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Discussion Starter · #1 ·
Since I've had few charging issues with my cars over the years, I do want to understand this battery voltage to generator output system. This question came up because my 59 2000 Spider needs to idle at 1100 + for the red light charging indicator to go out and the usual suspects have been ruled out. I do get the differential voltage issue as it relates to the red indicator but looking at the specs, I'm confused. The specs, from the Giulietta manual, indicate a Marelli generator rpm of 1350 to start producing 12volts(1050 - 1200 @13v for the Lucas). But I believe 700 rpm or so is stated for the idle speed (which won't happen with these Solex carbs). It would indicate that at idle the car would be draining the battery.
So am I to assume this generator rpm is not the engine rpm but the generator rpm on a test bed? Knowing this won't help but still interested in the answer. After researching the regulator function, I think I can rule out the original Marelli as any cause; or am I wrong with this assumption? Also, I have no intention at this time to convert to an alternator. Thanks, Mark


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I tend to believe both of your notions are correct:
  • Engine RPM generator RPM, as indicated by the difference in belt pulley diameters (and the ratio being defined by their circumferences).
  • Yes, at idle the warning light is typically on (or flickering), indicating the battery will be drained. 1,100 RPM for the warning light to go out completely seems quite reasonable.
That being said, generators must be properly polarized for them to deliver the specified power. Sometimes, they seem to lose some or all of that polarization (but I have no experience with polarizing them and there cannot provide guidance about how to do it -- this YouTube video may help).
 
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One of the historical arguments for an alternator was their ability to deliver a usable charge at lower RPM.

Don't worry. Your battery has plenty of reserve for the brief amount of time you'll be idling.

In my opinion, the bigger question is the antiquated design of the Marelli DN44 generator.

The front bearing uses two, metallic "shields" to hold the grease in, and the rear bearing is an oilite bushing that has an external lubrication tube that needs periodic oiling. If the front bearing fails, you'll need a lot of expensive parts, or a complete replacement generator.

My recommendation is to replace the DN44 with a little Denso alternator, and keep the Marelli in a box for entries in Concours shows or the future sale of your car.

I still have the necessary adapter bushings, and one alternator if you're interested.
 

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Discussion Starter · #4 ·
Thanks Ruedi and Don. Dealing with all the cars in my past I must say I've always assumed if the red charging indicator was on that the idle was too low and a little disconcerting while in traffic. Interesting that the(per specs) Lucas generator puts out 13 volts while the Marelli just puts out 12 volts. I just charged the battery with a charger that brought the battery full charge to just about 13 volts. It usually sits around just a little over 12 volts so maybe that differential keeps the indicator lit even more then what usually happens with this car. I also have a Pertronix 9MR-143 ignition sensor that needs min. 12 volts to work at all so I get a little sensitive when it comes to charging and maintaining the 12 volts.

As you mentioned Don about those Marelli generators, they are very feeble at best. I have one on my shelf from an early Giulietta veloce where the front magnesium plate has cracked in several pieces. The plate was welded once before but it didn't last too long, probably from the bearing not sitting correctly in the plate. It's too deformed now to fix but at one point was thinking about metal printing these but never found the time. And thanks Don for the alternator offer but I'm not driving it much at this point to be a major concern but I will keep it in mind.
 

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To effectively charge a 12V battery, the charging device must create more than 12 V. The details on the Marelli may say 12V as a way of indicating the nominal rating for the car's system. Or, it is stating the generator RPM to REACH 12V, above which the voltage will increase to whatever the regulator is set for.
 

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Discussion Starter · #6 ·
To effectively charge a 12V battery, the charging device must create more than 12 V. The details on the Marelli may say 12V as a way of indicating the nominal rating for the car's system. Or, it is stating the generator RPM to REACH 12V, above which the voltage will increase to whatever the regulator is set for.
That makes sense, as the regulator controls overcharging the system. I might check (as Reudi mentioned) the pulley ratios, when I get time, to understand the rpm issue and see if it comes out as noted in the specs.
 

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Some additional comments:
  • As the pulleys have V-grooves, the width of the belt has some impact on what radius they ride on (narrower belt rides closer to the hub, wider belt farther away) -- hence, this may have some impact on the pulley ratio. Belt tension also matters in context of actual radius and belt slip.
  • Although car batteries are referred to as 12 V, the actual voltage is actually between 12.3 V and 13.2 V depending on the material of the electrodes and charge level. Modern systems typically use a charge voltage of 14.6 V.
  • The type of battery and the resistance of the wiring between the generator and the battery (also on the ground loop of the battery and grounding strap of the engine) may affect whether or not the charge warning light stays on even though the battery is charged. The two pages copied below (from a Packard BB here) explain it as follows (for a 6V system):
Motor vehicle Font Parallel Engineering Paper


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Discussion Starter · #9 ·
Thanks for the info.The resistance in the circuit especially at the ignition switch is interesting.
 

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The pulley ratios are not a major factor. For a street car, the designers will turn the alternator/generator over a range of speeds that avoids over speeding the charging device, but which gets it up to a productive charging speed in a reasonably short amount of time. The speed of the charging device is not what determines the voltage or current capacity.
 

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I have lived with my red light on my 2600 at 1100 rpm for 14 years . Never had a flat battery, I'd be concerned if it didn't come on. I also live with zero oil pressure at 1100 rpm when hot , its an alfa . Press the go pedal and all comes good. I have a working relationship with my spider now. If I don't work on it it doesn't break down.
 

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I second what Redmerlin says. The generator on the Duetto has been trouble free for the 37 years that I've had the car. It has been rebuilt twice as precautionary measures. The Two-liter's generators have been trouble free for thirty two years -between two cars. Yes, I do give the 'oiler' Don mentioned an occasional drop or two of motor oil and I turn the 'greaser knob' on the distributor when oiling the generator. The generator light may come on for a few seconds when starting from a stop but no worries. My last battery in the TR from AutoZone lasted 10 years before it gave up -the new battery from NApa has a lot more kick to it then the old battery.

My advice, drive it every week or two and use Rec 90 or similar non-ethanol fuel (Ethanol = 💩) .
Mark
 

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For what it’s worth, I just dealt with a related issue on my 1960 2000. The car would die randomly. Sometimes when warm, sometimes after just a few miles. The problem was definitely electrical. Replaced the coil, removed an old kill-switch that had been installed, checked and cleaned the fuse box, etc, etc. Ran fine for a bit and then I decided to take it out at dusk. Lights seemed a bit dim. Then got dimmer. It took me six-seven jump starts to get the 2-3 miles back home (I have a patient wife—for now). The Marelli DN44 was spinning (I could run it backwards like a motor with 12v through it) but was only putting out 12v, even revving to 1,500 — 2,000. Voltage regulator seemed OK. Cut-out was open and regulator was closed—wasn’t sure how to test beyond that so I cleaned the points and put it back together. Then I took apart the generator. After breaking a copper connector and then cracking and losing a couple of the Bakelite insulators while re-assembling with a new copper connector I made from some copper flashing, I gave up. Took the generator and regulator to Rite Way Electrical in SF. $400 later, I have a rebuilt generator and a new voltage regulator. Reinstalled and am now getting 12v at 800 RPM idle and 13.5v at 2,000 RPM at the battery. My hope is that my electrical problems stemmed from the fact that the battery wasn’t charging and would eventually dip below the power needed for the ignition system. At least, that’s what I’m telling myself. I will let you know when I have the guts to drive it at night again.
 

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Discussion Starter · #14 ·
Thanks for the story, Chris. It seems the rebuild will work out unless there is an issue with the battery or some phantom ground problem. I'm originally from Buffalo, NY and very sensitive to battery charging issues. I've spent many a wet cold night in traffic with the heater on, wipers on, lights and radio on and the engine stuttering along. When I see a red charging light on or flickering, I guess I have some PTSD with it. My generator has been rebuilt but I probably need to check out its performance. I found the owners manual state that the red light should go out at 900 rpm.
 
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Thanks for the story, Chris. It seems the rebuild will work out unless there is an issue with the battery or some phantom ground problem. I'm originally from Buffalo, NY and very sensitive to battery charging issues. I've spent many a wet cold night in traffic with the heater on, wipers on, lights and radio on and the engine stuttering along. When I see a red charging light on or flickering, I guess I have some PTSD with it. My generator has been rebuilt but I probably need to check out its performance. I found the owners manual state that the red light should go out at 900 rpm.
So far so good. I haven’t driven it enough to have confidence in the charging system beyond what the meter says. We’ll see. I was born in Gloversville (between Albany and Utica) and spent most of my life sitting in the rain in traffic in the northeast before I had the sense to move west and sit in traffic in California. My PTSD is with window fogging and weak defrosters.
 
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