Mercruiser Bravo I Voltage Problem - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-06-2006, 06:59 AM
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Mercruiser Bravo I Voltage Problem

I know this is a hotrodder site but I haven't found any marine forum that comes close to this one. If there is a good marine forum someone please advise.

Anyway, here is my problem. I have a 1996 454 Bravo I Mercruiser setup in my boat. It has the "thunderbolt" ignition system. I am not sure what the "thunderbolt" is but it must be a coil that is hotter than stock. My volt gage in the dash reads really erratic voltage 10 to 13 volts. At idle it might read 13 volts. while accelerating it might read 11. at cruise speed it usually reads 12 volts. then while pulling the rpms down to stop it might bounce from 13 to 12 to 10 to 13 to 11 etc, etc. Sometimes while coming down from cruise speed to a slower speed teh needle will bounce from 10 to 13 and the engine will sound like it is sort of losing spark and hesitate for a half a second or so. It is weird as hell. At idle (about 800rpms) is the only time I can get it to read a steady 13. While this phenomenon is happening I am never using the power trim, trim tabs, hatch lift or anything. The battery has always started the boat and the power trim always works well. When the volt gage really gets erratic the other gages fluctuate slightly as well. Also the boat will tend to smoke black when the voltage really dives to around 10 volts or so. all and all the damn boat runs well at cruise speed and WOT.
I can not figure this one out though.

I had the alternator checked. It held 15 volts ( on the machine for about two minute) steady plus or minus .2 volts maybe. I watched the display the entire time. The alternator pulley did not show any signs of slippage. Belt is tight. I repalced the battery, I took all the battery cables, grounds, alternator wires, etc off and sanded the spade terminals and put star washers in to get everything to bite good and snugged everything up (ground side and hot side).

What is it!!!!!!!!

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Old 09-06-2006, 07:14 AM
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If your alt is working fine I would guess you have a problem with the volt gauge. Check with a hand held volt meter.
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Old 09-06-2006, 07:38 PM
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Doc here,

Never messed with boats at all..

Do I understand this is a GM 454 Version for Marine applications? (If I read it Correctly)

If so, What kind of Alternator Do you have, a CS series, an SI series? , 1 or 3 wire alternator? Internally or externally regulated output?

Notwithstanding That, sounds like the Alternator is doing it's job..My first line or recourse would be to inspect (Especially on a marine application) All connections from the Alternator, to the gauges, back to the battery, Vibration, I assume, is horrendous in a boat like that and connections can become loose..ALSO at each termination point inspect, clean and burnish for corrosion..

Check the Alternator & Mount , be sure the mount is not power white as well as the alternator..If so, you have electrolysis going on..remove them, and ALL the hardware and power wash or media blast it all clean..when you go to re~install, get some aluminum wire paste, and coat the bolts and mounts..everywhere the mounts attach to the engine..(it's steel, the mounts usually are aluminum..what were they thinking??) Run an additional ground from the Alternator to the ground buss system.

Try and duplicate the symptoms, and "Shake" the harnessing, to see if it has some effect on it..that will get you in the zip code..

If it all checks good, I'd suspect a device with an intermittent DEAD short, of very fast duration, causing the excursions. That would be a second avenue of approach.

Doc
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Old 09-06-2006, 07:47 PM
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actully from what I have heard the Ramjet 502 and ramjet 350's use the mercruiser Marine brain box's. so it could be a hotrod related issue also.
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Old 09-07-2006, 06:34 AM
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Quote:
Originally Posted by docvette
Doc here,

Never messed with boats at all..

Do I understand this is a GM 454 Version for Marine applications? (If I read it Correctly)

If so, What kind of Alternator Do you have, a CS series, an SI series? , 1 or 3 wire alternator? Internally or externally regulated output?

Notwithstanding That, sounds like the Alternator is doing it's job..My first line or recourse would be to inspect (Especially on a marine application) All connections from the Alternator, to the gauges, back to the battery, Vibration, I assume, is horrendous in a boat like that and connections can become loose..ALSO at each termination point inspect, clean and burnish for corrosion..

Check the Alternator & Mount , be sure the mount is not power white as well as the alternator..If so, you have electrolysis going on..remove them, and ALL the hardware and power wash or media blast it all clean..when you go to re~install, get some aluminum wire paste, and coat the bolts and mounts..everywhere the mounts attach to the engine..(it's steel, the mounts usually are aluminum..what were they thinking??) Run an additional ground from the Alternator to the ground buss system.

Try and duplicate the symptoms, and "Shake" the harnessing, to see if it has some effect on it..that will get you in the zip code..

If it all checks good, I'd suspect a device with an intermittent DEAD short, of very fast duration, causing the excursions. That would be a second avenue of approach.

Doc


Doc

Thanks for help

Internally regulated

The alternator is a 3 wire (i guess) It has a GRD, Bat +, and two 12 ga "control" wires. I guess one is to the volt gage in the dash and one is from the battery (maybe). There are 3 wires plus a grd (the ground goes from alt housing to bell housing bolt where most other elect components are grounded to). I can trace a wire to the dash. The other ust change colors on me in the harness somewhere becuase I can not find it down the line from the alt. What would the wire going to the dash do? Is it simply the voltage wire for the gage?

AS stated before the boat runs well 98% of the time but the 2% is when the damn voltage is going crazy and the motor idles a little rough, hesitates, and slightly smokes black. This may be a 2 or 3 second phenomenon. Then the voltage will stabilize and the boat runs well. Maybe I shouldn't worry about it but it buggs the hell out of me and I want to eliminate something that might get worse.

Thanks

hunter@matrixengineer.com
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Old 09-07-2006, 04:54 PM
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Doc here,

The brown wire should go to the idiot lamp..

The smaller red should go to 12 volt sensing,

The big red is output..

If the engine bogs down and runs bad when this occurs, I'd suspect a short somewhere that is robbing engine power..

Doc
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Old 09-08-2006, 12:37 AM
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My kids and I have always had boats and we like the "iboats" forums the best. Very helpful for the problems we have addressed there.

Its the black smoke with the other problems during this intermittent condition you have mentioned makes me think of a harness issue. Intermittent black smoke is usually from the computer that is confused from an input problem. It can command the injectors full rich. Possible short to ground or short to power. Perform the wiggle or shake test like doc outlined. Check the TP sensor for erratic reading while opening and closing the throttle. Most TP issues occur at just off closed position. Test is best made with an analog ohmmeter because its easier so verify the erratic movement of the needle than watching for erratic movement on a digital ohmmeter.
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Old 09-08-2006, 05:09 AM
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Thanks guys!!!!!!!!!!!!!
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