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Alternator tested

M

moondrifter

Regular Contributor
"Had my alternator tested. Man

"Had my alternator tested. Mando 65 on a 190 hp 4.3 Merc 2000 I/O
When on the test bench it did not put out until he took one of the 2 wires that came out of the plug in connector and touched it to the body of the alternator.Just a couple taps, Then the unit came alive with that singing sound and put out 70 amps.
When reinstalled it acts like it did before he did the touch/ground thing.
I am confused. Why will it test OK and not act like that when on the boat?
What did he do to get it to put out?
Moondrifter."
 
I'm guessing that was the

I'm guessing that was the excitation lead. Grounding it told the alternator to turn itself on.
 
"but thats not how to test it,

"but thats not how to test it, 12v is supplied to sense and excite,then measure output. grounding should do nothing unless residual magnetism turned it on."
 
"Use a DVM attached to the sta

"Use a DVM attached to the start battery. Read the voltage before starting, immediately after it starts and at idle. Turn on all lights, pumps and accessories at idle and see what the voltage reading does. Watch the meter as you increase RPMs. The alternator should put out more as needed and reduce its output as demand drops."
 
"Wow what results. Guy that te

"Wow what results. Guy that test showed a lot .
Battery before start 12.53 v.
At battery 1000 rpm 14.27v.
At battery 1000 rpm 14.27v EVERYTHING on.
At battery idle rpm 13.27 Everything on.
The volt gauge dropped to 10 v at the dash with everything on. I checked the leads at the back of the volt gauge, it was correct. 10.5 v. This is what made me be concerned. With all draws off, at best it has 12v at dash. The more rpm the lower it goes. This acts like a bad tach, so I removed the signal lead from the tach. No improvement. I have unplugged all the add on accys, one at a time with same results.
Any ideas??
At least I know the engine will not run out of juice. This is because of the help of your test.
Thanks but still stumped.
I must add the condition of this boat is like new kept covered out of fresh water, stored inside in winter. 2000 Crestliner 190 hp merc 4.3 I/O Had it since new.
Thanks
Carl, Moondrifter"
 
The volt meter in the dash get

The volt meter in the dash gets its power from the PURPLE wire wich is 12 volts when the key is at the RUN position.

Knowing this says that either the purple wire comming from the motor has a weak connection or the guage has a issue or possibly another connection where the purple wire is attached has a issue at the dash or possibly the engine harness.

If you have a shcematic you can repeat the test at any of the purple wire connections all along the harness back to the dash to see where the possible voltage drop is.

You can dissconnect the purple wire at any point ( do not short to ground!!) except the coil and check with the volt meter and take a reading at 2000 rpms and see if you get the 14+ volts you see at the battery.

If it reads correctly then it may be the guage.
 
Just so you know

Mine does

Just so you know

Mine does the exact same thing.

When I turn on accesories my volt meter drops way down even though I know I am charging correctly.

It is due to old wiring that has got a bit of oxidaition on it over the years.
 
Don't forget - the voltage

Don't forget - the voltage drop the gauge shows can also "live" on the ground side of the wiring
 
He was saying that it could be

He was saying that it could be a bad ground at the dash.....Black wire (s) through out the guage cluster.
 
"You check just like what was

"You check just like what was suggested above but you probe the different ground lead intersections/junctions. If you have a digital meter, the polarity will take care of itself."
 
"RE:" It is due to old wir

"RE:" It is due to old wiring that has got a bit of oxidaition on it over the years."

Another likely explanation, is that the diameter (gauge) of the wire is too thin to carry the actual electrical load that is presented and you are getting an excessive voltage drop over the length of the wire.

Manufactureres use the thinnest gauge wire they can get away with unless you have a top tier premium boat... and even then? Custom and semi custom built sportfish boats tend to have decent electrical systems. Few stock boats (Hatteras and Bertram and a few like boats) have decent electrical systems."
 
"I agree with the exception th

"I agree with the exception that it worked perfectly fine back in the '90's before age and moisture got to some of if not most of the wires over a 1-2 year spread when I had the wrong mooring cover.

I have had several weak connections on switches creating voltage drops to some of my accessories. ( horn) and other big draws.

My boat manufacturer used the appropriate wire size as I have looked many times and still do not believe how well it was all put together......

Nothing I can do at this point (nor do I want to) to clean/replace all of the oxidized connections unless I have a failure. But at least I know where to look...........You know the story, "the mechanics car is most always a piece of crap because he spends to much time fixing everyone elses problems to be concerned with his own""
 
"I should have been clearer...

"I should have been clearer... Instead of saying
"too thin to carry the actual electrical load"

I should have said. "too thin to carry the actual electrical load on the system at this time" Folks often add stuff to the existing system and they exceed any design margin that the wire size may have had when built.

I also did not intend my comment to mean, "no, its not poor connections", but to offer an addition item to consider. Historically, I've seen more problems related to poor connections rather than undersized wiring (for current needs...no pun intended), but its not unknown.

What I've seen a lot of in the poor connection field is improperly done crimps. If terminals are crimped properly to the manufacturer's design specs, they cannot corrode, at least at the point of the actual connection. However, if one uses cheap wire, i.e. copper wire with a high free oxygen content, then the wire itself over time and at elevated temps will, in extreme cases, literally turn to powder (I found a stretch of such wire on one of my boats once) and no amount of proper crimping (or soldering for that matter) will help."
 
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