When I turn the key the alternator light stays on while the truck is running.
I've replaced the alternator, starter, VR, and a few other things such as column (turn signal switch), battery. It seems the battery may be charging but the light won't go out while running. It does go out when I start it up then it stays on.
I've been replacing wires and connectors under the hood and also have grounded the engine to the body.
Any and all suggestions will be appreciated and looked into. Winter is coming and I'm just a driveway mechanic (but a good one) and I'd like to solve this mystery before it gets cold. I was hoping to be able to get out and run her once in a while during the winter months.
Thanks guys for reading my post and participating in this forum. It's always been a great help during my projects
The truck is a 67 F100 Stepside 352 headers/4bbl 3 on the tree 2wd

 

Have you had the battery checked for charge? I hate electrical, hang on a sec and I'm sure a scientist will be along shortly...

 

You need to get a voltmeter and check across the battery terminals. At fast idle with all appliances off you should see 14+ volts, dropping to 12.5 upon engine shutoff. Work back from that instead of throwing parts at it.
Remember, half the China junk is bad out of the box.

 

^^^^
What he said.

You need to read the voltage at the battery while the engine is running.
If the battery is charged completely, while it's running with the engine at a high idle (1500+ rpm) you should see over 13 volts.

I'd like to see somewhere between 13.5 to 14.5 volts normally.

You can turn on all the lights and accessories to verify the regulator keeps the voltage up.

Set the volt meter to DC (direct current)

 

The sum of the square roots of any two sides of an isosceles triangle is equal to the square root of the remaining side. This is wrong BTW

 

LOL

Gman...
It sure sounds good. Kinda like our current resident of the White House don't you think?
sounds good but isn't so?

LOL

 

and if I have the correct reading?
I started with a fully charged battery (as in new) most parts needed to be replaced anyway. where ever I could use NOS I did...
it had this problem when I got it and the wires to the alt are "new" so maybe I should check to be sure they're correct..
but input is always good and I appreciate you responding...and hope you'll continue this with me if and when you have time...
so where do I go from a good voltmeter reading? I'm going to check where the alt light plugs in in the back...since someone said a faulty/corroded connection could cause it. no tags yet so I can only ride up and down the road or idle in the driveway..

 

"....so where do I go from a good voltmeter reading?"

Buy a cheep volt meter. 5 bucks is good because you don't need an exact number.

Set the volt meter to VDC (volts DC)

Put the black (Common) wire on the negative side of the battery. Put the RED wire on the POSITIVE side. read the meter.
With the engine off you should see about 12.5 volts.

Start the engine.

The readings "IF" the system is charging properly should read over 13.5 volts.

Tell us what the readings are, running and not.

We'll work from there.

Keep the battery at full charge so we can eliminate it as a problem.

 

o.k. the vm reads 12 when the truck is off or running...
no difference in the reading. It's a new battery. I had taken it out for winter and fully charged it before I put it back in (recently).
I'll put the charger on it for that extra 1.5 while waiting for the next step.

I've rewired a car before and a jeep. But never had to work through like this. Thanks for your patience. I was hydraulics in the service. So even though I have a working knowledge and can read a schematic I haven't had to work through much electrical passed common sense fixes....

One of the bolts on the back of the alternator is touching the engine so I'm going to put another spacer in there and make sure everything is connected right also. I put it in last fall. This issue was present when I got the truck so I was hoping as I replaced worn out wires and electrics (like the alternator, regulator, etc) it would disappear.

 

OK...We know the system IS NOT charging. The light is working properly.

You said you replaced the voltage regulator and alternator. Did you replace them with new parts.?

Is there any wiring that you can obviously see is bad, broken or burnt?
Can you read a simple schematic diagram and chase the wiring to make sure everything electrical is connecting and/or connected properly?
You can find the wiring diagram for your engine here on this site. (ignition, starting, charging and gauges.)? try locating it and put a copy on your computer. (if not I can e-mail you a copy)


There are only 4 wires that matter here.
4 wires to the alternator and 4 to the regulator.

The power for the charging system comes from the BATTERY side of the starter relay on the firewall. One side of a split to the regulator and the other to the alternator.
Grounds at the alternator and regulator.
One wire from the Armature of the alternator to the Armature of the regulator and one wire from the FiELD of the alternator to the Field of the regulator.
A for armature, F for field, G for ground, B for battery.

If you accidently switched the Armature and Field connections at the Alternator the thing won't work.

Find the wiring diagram, open it up on your computer, zoom in on just the part for the alternator and regulator and put on your thinking cap.

This one is EASY man.

It's either the wiring is bad or incorrectly wired or the Alternator or Voltage regulator is bad.

Good luck

Bill

 

well we're on the same page anyway!
I'm going to check out the schematic and when I put the spacer on the alternator I'll be able to see better and check the wires.

all new parts went in. wires all look "good" from alternator, regulator, etc.
so I may have switched them when replacing the alternator...although similar, rebuilds don't always look the same...

it's raining now so I'll go through the schematic and get to it tomorrow morning....
and let you know how I made out...
I know! I know! It ain't that complicated...
Logical thinking...

maybe the old one was on wrong too and that's where the issue came from in the first place...don't know...but I've replaced quite a few wires and elements so maybe this will do it..

 

Sit there and study that wiring for the charging system. Don't beat yourself up here. It's easy when you know it.

Simple theory to help you understand.

ALTERNATOR

The alternator uses the principle of electromagnetism to produce current. The way this works is simple. If you take a strong magnet and pass it across a wire, that wire will generate a small voltage. Take that same wire and loop it many times, than if you pass the same magnet across the bundle of loops, you create a more sizable voltage in that wire.

There are two main components that make up an alternator. They are the rotor and the stator. The rotor is connected directly to the alternator pulley.

The stator contains 3 sets of wires that have many loops each and are evenly distributed to form a three phase system. (alternating Current)

Now, we have to convert this alternating current to direct current current. This is done by using a series of 6 diodes that are mounted in a rectifier assembly. A diode allows current to flow only in one direction. If voltage tries to flow in the other direction, it is blocked. The six diodes are arranged so that all the voltage coming from the alternator is aligned in one direction thereby converting AC current into DC current.

VOLTAGE REGULATOR

The voltage regulator controls the field current applied to the spinning rotor inside the alternator. When there is no current applied to the field, there is no voltage produced from the alternator. When voltage drops below 13.5 volts, the regulator will apply current to the field and the alternator will start charging. When the voltage exceeds 14.5 volts, the regulator will stop supplying voltage to the field and the alternator will stop charging. This is how voltage output from the alternator is regulated.

Effectively we "Excite" the Field side to modify the output of the Armature side of the regulator.

Insufficient Charging Output

If one of the three stator windings failed, the alternator would still charge, but only at two thirds of its normal output. It is more common for one or more of the six diodes in the rectifier to fail. If a diode burns out and opens one of the circuits, you would see the same problem as if one of the windings had failed. The alternator will run at a reduced output.

 

Make absolutely sure the posts on the alternator are now contacting the block. You could fry the entire system.

 

There is usually a "ground" post on the back of the alternator. Hopefully that is the one touching your block.
Wrhinehart, REPs to ya.

 

OOPS..
I typed it wrong.
I ment NOT touching..

Fat fingers, stupid man