Good alternator, battery, volt. reg - what's wrong?
 

1975 F250, 460.

Had the alternator bench tested (AutoZone) twice and it came up good. Battery was also tested and holds a charge. Brand new voltage regulator installed. New wiring harness throughout truck.

Motor turned off - multimeter shows 12.5V, When the truck is running, voltage drops to about 12.1-12.2.

Regulator is grounded, alternator is grounded (10 gauge wire from ground post on alternator to neg on battery), Block is grounded chassis, frame is grounded to firewall.

Why am I not getting 13.8V from the alternator?

There are two things to check:

1) There should be a fusible link from the output stud of the alternator to the battery side of the starter solenoid. Make sure it has continuity. It's very common for people to leave the battery connected while servicing the alternator, only to ground out a wrench behind the alternator and blow the link. Not saying that's what happened here, but check this first.

2) Check that the regulator is getting power. Remove the plug from the regulator and turn the key to RUN. If this truck has an ALT light in the dash, make sure you have power at the "I" terminal of the plug. If this truck has an amp gauge (ammeter), make sure you have power at the "S" terminal. The plug itself does not have markings on it, but the regulator does - so you'll have to key the plug to the regulator to figure out what conductors do what on the plug.

As a side note, you don't need to run a wire from the alternator body to the negative battery post. You just need to run your negative battery cable to the engine block. The alternator is grounded by physically bolting to the engine. That's the best ground you can get. You also need a ground strap between the block and the firewall. This is responsible for grounding the cab and the inner fenderwell which provides the ground to the voltage regulator. Some harnesses also have a small ground wire between one of the regulator mounting bolts and a ground post on the alternator to help close the sense loop between the alternator and the regulator.

He said he replaced the wire from the battery to the alternator output, so I would concentrate on checking for voltage from the keyswitch to the regulator.

x3 Not seeing 12v from the switch to the regulator.

If you have an idiot light instead of a gauge and it's burned out, it will not charge.

Thank you for the suggestions. It gives me a place to start.

The more I think about this, I have to ask a couple more q's just to be sure I do not overlook the more obvious possibilities.


--->I have the gauges completely disconnected and the cluster removed from the vehicle as part of this wiring harness installation. One of the gauges is an Ammeter. And, I do not have the signal wiring harness connected. (Where the wires connect at the base of the steering column) Basically, I have the keyed ignition switch and all of the under hood wiring connected.

Would the gauges not being hooked up cause the alternator not to work? :-huh I did even think to ask as I did not expect that to be a possible cause.


--->One other question, I replaced the ignition module with a new one - I have the 7 wire - green grommet module in my vehicle. Same as the old one. If there were some incompatible issues with the module, would that also be a possibility for the alternator not making power?

Thanks.

I also realized another factor that is likely a problem. The portion of the harness that has the regulator connector on it only has 4 wires coming out of it - including 1 yellow and 1 white wire. My old one and the ones that are sold by Napa and Autozone all have 6 wires - including 2 yellow wires and 2 white wires.

Napa:
http://partimages.genpt.com/largeimages/102400.jpg

AutoZone:
http://contentinfo.autozone.com/znet...211/9/image/4/

Painless harness has one less yellow and white wire than these pictures. :-arrgh

What would the two additional wires have been used for?

There can be several different issues.

One:
Trucks with ammeter and oil gauges are wired drastically different than trucks with idiot lights between the regulator and the ammeter and ignition switch.

Those connectors from autozone and napa are for alternator warning lamp equipped trucks. I'm going to assume your truck also had warning lamps from the factory because the plug matches these.

The connector from your painless harness is either for ammeter and oil pressure gauges equipped trucks, or it doesn't have the wires for the electrically assisted choke or the alternator radio supression.

Two: 1975 trucks use Duraspark-I ignition modules with green grommet. These are wired differently than the 1974.5's with Duraspark-I with black grommet, and the 1976 and later Duraspark-II.

The instrument clusters for 1975 can have a unique printed circut than the other years.

1975 was a one year wonder for wiring, so it's important to match it closely.

What year is your painless harness for? It has to be for a 1975 or there will be problems.

I'm going to need pictures of the connectors around the voltage regulator, ignition module, starter solenoid and alternator, to see what might be going on.

Also need to know if the truck has ammeter and oil gauges or warining lamps from the factory.

To fix this it's going to take pictures and questions as too many changes have been made to the original wiring to try and help you by just looking at the 1975 wiring diagrams. Need to see what shape the wiring is in now.

Truck has the factory gauge cluster with the following:
Brake light
Oil gauge
Fuel gauge
Speedometer
Temp gauge
Ammeter gauge
Low Fuel Economy light

Old module was Motorcraft w/7 wires (including blue wire) and green grommet. I replaced it with a new module that has the same wires and green grommet.

The Painless harness is kit #: 10118 - For Ford F series trucks 1967-1977. There sections of the manual for different year trucks.
http://painlessperformance.com/webca...manualIcon.jpg

I will take a bunch of photos showing all of the areas you requested. Thank you.

Ammeter gauge and oil gauge from the factory - not lights.

Thank you for the detailed info. I will get those pictures shortly. :D

Out at the regulator there should be a factory red/green wire. If you do not have this wire, you are in trouble right there. If you do have it, you should see if you have 12v on it when you turn the key to run. If you do, make sure this wire hooks to the "s" terminal of the regulator.

Would this be a direct wire from the ignition switch (at the key) to the "S" terminal on the regulator pigtail and is only hot when the key is in the run position?

Yes sir, that's it.

https://www.ford-trucks.com/user_gal...id=16603&.jpg=

That is odd - the last post shows I posted it with the wiring diagram - but I did not. ???

Anyway, to whoever posted it - my regulator has a 4 prong terminal. Would I still want that same wire going from the ignition switch to the S terminal?

I connected a wire from the ignition terminal in the key switch and ran it to the "S" terminal wire on the regulator.

Started it up and I am running 14.6-15.0V. Turn it off it drops back down to 12.9-13.0V (probably needs to charge)

I do believe this has solved this problem.

I do notice now that the regulator makes a single click when I turn the key on now. Is this normal? Is this due to the fact that the regulator is now getting power when the key is on? It is a brand new regulator - NAPA PREMIUM - if that makes any difference.

Thank you so much!

And to the two of you - thank you for the same suggestion. :D

I guess I would have thought for the hundred's of dollars that I paid for the wiring harness, it would have included this important wire. :-X15

Sorry, I hit the wrong button. I put the diagram in, but it ended up in your post instead of a new post by me.