Princess 420 Alternator problem

[Simon 420]

We have a 1997 Princess 420. Have just replaced both alternators on our TAMD63Ps. But have an issue with the starboard warning light/siren

Initially the port alternator gave up, causing port warning siren/light to sound continually. The alternator was not giving out any charge, so replaced it. That then led to an intermittent sounding of the starboard warning siren/light - usually when we were coming in and out of idle/neutral. If running at a constant speed there was no problem. Starboard alternator was giving a low voltage output (12v,) so changed Starboard alternator.

Now the warning siren and light sounds on starboard side, but only when we are running both engines. At start up it's OK, but it then sounds after about a minute or so. If each engine runs in isolation there is no problem -
each alternator is quite happy and putting out 14v. Engineer tells me that when the warning light comes on, the starboard alternator voltage output has dropped to 4v – triggering the warning siren/light. Port alternator is still putting out 14v. Have checked the warning units and they are not faulty. Also checked the splitting diodes and they are also not faulty

Engineer thinks the port alternator is somehow causing starboard to shut down. But no closer to solving the problem yet. Our charging set up is such that the alternators each charge both domestic and engine batteries. They are not split (so that one charges domestic and the other charges engine)

We have recently installed a new AIS transponder, but that's the only change in eletrics onboard

Any suggestions please?
Many thanks

 

[QBhoy]

I think you might have indicated at the issue. I don’t think you should have a charging system as described above.

 

[volvopaul]

sounds like the starboard alternator is loosing its battery voltage sensing signal as it’s dropping out , check the yellow wire on alternator for battery positive voltage both engine off then on, because Princess use split charge system on both engines you must be loosing the signal to that one for it to drop out .
I had a Fairline recently, different system but the port alternator was overcharging causing the starboard to oversense and shut down , what was thought to be at first the starboard one faulty actually was the port , a swap over found this . Running one engine at a time didn’t produce the fault .
I must state I rarely get problems with Princess wiring it’s spot on .

Check the split diodes for loose connections and that they are still connected to the battery banks on both start and domestic, then check the centre terminal before and after starting for voltage , you may have a burnt out diode but again it’s very rare .

I’ve never understood why Fairline had there system when it wasnt much more to build a proper system.

 

[QBhoy]

sounds like the starboard alternator is loosing its battery voltage sensing signal as it’s dropping out , check the yellow wire on alternator for battery positive voltage both engine off then on, because Princess use split charge system on both engines you must be loosing the signal to that one for it to drop out .
I had a Fairline recently, different system but the port alternator was overcharging causing the starboard to oversense and shut down , what was thought to be at first the starboard one faulty actually was the port , a swap over found this . Running one engine at a time didn’t produce the fault .
I must state I rarely get problems with Princess wiring it’s spot on .

Check the split diodes for loose connections and that they are still connected to the battery banks on both start and domestic, then check the centre terminal before and after starting for voltage , you may have a burnt out diode but again it’s very rare .

I’ve never understood why Fairline had there system when it wasnt much more to build a proper system.

Think the issue might be that (if I’ve read it properly) it’s now not as princess would have wired it. Princess would have had it as a split charge system. Reads as if it’s now combined and both alternators into the same system

 

[Simon 420]

sounds like the starboard alternator is loosing its battery voltage sensing signal as it’s dropping out , check the yellow wire on alternator for battery positive voltage both engine off then on, because Princess use split charge system on both engines you must be loosing the signal to that one for it to drop out .
I had a Fairline recently, different system but the port alternator was overcharging causing the starboard to oversense and shut down , what was thought to be at first the starboard one faulty actually was the port , a swap over found this . Running one engine at a time didn’t produce the fault .
I must state I rarely get problems with Princess wiring it’s spot on .

Check the split diodes for loose connections and that they are still connected to the battery banks on both start and domestic, then check the centre terminal before and after starting for voltage , you may have a burnt out diode but again it’s very rare .

Many thanks- I'l pass that on
Incidentally, would you think it better to have a split system anyway?

 

[volvopaul]

Oh

Think the issue might be that (if I’ve read it properly) it’s now not as princess would have wired it. Princess would have had it as a split charge system. Reads as if it’s now combined and both alternators into the same system

if you have done that you have also wired your separate start batteries interlinked with your domestics possibly , or your domestic batteries will now not be charged via the system as both engines are fed from the start bank , the domestics are only charged from the split diode units.
If that’s the case you will drain your start batteries when not on shore power .

 

[jrudge]

Volvo Paul. What is the charging set up on a Fairline please ? I have a s58 and have never given it much thought until I had an alternator failure this year

 

[QBhoy]

Hi. Can I just ask or confirm that you think that one engine charges starter batteries and the other charges domestic? Perhaps meaning that you have 2 starter batteries charged from one engine and perhaps 2 domestic batteries from the other ? This would certainly not be the original set up, I’d think.

 

[Portofino]

When one motor alternator fails .I would have thought it’s shown on the dash either by a red light or voltmeter or both .
Car or boat .

Then to determine what it charges on a boat , Qbhoys Q ^ ……you should see on the panel in the saloon the volts or charge Ah gauges to the respective battery banks(s) .
Figure out what said alternator charged by default.

The classic system on a boat is totally separate banks only connected via a dash parallel ( sometimes called a cross over) switch .
Both alternators or one first top up the engine bank it’s little starting juice then via the spit diode charge the domestic .

Or one motor charges the engine , then split diodes the doms , the other is doms only .This is not so good because if the eng batt motor isn’t running , or has an alternator failure the eng batts are sat there uncharging ,This may lead to halting of the running boat motor .
Modern engines require a heck of a lot of juice to run all the electrotwackery.They soon die if the engine alternator fails to keep up the discharge required to run the supporting electrotwackery.ECU s EDC , even injectors etc .

To find out what you have J Rudges Q simply start one up on its own .
Look at the panel volt / Ah gauges to see which bank is getting the charge from that particular motor .
Turn it off and repeat for the other motor .
Failing that ( unable to see the charge on the panel ) stick a voltmeter on the bat banks when running .You should see 14 + V if that banks getting alternator charge .Just like a car .

Some high end cars indeed have two batteries a starter bat and domestic bat……but only one motor .Split diode prioritises the engine start batt .Again with a voltmeter you are looking for 14+V across the bat terminals if the alternator is stuffing .