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Old 10-01-2012, 18:30   #1
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Alternator Stalls Engine ?

I've recently had a problem starting the engine (John Deere 4045DFM, less than 200 hours) in Boracay.

Problem was that the engine would initially fire, then struggle to pick up speed to idle. Sometimes it would not start.

Initially I thought it was the battery, as it was getting old (5 years). The battery is an N120 which is a big battery. So I replaced the battery.

Now the engine starts much better, but there is still a RPM drop just after it catches. I thought it could have been air in the fuel, so I've bled the system (much better at it now) but there's still a bit of a drop.

My opinion is that it's due to my battery charge setup. I've the alternator set up to charge the house batteries (4 x Trojan T105s) and they got used pretty heavily over Christmas. There's an Echo charge to top up the start batteries.

So I'm thinking that as the engine starts the alternator starts charging, and as it charges very well at idle RPM it drags the engine down and sometimes stalls it. The easy solution is to keep the house batteries well charged, but that's not going to be happening all the time.

One solution that comes to mind would be to check that the alternator is excited by the start battery and to put a switch on the positive cable to the house bank. Turn off before starting, on the charge after the engine is running smoothly.

But am I correct, and what should I do? There's been no complaints from management as yet, but that could change...
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Old 10-01-2012, 18:50   #2
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Re: Alternator stalls engine?

Typically a high output alternator and regulator will slow down an engine, especially at first startup. a 100 amp alternator might be putting out 85 amps for 10-15 minutes. But that is only maybe 1.5 -2 HP. How much HP is your engine and your alternator?
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Old 10-01-2012, 18:56   #3
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Re: Alternator stalls engine?

Does it run well if you advance the throttle a bit to take it back up to normal idle speed? Maybe the engine needs to be run hard to clear it out? You don't give us much to go by.
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Old 10-01-2012, 19:06   #4
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Re: Alternator stalls engine?

Hmmm, good point, is the idle set too low? I guess that's what.... 80 hp engine? Seems it should hardly know the alternator is there......????
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Old 10-01-2012, 19:10   #5
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Re: Alternator stalls engine?

Does it make a difference once the engine is warmed up? My engine doesn't like to be at idle until it has run a while. I never even start-up my engine with the throttle at idle.
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Old 10-01-2012, 20:00   #6
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Re: Alternator stalls engine?

G'Day Boracay,

It is a bit surprising that the alternator load is causing such a problem on a big JD. But, if it is there are some easy fixes. Expensive: many regulators have "soft start" features, where the field isn't excited at all for a short period (30-90 seconds) and then the field current is ramped up slowly over a minute or so. This helps avoid stalling. One such reg is the Balmar, and likely there are others. Cheap: put a simple toggle switch, rated at 10+amps, in the field circuit between the reg and the alternator. Leave the switch in the "off" position until the engine is stable, then switch it on and the alternator will start charging normally.

Good luck, mate!

Jim
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Old 10-01-2012, 20:38   #7
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Re: Alternator stalls engine?

I doubt seriously that it's the alternator. A 100A alternator at idle won't pull much load at all, certainly not enough to stall a bigg-ish engine.

However, what Jim said is a good solution anyway to avoid sudden alternator loads. The Balmar MC-612 and MC-614 regulators, among others, automatically delay the startup of the alternator to allow the engine to warm up a bit.

To see if it's really the alternator, just break the field current to the alternator before starting and see if the engine really behaves differently.

Bill
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Old 13-01-2012, 06:12   #8
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Re: Alternator stalls engine?

I had similar symptoms on a smaller (3GM30) engine. It turned out to be an engine problem. Had the fuel injectors and injector pump cleaned and pump re-calibrated and it runs like new. The alternator is a Mastervolt 90a controlled by a Balmar 612.

I can't imagine that an 80hp engine would have trouble running a large alternator at idle. The governor should simply provide the fuel needed to turn the alternator. After all, it can't require more than 1 or 2 hp.
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Old 13-01-2012, 07:01   #9
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Re: Alternator Stalls Engine ?

100 Amps @ 12VDC is 1.63 Metric HP (in the perfect world without losses) A metric HP is like 98% of a US mechanical HP

Crap, I finally beat Gord to one of these!
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Old 13-01-2012, 07:47   #10
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Re: Alternator Stalls Engine ?

I recall running my perkins 4018 engine at 2000 rpms minding my own business. I had recently installed a 2800w inverter to power ac circuits. Wife decides to make hot tea and puts into microwave. Engine drops to about 1800 RPMS and I see amp meter jump. So yes a large load on alternator can effect engine speed. Alternator I must add was standard 55 amp, nothing heavy duty.
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Old 13-01-2012, 22:28   #11
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Maybe not the alternator...

I did another test start yesterday, batteries looking pretty well charged.

It did it again. Struggled to fire, then finally did.

So, in desperation I did the first bleed at the filter. Lots of bubbles, but fired right up.

So I took the top off the Racor. Nothing. Poured in a fair slurp of diesel to top it up, fired 'er up again. So far so good.

Took the top off the Racor again, needs topping up. Poured in more diesel.

So I'm thinking that I've an air leak in the fuel line between the tank and the filter, or in the filter. Or maybe even a blockage in the fuel pick up. Any thoughts?

Next step. I dismantle half the boat, pull out the fuel line and pickup and rebuild to totally airtight.

Fingers crossed.
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Old 13-01-2012, 22:35   #12
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Re: Alternator Stalls Engine ?

Quote:
Originally Posted by S/V_Surya View Post
I recall running my perkins 4018 engine at 2000 rpms minding my own business. I had recently installed a 2800w inverter to power ac circuits. Wife decides to make hot tea and puts into microwave. Engine drops to about 1800 RPMS and I see amp meter jump. So yes a large load on alternator can effect engine speed. Alternator I must add was standard 55 amp, nothing heavy duty.

If your tach was alternator driven, the engine probably dropped to 1950 rpm and the rest was belt slippage.
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Old 14-01-2012, 02:14   #13
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Re: Alternator Stalls Engine ?

Quote:
Originally Posted by donradcliffe View Post
If your tach was alternator driven, the engine probably dropped to 1950 rpm and the rest was belt slippage.
I concur -- at 2k revs your engine would hardly notice the load, but the skinny belt on a stock alternator might well not like the sudden extra load.

And for the OP, glad that you seem to be on the right track now. Air leaks in the suction side of the fuel system are a bugger to find unless you can work out a way to pressurize it.

Cheers,

Jim
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