High output alternator

Maine Sail · Dec 15, 2014

kgodfrey said:
Peter,

Do you have a Yanmar engine? If so, the tach does NOT get it's signal from the alternator.

Those reading this, for future reference, should bear in mind that many, many, many Yanmar models that do use an alternator AC tap for the tachometer. Not all Yanmar's use a flywheel sensor...

However, if this is the typical 75HP 4JH4 XX found in many Beneteau 46's then it is a flywheel tach sensor not an AC tap on the alt... The standard alternator for the 4JH4 XX is the 55A LR155-20 Hitachi but Beneteau may have spec'd something different.

It seems the alt is working but the tach is not. You need to trouble shoot the tach sensor and tachometer... Yanmar tachs of this generation are fairly prone to failure but occasionally it is the tach sensor... Both will need trouble shooting... Sender is both resistance and voltage and tach is straight voltage measurements plus a continuity test of all wires between tach and sender...

kgodfrey · Dec 15, 2014

MS,

Thanks for straightening me out. Question, what damage if any would have occurred to Peter's alternator if he grounded the stator tap while the engine was running?

Ken

gldndrm · Dec 15, 2014

Wire sizing

This was briefly mentioned above but I'll add this anyway. The first thing that was noticeable in the picture of the back of the new alternator was that the positive output wire was undersized. As noted above it should be at least a 2ga for a short run and larger for a longer run. AND - whatever the ground path is, it has to be able to carry the equivalent current as the positive red wire. Often this is the case ground through the engine but that isn't the best use of your engine. Especially since you are more than doubling the current involved. So you should have a same size negative wire from the case ground point on the alternator to a good ground point. I have seen it put on the top of the main engine ground wire termination but in general that is not good to gang up terminals on top of each other subject to lots of vibration. The best place to take it is to your main ships ground bar (if you have one) or to whatever serves that purpose.

Maine Sail · Dec 15, 2014

exmaggiedrum said:
This was briefly mentioned above but I'll add this anyway. The first thing that was noticeable in the picture of the back of the new alternator was that the positive output wire was undersized. As noted above it should be at least a 2ga for a short run and larger for a longer run. AND - whatever the ground path is, it has to be able to carry the equivalent current as the positive red wire. Often this is the case ground through the engine but that isn't the best use of your engine. Especially since you are more than doubling the current involved. So you should have a same size negative wire from the case ground point on the alternator to a good ground point. I have seen it put on the top of the main engine ground wire termination but in general that is not good to gang up terminals on top of each other subject to lots of vibration. The best place to take it is to your main ships ground bar (if you have one) or to whatever serves that purpose.

I agree the 10GA wire should be larger but it is unfortunately standard protocol when the alt B+ runs 10" and picks up the large GA wire at the starter B+...

Works okay for a car or small starting battery but can inhibit performance when trying to drive large current for long durations to a cycling bank... The 10GA wire was also sized for the stock alt not an upgraded 100A+ alt...

Best practice is as you described but unfortunately is rarely done....

gldndrm · Dec 15, 2014

Maine Sail said:
I agree the 10GA wire should be larger but it is unfortunately standard protocol when the alt B+ runs 10" and picks up the large GA wire at the starter B+...

Works okay for a car or small starting battery but can inhibit performance when trying to drive large current for long durations to a cycling bank... The 10GA wire was also sized for the stock alt not an upgraded 100A+ alt...

Best practice is as you described but unfortunately is rarely done....

You're absolutely right about the best practices. Most boats are not wired with a separate ground to the alternator case. The alternators can still work but the connection from the alternator foot to the engine has to be good electrically and sometimes is not.

Also, I made a big assumption in thinking the alternator output would go off to the house batteries and not to the start battery which is where (usually) the start battery is directly wired (via a start switch hopefully). Not every boat as two separate means to charge the start and house batteries or just one battery bank for both uses. But wire harnesses are built for stock installations and the wire sizes are whatever they are. In this situation I would run a separate wire to wherever and remove or disconnect from both ends the old 10 gauge positive.

ALNims · Dec 16, 2014

PeterBreit said:
Yes I have the 2000 Yanmar 4JH3 TE turbo charged engine with the Hitachi LR155-20B
IC REG 12V-55A alternator.

Thank You
Peter

Peter,
I looked up the engine specs and this engine showed that it came with a 125A alternator. Yanmar has its own wiring system. They should be able to tell you which alternator to use. If the alternator you have is fully compatible you should not be having these problems.