I am wondering what would cause the amp gauge to never read anything? The charging system works great As I have a aftermarket Voltmeter installed under Dash and it reads most of the time 14 volts when truck is running. I read a post some where that says, The gauge measures voltage across a heavy shunt wire in the harness
between the alternator and starter relay.OK so if that's true where is this Heavy shunt wire what is it and how do I fix it? Any Ideas Guys? I just replaced the circuit board and IVR and that cured all my other gauge problems. Just not sure why this one (amp) don't read. My Dad had a 72 F100 and it never read either. Is this a Common Problem With Fords?

 

Oh yeah! There are wiring diagrams in one of the sticky threads, which can help you find the wires, that's how I did it.
There is a two-pin connector near the starter relay which goes to the gauge.

One of the shunt wires (I think it's the shunt) goes through a rubber block with a thicker black wire. I think the ammeter wire might be red on one side of the rubber thing, and yellow on the other. Mine had a cut wire there, which I reconnected, restoring my ammeter function.

Sometimes the ammeters do work, but the gauge movement is so slight that it's difficult to see.

 

I understand why they went away from the direct-reading type (to keep the main charge wire under the hood and out of the cab) but geez, I miss those very functional and consistent gauges!
The one from the old Broncos was great, in that it read the current flow through an inductive loop on the back of the gauge. Rather than having the current actually pass through the wiring of the gauge itself. Just like a modern clamp-on amp meter.
Worked sweet. Kind of a shock when I got into my '79 and found out that it was more common for them not to work than to work.

That's good to know about the information here meangreen. Do you have a link by any chance? I can search, but sometimes my search-fu is weak.
The one on my '79 worked well for about a week after I changed the voltage regulator. Once it stopped though, it never read a tick of current again. Think I'll go digging soon.

I'll fix it just to say I did. And then just buy a Dakota Digital VHX cluster!

Paul

 

If you'd like to convert your ammeter to a voltmeter with much needle movement try:

Rocketman's Classic Cougar Innovations

 

I'll try to remember to snap a pic of the two-prong connector today, as well as where my cut wire was

 

I'll do some more looking myself today. Those pics would really be appreciated!

 

Yes, a picture would be nice. I always wondered what that little box next to the relay was. I suspect the picture will let me know. My truck didn't come with the gauge for charging just the warning light.

 

On many trucks with the horn relay, that's what the little box next to the starter relay was. Smaller, silver (usually) and grounded through the case to the body, with a 3-wire plug.
The main power feed was usually Yellow and went straight over to the battery side of the starter relay. Where the Yellow wire to the voltage regulator was usually spliced into the the supply wire and all taped up nicely in the harness.
The horn relay was kind of a later add-on, but I think they kept it separate mostly because right up to at least the '79 model year, did not come on every truck.

Maybe it was the difference in gauges and cruise-control or not?

My '79 had the ammeter but no horn relay or cruise-control. So I put my own horn relay in to take the load of the horn off of the turn signal switch. I don't think they ever intended people to use their horns as often as I use mine maybe?

Paul

 

I guess there's three pins...
My broken wire was where the tape is. The red part was off. I ended up cutting off the rubber blob which was covering the junction. I think that's the charge wire that the yellow and red are bundled with. It becomes the gray wire with my newer fusible link.

My truck was missing its horn relay, and I could see the spot where it had been bolted up, so I wired in a newer one, which you see above the taped junction.

 

OK thank you for the info I'll go check out mine now

 

IF you're not stuck on the idea of an ammeter gauge (how is one useful to what you alternator, regulator & battery is doing?), I'll throw in a second vote for Rocketman's conversion of a stock gauge to read voltage. I did it and have NO regrets.

 

I think you are correct! The Volt gauge is the way to go. I currently have a aftermarket gauge mounted under dash so I can see that it is actually charging.Thank you for the info!

 

You can source these wires on your truck along with the photo from Meangreen92 it shows most of them. Get some WD40 or simple green and clean the wires so you can see the true colors. I pulled my full charging harness the other day and included a picture here.
You can find a png file of the wiring harness on the web. Might be one on this site.
There is a black red wire that comes from the alternator that enters a splice with a yellow black and black yellow wire. The second color is the tracer color.
The yellow black wire from the splice enters a connector with a red orange wire.
This pair runs to the two connections on the ammeter. You can see the connector in the picture
from Meangreen92 center bottom of the photo.
The black red out of the above splice enters another splice with a yellow wire
from the regulator and the red orange. This black red wire between the two splices is referenced in the drawings as the shunt.

The black red wire from the second splice then terminates on the battery side of the starter motor relay.

I have gotten my ammeter to show charge just by cleaning all of the terminations in this harness. it works for a little while.

 

Yep. Large Black w/yellow is the main charge, the Orange is the Field wire from the regulator to the alternator, and I believe the loop created by the Yellow w/black and Red w/orange wires is your finicky ammeter shunt.

Good thinking. I prefer the easier to find (usually) and less expensive (usually) standard relays anyway. And you even kept it in the Ford family!

I think voltmeters are great. But I started with ammeters and never thought of them as giving less information than a voltmenter. It's just in a different format.
Some handy things they tell you for instance:

1. If you see a negative reading even with the key off you can tell something is drawing current. This shows immediately, where you won't know a battery is being drained by a voltmeter for some time while the battery is being depleted. And in the case of the older "always on" ammeters, you would be able to see the drain even with the key off. Where a volt-meter would need the key on before you noticed something.
You get the same information eventually. You just get it immediately with an ammeter.

2. As with a voltmeter, if you don't see it go into the positive range with the engine running, you know instantly that your alternator is not charging.

3. Like volt-meters, some ammeters have full range of readings, while others just say "D" and "C" or give you a "+" and "-" range. Nether is very helpful until you get a full range scale in my opinion. Yes, you can tell something is going on with either, but without a full scale you don't know the full details.

4. When the engine is first started, with an ammeter you can see the needle go up quite high during the initial charge. As it slowly goes back down to just above zero while running, you can watch as the system brings the battery back up to it's full charge. With a volt-meter you simply see it maintain 14.5 volts or thereabouts, without really knowing the condition of the charging system or the battery. Only that it's charging. This is fine for most of course, I just like seeing the little details.

5. If your alternator ages and deteriorates, you can see with an ammeter that it's not able to put out it's higher amperages. For instance a 70 amp alternator that's only able to put out 30 amps due to old age or poor connections is still going to show 14 volts on a volt-meter and not let you know that it's got issues. With an ammeter that you've viewed over time, you can see that what used to read 45-60 amps after starting, now only reads 25-30 amps. Letting you know that either the charging system is weak, or the battery can't take it's full charge for some reason.

There might be other examples of the differences, but none of them take anything away from a volt-meter still being handy. Just that I feel an ammeter can be quite handy in and of itself.
They were just super finicky and unreliable in the later years, and literally unsafe in the early years when either poorly thought out, or produced on the lowest possible budget. Simply using larger gauge wiring (more money), better designed ammeters (even money?) and fewer connections to get rusty and unreliable (probably harder to install, therefore more money) and keep the longer wires (definitely more money) the old ammeters would still have been a handy unit of measurement.
Obviously someone back in the day thought an ammeter more handy than an easier and less expensive volt-meter or they would not have been installing them instead. Unless someone has heard other reasons for the inevitable change-over from amps to volts with the gauges?

Anyway, I don't mind having a volt-meter. I just like to defend the old ammeters (the ones that worked anyway!) when I can.
Probably just some old guy fondly pining for the "good old days" and
bemoaning their demise.

(edit: Sorry, I forgot to make clear that all of the above are regarding an ammeter that actually works as expected!!!)

Paul

 

Well since I replaced the circuit board I noticed today that the Amp Gauge was WORKING! YAY! With Lights on engine at slow Idle the rev up slightly I noticed that the gauge jups upward somewhat not alot maybe an 1/8 of an inch.