Possibly a diode of unknown location. Expert will need a wiring diagram for my dozer or has it memorized??
And it is a complicated diagram.Have Komatsu dozer D37E5 approx 1995 yr model--having trouble troubleshooting starter issue--starter OK as can activate solenoid manually but not thru switch--strangely enough by grounding the (white /grn) R on batt relay either at batt relay or starter sw it will work-- My main question is looking at wiring dia it would appear the batt sw disconnects ALL from neg side battr--I know this couldn't be as it would require a ground for the batt relay to work. so I'm assuming the batt relay just controls the ground to the starter. I replaced the batt switch but did not solve. Also when turn engine off, the batt relay remains activated and no”click”—batt lite on dash remains on—I can again ground the same wire above and all goes off—batt switch disengages. I need to know detailed info on how the starter, starter safety relay, starter, neutral safety sw, alternator and battery neg disconnect all work together—meaning very detail analysis of wiring diagram.
I'm confused.
Submitted: 6 months ago.
Category: Construction and Road Equipment
Expert:  Dan replied 6 months ago.

Hi Jim, My name is***** will be glad to try to help. I worked for Komatsu, Cat, Volvo, and Deere before going independent. I can assure you that in all my years as a heavy equipment technician that no Dealer technician anywhere has memorized schematics on a machine from 1995 and wiring changes constantly between serial number breaks so anyone helping you will need the diagram to look at for your serial number break. To speed things up do you have your diagram in PDF format and if not what is your serial number so I can see if I have the correct diagram for your serial number?

Customer: replied 6 months ago.
Hello Dan, I was only making a point in the diagram issue. I can send you the diagram. FYI --the dozer ser # is 3133. My machine has the 25 amp alternator. It does not have elect fuel shutoff ( just old fashion throttle to stop engine). I'm almost certain the problem is with the portion of the system that prevents the starter from being engaged while engine is running. One of the things I'm puzzled about is the workings of the main batt disconnect which controls the ground to chassis. There is a 4th terminal that is inner twinned with the system that prevents me from turning off the relay until it is manually temporarily grounded?????
Customer: replied 6 months ago.
FYI the drws have some added comments (by me ) in red. If you open with an iphone, they will not be visable as Iphone does not use ADOBE as a pdf reader and I created those notes with ADOBE ACROBAT. If this is the case I can send another set of files and use a diff method to show my notes.
Expert:  Dan replied 6 months ago.

Thanks for the diagrams. The battery relay on this machine and others should be energized to connect ground at keyswitch on. This then powers the relay and accessories to the the machine cab or dash etc. if open ROPS. That signal is sent to the battery relay in the ignition switch on position.. The R terminal on the alternator excites the alternator for charging when the ignition switch is on. R is also powered at the safety relay and as marked on the battery relay. All should have power and be energized with the switch on. Then in the cranking position C on the IGN switch sends power through the Limit switch if "closed" or engaged to send power to the safety relay S terminal at the starter motor. If so the starter should crank under these conditions existing. Let me know

Customer: replied 6 months ago.
My problem is not that simple. Kamotsu has a method of disabling starter when engine is running. I've investigated enough to know any description would have to include this as a part of system. I'm certain the start system involves:1) batt 2)key sw 3)batt relay 4) starter 5) starter safety relay) ( which is electricaly in front of starter solenoid) 6) neutral safety sw and & 7) altThis is present situation: I have a wire (lets call it x)connected to the R terminal on Batt relay with a wire nut on other end. If I want to run dozer, I turn key on and touch x to a ground. Main relay turns on and starter works. I put my wire nut back on x for later. I can operate machine and everything works great incld alternator. When Im ready to stop engine, I de-throttle --engine dies. then I turn off key--it does nothing ( dash warning lights stay on and batt relay stays engized. I then touch a ground with x and batt relay deengizes--dash warning lites go off --all is well until next time I want to use.The alt is the key to my problem and it would really help me if I knew exactly how the batt relay operates.It has 4 terminals and i'm not sure what is happening inside the relay. the one thing I'm sure of is that grounding wire x at proper time fixes all.
X can't be grounded all time as the "charge caution light" would stay on 100% of time.It would appear B terminal in alt is hot all time--It would also appear that R terminal is B+ when alt is running but is B- when alt is not running??? that is the only thing that makes sense.There must be a diode involved some where.the Alt controls the circuts that disable the starter when alt is running.
The 4th terminal (R) on the batt relay is confusing. The BR terminal on batt relay comes from the key switch--this is what turns on the relay but it wont work without a ground to R. Also wont turn off without a ground to R ???The mystery is exactly how the starter safety system works. R has to be grounded when not running and have a positive voltage when running--otherwise the service meter wouldn't run. If you look at the wiring for the service meter and charge caution lite you will see that the wire from R on alternator provides a lead to both items--one needs a neg and the other needs a pos. which means sometimes R is neg and other times R is pos. The charge caution lite has B+ 100% time and hr meter has B- 100% of time.If all is working correctly I think this is what would be happening:
KEY OFF--R is not neg or pos ( R is not connected to batt at all)
KEY ON --Engine not running --R is grounded ( B-)
KEY ON --Engine running ---R is B+ (24V)
that would make everything work as well as dash lights and and hour meter--not counting the the starter and batt relay would work fine. That is what I'm doing with my jumper wire--I just need to do away with jumper wire-
I could tie X to ground thru a momentary push button and use it this way forever ( been doing it for a couple months)
Customer: replied 6 months ago.
I think I could do some re-wiring and all would be well but would loose the safety that prevents me from engaging starter with engine running. Wouldn't be end of world but would still prefer it to operate as designed. Just have problem understanding this "DESIGN"
Expert:  Dan replied 6 months ago.

Okay. Let me back up. Once the alternator is turning with engine running the R terminal produces voltage to the safety relay to not allow starting. So if you have R terminal voltage, try disconnecting the R terminal wire from the alternator and see if it eliminates the voltage and allows the engine to start. If so, test the R terminal and see if there is voltage there with the R terminal wire disconnected. If there is, you have a bad diode inside the alternator and power backfeeding to the R terminal. Sorry for the confusion. Some of the Komatsu machines have inline diodes in the harness but yours should be internal in the alternator. Disconnect the R terminal and let me know if it starts.

Customer: replied 6 months ago.
My original purpose was to determine 100% if the fix is outside the alternator and hoping not to buy new alt.I do know that disconnecting R will do nothing. R must provide a ground to the effected item for starter to work. Example : Starter relay gets 24v+ from key thru neutral sw—R is connected to other side and is needed to provide the ground to activate starter. Disconnecting R would not help—R is very much needed but it must play dual—no triple role—sometime provides ground, sometimes provides 24v +, and at night it does neither ( not connected to ground and not connected to Pos).R must provide ground to starter safety relay for starter to work ( not just remove pos voltage )
The dual role of R can also be seen by a careful study of the hour meter and charge lite on dash. R is the control for both and R must have 3 personalities ( not just on or off)The starter relay is sort of like the red alt warning lite on dash. The lite has +24 volts applied to it 100% of time. R supplies the other side of lite. If R is neither neg or pos lite is off.—If R is pos , lite is off---if R goes to Neg lite come 3 on.I still think the key to my confusion is the inner workings of the main batt relay. My task will be to look at inside of this devises and understand what is the purpose of each of the 4 terminals. The R terminal is the most confusing knowing it can be one of 3 ways at diff times.I appreciate your help but I assumed from the start that I needed someone who has had lots of experience dealing with this specific Komatsu issue. I was hoping that this person could verbalize ( from memory) how all 7 items work together. I need to find a source that can explain in detail what is happening inside all 7 of the devises involved in starting the dozer (especially the main batt relay which connects the chassis ground)
Expert:  Dan replied 6 months ago.

The diode is inside the alternator. You can save money by testing the alternator diodes yourself after pulling it apart or have an alternator shop do it. You may get away with just replacing the diodes in the alternator.

Customer: replied 6 months ago.
Thanks again but I have a parts book ( see attch) and had already decided a new or rebuilt alt would probably fix the issue. But my original question was not how to repair it. I wanted to know what was happening inside the 7 parts that worked together during the starting procedure. And the battery relay was the most confusing issue. I've decided to "dissect" the old batt relay ( tear it apart) and with a multi-meter find out what makes it tick.
I've searched the web for the info as well as all the Komatsu manuals and could find nothing that would answer my questions. I'll send you my findings and maybe you can use it to help the next guy.
Customer: replied 6 months ago.
alt parts --attch
Expert:  Dan replied 5 months ago.

Hello Jim, I wanted to check in and see where you were with this? the rectifier in that alternator serve as the function of the diodes. Let me know

Customer: replied 5 months ago.
The pic in parts book shows what I'm assuming is probably a normal a full wave rectifier which has either 3 or 6 diodes that couldn't possibly do double duty since an alternator is in realty a 3 phase AC generator using the rectifier to convert to DC.
I would also assume this special safety function is not in the normal rectifier assy. Just a guess ?? I would also bet if I took it to an alternator shop they would test it and tell me the alternator is working fine--which is a correct statement. I'm not even sure the starter safety function is in the alternator. One fellow told me his was under the seat???The weather has been terrible so I haven't worked on dozer. Specially since it is fully functioning other than my having to temporarily ground the main batt relay to make it turn on or off.But I will certainly let you know when I have uncovered the secret !!
Expert:  Dan replied 5 months ago.

thanks for the update. Click on this link... Im showing a fusable link and diode in harness #6 here but doesnt designate what circuit it is in.!0!2!1269!65!77

The safety relay location is number 37 below!0!2!1269!65!78

Expert:  Dan replied 5 months ago.

The relays/components he might have been referring to are C and D here!0!2!1269!65!69

Customer: replied 5 months ago.
The diagram you sent is not my machine- see attch-- my dozer is "without" stop motor ( doesn't not have D)--the one you looked at is with a stop motor. C (which I have ) is the neutral safety switch, a simple sw like a light sw ( only on or off)- and it is working fine.
I know you are really trying to help but we need to go back to my original question. I didn't ask how to fix it. I asked for a detailed narrative of how each of the items involved in starting system worked.I could then solve the problem. I knew from the start that whoever answered the question would not only know the wiring diagram but would have spent many hours working on this specific komatsu issue. As you can see the answer is not in the wiring diagrams but knowing what is going on inside the various elect components. I would venture only a handful of Komatsu techs could verbalize the entire workings of their starting system on this machine.
I still plan to share with you my findings. But just shooting from the hip won't answer my original question. The complicated part of the system is the circuity that prevents the starter from being engaged if engine is running.
when I'm done I will know what is happening deep inside every component involved in starting this machine ( that was and is my question. I feel I'm wasting your time .

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