Yanmar 3JH5GE Wacky Electrical Problem

[SeamanStaines]

I have a pair of Yanmar 3JH3GE engines, they are both brand new and I sorting out getting the electrical loom through (dont even start me on why Yanmar decided to have exactly the same cable but changed the plugs!). Whilst doing this I noticed a spark when I disconnected the engine starter battery which I diagnosed to a faulty blocking diode. I replaced the diode with a VSR and decided to also replace the port side diodes at the same time. The Starboard engine does not exhibit any problem.

Port engine was running perfectly before this. I removed the diode and connected the Port battery positive and the alternator output to the input side of the VSR. As soon as I did this (ie connected the battery and the alternator output together) the fuel pump started on the engine. I removed the engine control panel in case it was an issue there which made no difference. Put the blocking diode back and its fine.

Now I can kind of see this. With the blocking diodes in place the output from the alternator can go to the batteries but power from the batteris cannot get to the alternator. With the VSR then it can (which is exactly the same situation as with a single battery).

I cant see what is going on here, obviously I will get the engineer back who fitted it to look, but I suspect he will just tell me I have to leave the blocng diodes in place, whch is clearly not an answer, its just fixing the symptom.

Any help to understand this would be very useful

Many thanks all

 

[VicS]

Look at the wiring diagram

#44 is the power feed to the fuel pump. Power can come to it two ways. From the panel via the red/ black (RB) wire in the harness or via the RB wire that terminates near the alternator (#40)

Power presumably should only come from the panel after pressing the power button. If you have removed the panel it cannot come from there.

Therefore it must be coming from the RB wire that terminates near the alternator. Take a look at this. Has it been connected to the alternator B terminal, or anything else in the vicinity ?

 

[SeamanStaines]

Absolutely right, many thanks

The wire was connected on one engine and not the other. I wonder why its there though, I cant think why you would ever want it connected?

David