Entire electrical system dies when under way

[peters715]

I wasn't clear, apologies. Reviewing the Mercruiser Service Manual 1, there is no reference to a circuit breaker.

The manual covers 1963-1973, my Mercruiser being a 1969.

That is an old picture. The power cable is brand new and all connectors are bright and clean.

 

[99yam40]

does this happen every time you hit the T&T?
Or only while underway?
I would be checking to see how much current the T&T pulls and what the voltage drops to on everything while the T&T unit is running

 

[peters715]

does this happen every time you hit the T&T?
Or only while underway?
I would be checking to see how much current the T&T pulls and what the voltage drops to on everything while the T&T unit is running

After replacing battery cables and battery switch I am not experiencing the problem now. I can run the trim, blower and sump pump without any issues.

But my alternator now seems to be acting up. When I start the engine the volt meter shows +12.5~ for a bit and then drops to zero. So break out the multi meter and start checking voltages.

 

[99yam40]

You need to monitor the battery voltage and the voltage at different places before starting and after.
with motor running you should be reading over 12.5VDC probably around 13.5 or 14

I do not believe the battery voltage your gauge shows is correct if it goes to 0 and RPMs drop
I was wrong it was the bilge pump that you said it affects the systems

"The alternator output drops and engine rpms drop (probably less than 50 RPM) when I turn on the bilge pump, system doesn't die, but it's noticeable."

 

[peters715]

You need to monitor the battery voltage and the voltage at different places before starting and after.
with motor running you should be reading over 12.5VDC probably around 13.5 or 14

I do not believe the battery voltage your gauge shows is correct if it goes to 0 and RPMs drop
I was wrong it was the bilge pump that you said it affects the systems

"The alternator output drops and engine rpms drop (probably less than 50 RPM) when I turn on the bilge pump, system doesn't die, but it's noticeable."

Well, I went through every connection in the engine compartment. Polished every single point of contact and measured voltages before starting the motor. Reading 12.84v DC everywhere.
Started the motor and rechecked voltages. 14+ volts everywhere except off the alternator. The alternator is putting out a little over 16 volts.

Volt meter in the panel holds steady now somewhere around 14 volts.

So all looks good again.

And thanks for the electrical varnish tip. A bit spendy but that little bottle will last longer than I need it to last.

 

[peters715]

Well, I thought I had this dialed in.

Took it out today and volt meter was showing in the positive but over the course of an hour or so under way, mix of WOT and low speed cruising, it dropped down to zero.
When I hit the trim switch the volt meter would jump up to where it was on start up.

Didn't have my multimeter onboard so couldn't measure anything. I'll take a look tomorrow but it seems to run contrary to everything I thought I understood about charging systems.

Anyway, any thoughts would be appreciated.

 

[redneck joe]

with my luck and 'skill' with electrical i cannot offer you any thoughts, just prayers...

 

[jlh3rd]

I dislike electrical intermittent problems because you can't see it.
Took my boat out in the spring, everything showing good on gauges, head out on the water, look at the gauges....and they're sitting at zero. WTF.
Hit the trim button, everything comes back. My uneducated but common sense said I guess the high draw from a trimming selection forced the current past the weak temporary break and "fixed" it.....dunno.
Sometimes just unplugging/plugging connections cleans them up. I believe the purple wire from the ignition switch feeds the voltmeter.
I just also read that the ground/s need to be checked if a gauge suddenly goes to zero.
Anyway, went all season with no repeat.

Was it just the voltmeter? Were there any low voltage warnings?
I don't believe it was the charging system. You still have a weak connection somewhere.

 

[peters715]

I fired it up last night and monitored the voltage output at the alternator, at the battery and starter.

On cold start up the alternator output was 16.4 VDC. After about five minutes it dropped to 15.2 VDC at alternator, battery and starter.

Meanwhile, the volt meter in the dash dropped to zero even though the alternator was putting out voltage. So need to look under the dash.

When hitting the trim, meter voltage jumps up until I release it. That part is a mystery to me. Also of note when trimming up solenoid makes a loud initial click while trim down does not make click of any note.

 

[jlh3rd]

so the volt meter read the cold startup voltage THEN dropped to zero?
Then jumps up while trimming, then drops to zero again?
If you just turn the ignition switch on, not start, what does the volt meter do?
Then what does the voltmeter do if you hit the trim switch?

 

[alldodge]

Behind the helm is a good place to look.
Would also pull the engine connector apart, those pins inside will corrode. Pulled an old one apart years ago and had to cut the plug out and solder all the wires together, they were bad inside

 

[peters715]

Battery online, ignition switch in Run: Volt meter is zero or drops just slightly below zero. Trim motor does not spike volt meter.

 

[Scott Danforth]

Your alternator feeds the connection at the starter motor. You bad connection is between the starter and the helm

 

[alldodge]

There is a bad connection feeding the sense leg on the ALT, and this is why your measuring 16V at it. The 16V is enough to boil your Bat

 

[peters715]

There is a bad connection feeding the sense leg on the ALT, and this is why your measuring 16V at it. The 16V is enough to boil your Bat

Not sure I'm following. Bad Sense leg on the ALT?

 

[alldodge]

The ALT has 3 legs
Output
Sense
Excite

The Output is connected direct to Bat
The Sense is connected to the motor electrical to measure what sensors are seeing
The excite is the purple wire that is energized when key is turned ON

If sense does not see what output sees it increases voltage until it matches

 

[peters715]

OK, the red and wire goes to battery. The green wire (field) goes to the voltage regulator mounted on the transom. That leaves the yellow/black wire to be the sense wire.

All the connections are polished to near mirror finish and then coated with electrical varnish.

It sounds as if the yellow/black wire is the one I need to follow, correct?