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65 Mercury Marauder 390 - After messing around with a couple voltage regulators and stock alternators, in an attempt to solve an overcharging issue, I finally went with a Tuff Stuff 100-amp single-wire alternator.

I thought this solved everything, as the new unit is charging the battery consistently between 14.6-14.75 volts at idle.

I happened to check the battery with a voltmeter with the engine shut off after a 20-minute drive tonight and noticed it read 13.1, dropping to about 12.9 within 10 or so minutes. The battery is about a year old and normally reads 12.75 when "cold." Does this suggest an overcharging issue with the one-wire unit? Thanks.
 

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Sounds about normal to me when off. That almost 15 at idle, is that with nothing on. What is reading with everything on high, headlamps heater wiper brake lamps at idle and at average rpm. I do hope you have the 100-amp alternator hooked directly to the battery or starter solenoid and not trying to run it through the regular harness. The factory harness is hard pressed to cover the 37 or 42 amp original.
 

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Thanks for the reply. With nothing on at idle, it's running at about 14.75. I haven't tested it with all accessories on, but with just lights, it drops to about 14.63. I have the alternator connected to the starter solenoid via a 6-gauge wire.
 

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Thanks for the reply. With nothing on at idle, it's running at about 14.75. I haven't tested it with all accessories on, but with just lights, it drops to about 14.63. I have the alternator connected to the starter solenoid via a 6-gauge wire.
Hello Merc65,

14.75 volts at hot idle is on the high side. Automotive electronics are specified to work between 9 and 16 volts from the OEM. All OEM voltage regulators, be it internal or external, are temperature compensated. That means when you start a cold system the regulated voltage is purposely set higher, but as the voltage regulator heats up, either by engine temp or ambient temp, it lowers the regulation set point. On the old Ford 1G electronic regulators it is typical to see around 14.5 cold and around 14.0 hot with no heavy loads on.

Our Grand Marquis's with the 4 and 5 G alternators were set around 13.7 hot. Having a higher system voltage will effectively allow more current into the battery, generate more internal battery heat and increase electrolysis of the water in the electrolyte solution. It will shorten the battery life but also the escaping hydrogen and oxygen gasses take some sulfuric vapour with them and can rust out the area around the battery quicker as well.

Just thoughts.

Cheers
 

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Appreciate the information! I suppose I'm willing to deal with a shorter battery life in exchange for consistently bright headlights and functioning blinkers...
Hello,

Well having a higher system voltage is one work-a-round the shortcomings of the factory harness. I have noticed on Ford's 3rd gen full size that they really skimped on the wiring. Not less forgetting the horrible main connectors (by today's standards) they used from the firewall ones ('65/'66) to the ones in the dash for '67/'68. I have seen many burned pins in these connectors from corrosion to undersized pins.

My '68XL was just an electrical mess, as well as many other areas. I replaced 95% of the wiring in the engine bay and that made a significant difference. I still use the Ford 1G 55 amp alternator and external regulator, but I do use the later electronic version rather than the factory oscillating point gap type. My system voltage stays very consistent. The only time the alternator cannot keep 14.1 volts hot at the battery is when I'm at idle with the A/C on high, wipers on, radio on and 560 watts of high beams on, emergency flashers on and interior lighting on. If I click back to 200 watts of low beams or just raise the RPM's a few hundred the battery voltage shoots right back to 14.1 volts with everything else still on. That's actually pretty good for the original charging system.

One more note about 1 wire alternators. They do have considerable parasitic draw all the time. The reason for this is to maintain a strong enough residual magnetism in the rotors soft iron core so the alternator can self excite once the engine is started. Factory charging systems trigger the alternator by the ignition switch to either allow full field (3-4 amps) or a partial field limited to a few hundred milliamps. The 1 wire alternator has no clue when the ignition switch is on and needs to self excite so the manufacturers play the odds with a lower partial field and hoping the engine RPM's will jump high enough to self excite. But there is still a small constant drain when everything is off.

The Ford 1G system with the point gap regulator would only allow the current through the "ALT" lamp and a parallel resistor (another resistance wire buried in the dash harness along with the ignition resistance wire) to flow through the alternators field when the key was either in ACC or RUN. When the engine started and the alternator started producing voltage, that was sensed on the 'Y' centre point (Stator connector) by the voltage regulator and it would switch to full battery voltage for the field and the "ALT" lamp would go out.

Just more food for thought.

Cheers
 

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Random thoughts on this topic: Always test with a full charge on the battery, or readings will be off as the system attempts to recharge it. Keep in-mind, where you connect the alternator is what the voltage sensing reads, not battery voltage. Unless of course, you connect the alternator sense wire directly to the battery. If testing voltages, test on the battery terminals (not connections), but also the junction where the alternator connects, and from the Bat(-) to the engine block and chassis, as that's part of the resistance circuit. It will be lower, and so the alternator thinks it needs to charge more, when it's reading voltage drop from the battery through the cables. This makes your cables and connections (all of them!) a very important maintenance item to avoid over-charging.

Alternators with a separate sense wire can compensate automatically for this, reading the direct battery voltage instead of after voltage drops. Some systems charge lower voltage to compensate for the voltage drops, such as classic GMs that sensed at a firewall junction assumed to be lower. 14.7V is considered a standard "fast charge", good to help quickly rebuild battery charge on local short trips. As the battery increases toward 100% charge, it takes a while but the voltage should drop, using a constant current (value of ammeters) rather than a constant voltage (Ohms' Law). In a similar fashion, higher loads will drive-up voltage to deliver current (amps) if it can. If your voltage drops under high load, your alternator has maxed the volts to deliver the amps, but can't keep-up at that alternator speed. Mr. Ohm again.

15V is fine just after extended cranking or under high loads (that pesky Ohm's Law), and higher is OK if very cold out, up to ~15.5V. Yet higher is becoming a concern if there is no known reason for it. Likewise, highway cruising all day across America should show the voltage slowly decrease to slow-charge (~14.2V), and lower once the battery hits full charge (~13.8V), depending on temperature and loads.

The battery should stabilize at ~12.75V at rest, after the surface charge depletes. Watch your voltage and it can tell you how the alternator is responding, and if that makes sense for the conditions.

BATTERY CONDITION - Volts and Specific Gravity (RESTED 12 HRS.)
CHARGE VOLTAGE SG (<25°C) SG (>25°C)
100% 12.75 1.250 1.240
90% 12.65 1.235 1.225
80% 12.55 1.220 1.210
70% 12.45 1.205 1.195
60% 12.35 1.190 1.180
50% 12.25 1.175 1.165
40% 12.10 1.160 1.150
30% 11.95 1.145 1.135
20% 11.85 1.130 1.120
10% 11.75 1.115 1.105
<10% 11.65 1.100 1.090
 

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I just did a 500 + mile round trip with mine last week, and towing at that. I also ran the ac (aftermarket under dash) most of the time. I have the exact same alternator, also run 2 12" electric fans. I typically saw 13.9 V with everything on (lights too) over idle on the highway at 70 ish mph, 14.1-14.3 at idle w no lights on. I have the volt meter way down the electrical system to take voltage loss into account. I also have the trigger wire sensing off of the same end of system wire, not looped back into itself. (i think i used the heater fan wiring, when keyed on has power). zero issues.
 

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