Hi guys
I have a 1970 F-100 ½ SWB, 4 speed with a 360, I purchase this pickup new in 1970 and she has live in Arizona for the last 32 years. She been stowed for 12 year is my garage. I’m presently restoring her from front to back.
My question is, all the years I drove this pickup the AMP meter would hardly move, I was told this is normal for this model, because the meter is looking at the voltage difference between the battery (+) and the alternator (+). Does someone know a way to get more movement out of this meter?

Jellobean

 

[updated:LAST EDITED ON 09-Mar-02 AT 11:58 PM (EST)]The Ammeter (AMP) is actually across a long piece of wire. This wire acts as a shunt (it carries almost all the current; the ammeter sees very little current).

I worked as an electronic technician and an electrician from 1956 through 1992. So I know a little about electricity. I have driven these old Fords for about 30 years, and have told myself time and time again that I was going to experiment with the ammeter to see if I could get more needle movement. But I've never got around to it.

What I had planned on doing was to locate the end of the wire from the ammeter (gauge) where it connects to the wire from the alternator and disconnect it. Then I was going to carefully move it down the alternator wire, attaching it with an alligator clip at various places (in very short moves along the wire) until I had about a quarter needle movement with the engine at 1500 RPM (with a well-charged battery). The amount of current passing through the ammeter will increase as the length of the wire it is across is shortened (done by moving the ammeter wire, not by cutting the alternator wire).

I would have to be very careful, because if I went too far, the ammeter (which is really a voltmeter in this application) would "peg out" and burn up under heavy charging of the battery (or any heavy current drain).

Before I tried this, I planned to have a spare ammeter, just in case.

OldFords Bad attitudes

 

Hey, OldFords great minds think a like I had planned the same thing but I also never got around to it. And being in between trucks right now I do not even have a "patient" to practice on. I also had thought about splicing to shunts together in series to see if that increased meter movement.

Sparky

 

Thanks Sparky, but I may be wrong.

Jello, don't do anything that will ruin your wiring harness.

Sparky, The D'arsonavol (SP) movement in the gauge has its own shunt. The meter is actually a voltmeter, sensing the voltage drop from the alternator to the battery. This came to me in the middle of the night (I hate being woke up by my brain 'cause its usually to tell me I'm wrong). The meter shunt, the meter movement, and the alternator/battery wire are in parallel; meaning that there is a certain amount of current flow throught each one depending on the voltage difference between the battery and the alternator.

Moving the gaugae wire along the alternaor/battery wire will only lower the amount of voltage the gauge senses. It is the same as having several resistors in series and moving the meter to a resistor lower in the string (closer to the "negative" represented by the battery).

Right now my brain is telling me that the only way to increase needle movement is to change the shunt in the gauge, which is not feasible for us "shade-tree" electrical engineers.

What you think, Sparky?


OldFords My BAD memory

 

Ok this is the way I thought it worked:
The ammeter, which is a milli-voltmeter, measures the small voltage drop across the shunt wire. The ammeter has a huge resistor in it that causes most of the voltage/current to flow thru the path of least resistance being the shunt wire in this case. The shunt wire has a small resistance in it and is in series with the large wire between the alternator and the battery, with the ammeter wires connected to each end of the shunt wire.
So if you increase the shunt wire size/resistance you drop more voltage and thus have more meter movement.
Also if the battery has less voltage than the alternator output the meter reads towards charge, and if the battery has more voltage than the alternator output it reads towards discharge. The greater the difference the greater the needle movement.
So in a perfect system, ammeter actually has movement , when the truck is at idle with minimum eletrics on, fully charged battery, charging system working correctly, the meter should be dead centre. No charge no discharge of battery. The as you turn things on, at idle, the meter would drift towards discharge as the battery is supplementing the alternator output to supply the eletrics demand. Then if you rev up the engine the alternator output increases, which causes the meter to move towards charge as the alternator is now suppling the voltage, actually amps but we wont go there this confusing enough , the eletrics are demanding.
How does this sound? Am I out in left field or what

Sparky

 

The big problem is, you can have an alternator die on you and the ammeter really doesn’t change, “no warning” until the battery is too low, at least the light type warning did work but I don’t like it.

The movement is so small maybe a half needles the most.

So the wiring between the alternator and battery is acting as an external shunt. Really the ammeter is a micro-voltmeter which can read + or – voltage, and the voltmeter scale is too high. So if I shorten the wires or increase the wire size, this would lower the resistance, which lets more current flow through the shunt. I think your right Sparky and it may work.

 

If my battery gets Drained,When I get the engine started the needle will move up to 3/8" before it settles back to *Just Off* the centre line after the battery is chraged back up.

Dennis

 

[updated:LAST EDITED ON 10-Mar-02 AT 06:29 PM (EST)]OK Guys, you forced me to do something I've been putting off for 30+ years.

I found an old AMP (ALT) gauge out of a '77 Mustang II that had given up the ghost and took it apart to see if there is a resistor in line with the meter movement to limit the voltage reading. There is not. One wire goes into the gauge, through the coil (winding) and back out thru the other wire. Also, I could not find any sort of shunt resistor across the gauge coil (winding). Now, before you tell me it's not a gauge from a '69 thru '72 Ford truck, I agree that it is not. But, according to my wiring diagrams for the '73 Ford truck and the "77 Mustang II, the circuitry is identical. The pickup uses a red/black wire for the shunt while the Mustang II uses a large yellow wire for the shunt portion of the wiring from the alternator to the battery. Both use red/orange and yellow/black wires from the gauge to the shunt. I have the factory tape removed from the wiring harness that runs from the alternator to the battery on my 1970 F100, and the circuitry is the same.

What this tells me is that the shunt wire is actually just that, a shunt. It shunts current around the meter coil, leaving only a small amount to flow throught the coil itself.

Assuming this to be true, I believe that the resistance of the shunt would have to be increased to force more current to flow (voltage drop) through the meter coil. I can see a couple of problems with doing this. 1) There is a greater possibility of the AMP coil burning up before a fuse blows if a short occurs in one of the circuits (lights, horn, radio, etc.). 2) If the meter coil were to open for some reason (above example in #1, old age, or corrosion, there would be less charging current to the battery.

I think the physical movement of the gauge needle can be increased, but you take a chance of burning it up. This may be why the ??Ford engineers?? designed the circuit the way they did; to protect the meter as much as possible while showing some movement.

My thoughts, for what they are worth.

OldFords

 

Old Fords that makes me go hhhmmm.
I think I am off to buy an old truck with an ammeter in it and disect it Enquiring minds want to know how it works Now if I can just convince my wife that is why I really need to buy another old Ford truck

Sparky

 

Well,
Just be thankfull that yours moves at all. I was out off roading with my '67, when she died. Couldnt crank her over. Jumped her and she ran for 5min before she died agian. Tried comp. starting on the hill, no go. Jumped her agian, ran for awhile then died. Come to find out my alternator wasnt working at all. Would have been nice to have some warning. It was a pain to get home that night. 10 min charge the battery to run for 20, then repeat.


D

 

67Ford250, I've driven these old Ford pickups since the early '70s, and I have never trusted the "ammeter" since the second time my alternator died and I was stranded; both times in the dark several miles from home and any phone (no cells then and I hadn't installed a CB yet. Every since then, I have hung an after-market voltmeter somewhere under the dash and ran a + wire from it all the way to the battery.

When I hung the overhead console from a semi in the '67 F100 I have now, I put after-market oil pressure, water temp., and volt gauges in it along with my CB, switches for my trailer lights, trailer brakes, camper lights (when I have the camper on the truck), battery select switch (with the exception of one alternator, I have two separate and complete electrical systems), and other misc. stuff. I have a '67 instrument panel, and the only gauges I have in it are two fuel gauges for the behind-the-seat tank and the extra one I hung under the frame in place of the spare tire.

I like to have my truck as original as possible, but the two "original" walks into town when the alternator died forced me to make some compromises.

OlfFords Bad attitudes

 

 

There used to be a tech article on this subject. It involved making a new shunt, might have been by Steve Delanty? This was a few years ago...

 

Yeah, that’s one way to do it. The amp meter is a millivolt meter that measures the voltage drop across the shunt wire that is also the main feed wire that goes between the alt. and battery just before the fuse link. The amp meter tap points are already as far apart as they can be across the shunt, can’t spread it any further. I don’t like the idea of changing the shunt, it would have to go up in resistance in order to develop a larger voltage drop, something you don’t want happening. This would reduce the available voltage going in or coming out of the battery. The shunt would also develop more heat from resistance heating.

I’ve found that all the connections in the amp gauge wiring can add up significant resistance to impede the meter movement, especially the connections directly on the meter. After going through them in my truck, the gauge works as good as expected. With a fresh battery, the initial charge surge after starting the engine will deflect the needle a little over two needle-widths into the positive range.

To get more deflection, you could rewind the amp meter movement. Now that’s a project for another day.

Barry

 

Hay guys
A good friend of mine, who has been an electrical engineer for 35 years, fixed his Ammeter on his 72 F100. He said to add two new wires (20 AWG) in parallel with the old wiring. Make sure not to cross the wires at the ammeter. Go to radio shack and get a two-pin connector so you can remove dash, without undoing the nuts on the ammeter studs. He said if you going to remove the old wiring, put in 18 AWG wire. The larger the wire size the more meter movement.

I want to really thank you guys for all your help.