Alternator Rating Question

Doug R · #1 (**permalink**) 03-13-2017, 11:28 AM

I own a 427 cobra which has developed a problem in the electrical system which I think may be related to the alternator not producing the required output. I cannot find any markings on the housing to indicate the min and max amperage ratings and hoping that someone could provide that info. The car has two push fans on the front side of the radiator and one large pull fan mounted on the inside , two electrical fuel pumps and a MSD pro billet distributer, coil and 6AL. Thanks

KevinW · #2 (**permalink**) 03-13-2017, 12:44 PM

maybe useful to state what the problems are. Is it the charge light failing to extinguish?

Dwight · #3 (**permalink**) 03-13-2017, 08:22 PM

To check the output of the alternator use a volt meter.

Check the voltage before you start the motor at the battery. Should read about 12 -12.5 v.
Then start the motor and check the voltage at the battery. It should read more. 13-14 volts.

Tommy · #4 (**permalink**) 03-14-2017, 04:38 AM

Doug,
I have two suggestions. First is to look for any identifying part numbers that you could use to research the capacity of your alternator. They would be the same numbers you would use if you wanted to go to the parts store to replace it.

Second, think of the battery as a reservoir of electrical power that fills in for the alternator when it is not making enough power (e.g., when starting the engine), and takes back power from the alternator to refill the reservoir when the alternator is making more power than the other electrical systems need. That means an underproducing alternator would leave the battery power lower after running the engine than before. As Dwight said, you can read that effect with a voltmeter. ... Here's what I would do. First, fully charge the battery with a charger. Let is sit for a while to reach its steady state and then check the voltage (e.g., 12.4 volts). Then start the car, let it warm up a bit and then check the voltage at the battery with only essential electrical systems on. Check it at idle and at about 2,000 RPM. It should be above 13 volts. Now drive the car for 30 minutes or more without stopping or restarting the engine. When you do stop, turn off everything and check the battery voltage again. If it is less than the the voltage of the fully charged battery (e.g., 12.4), then the electrical system is not sufficiently replenishing the power reservoir in your battery. This could be due to a bad battery, failing alternator, undersized alternator, or bad voltage regulator. If I had these symptoms I'd first have the battery checked and, if it passed, I'd have the alternator output checked. Most chain parts stores can do this.

Doug R · #5 (**permalink**) 03-18-2017, 01:42 PM

Thanks the comments were very helpful and the testing as described show the voltage under load at about 1800 rpm was less that the basic battery voltage of 12.65v engine not running. At idle no load was 12.95V, Idle under load at 12.35v and at 1800 rpm the voltage was 12.52v . So I guess the alternator is suspect.

Phx Mike · #6 (**permalink**) 03-18-2017, 05:46 PM

Quote:

Originally Posted by Doug R

Thanks the comments were very helpful and the testing as described show the voltage under load at about 1800 rpm was less that the basic battery voltage of 12.65v engine not running. At idle no load was 12.95V, Idle under load at 12.35v and at 1800 rpm the voltage was 12.52v . So I guess the alternator is suspect.

If you have an external voltage regulator then suggest you check those connections to ensure they are correct and secure. Not sure what setup you have but the old style arrangement has 3 wires from the alternator marked Field (usually a green wire but not always) Stator and Alternator. The Field and Stator wires run between your alternator and voltage regulator - make sure they are secure and not reversed. The Alternator wire also runs to your voltage regulator, via the starter solenoid and possibly via the ammeter if you have one. Make sure those wires are correct and secure. Your voltage regulator will also have a wire marked Ignition and it runs to your ignition switch. If all these wires look like they are routed right and not loose, then suggest you pull your alternator and take it to Autozone or O'Reillys and they can test it.

Doug R · #7 (**permalink**) 03-19-2017, 04:04 PM

Thanks
Yes - My setup is the old style with the external voltage regulator and my alternator has three wires so I appreciate the information on the connections.

cycleguy55 · #8 (**permalink**) 06-10-2017, 03:06 PM

I just completed a 100 amp 1G to 130 amp 3G alternator conversion - the difference is night and day. The 3G was introduced by Ford in 1994 and produces more output at idle than the 1G does at 2,000 RPM. There is minimal dip in my voltmeter when the cooling fans or headlights are switched on, unlike before. 3G alternators are internally regulated and the conversion is easy to do. The physical mounting is pretty much the same too, as the 3" spigot on the mounting ear is the same as the 1G. Easy swap.

3G Alternator Wiring Diagram:

Notes:

Connect the 'I' wire from 3G alternator to: a) ignition switch if you're running a voltmeter; or b) warning lamp if you're running an idiot light.
If you're running a warning lamp the alternator will not charge if the lamp burns out. The alternator will charge if you wire a resistor in parallel with the lamp filament.

Worth noting is that many, perhaps most, people replacing installing 3G alternators connect the yellow wire to the alternator output stud (red wire) - after all, it's right there and ultimately connects to the same place. My installation utilized a factory harness, so I connected the yellow wire with one that was an always 'hot' wire that connected to the old external regulator.

Also worth noting is because the 3G alternator has much higher output at low RPM, it's more inclined to give you belt squeal upon startup. I have not had that problem, but I installed a slightly larger, billet aluminum pulley that may have helped prevent belt slippage and squeal. Aluminum is less 'slick' than steel pulleys and better grips the belt, plus the slightly larger pulley: a) turns the alternator a bit slower, thus reducing the load; and b) provides a greater gripping surface for the belt. I'm also using a turnbuckle-type tensioner, allowing me to easily get the belt tension I need - much better than a pry bar and a slotted bracket and bolt!

Keep in mind the 3G doesn't look anything like the 1G, so if 'period correct' appearance is what you're after, you'll need to hunt down someone who produces 3G alternators in 1G cases - they're out there.

Here's one of the many write-ups posted on these upgrades - this one is better than most: https://www.ford-trucks.com/forums/9...g-upgrade.html BTW, I prefer the wiring diagram posted at the top of this over the one in the intallation thread - it just seems cleaner.

Please note:

the diagram in the thread shows how to wire the installation if you have a charging lamp (a.k.a. idiot light), while the 1st one does not
a resistor is NOT required in the switched power source if you don't have an idiot light - or if you're okay with your alternator not charging if/when your idiot light burns out!
it's highly probable that, if you have an ammeter, it WILL NOT WORK after installing a 3G alternator. This is probably moot, as I suspect most would be running voltmeters in Cobras (they work fine with 3G alternator)