Intermittnent Alternator

[eriefeelin]

Is it possible for a Mando alternator on a 1990 260 MerCruiser to just work here and there? I took it off and had it checked out at a reputable shop. It tested out just fine.
My volt gauge reads about 12 volts 90% of of the time. At times it jumps up to about 13.7 volts. The gauge is working correctly as my fish finder backs up the voltage reading. Would having 2 batteries on board make any difference. They run off a 3 way switch. 1 all 2 off

 

[dubs283]

Re: Intermittnent Alternator

you may want to check all your alternator wiring, from alt to starter and batteries, including the grounds

check battery connections, positive and negative

it would be a good idea to check alternator output at the alt, starter connection, and batteries

 

[Searay87]

Re: Intermittnent Alternator

Most voltage problems like yours are typically traced back to the alternator. Where you've had your checked out it could possible be intermittent and related to temperature. Possibly a defective regulator (diodes) or slip rings? I'm not familiar with how the 2 batteries are wired. Do they run in parallel?

 

[eriefeelin]

Re: Intermittnent Alternator

Yes they are in parallel and are connected to a Perko switch. I've run this boat for 13 years and my volt reading have always been like this. The batteries are brand new as well. Would the volt gauge only read higher when the regulator senses the battery needs it's charge topped off? I usually only run a Lowrance unit and a VHF radio while out.

 

[zbnutcase]

Re: Intermittnent Alternator

Yes they are in parallel and are connected to a Perko switch. I've run this boat for 13 years and my volt reading have always been like this. The batteries are brand new as well. Would the volt gauge only read higher when the regulator senses the battery needs it's charge topped off? I usually only run a Lowrance unit and a VHF radio while out.

You say you have ran this boat 13 years like this; has battery charging ever been an issue? Or have you had a new and recent issue? Not sure I'm really understanding what your asking about. But on the surface, yes a bad regulator/alternator could work intermittently, but for 13 years? With no issues? I highly doubt it.

 

[eriefeelin]

Re: Intermittnent Alternator

Yes sir the voltage readings have been this way for 13 years and never any battery issues unless they go bad. The last set of batteries only lasted a little over 3 years. The set before lasted 5-same brand.
People say your volt readings should be close to 14 volts while underway. I only get those readings from time to time. My Lowrance shuts off at times when starting up. Just concerned my 20 year old alternator might not be up to the task anymore. It tested fine at the shop and my onboard tests and checks on the wiring looked fine.

 

[Don S]

Re: Intermittnent Alternator

Put your DVM directly on the battery and see if it's the same thing. If power remains at 13+ at the battery, it may be the harness connections going to the helm.

 

[Searay87]

Re: Intermittnent Alternator

RV/automotive Batteries lasting 3 to 5 years is normal, high temperatures will kill any battery. As part of your onboard checks I presume you've had a digital multimeter clipped on to the +ve and -ve posts and had someone watch the meter? If it reads steady at around 13.5 to 13.8 volts while your dash board meter fluctuates then you could have a bad connection on your panel meter or possibly a bad voltmeter.

You could also put the same digital meter across your panel voltmeter and watch while the engine is running. It should read exactly the same as the output of your alternator.