Alternator Voltage?

Dane Brahler · Oct 17, 2017

Noticed my 86's dash turn signals keep getting dimmer and barely light up! (LED bulbs)
I checked my voltage at my alternator and battery and I'm only reading 11.5V!!
The alternator is from summit #SUM-810301 it's a 140 amp, 1 wire (new 9/12/16) just out of warranty-____-
I am using 6ga wire back to the battery in the trunk. Is this maybe too small to run that far for 140amps?
the battery is a new interstate that i got in May of 2017. starts the car fine but 11.5 volts isn't ideal obviously...
Any thoughts out there?

General karthief · Oct 18, 2017

Is there sufficient voltage at the battery, meaning is your battery fully charged?
You can take the alternator to any parts place to get tested, that is where I would start.
As far as the wire size to the alternator, maybe @jrichker can chime in, I'm only an apprentice at wiring stuff.

jrichker · Oct 18, 2017

Wire size current table:

A 140 amp alternator needs a 4 gauge power feed to the starter solenoid..
2.) The secondary power ground is between the back of the intake manifold and the driver's side firewall. It is often missing or loose. It supplies ground for the alternator, A/C compressor clutch and other electrical accessories such as the gauges. The clue to a bad ground here is that the temp gauge goes up as you add electrical load such as heater, lights and A/C.

Any car that has a 3G or high output current alternator needs a 4 gauge ground wire running from the block to the chassis ground where the battery pigtail ground connects. The 3G has a 130 amp capacity, so you wire the power side with 4 gauge wire. It stands to reason that the ground side handles just as much current, so it needs to be 4 gauge too.

The picture shows the common ground point for the battery , computer, & extra 3G alternator ground wire as described above in paragraph 2. A screwdriver points to the bolt that is the common ground point.

The battery common ground is a 10 gauge pigtail with the computer ground attached to it. It is located by the windshield washer filler neck on the driver's side.

Correct negative battery ground cable.

Dane Brahler · Oct 18, 2017

Okay I will switch out the 6 for 4 ga. but still, shouldn't I be pushing around 14 volts at the terminal posts of the alternator? At idle (900rpm) I am only getting 11.5 exactly, but when i hold the throttle at 2000rpm I get the 14 volts. do you think it could be the design of the alternator? the only things I am powering are an msd box and 16" electric radiator fan and walmart radio. no aftermarket speakers...

General karthief · Oct 18, 2017

Underdrive pullies?
Yes you should be more at idle,
What size is your alt pulley?
And I would get that alt checked just to be sure, hell, with a electric fan I'd up grade to a 3g
Stock alt is barely enough to power stock stuff

mikestang63 · Oct 18, 2017

check the volts at the battery, the back of the alternator at idle and 2000 rpm. See if there is a significant drop Also you need to have at least 1/0 ground wire for the battery in the rear and also a 4 gauge ground wire from the block to the chassis up front.

Dane Brahler · Oct 18, 2017

All i am left with is taking off the alternator to check it at a parts store.... all the pulleys are stock diameter. idle voltage is 11.5 2000rpm it is 14v
voltage drops to 10.3 with all accessories on; wipers, headlights, hazards, radio

Just to verify:
this 1 wire alternator only has two terminal posts on it. positve and negative
positive should be 4ga running to starter solenoid
negative should be 4 ga or bigger to engine block? or chassis ground? its mounted behind my drivers seat and i have it grounded to the old rear seat belt bolt with 2 ga wire.

General karthief · Oct 18, 2017

Wtf?
I don't think thats good. Wheres your battery located?
Ok I missed the battery in the trunk thing,
Follow this guide, courtesy jrichker
jrichkerStangNet's favorite TOOL
SN Certified Technician
The following cut off switch is a requirement for most drag strips...

For a battery cut off switch, see http://www.moroso.com/catalog/categorydisplay.asp?catcode=42225
is the switch http://www.moroso.com/catalog/images/74102_inst.pdf is the installation instructions.
Use the super duty switch and the following tech note to wire it and you will
be good to go.

Use the Moroso plan for the alternator wiring and you risk a fire. The 10 gauge wire they recommend is even less adequate that the stock Mustang wiring.

There is a solution, but it will require about 40' of 18 gauge green wire.

Wire the battery to the two 1/2" posts as shown in the diagram.

The alternator requires a different approach. On the small alternator plug there is a green wire. It is the sense lead that turns the regulator on when the ignition switch is in the run position. Cut the green wire and solder the 40' of green wire between the two pieces. Use some heat shrink to cover the splices. See http://fordfuelinjection.com/?p=7 for some excellent help on soldering & using heat shrink tubing.

Run the green wire back to the Moroso switch and cut off the excess wire. Try to run the green wire inside the car and protect it from getting cut or chaffed. Crimp a 18 gauge ring terminal (red is 18 gauge color code for the crimp on terminals) on each wire. Bolt one ring terminal to each of the 3/16" studs. Do not add the jumper between the 1/2" stud and the 3/16" stud as shown it the
Moroso diagram.

How it works:
The green wire is the ignition on sense feed to the regulator. It supplies a turn on signal to the regulator when the ignition switch is in the Run position. Turn the Moroso switch to off, and the sense voltage goes away, the voltage regulator shuts off and the alternator quits making power.

The fuse & wiring in the following diagram are for a 3G alternator. The stock alternator uses a dark green fuse link wire that connects to 2 black/orange wires. Always leave them connected to the starter solenoid even if you have a 3G alternator.

Rear mounted battery ground wiring. Follow this plan and you will have zero
ground problems.

One 1 gauge or 1/0 gauge wire from battery negative post to a clean shiny spot
on the chassis near the battery. Use a 5/16” bolt and bolt it down to make the
rear ground. Use a 1 gauge or 1/0 gauge wire from the rear ground bolt to a clean
shiny spot on the block.

One 4 gauge wire from the block where you connected the battery ground wire to
the chassis ground where the battery was mounted up front. Use a 5/16” bolt
and bolt down the 4 gauge engine to chassis ground, make sure that it the metal
around the bolt is clean & shiny. This is the alternator power ground.

The computer has a dedicated power ground wire with a cylindrical quick connect
(about 2 ½”long by 1” diameter. It comes out of the wiring harness near the
ignition coil & starter solenoid (or relay). Be sure to bolt it to the chassis ground
in the same place as you bolted the alternator power ground. This is an
absolute don’t overlook it item for EFI cars
Note: The quick disconnect may have fallen victim to damage or removal by
a previous owner. However, it is still of utmost importance that the black/green
wires have a high quality ground..

Picture courtesy timewarped1972

Crimp or even better, solder the lugs on the all the wire. The local auto stereo
shop will have them if the auto parts store doesn't. Use some heat shrink tubing
to cover the lugs and make things look nice.

Dane Brahler · Oct 18, 2017

The write-up doesn't really do much for me because I don't have that style 3g alternator that uses the factory regulator harness. Apparently I'm probably going to be swapping to one here soon from what I can tell....

General karthief · Oct 18, 2017

Yes, yes you will.

mikestang63 · Oct 18, 2017

If you have a ONE wire Powermaster alternator, that is different than a 3G. The one wire alternator is dependent on rpm to excite and charge,which cause idle output problems especially if underdrive pulleys are used. .They will not charge at idle when you first start the car up, unless you bring the RPM up over 2000 to trigger the exciter, but once you do this, then they charge down to idle. You can also run the Powermaster one wire as a standard 3G wiring setup, which is what I would recommend doing. That should solve your charging issues at idle.

Personally, I had one of those 20 years ago and hated it for the same reasons. I sold it and put in a 94 mustang 3G.

Mustang5L5 · Oct 19, 2017

I never liked those 1-wire setups much. Wire it like a 3G.

You also need to pay attention to pulley diameters to ensure adequate idle rpm to allow the alt to make full charge at idle.

amperage isn't tied to idle voltage output. Two different things and most people think the cure for low voltage at idle is to stuff a bigger alt on. Not quite

Dane Brahler · Oct 19, 2017

Thanks for the replies. Does anyone know what the stock pulley diameter is for fox cars? What if I'm able to just swap out the pulley for a smaller one to speed up the alternator?
I'm a machinist. I'd rather turn a new pulley than buy one lol

General karthief · Oct 19, 2017

I have found 2" and 2 1/4" not sure which is stock

mikestang63 · Oct 19, 2017

i thought 2 1/2 " was stock

General karthief · Oct 19, 2017

It could be, I had to change mine do to charging issues after a alt swap.

Dane Brahler · Oct 20, 2017

Update, rewired alternator to starter solenoid with 4ga wire . Still no luck . Had to jump the battery to even fire it up but charging systems not charging the battery.. gotta yank everything off and get it all tested

Dane Brahler · Oct 20, 2017

The way the whole car is acting it is like the whole charging system is drawing from the battery instead of the alternator... The car will run and then slowly slowly slowly die off until I can't even fire the injectors

Blown88GT · Oct 20, 2017

If you do not get enough output voltage at idle, replace the standard 60mm pulley with a 1 3/4" (44mm) overdrive pulley from Auto Specialties.
P/N ASP520004

mikestang63 · Oct 20, 2017

Dane Brahler said:
Update, rewired alternator to starter solenoid with 4ga wire . Still no luck . Had to jump the battery to even fire it up but charging systems not charging the battery.. gotta yank everything off and get it all tested

did you hook it up like a 3G?

Alternator Voltage?

Dane Brahler

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General karthief

wonder how much it would cost to ship you a pair

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wonder how much it would cost to ship you a pair

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General karthief

wonder how much it would cost to ship you a pair

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That is…until I whipped out my Bissell

Dane Brahler

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wonder how much it would cost to ship you a pair

mikestang63

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wonder how much it would cost to ship you a pair

Dane Brahler

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