View Full Version : How do I check alternator?

ProStar Slalom
07-28-2008, 08:47 AM
'84 Stars and Stripes with PCM 351. The last couple of times on the water, the battery has struggled to crank the engine. I pulled the battery, checked water level and placed it on a trickle charge overnight. Cranked just fine the next morning, but seemed a little weaker later in the day. So....I'm assuming the alternator is going bad. Can someone talk me through testing an alternator? Thanks....

07-28-2008, 09:02 AM
If I suspect an issue, I take both to Autozone and have them test them... free.

07-28-2008, 09:10 AM
While you're taking the alternator in, I'd take the starter with you too and get it tested. If it's the same age as the boat, it may now be drawing more current than can be readily supplied by anything other than a freshly charged battery.

07-28-2008, 09:32 AM
How old is the battery? Has it gone completely dead? If it did, was it extremely cold when it happened?

If it's older than 4-5 years, you probably need a new one. The yellow case batteries at WalMart (or the same brand at other stores) are good and have a 2 year exchange warranty.After that, it's pro-rated.

Have the battery load tested and if you can, take the alternator in, too. If they can't test it, you may have a rebuild shop near you who can. If the battery checks out OK for load, it's usually an alternator/cable/terminal/clamp issue.

07-28-2008, 11:06 AM
One quick way to test the alternator, is to start the boat and while running disconnect the positive terminal, if the boat dies it probably is your alternator. Also, I agree with the previous comment on the starter going bad, my 79 did the same thing and the starter was pulling to many amps, replaced it and everything was fine.

07-28-2008, 11:31 AM
Another easy way to test the alternator (and regulator at the same time) is to put a volt meter on the battery, and start the boat. The voltage should go up to about 14 volts with the boat running. If it does not go up, the alternator, or regulator (if seperate) are suspect. I believe in '84 the regulator would be seperate from the alternator.

You might even be able to see this directly on the voltmeter on the dash without doing anything other then watching it before and after starting.


07-28-2008, 07:53 PM
I think a better way would be to disconnect the charging lead from the alternator and start it. If it puts out more than 14 volts, it may be good but it's not under any load. On a 20 year old boat, I usually start with the terminals, contacts, like the engine's ground connection at the rear of the motor, etc. The starter gets its ground through that bolt and while it may not look bad, it can be bad enough to cause this problem. The actual starter ground is through the mating surface on the motor, where it mounts. If the starter hasn't been removed for lubrication, it's likely that it's due to this and not the alternator or battery.

07-28-2008, 11:38 PM
ProStar Slalom I am currently have the same issue. Keep me posted on what you find and how you fix it. Took both my alternator and battery in today and both checked out to be running perfect.

07-29-2008, 12:02 AM
If the alternator and battery check out OK, it would have to be:

The starter

The solenoid

The Starter Bendix/gear binding

The ground- the starter grounds through the mating surface at the block and uses the bolts, too.

The battery cables/clamps/dirty battery terminals

That about includes everything from battery to starter.

ProStar Slalom
07-29-2008, 07:43 AM
Thanks for all the ideas. I'll do some troubleshooting this coming weekend and let you know.

07-30-2008, 11:38 AM
Replaced the battery, even though it checked out fine and that seems to have fixed the problem. Went out and it started on the first turn of the key and didn't stop the entire time. How old is your alternator prostar slalom?

07-31-2008, 08:49 PM
If you have anykind of stereo in your boat, just take the time to remove it and have an autoparts store check it. After talking with some friends who install high end stereo systems they siad the quickest way to fry an amp is check your alt. by disconnecting while running.
just a thought.

07-31-2008, 10:20 PM
An amp doesn't like the spikes associated with removing the battery cables and neither doesn any of the other electronics in a newer boat. Anything with a microprocessor is at risk from removing the battery cable while it's running.

Disconnecting the battery while the motor is running is OK with an old motor and old electrical systems but not with anything new. Someone I worked for just fried the voltage regulator on his Mercruiser 170 motor (if anyone knows the 4 cyl Mercruiser motor with the stator behind the crank pulley and a big, unstable & expensive voltage regulator on the side, that's the one) this way because he wanted to jump start someone else's boat. He has worked on cars for decades but doesn't quite get it. The motor connected to a donor battery should NOT be running when the one with the dead battery is cranked. This is a huge load for the regulator to handle and if the donor alternator is rated at 95A but the dead motor cranks at 250A, it's still gonna kill the regulator.

08-01-2008, 02:20 PM
I know it's been mentioned on this thread........I have an 83 model. I was having difficulty cranking also, and found the cables, both pos and neg needed to be replaced. It wasn't expensive, was easy to do, and it sure made a huuuuuge difference. Hope this helps you.

amber 6
08-01-2008, 02:34 PM
on the 83 and 84 stars and stripes where is the regulator located please

Bruce Carr
08-01-2008, 02:41 PM
On the '83 (probably '84 also), the regulator is attached to the aft side of the alternator.

ProStar Slalom
08-03-2008, 09:58 PM
I pulled out the battery and alternator this weekend and took them to autozone for a test. Battery load tested fine. The kid was puzzled by the alternator (wasn't sure what to do with the wire coming out of the voltage regulator) and it only read 3 volts on the bench. I took them both home, trickle charged the battery overnight, and re-installed both. On the water this morning my analog voltmeter was reading about 13 volts at the battery terminals with the engine off, and about 14 volts with the engine running.

So...I trust the battery test they did and it charges fine on a trickle charge. I'm still not sure about the alternator, so I think I'll try to find a re-build place that knows how to test it and go from there. Any other thoughts? Thanks....

08-03-2008, 10:09 PM
What color is the wire and what was it connected to in the boat? If it was connected to a purple wire, make sure that wire has 12V on it with the key ON. The kid may have been thinking it was a one-wire alternator but if he was stumped, he should have asked someone about it.

ProStar Slalom
08-04-2008, 08:30 AM

The positive and ground posts are standard posts; remove the nut and the wire comes off. The wire coming off the alternator is about 5" long and has a quick-disconnect fitting. Looks to be yellow in color, but they're all pretty faded. From the markings on the voltage regulator, it has something to do with A.C., but I'm not sure where it goes....just into a taped wire bundle.

ProStar Slalom
08-11-2008, 06:16 PM
I finally got the alternator to a place that knew how to check it. Checked out fine. So...time to move on to the starter, I guess. It was rebuilt about 5 years ago, but hasn't been removed or serviced since then. How do I know if the cables are bad? Is there a way to measure resistance in them? Thanks....

08-11-2008, 06:40 PM
Before anything else, I would charge the battery overnight then take it to get it load tested.

08-11-2008, 07:10 PM
get a volt meter and start taping wire connection point. if you have a suspect wire you can check the current at one side of the wire and the other. they should be the same. if there is a signicant drop, you can figure that wire has gone bad. volt meters are also good for testing devices. your alternator should be putting out 13+ volts (mine pushes out 14 or so). I recently learned that alternators typically fail in thirds. so if max output is 14, then the first failure would drop it to 9-10 volts, then the next would fall to 4-5, then the last would be 0. as long as you are correctly identifying connection points, and you know what you reading you should be getting, you can quickly trace out your engine's electrical circuit and find where the problem is.

08-11-2008, 07:17 PM
one more thing......i've been having similar issues and i'm convinced my issues are related to a combination of a bad starter solenoid and/or incorrect/bad wiring. i'm trying to obtain a more thorough understanding of the wiring (how it should be) because i think that a previous owner may have made a mistake somewhere. I think it's either that or I've purchased 2 bad starter solenoids in a row. and while that seems unlikely to me, i've been told that it may not be so unlikely as manufacturing standards for those things are pretty low. so what i'm taking too many words to say to you is, check your starter solenoid and wiring. if either are bad, or if things aren't wired up correctly, your problem may not be with a major (expensive) component like the starter, but with something cheap and minor (easily overlooked) like a solenoid or some wires.

ProStar Slalom
08-12-2008, 08:36 AM
Thanks, Ryan. I'll continue to tinker this coming weekend.

ProStar Slalom
08-22-2008, 09:29 AM
Ok, all of the big components have checked out fine by qualified guys....alternator, battery, and starter. I'm thinking about swapping the starter relay since it's probably the original which would make it 24 years old. Question....is there such a thing as a marine starter relay or can I get one at any auto parts store? I know the deal about spark hazards with alternator, starter, etc., but would think that there isn't a concern with the relay. Can anyone confirm? Thanks.