Delco externally regulated alternator

I have a Perkins 4-108 in my sailboat. I wanted to upgrade the alternator from a Delco (12si, I think) to an externally regulated unit of about 100 amps. I obtained a Balmar ARS-4 multi-stage regulator and I was all set to replace the delco with that, and a Leece-Neville alternator(2" delco mount). But I ran into problems: the L-N had a different pulley mounting distance and also (according to the alternator shop), would not work properly with my tach. So the shop recommended that they build me a new Delco 100 amp with a dummy reg. so that the external Balmar would work. INSTEAD, they rebuilt my old Delco (with an expensive 100 amp stator) and sent me on my way.


I was not pleased (the old delco was to be my spare) but I installed it any way. But it's performance sucks. It does not show any inclination to start charging until about 1,400 engine rpms (works out to about 2,900 alternator rpms). If I reduce rpms, the voltage falls off quickly to battery voltage. I went through the system (full field test etc.) and I think the problem is the alternator. So I went back to the shop and asked them to re-test it. I watch and notice that their "test bench" (home-made junk) spins about 4,000 rpm. No wonder they got 100 amps! But they claim they did what I asked (BULL) and so I have to be satisfied with the result.

My question(s) are;
Has anyone else been able to successfully regulate a Delco 12si externally? Or do all delco 12si refuse to charge below 2,600-2,900 alternator rpms?

Comments for Delco externally regulated alternator

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Apr 08, 2009
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Give it a shot of throttle
by: ASA Instructor

These regulators need to get above a certain speed to excite the system and begin charging. Start the motor and after the first minute to make sure the oil is circulating through the motor, goose the throttle to about 2,000 and bring it back down to idle. You'll see your voltmeter jump from 12v to 13.8v and the ammeter go to charge. It will charge just fine at idle...once you put it into the charging mode.

No need to change pulleys or anything else...that's the way it works. Some people will change it from a 3 wire to a 1 wire to avoid the need to accelerate the engine but it's a simple thing to remember.

For training, buy yourself a VW with an alternator...it soon becomes second nature to give it a shot of throttle right after you start up.

Sep 08, 2007
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Delco Alternator Problems & Modifications
by: Anonymous

Yes, Delco integrally-regulated alternators can be modified to use external regulators.

No, it should be producing current with a shaft speed of 1000 +/- RPM.

Most integrally-regulated Delco models must have battery voltage present at the output terminal to work. If a multi-battery isolator has been installed, additional circuitry (a resistor, diode, and relay) is needed. This is applicable mainly to the Leece-Neville units with integral adjustable regulators.

The modification for your unit is a little tricky, hard to lay out without photos, which I don't have. Try these folks for a solution first: DB Electrical, 800-753-2242. They are a big alternator shop in Tennessee.

Most alternator rebuilders and their workers understand as much about how the thing works as a hamburger cook knows about raising a cow. They just put the parts together.

When determining whether your system is working properly, you can't go by a voltage measurement alone. If the voltage is lower than expected, you'll need to know the current, right at the alternator output, to make a clear determination. Alternators are usually rated for output at a shaft speed of 5000 RPM.

Yours is a fairly common challenge in setting up a smaller sailboat for cruising. It is generally best solved by bracketing and fitting a complete installation from the start

Sep 08, 2007
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More on Delco Problem
by: Matt

I have no experience at all with the Delco. My only experienc was with a now obsolete alternator body, i.e. hard to get so
I sympathise with you since I know it can be a big hassle to try and upgrade an alternator.

I took my alternator to a auto elec to get a new one, after this turned out to be impossible I sent the old one of to get rewound and set up for external regulation. In the end it never got rewound and since it seems practically
impossible to get a more powerful alternator with an identical body, I am faced with modifiying the attachment points on
the engine, i.e. welding on new brackets. All a bit much at present. Sounds a bit similar to your probs.

Few things that might be helpful.
1. If you give all the measurements to an auto electrician (or take the alternator to them)they have books and books on different types of alternators that could fit the specs.

2. I have two pulleys of different sizes on the front of my engine. One is for the water pump and one for the engine driven compressor.
But you might be able to do the same for an alternator that runs at a different RPM. I investigated pully spacers for a new alternator
but in the end have not gone down that road.

3. Some external regs are very sensitive about how you set them up. I know my Ample Power Next Step had all sorts of warnings about which wires to connect first etc. It also has a bunch of LED codes to debug it, a bit primitive but it works

I know there is quite a bit of stuff in the book about this type of thing, but perhaps not on your specific alterantor.

Sorry I couldn't be of more help.

Sep 08, 2007
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Pulley Sizea
by: Anonymous

I'm aware of the relationship between pulley sizes and rpms. My crank pulley is about 5.5" and the alternator pulley is 2.625" giving me a ratio of about 2.1. That's why in my post I noted the alternator rpms of 2,900 with engine rpms of 1,400 before the alternator starts charging. The smallest pulley one should use is 2.5" so I won't gain anything much there. A larger crank pulley would help but would also drive my water pump faster; probably not desirable. It all comes back to an alternator that just not seem to perform properly. I'm wondering if the rotor is a dud or if the rotor is not compatible with the 100 amp stator they installed (I think the stock stator was 60 amp).
I have of course considered using it as a spare. In fact, before the dodos rebuilt it (w/o my consent!), that was exactly my plan. But I would still like to hear from someone who has converted a Delco 10si or 12si to an external regulator, and how it worked out; if it worked at all

Sep 08, 2007
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Change the Pulley sizes
by: Anonymous

I have been going through similar issues.
You have to measure the pulley sizes (radius, circumference) to figure out the RPM the alternator will be driven at for different engine RPMS.

So looks like one choice is to change one or both of the pulleys to get the alternator rpm range right.

Alternatively get another alterantor and use the incorrectly set alternator as a spare. Live and learn.

To install a more powerful alternator I broke the problem down into smaller steps. i.e. step 1 get the external reg working with the existing alternator. step 2. either modify alternator
or change alternator attachemnt point to allow larger alternator or install another bracket for a second alternator. But I hear that a second alternator can sometimes place too much pressure on engine bearings. I have done step 1. For step 2 I think I will install a second alternator

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