View Full Version : Alternator causing engine miss

Mahi 1
09-13-2009, 11:12 PM
So I wired my Mando alternator today:

1) E wired to bolt on the intake manifold
2) Excite wired to Positive terminal on the coil
3) B wired to positive terminal on second batt

I didnt have the proper wire to run the B terminal over to my Perko battery selector. So I just ran a 16 gauge wire over to the Positive post like I said.

Today the Voltmeter was reading what looks to be over 14 volts at idle. The Voltmeter read 13 volts with the Alternator output disconnected.

Trouble is, once I wired up the B terminal the engine misses and losses power under load. Ask me why I know its the B terminal? B/C I ran the boat last sunday with charged batteries for 30 minutes without the alternator's output post wired to the battery. And I disconnected wire from the Positive post on the battery and the boat ran smoothly again.

ANY IDEAS why hooking up the charging output to one battery would destroy the running quality of my engine??? Not aware of Alternator's age, but the engine is a 2004. It appears to be the same age as engine.

Thanks again in advance!

Mahi 1
09-14-2009, 12:06 PM
Anyone suspect that the 16 or 18 gauge wire I temporarily ran off the alternator's output post to the battery is the problem? Perhaps it was too small a diameter to carry all the amps the alt was producing?

Like I said, I was out on the water without my electrical goody box and all I had to rig an output wire was that thin stuff.

Havent gone back to rewire with a thicker gauge yet, but hoping that solves it.

Bite Me
09-14-2009, 01:36 PM
I'd say you need to remove the alt. excite wire from the coil, and find another source of switched voltage to feed to the alt.
And yeah, you flat gotta go to heavier charging wire!
Good luck!

09-14-2009, 04:02 PM
I'd say you need to remove the alt. excite wire from the coil, and find another source of switched voltage to feed to the alt.
And yeah, you flat gotta go to heavier charging wire!
Good luck!

I concur with Bite Me on the coil wire and I dont think the 16 gauge wire is your problem but it needs to be changed to the correct gauge or it will be your next problem when it burns up.

Mahi 1
09-14-2009, 06:08 PM
Finding a new ignition source was my original idea until I searched and found all the recommendations to excite from the coil's + post. Then I figured that all those folks couldnt be wrong, plus I'm hooked up that way on my 20 footer.
But it does make the most sense to disconnect my alternator from the coil, to eliminate the interference that appears to be going on....And run a fatter wire to the batts.

09-15-2009, 04:09 PM
If the coil has a ballast resistor between it and the +12V ignition wire, that may be the cause of the problem. If there is a resistor, you need to connect to it at the terminal that is NOT connected to the coil's (+) terminal to avoid loading down the coil side.

The wire from the "B" terminal to the battery or battery switch should be AT LEAST a #10 (the Mando is supposed to be able to deliver 55 Amps, max, so a #8 would be more suitable.

Also, there should be a jumper (wire or link) connected between the "B" terminal and the "S" (sense) terminal otherwise the internal regulator will not know what the battery voltage is. Take a look at this Mando and you can see the jumper link from the "B" to "S" terminals:


Mahi 1
09-16-2009, 10:02 AM
No Ballast Resistor Kurt. And my Alt looks identical to the one pictured, jumper wire and all.
Which leads me to conclude that I ran too skinny of a wire to my port battery. I did it on the water, without the bebfit of my wiring kit. So I will run an 8 gauge wire off the Alt.. BTW, somewhere I read that there's a post on my Perko switch that is specifically designed to accept the Alternator's output wire (B post). Am I correct? Otherwise, I could use some guidance in the best method to get both batteries charged off the B post. :)

09-16-2009, 11:49 AM
Then, I'm at a loss as to why there should be an ignition problem because of where you connected the "EXC" terminal. On my engine, there are separate wires, in the harness, that go to the coil (+) and to the"EXC" term on the alternator BUT they are made common in the harness.

The field does not draw that much current (when all is working right) and should not pull the ignition feed voltage down. The #16 wire to the battery from the "B" terminal should also not have been the culprit either. The only thing that would have done would be cause the thin wire to get (dangerously) hot if the alternator was charging at near full capacity. Also, it would run hot if you had a decent amount of accessories/electronics on while running. The resistance in the #16 wire would not have let the alternator supply full current to the batt(s) and other items on the system.

Also, to prevent problems if the diodes short, the wire from the "B" terminal is usually connected to the ignition switch side of the engine breaker (away from the battery) to protect the wiring harness from overcurrent from the battery through the shorted diode(s).

Diagram of a carbed Ford engine:


The item you mentioned about the battery switch with an alternator connection, I think, refers to a set of field interrupter contacts that removes the field excitation voltage (from the "EXC" terminal) when switching the batteries with the engine running. This is to protect the alternator diodes from surges if one battery disconnects before the other connects. My switches do not have those contacts since they are what is called "make before break". The only danger there is turning the switch to off with the engine running - that can take out the diodes. It's not a recommended practice.

09-17-2009, 03:52 PM
The under sized 16 gauge wire to the battery is not the problem. I am starting to think based on what you have said there could be intermittent grounding in the regulator causing this. Is it possible you can get the Alt tested or switch it out with someone else to test it.