Testing alternator.....

[HOWARD O]

Is there a more advanced way of testing your alternator while installed other than checking it's output with my multimeter? Would it help to do it at various RPM's?a

I have always been suspicious because my gauge is almost always pinned, putting load on it drops the gauge quite a bit. If I hit the trim switch, it drops a lot. I ask about the RPM's too, because the engine sounds bad up past 4k and I was attributing that to perhaps it's age.....maybe valve float, whatever. Today I was running it fairly hard and I popped the 50 amp breaker. Boy, that stops you quick! Pressed the button and I was off and running again. Back at the dock, I inspected all the wiring, especially the big ones, to see if anything was rubbing raw, etc. It all looked good.

Can an "over achieving" alternator pop a 50 amp breaker? Could it perhaps play havoc with ignition? How about a trim pump? I've sprung a leak in one fitting and burst a hose a couple months ago. I know I need to rewire the whole boat. It's the original, factory mess.

Maybe I'll just install a new alternator for piece of mind, if nothing else. I sure don't feel like rewiring at the start of the summer. :frown:

[zelatore]

Is the gauge you're referring to voltage or amperage?

If it's voltage, I suspect you've got a bad regulator.

If it's amperage, you've got a short somewhere. It's possible the fault is in the alternator, but most likely it would be a wiring issue. I doubt this is the problem though, or you'd likely end up with dead batteries on a regular basis.

Either way, it's a simple job to pull the alternator and take it to a local auto parts store where they do free testing.

[HOWARD O]

It is a voltage gauge and yes, the battery always seemed to be charged very well. I've gone through the wiring, cleaned grounds and all that good stuff.

Here's a for instance. I have a photo that Joe Cuda took of the gauges when he was running wide open. Clearly the voltage gauge in the top left appears to be in what I think is a normal position. When I am running at any speed, it is almost pinned and like I said, if I hit the trim, up or down, that needle drops a good way down, perhaps to half.

I'll take the alternator in and have it tested. I believe the last time I checked at the battery at idle, it was putting out 14.7 volts. From what I've read, it may be on the high side but still in spec.

Thanks zelatore for the advice. :)

[VetteLT193]

It's not like a car where the voltage regulator and alternator are one unit. Merc used an alternator with an external voltage regulator. When the VR goes bad it will do what you are describing.

[MOP]

Vette almost all Merc units are one wire totally self contained!
From what you are describing thinking the gauge is reading Ok, my first suspect would be the diode bridge. The diode bridge controls the voltage out put, not sure how handy you are but once you get it off the bridge can be swapped out fairly quick. But it is always a good idea to do brushes and bearings at the same time.

[VetteLT193]

Vette almost all Merc units are one wire totally self contained!
From what you are describing thinking the gauge is reading Ok, my first suspect would be the diode bridge. The diode bridge controls the voltage out put, not sure how handy you are but once you get it off the bridge can be swapped out fairly quick. But it is always a good idea to do brushes and bearings at the same time.

mine isn't, and I thought all of the old ones were like that.

this is like mine, part 18.http://www.mercruiserparts.com/images/COMMON/8186.png

also found here... http://www.go2marine.com/product.do?no=158066F

[MOP]

No the majority a re one wire, I am pretty sure Part #11+12 will fix his problem!

[HOWARD O]

Mine's an old one like yours, Vette. It has the VR piggy-backed on it with 2 wires coming out. I guess a sensor wire and what's the other, an exciter? Man, I don't remember these things....it's been too long.

I'll have this thing tested, see what they come up with anyway. I did find a replacement VR on the internet that cross matches to mine.

Also, I read in one of my old manuals, don't remember when or for what boat, but if an alternator output is too high, it could be a faulty ignition switch too. Does that make sense to you, Phil?

[zelatore]

I can't see how any external component would cause an alternator to go high. The regulator (either built in or separate) does just that - regulates the output voltage of the alternator which would otherwise put out much too-high voltage at high rpm.

The only thing I could think of that a bad ignition switch might do is if there is a feedback to the regulator run through the switch. No feedback, and the regulator will keep trying to increase the voltage.

I'm used to the one-wire set-up, which wouldn't have that feedback running through the switch. (as an old Ford guy, I was always envious of the GM one-wire alternator).