Yanmar 3jh4e alternator overcharging

[Henry Walker]

I have a Yanmar 3jh4e 15yo 4500 hrs. Returned after 4 month absence. Battery good fully charged. On moving notice battery at 15v. Done all the usual. Check connections and resistances. Serviced alternator. Got a spare. Checked with both. On running battery 14.75 to 14.9v both at battery terminals and alternator terminals. No voltage drop there. Loading battery shows variation in amp output so regulators working. When output drops to about 1 amp or lower then tacho wanders and battery light flickers slowly. Both alternators work on bench test. Have hardwired exciter to starter panel and also alternator to battery. No change. If disconnect positive of exciter. Volts shoot up to +18v. With engine running voltage at exciter wires at both panel and alternator about 0.2 or 0.3v. Connections in panel seem fine but with battery fully charged did notice readings in it of about 14v when both battery and alternator terminals showed 14.8v. Resistor in panel at back of battery light reading 47 ohms as it should. Given I have hardwired the rest my feeling is the fault is in the panel. It is a simple switch panel with tacho on left key and stop button on right and four warning lights on the top right. About 7 x 5.5 inches. I think it is a Yanmar type B. I am not in a place where I can easily get another. Suggestions on what is going on here and how I can fix it appreciated.

[sailormed]

If I understand it correctly you have tried with 2 alternators and get the same result.
The only thing you have not swapped out is your battery.
Put in another battery and see if you get the same results.
Anyway I don't think the fault is in the Yanmar tacho panel.

[GordMay]

Greetings and welcome aboard the CF, Henry.

[tkeithlu]

The regulator is working, but could it be over-charging the battery? Sailormed knows more about this than I do, and I agree - there's nothing in that panel to create such a problem.

[Scubaseas]

Doesn't sound like panel to me. Either dead cell in battery or bad ground from alt to engine or engine to neg. battery ground would be my first guesses/things to check.


By positive of exciter do you mean the filed wire? Picture perhaps?

[NorthernMac]

Do many boats have a over voltage relay?

[Dsanduril]

Originally Posted by Scubaseas

...or bad ground from alt to engine or engine to neg. battery ground would be my first guesses/things to check

Second this thought. The negative wires make up half the circuit and usually pass through the engine block and alternator mounts. If you have a spare battery capable (something suitable for the alternator's output) you can try connecting direct from the alternator case to the battery negative. If that solves the problem then you need to chase through the negative connections.

[Edit]one more place to check, does your battery switch cut both pos and neg? Many Euro boats do. We had the exact troubles you describe, found that a weakened spring in the switch was not holding the contacts together well and caused a 0.5V error.[/edit]

[Henry Walker]

No voltage drop between battery and alternator. Have hard wired both alt pos and neg to battery and no change. Have hard wired exciter. Not sure if also called field wire. It is the one that supplies initial charge to alternator to get it charging. Battery seems good.. Multiple starts trying to sort this out and holding charge at 12.85v. Have tried to load it a bit prior to starting to run it down to see what happens but does not drop voltage quickly. Will try find another though to check that out.

[Wotname]

Welcome aboard CF, Henry Walker. You first post is a curly one!

What meter are you using to measure the voltages and where exactly is it connected?

If you are also using a handheld DMM, where are you placing the negative probe?

[donradcliffe]

The first question is whether the alternator is the stock internally regulated one, or something else. I had this overchargning problem with an off-brand alternator, and finally ended up rewiring the alternator for external regulation and putting in a $25 NAPA regulator. Its been working fine for 3 years now.

[Wotname]

Originally Posted by Henry Walker

I have a Yanmar 3jh4e 15yo 4500 hrs. Returned after 4 month absence. ..........

Connections in panel seem fine but with battery fully charged did notice readings in it of about 14v when both battery and alternator terminals showed 14.8v. Resistor in panel at back of battery light reading 47 ohms as it should. Given I have hardwired the rest my feeling is the fault is in the panel. It is a simple switch panel with tacho on left key and stop button on right and four warning lights on the top right. About 7 x 5.5 inches. I think it is a Yanmar type B. .....

After re-reading the OP, perhaps the following can help...

I am not familiar with the 3JH4 but many other smaller Yanmars of that vintage have in line fuse in the engine loom that supplies power to the panel. It is usually wrapped up in the loom near the alternator / starter motor wiring and the fuse terminals do get somewhat corroded and will cause voltage problems in the panel. Perhaps after sitting for 4 months possible corrosion has occurred and just maybe is part of the problem.

[smac999]

do you have an external regulator, or internal?

both those are normal with external reg. 15v charging in winter is normal if you have a temp sensor on the batt.

also ext reg cuts and tach cuts out when batteries full also normal.

if you have an internal reg you need to see if it has a voltage sense wire, and see if you are losing voltage on that wire. low voltage on the voltage sense wire would increase votage at alt output and battery. I don't think yanmars use a voltage sense input though. normally just the ignition and lamp

[Henry Walker]

Have found a 4.3 ohm resistance on neg wire to panel. Tracked it to this thing at the the back of the engine. 4.3 resistance between the two terminals marked red. Is this significant? What does this thing do?. Starter relay??

Alternators both internally regulated. Both work fine. Pretty sure they are not the problem. No access to external regulator here.

[Henry Walker]

Seems it os a starter relay. Like a solenoid for the solenoid. How do I bypass this?

[Scubaseas]

Terminal 30 to terminal 87. 30 should be a yellow/brown wire. 87 goes to starter. This would be for a DIN relay. YMMV. Relay is probably after market to compensate for low voltage to starter solenoid wiring. Better solution would be to use bigger wire from starter switch to starter solenoid.