Another blown alternator diode

[TomG]

Late last fall, I found myself owning a new old boat. She had a few warts and hairs, but we've slowly and methodically worked through many of them. One of the minor problems involved a bad alternator. I took the alternator to a local shop here in Annapolis and they had the alternator repaired in short order. I few weeks ago, I finally got the boat in the water and things have gone well with the boat and the Moyer rebuilt A-4.

On a whim last week, I checked the alternator output and found that, lo and behold, there wasn't any. Output voltage equaled battery voltage. "This can't be!" I said to myself. "I just had the alternator rebuilt!" I exclaimed to no one. I pulled the alternator and dropped it by the shop today and sure enough, the internal diode(s) were blown.

What I have discovered is this and I wonder if the Brain Trust can shed some light. Apparently, the boat was at one time wired with an ammeter (8 gauge red wire from the starter solenoid to the cockpit instrument cluster) that has since been replaced with something else (probably by looking at the instrument cluster, a fuel gauge). The alternator output wire (10AWG Orange) goes to the instrument panel as well. What was apparently done by someone in the past was, when the ammeter was removed, the ammeter circuit was simply spliced together with the alternator output wire which in effect sends the alt output from the alternator to the ignition switch (about 15 feet on my boat) then BACK down the old ammeter circuit to the starter post THEN on to the battery.

My question is, would this long wiring run and subsequent voltage drop (and perhaps a little corrosion resistance) be enough to cause alternator problems?

Would I be better off sending the altnernator output right back to the starter solenoid?

Would love to hear some insight before I put the alternator back into the system.

[hanleyclifford]

In order to solve this problem we need to know where voltage is being sensed. I don't like the sound of that charging circuit. Do you have internal or external regulation? The point is this: If you have internal regulation and sensing is being accomplished at the output terminal of the alternator, you must provide a clean, straight path back to the batterys for this wire. You cannot "sense" on a wire that is being loaded by a user without confusing the alternator and causing it to put out more than the batterys request.

[ndutton]

My question is, would this long wiring run and subsequent voltage drop (and perhaps a little corrosion resistance) be enough to cause alternator problems?

Oh yeah, long run, marginal wire size, a dose of corrosion, no doubt several connections, any of which could be loose, all of which can wreak havoc with your system and make troubleshooting difficult. But that's a general statement. As Hanley said, to be specific we need to know more about your alternator and its regulation.

Would I be better off sending the alternator output right back to the starter solenoid?

That's how mine is, a single #4 wire maybe six inches long, alt output to starter post. But know that I have a single wire, internally sensed and regulated alternator. What works for me may not work for you.

[TomG]

Hanley and Neil,
This is an original 35A Motorola alternator with internal regulator. There is only the output terminal and the exciter wire to connect. The exciter wire is connected to the + side of the coil. The output wire (orange) goes from the alternator all the way to the instrument panel in cockpit where is was (very poorly) spliced in with the old ammeter circuit.



My intention is to wire the alternator output directly to the starter solenoid.

[ndutton]

Sounds like a plan. It will certainly be better. I'd scrutinize the exciter wire at the same time, termination and plug connector. May as well be thorough.

[TomG]

Neil,

Thanks for the help. I checked the exciter wire and it looks fine. No kinks, no corrosion, no loose connections. I also replaced the original connector lug on the output wire when I worked on the alternator this winter. What do not want to do is get the altnernator repaired (again) only to put it back in the same system thereby having the same thing happen again. I figure running the output to the starter will mostly eliminate circuit issues downstream from the present setup. I just want to tap into the wealth of experience here to see if there is something else to consider before blowing yet another diode or two.

Thanks for the help!

[smosher]

Its opens that blows the diodes. So either a connection is opening up or there's a break before make battery switch.

Steve

[TomG]

Steve,

Good point. I originally thought it was somoeone going through "OFF" on the switch that had blown the diodes the first time. However, since that hasn't happened since the repair, I am thiking it is something more sinister and hard to find. Like a broken wire, resistance, or corrosion or a combination.

[hanleyclifford]

Here's what has been happening: The alternator has ben charging thru the ignition switch and a whole lot of wire of questionable size and conductivity. The ignition switch has been feeding loads from the same circuit while the voltage regulator has been trying to figure out what is going on because it has been sensing from the same circuit. The voltage has been all over the planet. Voltage in a dynamic system (unlike a static one) is variable depending on where you sense it. That is why voltage meters with rotary switches were invented. It is the battery voltage that is important and must be maintained. Other voltages in the system are also important for different reasons. Hope this helps.

[tenders]

And also make sure that you don't change the position of the OFF-1-ALL-2 battery switch while the engine is running. Especially to OFF -- that will blow the diodes in a hurry too.

This story reinforces my view that for most engines the hassle of running proper wiring for an ammeter on the instrument panel isn't worth the information the ammeter provides and the frequency with which you need it, compared to what you need to run for a voltmeter and what you get from it.

[TomG]

Thank you all for your thoughtful replies. I had hoped to avoid a rewiring project during sailing season, but I think I will go ahead with my plans to rewire the alternator, the ignition switch, and the DC panel as I also install the new Blue Sea ACR and battery switch. We'll see how it goes!

Thanks again for all your help.

[hanleyclifford]

Two ideas are growing in my head as a result of this discussion: 1) The instrument panel mounted ammeter is becoming suspect wherever big loads are being added to a boat's system and being placed in the loop that goes from alternator to battery. 2) Internally regulated alternators which get their sensing signal from the output terminal are especially vulnerable to voltage variations induced by long ammeter runs and large intermittent loads. It's easy to see why Neil and others recommend going to a voltmeter for battery monitoring. The complexity of our electrical systems has outstripped the practicality of the in- line ammeter. If we want to have an ammeter and have large loads it is best to go to the shunt type ammeter such as the Blue Sea Systems units. The very minimum consideration especially where internally regulated alternators are concerned: the output from the alternator must run to the battery bank in the most direct and unimpeded manner possible with the largest practical conductor.