Engine won't run in gear with alternator

[svsonora]

My starting bank is on its way out, and while it somewhat holds a charge, its not holding 100% charge. (75ah battery) House bank is primarily used (150AH) until I get a new starting bank.

I noticed today when I'm running the engine off the house bank, it runs no problem. At idle (750RPM ish currently) it'll go into gear, run, idle, rev up, etc. When I run it in my starter bank, the additional load on the alternator seems to preclude this. As soon as I knock it into gear, it turns over a few more times and dies. It also seems to have trouble revving up.

My initial reaction is that the engine should be able to run in and out of gear no problem regardless of alternator load. However, I'm also wondering if the battery is so bad that its not getting enough juice to the coil, producing a weak spark.

Engine received a new oil change, new plugs, new cap and rotor recently. My old cap looks pretty good so I'm keeping it as a spare.

As I said, it seems to run just fine on the house bank. I ran it in gear at the dock for about 30 minutes. Re-starts just fine too.

I think my idle mixture is set decently well but my tacho doesn't work very well. I received a new tach today from MM so tomorrow, I'm going to install it and re-try setting the idle mixture and idle.

Any thoughts?

[ndutton]

I'd like to eliminate the alternator as a potential problem before doing anything else. The symptom MAY indicate a dead or dying battery with no help from the alternator. If you're charging your house bank via shore power its reserves will start and run the engine for hours without the alternator, possibly explaining why the house bank works and the engine battery doesn't.

Easy test: start the engine on the house bank, rev it up to at least 1500 RPM (out of gear is fine) and measure the voltage between a solid block ground and the alternator output post and report back. To double check, repeat the test with the engine start battery.

[svsonora]

I'd like to eliminate the alternator as a potential problem before doing anything else. The symptom MAY indicate a dead or dying battery with no help from the alternator. If you're charging your house bank via shore power its reserves will start and run the engine for hours without the alternator, possibly explaining why the house bank works and the engine battery doesn't.

Easy test: start the engine on the house bank, rev it up to at least 1500 RPM (out of gear is fine) and measure the voltage between a solid block ground and the alternator output post and report back. To double check, repeat the test with the engine start battery.

Thanks. I am a bit confused. What would I obtain from this test? Aside from checking if the alternator charges or not? (I believe it does, because it does charge up batteries faster than my solar array does judging from readouts on the victron moniter)

[svsonora]

Sorry, I think what I'm not understanding is -- is it normal for engine to run that roughly (e.g. die in gear + idle throttle) if the alternator is under load to try to charge up a dead battery?

[ndutton]

If there is insufficient voltage to the coil, ignition spark wanes and engine performance suffers. It takes less spark energy at idle than running under load explaining why it may idle - even poorly - and not run at all in gear.

I'm not saying I magically know the alternator is dead, just that it would be good information to confirm proper operation before looking elsewhere. We don't know what the problem is, this test will either condemn or eliminate the alternator as the cause and if the latter, we can move on to other tests.

To respond directly to your alternator under load question, figure every 25 amps of alternator load takes 1 horsepower from the engine.

edit: more on alternators in general (from 17 years ago)

https://www.moyermarineforum.com/forums/showpost.php?p=356&postcount=2

[svsonora]

Thanks. I think I'm figuring this out. I will have to test my alternator tomorrow to make sure, but I think what is going on is the alternator is trying to charge the dead battery (several years old now, lead acid.. vs my house bank is AGM and always topped off with solar panels). Dead battery won't output enough juice to maintain healthy spark under load. Either way, gotta find another battery to replace the dead one.

Much appreciated!

[ndutton]

Thanks. I think I'm figuring this out. I will have to test my alternator tomorrow to make sure, but I think what is going on is the alternator is trying to charge the dead battery . . . . .

You may be right but at this point it's guessing. Solid troubleshooting is not guessing and nothing is determined to be good until it tests good. If this turns out to be a simple dead battery - - and it might - - the question will become WHY is the battery dead? Age or something else?

[svsonora]

Battery is dying for sure. Tested the battery a while back. (and if I had this knowledge you gave me on this thread before, I prob wouldn't be asking this question! ) So the only thing left to do is to test the alternator, and see if its charging up my house bank properly. Thanks again!

[ronstory]

Neil is guiding you well.

Don't forget to record the voltage for the battery at rest before starting, the coil voltage when running and voltage at the battery terminals when running. You need to use the 'typical' battery used for starting/running. If you switch it to another bank, it adds addition variables that makes the debug more challenging.

[svsonora]

Cleaning the points did it. Everything downstream of it has been replaced, in terms of ignition. I'd replace the points and condenser too (have the parts) but I'm just gonna go with the MMI offering of the electronic ign kit. Be done with this problem forever. Thanks so much for getting me to see outside what I thought was the problem.