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Voltage drops as RPMs increase |
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uncle-buck 
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Joined: June-14-2004 Location: United States Status: Offline Points: 337 |
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 Topic: Voltage drops as RPMs increasePosted: April-02-2025 at 11:08am |
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Took my boat out on Sunday, for the first time this season. Noticed that voltage shown on the dashboard volt meter decreases as engine RPMs increase. Haven't seen that before.
Have done the following troubleshooting steps so far: Checked the alternator belt: it's tight Checked voltage output at orange alternator wire: 13.9V to 14.0V Checked voltage at battery with engine running: 13.9V to 14.0V Checked and cleaned ground wire connection on the alternator. Cleaned and cleaned ground wire connection on the top of intake manifold.
What other troubleshooting steps I should take? Thanks |
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1990 Ski Nautique (original owner)
PCM 351W with D.U.I. |
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GottaSki
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Joined: April-21-2005 Location: NE CT Status: Offline Points: 3372 |
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Posted: April-02-2025 at 12:35pm |
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I suspect you could be seeing unloading of the ignition wire (purple) as rpms increase.
yes, more sparks at higher rpm, but maybe that also means less time with the coil drawing current energizing the field, because it sparks when the field collapses, not energizing when your purple wire gets ohmic due to age, the quick disconnects, connections, keyswitch, breaker/fuse, etc, build series resistances that can add up, and odd things happen. Lots of components are pulling current through that one, undersized wire. Sometimes If you look how the factory daisychaned off the buss bar to eventually get the the switch and beyond, usually with undersized wire, you may be aghast
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole |
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uncle-buck
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Joined: June-14-2004 Location: United States Status: Offline Points: 337 |
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Posted: April-02-2025 at 12:44pm |
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So, is a possible solution the replacement of that purple wire with a heavier one?
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1990 Ski Nautique (original owner)
PCM 351W with D.U.I. |
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GottaSki
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Joined: April-21-2005 Location: NE CT Status: Offline Points: 3372 |
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Posted: April-02-2025 at 1:13pm |
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somewhat, never hurts. but every connection prior is even more suspect as well . to affect your gauge, extra resistance has to also exist prior to where the voltmeter and purple to engine diverge
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"There is nothing, absolutely nothing, half so much worthwhile as messing around with boats...simply messing."
River Rat to Mole |
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KENO
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Joined: June-06-2004 Location: United States Status: Offline Points: 11266 |
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Posted: April-02-2025 at 6:54pm |
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I'd probably take your meter and hook it to the back of the dash voltmeter and see what it reads...
when the engine is idling_________________ when it's at higher speed_______________ compare those numbers to the dash gauge when the engine is idling.................... when it's at higher speed................... You mention 13-9 to 14 volts at the alternator and at the battery, is that both idling and at speed ? There could be more questions, but if you answer these it's at least a start Meanwhile I have to try to figure out this ohmic thing. mentioned earlier It must mean that a perfectly good wire and connections is called something else like non ohmic or something if it turns ohmic with age  |
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uncle-buck
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Joined: June-14-2004 Location: United States Status: Offline Points: 337 |
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Posted: April-02-2025 at 11:27pm |
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Will check soon and reply.
Thx! |
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1990 Ski Nautique (original owner)
PCM 351W with D.U.I. |
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uncle-buck
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Joined: June-14-2004 Location: United States Status: Offline Points: 337 |
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Posted: April-03-2025 at 11:42am |
Ken, I *think* the mystery has been solved.
Am getting good voltage numbers at the back of the voltmeter, alternator, and on the battery at both idle and 2,000 rpms. But the voltmeter failed during the diagnostic process. Have ordered a replacement and will report back after installation. Thanks |
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1990 Ski Nautique (original owner)
PCM 351W with D.U.I. |
|
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|
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uncle-buck
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Joined: June-14-2004 Location: United States Status: Offline Points: 337 |
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Posted: April-04-2025 at 12:21pm |
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Installed a new Faria Beede voltmeter. It reads about one-quarter volt lower than my multimeter, which is close enough.
Am still getting some voltage drop at higher RPMs, but much less than previously. I assume there's a fair amount of resistance in the daisy chain of 35 yr. old wires in the dash pod. Am going to enjoy the boat for now, and consider rewiring the dash pod next winter. Thanks for your helpful replies.
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1990 Ski Nautique (original owner)
PCM 351W with D.U.I. |
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KENO wrote: