How do I test my alternator?

[ZaQ]

I have a 91 Merc V150. My battery is loosing it's charge every couple of weeks. I don't think the big motor is charging the battery anymore. How can I test the alternator to see if it needs to be replaced?

 

[ZmOz]

Re: How do I test my alternator?

I don't know what the spec is for that alternator, but when the engine is running the voltage should be about 13.5-15 volts.

 

[ZaQ]

Re: How do I test my alternator?

My fish finder tells me what the battery voltage is. Would that be what the motor is putting out too?<br /><br />I trickled charged the battery last night, and it is up around 12.7. I was fishing the bouy on Saturday and noticed my battery was 11.6. On Sunday, I fished 10 miles from port and when it was time to come home, no juice to turn the motor. This was never a problem until about a month or so ago. I have the battery now hooked to my kicker motor, but would like the convenience of the big motor charging the battery instead.

 

[Dunaruna]

Re: How do I test my alternator?

Originally posted by ZaQ:<br /> My fish finder tells me what the battery voltage is. Would that be what the motor is putting out too?<br />

Yes. When testing with a voltmeter, the test cables are connected to the battery posts. Your fishfinder has a voltmeter display built in.<br /><br />If you want to double check the readings connect a seperate voltmeter to your battery, the volts should begin to climb and peak out at about 1500rpm. At 1500rpm you should be looking at miniumum 13.8v.<br /><br />In the absence of a volt meter you can do a very basic test of the alternator - start the motor (idling)and switch on a light (one only - anchor/dash/spot/running lights). Rev the motor up to 1500 but don't take your eyes off the light. if the alternator is charging the light intensity will increase, not blindingly increase but there will be a dicernible increase. BTW, if you ever suspect an alternator problem with your car this test works great with headlights.<br /><br />Aldo

 

[ZaQ]

Re: How do I test my alternator?

I was going to say, that is how I used to test the generators in my beetles.................good idea, I will try that today on my fishing trip. Thanks!

 

[dajohnson53]

Re: How do I test my alternator?

Originally posted by ZaQ:<br /> My fish finder tells me what the battery voltage is. Would that be what the motor is putting out too?<br /><br />I trickled charged the battery last night, and it is up around 12.7. I was fishing the bouy on Saturday and noticed my battery was 11.6. On Sunday, I fished 10 miles from port and when it was time to come home, no juice to turn the motor. This was never a problem until about a month or so ago. I have the battery now hooked to my kicker motor, but would like the convenience of the big motor charging the battery instead.

I'm no battery expert but a few things come to mind, based on trouble shooting I've learned the hard way.<br /><br />Alternator - you've already gotten advice on how to check the output. Your fish finder should do it, but it's easy enough to test with a multi meter or similar.<br /><br />Battery condition - it seems to get charged to the right level, but I'm wondering if it holds the charge. Have you checked the cells with a hydrometer to see if any of them are sub-par? Have you let the battery sit for a couple of days disconnected to see if it stays at the fully charged level? Maybe it just isn't holding a charge. Also, it probably doesn't need to be said, but clean all connections.<br /><br />Small current leak - you can test to see if there is a current leak. Basically, you turn *everything* off, so there should be zero current running out of the battery. Then you test to see if there is any current. If there is a current then something hidden is pulling it - either something that is on even though you think it's off or a short somewhere. In my case, a new lift gauge I'd installed was "on" when the key was off. It pulled very little current, but over several days, it would discharge the battery. I've also left things on without knowing it - on my boat, fishfinder, GPS, radio can be left on - DOH! These are just examples of the tiny things that could be giving you grief.<br /><br />To my limited experience, the second two (condition, leak) could possibly be the problem, not the alternator. <br /><br />Good luck finding the problem. If it was me, I'd spend the $50 on a new battery and case, charge it, and carry it in the boat as a spare until I figured out what was going on. (BTDT!)

 

[Bearcamp]

Re: How do I test my alternator?

Start it up and remove the possitive cable from the battery. If it's not charging the motor will stall. Other than that it's running off the battery and that's why your battery would be getting low.

 

[emckelvy]

Re: How do I test my alternator?

I you pull the cable when it's running, if the electronics are not fried before they will be afterwards! <br /><br />DON'T DO IT MAN !!!<br /><br />This is not the correct way to troubleshoot a charging problem. The proper way is with a volt-ohm meter, you can pick up a decent one at Sears or Rad Shack for under $20.<br /><br />BTW, your Merc's ign is self-contained and will run regardless of whether there's a battery hooked up or not.<br /><br />But if you lift the battery cable while it's running, you stand a chance of blowing out the rectifier/voltage reg assy.<br /><br />HTH and Good Luck........ed

 

[tallcar]

Re: How do I test my alternator?

Just a few questions for this topic. I have a 1974 Evinrude 25 hp Sportster Electric. Now I know this motor has no alternator, but it does have a " lighting coil", is this sufficient to recharge battery or do I still need to recharge on occasion?. and will a multi-meter tell me the output of the lighting coil if checked at the battery terminals? If so what should the output be?<br /><br />Thanks<br /><br />Richard

 

[Ben Konopacky]

Re: How do I test my alternator?

saltydawg; the lighting coil is an A.C. circut for running lights.you need a rectifier to change A.C. to D.C. to charge the batt. the conversion i would say is not werth the effort,its only a couple af amps of charging at most.

 

[MURX]

Re: How do I test my alternator?

"I have the battery now hooked to my kicker motor, but would like the convenience of the big motor charging the battery instead".<br /><br />Huh? you wanna run that by me in Spanglish

 

[pabalite1]

Re: How do I test my alternator?

Originally posted by ZaQ:<br /> I have a 91 Merc V150. My battery is loosing it's charge every couple of weeks. I don't think the big motor is charging the battery anymore. How can I test the alternator to see if it needs to be replaced?

 

[pabalite1]

Re: How do I test my alternator?

Take a voltage reading across the battery terminals while it is off. Then start the engine. Take another voltage reading across the battery terminals. The voltage should go up. If it does, then your alternator is working. If it doesn't, your alternator is doing squat.

 

[ZaQ]

Re: How do I test my alternator?

MURX, my kicker motor has a cable that runs to the battery to charge it while I am trolling. I think the big motor would be more efficient at charging the battery though. I am going to take my voltmeter with me on Saturday.