78 f150 with 351m is overcharging, was at 18.5volts replaced regulator and went down but satill wayy too high at 17.5, alternator is a month old same with battery. any ideas?

 

Run a jumper wire from one of the regulator mounting bolts to the GND stud on the back of the alternator, and report whether or not the voltage changes. If you can't find the GND stud, try to get a good, clean, and unpainted ground somewhere on the block.

Does this truck have an ALT light, or an ammeter (amp gauge)?

 

it has both, neither of which are hooked up, im going to try the the jumper wire now

 

just tryed the jumper wire and no change, could it be the altenator itself? somehow stuck in full field?

 

What do you mean it has both? In the factory instrument cluster, there is either a RED light that says "ALT", or a needle that goes between "D" and "C." Please specify which setup you have.

Thank you for the update. The alternator cannot get stuck in full field, only the regulator, unless there is a short in the wiring somewhere. The wiring is different depending on which instrument cluster you have, hence my asking the question. I need you to answer this before I can help you any further.

 

im sorry it has an aftermarket amp guage, but on the cluster it has the d to c guage

 

Got it, thanks. Assuming both the alternator and regulator are good, you are left with these possibilities:

1) Field short to battery. Make sure that you have a solid connection between the F terminal of the regulator and the FLD terminal of the alternator, with nothing else touching either end.

2) Poor sense connection. Make sure the output stud of the alternator has a clean connection to the A terminal of the regulator. Make sure the negative battery cable is disconnected when you're poking around this stuff; it's live.

3) Regulator ground issue. It's possible that the ground that the regulator is seeing is not true ground. That was the purpose of the first test I had you try, but if you ran your jumper wire to a poor ground (like rusted or painted metal on the block, or somewhere else beside the block), the results would be invalid and you would need to try again.

4) Measurement error. Check with a second meter, and use that meter to check the batteries in the meter you're using. Yes, it does happen.

It also wouldn't be impossible for your second voltage regulator to be bad, either.

 

It's definitely over charging, the battery was boiling and swelled up, I disconnected the regulator and as expected read battery voltage, is there a way to test the regulator? Should I try simply running new wires from the regulator and alternator?

 

A parts house can bench test the voltage regulator.

I don't know if you should try running new wires, because I don't know what's wrong yet. That's why I gave you the list above; you still need to check these items.

 

Well I've checked everything on the list and everything is good, the guy I bought the truck from did a real hack job with the wiring, there's bare splices showing for the gauges he wired in, there's probably 50 feet of unused wire I've removed, just a mess, I should add I was driving a bit hard the night before the overcharging started if that makes any difference

 

In case you want to replace the alternator wiring harness, that's the one that goes from the alternator to the regulator.

Harness, alternator wiring
Fits 78 f100-350 w/ amp and oil press. gauges all engines and 70 amp Ford alternator
D8TZ-14305-B
Green Sales, Cincinnati, OH has 40 (800) 543-4959

Fits 78 F100-350 w/ gauges and 40 or 60 amp Ford alternator
D8TZ-14305-C
Here's where I ran into conflicting info. According to one parts locator Bob Allen Ford has 3 of these and none of the other. On another locator it says he has 3 of the first one and none of these.
Maybe the "B" supercedes the "C" part number and replaces it.
Personally, I wouldn't deal with Bob Allen even if they are the only ones that had a part I needed.
They totally screw the customer pricewise on obsolete parts.
If you decide to go this way, let me know and I'll send Bob Allen's contact info.

 

Regulator needs a good ground and I strongly suggest using the newer style electrinic regulator like the Motorcraft GR540B or ACDelco F662 or comparable. Make sure to securely ground the new regulator before plugging plug in.

Also, I'ld say better than an amp gage is a plain and simple Voltmeter wired through a relay direct to battery so it reads system voltage ... and the relay for it activated through the ignition accy switch term.

Back in late '80s I had trouble with my '77's charging system, went through a couple regulators, they were OEM style electro-mechanical units. I then switched to the electronic updated style (plugs in) and the truck's charging system has been trouble free since, including the now 9yr8m old battery. Only downside was amp meter barely moves no matter what as it doesn't work so well with the electronic VR but my buddy who recommended this back then said that would happen. Since then I have grown to prefer voltmeters anyway .... as has FoMoCo it seems.

 

S that what you suspect a bad harness? I removed what seemed like a hole roll of electrical tape that had the slack wrapped up and didn't see any obvious problems

 

Hold the phone people!

These connections only apply to an ALT light setup. The OP has an ammeter (amp gauge) setup, which uses different connections at the regulator in the absence of an ALT light. Please refer to my description below, as these are what must be used, not what has been posted above.

Here is the wiring diagram that applies to the ammeter gauge setup in your truck: http://fordification.net/tech/images...aster_2of9.jpg

It's a little confusing to make out because it consolidates several different options into a single diagram, but here are the main points:

Voltage Regulator:

I terminal: Unconnected.

F terminal: Goes straight to the FLD terminal of the alternator. No other connections.

S terminal: Connects to the GREEN with RED stripe wire coming from the truck. This is essentially a hot-in-RUN connection. No other connections.

A terminal: Connects to the alternator output stud (the connection is made in the harness).

Alternator:

GND: Optional, but recommended; runs to one of the regulator mounting bolts.

FLD: Goes straight to the F terminal of the regulator. No other connections.

STA: Goes straight to the electric-assist choke, if equipped. No other connections.

BATT (B+, A): Connects to the battery side of the starter solenoid through a fusible link, AND the A terminal of the voltage regulator. Note that with the factory amp gauge setup, along a short length of the connection back to the solenoid, a tap is made to run to the ammeter, which is tapped back into the harness a short length later. This is reflected in the wiring diagram. All of this is done within the wiring harness itself; you won't see these connections unless you peel back all of the harness wrap.

Anything in addition to this that a previous owner added must be removed and taken out of the equation. Anything missing must be restored.

If the previous owner added an aftermarket ammeter, I recommend removing this as it adds too many variables. If it's actually an aftermarket voltmeter, this is independent of the charging system because it just "snoops" the voltage and isn't inline with anything. This can be left alone. I have a feeling you actually have the latter; the former is less common. A voltmeter says "volts" and will have numbers in the 10 to 15 range on the dial.

 

It's an amp meter it goes up to 50, it's not hooked up to anything though it's just surround in the guage housing no wires are run. All connections are good, the regulator is securely grounded, although I must say when I purchased the truck it wasnt bolted on it was laying on the fender and the charging system worked perfectly, I just don't know if it's the wiring bad or the regulator. How do I go about testing them? Or is it better to get yet another regulator and try it?