Alternator Exciter

[JLR]

I am trying to chase down a problem on one one of my alternators on my 1292 DDEC's. I pulled the alternator from my port engine and had it (and its bad diode) repaired. When reinstalled, the alternator as bench tested seems to be working fine but does not appear to be called upon to put out. I am guessing that I need to trace down the parts of the wiring that excite the alternator. I also have a steering cooling pump driven by the same alternator that needs the same exciter to energize the pump. Any ideas what I should be looking for?

 

[bostonhatteras]

fuel pressure switch perhaps. located on secondary housing.

 

[JLR]

Fuel pressure switch is a good guess but my hour meter is working properly and that is usually driven by the fuel pressure switch.

 

[bostonhatteras]

Fuel pressure switch is a good guess but my hour meter is working properly and that is usually driven by the fuel pressure switch.

What wires connect to the alternator? Trace them back and youll start to see how it works. External regulators? alternator model number? Self excite? one wire connection? There is the turn on speed to consider too alternators produce electricity based on the speed they rotate. at idle some produce little to no juice.

 

[JLR]

I will have all of that information later on in the week. As for wires, I believe that there are three or so coming off of the alternator. Two of those wires appear to be relatively heavy gauge on the other is about a 14-16 gauge wire. It is a 65Amp 24V alternator (Leece) I believe. I increased rpms as high as 1500 and saw no increase in output from the alternator.

 

[bostonhatteras]

with a multimeter, run the engine and check the voltage at all the points. leece Neville/prestolite all have isolated ground so the two large wires must be 24v + and -. The remaining wire will be the voltage sense wire. in on the road situations the voltage sense will often be connected by means of a small jumper wire to the positive lug on the alt. boats often have this removed and voltage switched via an external means. You could make a temporary jumper wire to feed 24v from + lug to the voltage sense (field) terminal to see if alternator turns on. Now I don't know which alternator you have but if I am right it will have a stamped metal case bolted to the back that houses the regulator and the two big lugs will be on either side of that. The top of the alternator has two smaller terminals the left side is voltage sense and the right is a lamp connection. The lamp connection is unnecessary for operation but the voltage sense (field) is critical. measure the voltage of the voltage sense on and off the terminal, with it running. if there is voltage present in both situations then the problem most likely is the alternator. if there is no voltage present off the alternator then look to the switching you asked about in the first post. like I said you can add the jumper wire to check the output of the alternator.

 

[JLR]

Thanks. It sounds like tracing the voltage sensing wire is the way to go. By the way, on the two larger wires, they appear to have rubber boots, like spark plug wires.

 

[JLR]

Sorry to keep this thread going but I am just not that familiar with the DDEC engines. If the fuel pressure sensor is what excites the alternator, and hypothetically, that fuel pressure sensor is not working, wouldn't I get a fault code of some sort on the EDM for that engine? I am just trying to be as efficient as possible in tracing the source of the voltage sensor to the alternator. Thanks.

 

[JLR]

Still playing around with this and still trying to figure out what excites the alternator. The exciter wire is bundled so it is impossible to trace. Sorry for a really silly question but here goes. Is there any reason why I can't simply run a small wire from a source that I know turns on with the ignition and run that wire directly to the voltage sense? I know it would mean that the alternator would be on any time that the engine is on but is that a problem?

 

[JLR]

Any ideas?

 

[doc g]

Maybe the 24 volt batty charger is on at the same time with the engine running ,so the alternator does not see a need to put out??????.................Pat

 

[JLR]

Pat - that makes a lot of sense and I never thought of it. However, it still does not explain why the Detroit Diesel readout on the EDM shows a declining number getting to as low as 11.3 volts on the readout. The analog charging gauges show a charge of upwards of 24-25 volts so something is happening, just not sure what. My best case scenario is that the alternator is working fine and the battery bank is just shot. Replacing all 8d batteries next week and checking altnator output at the same time. Roger mentioned that the altnator is self exciting above about 1250 rpm's with an OEM dropping (something) which drops the voltage by 1.5 volts to keep the alternator working. Now on to another post about electrical gremlins.

 

[dsharp]

I would think that there is a wiring diagram for that engine floating around on the internet. They are usually triggered by a "hobbs" switch on the fuel or oil pressure.

 

[JLR]

According to the Hatteras blueprints, the alternator is excited by the fuel pressure switch. Been tracking down why the exciting wire on the fuel pressure switch has 26volts even when the boat is off. I also learned yesterday that the alternator is warm to the touch even when the boat is not running. So, the first problem is why is the alternator warm when the boat is not running. The second inquiry is the possibility that the exciting wire is being back fed from the ignition post on the alternator because the alternator must be running at rest. I've received some great help from Karl who, as usual, is very gracious with his time. Any additional help would be appreciated. Also, with this 24v 65 amp Neece alternator, is the regulator external and if so, easily replaceable? I do not want to trash the new batteries I just put in. Finally, since I have been playing around with this for too long, what is th cost of just replacing the alternator with a new one. I am having a hard time differentiating between prices of what appear to be similar alternators running from $140 to 2K.

 

[davidwigler]

Put in a good battery monitoring device like this:

http://www.amazon.com/Victron-BMV-7...53850&sr=8-2&keywords=victron+battery+monitor

In addition to providing useful information about the condition and State of charge of your batteries, it will help diagnose any current draw with the engines and chargers off. It will also tell you if your analog voltage meters are malfunctioning.

 

[JLR]

Can anyone verify that this is the replacement for the alternator I am talking about:
R0459153 DETROIT DIESEL Alternator DR 27SI/200 65A/24V

 

[Reefgeorge]

There are some good thoughts here but perhaps some clarification might be helpful.

There are two basic types of alternators: internally excited (usually "one" wire) and externally excited "three wire". The literal number of wires can be confusing because as one poster said the ground may have its own wire as apposed to the alternator being grounded to the frame/engine.

An internally excited aternator is the kind that might need the rpm's to increase before the exciter circuit turns on and alternator produces output then it should output at all rpms until it is turned off and you start all over again.

The concept that some alternators produce little "juice" at low rpms can mean two things. 1) the process above has not occured yet and 2) the alternator is on but not producing its rated output current like say 80 amps. In this case the voltage will be a charging voltage unless the batteries are depleted and the voltage will not rise until the batteries start replenishing. In the case of healthy batteries, wire connections, and an excited alternator you will see the expected 28.6 or so volts at idle, just not 80 amps.

You most likely have an alternator that is not internally excited and therefore not dependant on getting to a high rpm before it will turn on. In this case there are four possible wires. 1) the output wire which will be the largest gauge, usually red,usually booted which sends the alternator output to the batteries. 2) possibly a ground wire, usually black, usually booted that grounds the alternator either to a lug on the engine or goes back to the batteries (also large gauge). 3) An exciter wire which presents system voltage to the regulator only when the engine is running or the ignition switch is at least on so that the alternator only tries doing its thing when its supposed to. A fuel pressure switch or other method may be used to idicate a running engine. If the engine is not running and the ignition switch is off there should not be system voltage at this wire. Its color is pot luck but probably red, purple, brown, etc. 4) a remote sense wire. In this case the alternator does not determine the state of the battery charge at its own output as this can cause many problems. Instead a wire is run from the batteries or near them like at the big system breaker back to the regulator sensing terminal. This way the alternator attempts to deliver the appropriate charging votage to the batteries not the alternator output post. This wire may have battery votage on it all the time. Color is also pot luck. The last two wires are much smaller gauge as they carry little current.

If you want to see if your alternator will produce the proper output start the engine and measure the sensing voltage which should be very close to the battery voltage and the exciter voltage which should be very close to the ignition voltage. You can disconnect both and present battery voltage to see if you get the proper output. Start this test with batteries that are properly charged and sitting near 100% charge which is around 25.4v when you start the engine. Also do all your wire swapping first. Do not ever preturb the wiring while the engine is running because this can cause a load dump scenario and damage the alternator.

 

[JLR]

George - thanks for the help. After looking closely today, here is what I have. First, I have an alternator that is warm to the touch when at rest. That can't be normal especially since my other alternator is not. Second, I have three terminals. One is positive carrying a large gauge wire, pressumably to the positive on the battery. The second terminal also carries a large gauge wire and is the negative post. The final post says ign( presumably ignition). It has two wires of a smaller gauge attached to it. One goes to my fuel pressure switch on the side of the switch that is dead until the fuel pressure gets above 4psi. We will talk about this wire (I'll call it the exciter) in a moment. The second small gauge wire on the ignition terminal goes to end end of a mounted power resistor. The other end of the power resistor goes to the negative terminal. Here are my questions. First, why is the alternator warm at rest. Second, why does the fuel pressure switch end of the exciter wire carry 26 volts at rest. I am guessing because the ignition post now is putting out 26 volts at rest. Third, what do I do to get this fixed, including whether I should just replace the whole alternator since it was rebuilt once already. Fourth, if the regulator rather than the alternator is the problem, how to I get that fixed. Thanks all for your continued help.

 

[Reefgeorge]

I suggest that you look at these items and compare the results to the other engine as a check:

1) With everything at rest, what is the voltage at the ign terminal of the alternator and both sides of the fuel pressure switch?

2) If there is ignition voltage at the ign terminal remove the two wires and measure the voltage on the empty ign terminal post

3) If there is ign voltage on the empty ign terminal of the malfunctioning alternator and not the good alternator under the same conditions you likely hqve a bad alt/reg. A check would be a clamp ammeter on both +V batt cables to verify that the bad alt is drawing current with an open ign term.

Is the regulator external and removable?

 

[JLR]

George - thanks so much. It appears as if the regulator is external and removable. At least it has for nuts on it . and houses the ignition terminal only. It also has a separate part number and an indication that it is 28v. That part is readily obtainable for about $125. From what I now see, the exact alternator, including the regulator can be bought new as a Detroit Diesel part for just under $3,000. Another site has it listed for just under $2,000. Is there an easy way to tell whether the regulator is shot versus the alternator. It sure would be an easier fix? Finally, and under a short term worst case scenario, what would happen if I were unable to get it fixed in the near term and just removed the main red higher gauge wire to the alternator. The only reason I am so concerned about this is because I have 1292 TA DDEC engines and I have been told that the DDEC's do not like fluctuating voltage. I always run with my battery charger on. Thanks again for the help. By the way, to answer your first question, there is voltage on one side of the fuel pressure switch at neutral and not on the other side. Unless, I connect the exciter cable, even at rest, and that wire does have voltage. Of course the other engine has one hot side at rest and two wires on the other terminals at rest with no voltage on either wire.