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Discussion Starter · #1 · (Edited)
1989 VN750 here. Back at the old problem after a year of not riding it much. My friends and I have already replaced the stator, R/R, and the ignition switch, and I think we can rule out either of these items as the cause. We also did the headlight relay mod (Blue wire bypass - headlight relay mod). The problem of parasitic battery drain was never fixed and the battery still keeps dying after a couple hours of riding.
My question is this: I am observing a voltage drop from >14V to about 12.4V when I turn on the ignition, while plugged into the trickle charger and with the engine off and the head and tail lights disconnected. The voltage does not go up when the engine runs and always stays in the low 12V. This is clearly not expected, but I am curious what the expected voltage drop is from the ignition alone, without any lights.
I am looking for the source of the drop. I disconneced the R/R and the stator and did not notice a change in the drop when the engine is off. The cause is definitely something on the main circuit. No drop with the main fuse removed (but this circuit just powers the ignition switch and removing the fuse amounts to turning it off, so no new insights from this).
I would like to start disconnecting the most likely consumers one by one to identify the culprit.
We noticed that the battery would charge when the engine is running and the power is disonnected from the ignition switch. This will disable the headlight and the instruments, but blinker and tail light work and the charging voltage goes up to where it should be, in the >14V range.
Is there a procedure for eliminating possible causes? There is a manual somewhere, but I don't have it here at the moment and don't remember seeing this for general diagnostic steps.
One other thing to be looked at is that the fan comes on immediatly I press the start button, even when the engine is cold, and when it does not start right away the fan will keep running as long as the ignition is on. I understand that the fan relay is inside the junction box and cannot be replaced individually. Should I just replace that damn thing as well before we look any further?
I had the tank off, headlight open, and checked the wiring, removed all connections and made sure nothing is loose, and I am running out of idea.
 

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One huge question: "when the engine is running and the power is disonnected from the ignition switch" WHAAAT? laugh How did you go about removing power from the ignition and still have the bike running? Pulling the white wire at the ignition box should behave like hitting the kill switch. It also powers the headlight. It would NOT remove power from the instruments (which are driven by the white/black)

During troubleshooting, leave your R/R disconnected just to make sure it isn't providing a path for current, even if it is 100% working.
Pull the headlight fuse out, which should remove power to your instrument cluster and tail light.
Pull the tail light fuse (which, after the blue-wire mod, will kill your headlight) just to see. Sounds like you did this already.
Try flipping the ignition to 'park' and see if that still has the drain or not. This will cut out the dash cluster and the head & tail lights.

I don't think your fan is the related to your ignition voltage drop issue, since it isn't connected to the ignition in any way. Still, best to unplug when troubleshooting, since it is known to be acting weird. It also might be a major player in your overall lack of charging, since the fan doesn't normally run when you're on the highway since there's already good air flow. If it's turning on when the engine isn't hot, it's probably also running the entire time during a long ride.
 

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Discussion Starter · #3 ·
One huge question: "when the engine is running and the power is disonnected from the ignition switch" WHAAAT? laugh How did you go about removing power from the ignition and still have the bike running? ...
There are 2 wires that run under the tank to the ignition switch. There are 2 connectors right next to where all the molexes are, but they are just wire connectors. They are separate from the main harness but appear to be original. We had a bad connection on one one of them, Obviously it did not disconnect the solenoid, but the headlight and instruments were out. Blinkers and tail light were working. When I disconnect one of these wires while the engine runs, the charging voltage returns to the normal range and the battery charges. In this state, the engine does not start and turning the ignition key does nothing.
I will try your suggestions on the weekend - thanks for taking the time.
 

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Ah, that was probably the brown/white wire feeding "the back half" of the JB from the ignition. That means you'll probably see the drain disappear when you try 'park'.

I'll be interested to see which fuse has the draw on it. If it is the tail light fuse you could try bypassing the RLU. If it's the headlight fuse, it's something in the instrument cluster.
 
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