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Old 03-05-2021, 10:07   #1
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Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Hi all,

I have a 1988 Wellcraft St. Tropez 32' cruiser with twin 1988 Mercruiser V8 454's 340HP inboards, Borg Warner transmissions, Thunderbolt IV Bluewater ignition system, distributor (not points).

The starboard engine has perhaps one of the strangest issues I've ever encountered, and I've come across some doozies! Hear me out:

Whenever the starboard engine is running, every 36 seconds exactly, like clockwork, regardless of RPM, in or out of gear, or other conditions, the voltmeter gauge drops to zero and then immediately comes back up to "normal".

And the cycle repeats EVERY 36 seconds.

Every.
36.
Seconds.

Port side does not do this.

Anyone ever encountered anything like this or have any thoughts? This has me scratching my head, and has all of my salty dog and mechanic friends quite puzzled...

Any help or ideas on how to diagnose this would be greatly appreciated!

Thank you in advance!
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Old 06-05-2021, 05:21   #2
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

OK. No promises on this one. Pure speculation.

Voltmeters drop like rocks when either a whopping current goes through the circuit or when their supply of volts is interrupted. In the first case, I'd expect to see a coreresponding ammeter jerk upward. A temporary meter attached to the battery should show that. The mystery would be where and why.

Interruption of the voltage sounds like either the alternator or more likely the voltage regulator.

Either way, it's an electrical problem, not likely to involve the engine itself.

Further speculation will require another cup of coffee.
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Old 06-05-2021, 05:42   #3
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Put a voltmeter on the battery and see if the voltage is changing there. If not, put the meter on the terminal feeding the ignition switch. Then try the "run" output from the ignition switch. If it's normal there as well, open up the instrument panel and put a meter on the wires feeding the voltmeter. That'll help you narrow down if there's something weird going on or if the gauge is failing.

Does the engine hiccup like a momentary loss of spark or anything when the gauge drops? If not, then the issue is likely not an actual voltage drop in an important part of the system (but poking around with the meter will confirm).

If you need a hand figuring out any of the wiring, let me know. I've got the same engines in my boat, just 2 years older. So some of the wiring diagrams (or me physically looking at the harnesses) will translate over to your setup.
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Old 06-05-2021, 11:53   #4
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Quote:
Originally Posted by rslifkin View Post
Does the engine hiccup like a momentary loss of spark or anything when the gauge drops? If not, then the issue is likely not an actual voltage drop in an important part of the system (but poking around with the meter will confirm).

If you need a hand figuring out any of the wiring, let me know. I've got the same engines in my boat, just 2 years older. So some of the wiring diagrams (or me physically looking at the harnesses) will translate over to your setup.
First off, thank you for the response and for the offer to chat more about the wiring!

To answer your question, the engine does *not* "hiccup" every 36 seconds when the gauge does. (Side-note, I think I'm understanding that perhaps I miscategorized this under engines as opposed to electrical. Hmph. Any way to recategorize a live post or do I have to post again?)

Regardless, I will try some of the measurements you suggested to try to isolate the issue, and get back to you with some answers. May be more than a few days until I can get to it.

Since you have the same engines, perhaps you can help answer a different and possibly related question that I just a few moments ago posted here:
https://www.cruisersforum.com/forums...ml#post3402048

Thanks again!
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Old 06-05-2021, 11:55   #5
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Quote:
Originally Posted by tkeithlu View Post
A temporary meter attached to the battery should show that. The mystery would be where and why.

Interruption of the voltage sounds like either the alternator or more likely the voltage regulator.

Either way, it's an electrical problem, not likely to involve the engine itself.
Thank you -- I'll check it out!

(And, yes, I'm realizing this would have been better categorized under electrical systems. I'm wondering if I should re-post?)

Also -- since you mentioned the alternator -- what do you think of my situation that I outlined in this post?
https://www.cruisersforum.com/forums...ml#post3402048

Thanks again!
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Old 07-05-2021, 01:22   #6
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Quote:
Originally Posted by Aquasition View Post
.....
(And, yes, I'm realizing this would have been better categorized under electrical systems. I'm wondering if I should re-post?)

......
You can always ask a moderator to move it. Use the 'report post' icon on the lower LHS to contact a moderator. It's the red triangle thingy.
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Old 07-05-2021, 08:29   #7
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

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Originally Posted by Wotname View Post
You can always ask a moderator to move it. Use the 'report post' icon on the lower LHS to contact a moderator. It's the red triangle thingy.
Thanks - great to know. Just made the request!
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Old 07-05-2021, 09:24   #8
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

36 seconds is a weird time interval (not 30 seconds, not 60 seconds), however something so consistently recurring is usually by design.

Do you have bilge pumps with internal sensors on them? Some of them actually start up for a 1-2 seconds to detect water. If water is detected, they keep running. If none is detected then they stop. A large pump and wiring or connectors with heavy corrosion could cause a voltage drop. Those things live in wet environments.

Check the fridge. I had a boat that when the 12VDC fridge kicked on, my voltage dropped to 10V and my temp guage went up. When the fridge shut down, the voltmeter would go back to normal and the heat gauge would drop.
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Old 07-05-2021, 09:33   #9
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Thanks for the brainstorming! Some answers:

Quote:
Originally Posted by Shrew View Post
36 seconds is a weird time interval (not 30 seconds, not 60 seconds), however something so consistently recurring is usually by design.
Agreed that it is weird.
And while it maybe a result of something that is by design, I think we all agree that it shouldn't be doing what it is doing.

Quote:
Originally Posted by Shrew View Post
Do you have bilge pumps with internal sensors on them?
No.

Quote:
Originally Posted by Shrew View Post
Check the fridge.
Not the fridge. (Compressor doesn't turn on every 36 seconds, AND the problem occurs even if the fridge is entirely off/unpowered.
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Old 07-05-2021, 09:38   #10
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Is the voltmeter mechanical or digital? If it's digital, blame the magic of computers.
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Old 07-05-2021, 09:41   #11
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

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Is the voltmeter mechanical or digital? If it's digital, blame the magic of computers.
Voltmeter is mechanical... needle drops down, then swings back up. (I like the way you think, though... my first thought was to REBOOT!)
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Old 07-05-2021, 09:51   #12
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Quote:
Originally Posted by Aquasition View Post
Thanks for the brainstorming! Some answers:

Agreed that it is weird.
And while it maybe a result of something that is by design, I think we all agree that it shouldn't be doing what it is doing.
Mutually exclusive. I agree the results are undesired. I'm not as concerned about how the issue manifests itself as what is causing. Hence why giving suggestions of things to think about. Commonly, when something behaves so consistently precise it is some other variable in play. Such as the suggestions I was giving.
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Old 07-05-2021, 10:13   #13
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Do both engines charge the same battery bank? (Is there a way to switch so it doesn't?) If so, one alternator is sensing the output of the other alternator so the regulator is kicking out of "charge cycle". Then tries again, then tries again, then tries again, then tries again,
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Old 07-05-2021, 10:16   #14
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Re: Every 36 Second Engine Electrical Mystery 1988 Mercruiser V8 454

Quote:
Originally Posted by Cheechako View Post
Do both engines charge the same battery bank? (Is there a way to switch so it doesn't?) If so, one alternator is sensing the output of the other alternator so the regulator is kicking out of "charge cycle". Then tries again, then tries again, then tries again, then tries again,

That might cause a flicker in the voltmeter, but it shouldn't cause it to drop to 0 and back. Assuming he's got the original alternators, they should be 55A Mando / Motorola internally regulated units. I can confirm that mine don't misbehave when both are charging the house bank (via ACRs).
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