Shunt pictures

[surubet]

This is all great information about shunts! I never even knew why the factory would splice into a wire way out in the harness somewhere, but through all your efforts I am beginning to understand. Have to figure this information will help keep me from making a mistake during some rewiring project (I probably would have just moved all the wires onto the same terminal and called it good ... and then wondered "how come?"). Anyhow, I am looking forward to the finished tech article.

[bluef250]

Looked at some more sites regard meters and shunts. Ford is not the only one that used the shunt theory.
http://www.bracketracer.com/nova/shunt/shunt.htm

In the following article, the second to last diagram shows a diagram of the Ford circuit. The resistance, R shunt, is created by wire from the splice to the starter solenoid. As far as I know, Ford did not use a special shunt device similar to the Chevy on the 67-72 trucks.
See http://www.allaboutcircuits.com/vol_1/chpt_8/4.html

[FORDification]

Hmm, that is strange. The field is going to the starter solenoid? It should be going to the voltage regulator. The ammeter is measuring field current? Gotta think about that.

Actually, it's going to both....one of the three wires coming from the splice at the end of the shunt goes to the voltage regulator.

OK, I think I may be beginning to get a few flickers of understanding here...let me think out loud, and those of you with a bit more upstairs can tell me if I'm headed in the right direction.

When we had the wiring harness out on the floor, hooked up to a battery and a full-instrumention panel, we measured the ammeter leads at the gauge, and both read the exact same voltage as the battery, which led me to believe that there wasn't any resistance wires specifically inserted into the circuit. We measured the resistance of the 10 AWG cable which extended from the starter solenoid to the splice, and at the most sensitive setting on the ohmmeter we read .10Ω resistance, which I figured was negligible and thus immaterial. However, it appears now that this amount might be all the ammeter needs to operate correctly...that is, it really doesn't matter for use with this specific Ford ammeter WHERE the hookup was or the distance apart the two ends of the shunt were located. The power is actually being being 'metered down' by the smaller wire size of the leads going to and coming from the ammeter.

The alternator-to-solenoid wire was 10AWG....if someone were to simply replace the two smaller ammeter leads with 10AWG cables, for example, it would read full voltage and more than likely fry the ammeter.

So why didn't the factory engineers simply hook the shunt between the alternator and starter solenoid? From talking with Earl, he believes it was simply because the engineers wanted fewer connections at the alternator. Having them spliced into the cable and wrapped up would mean it would stay much cleaner and thus the gauge would remain more accurate, since obviously dirty connections would affect the gauge reading....and splicing it into the alternator-to-solenoid cable served their purposes just fine, and it was convenient.

[68F250]

Well, I still don't understand how that works. If the field is connected to the battery, the alternator is forced into full output. BTW, that is one of the ways to test an alternator, it's called a full field test.

Keith, see if the ammeter circuit in this scan matches your harness. Yeah, the regulator is different with an external field relay but the ammeter part of it should be similar, I think.

[68F250]

If it helps,here's the color codes and descriptions of the alternator wires:

38, 38A - Black (Output)

26, 26A - Black w/ Red Stripe (Ground)

35 - Orange (Field)

37 - Black w/ Yellow Stripe (Ignition Feed)

[FORDification]

Well, I decided I'd better double-check the alternator connection before answering, and I just got back from the shop, and found that I'd mislabeled that connection. The cable doesn't go to the FLD terminal, it goes to the BAT terminal instead. (Makes sense now that I think about it! ) I've corrected the graphic to show this. (sorry about that!). I also followed the larger wire (black with yellow tracer) coming from the factory splice and found it does go to the ignition switch.

So now...yes, from what I can tell, your '68 diagram is identical to the '67 harnesses I have...aside from separate field relay and the two missing fuses. I got to wondering though, on the subject of the fuses...are they actual cartridge-type fuses or fusible links? Here's a picture of the solenoid end of the solenoid-to-alternator cable. Is that a fusible link hiding in there? I followed the other yellow wire which goes to the ammeter and wasn't able to find anything else that looks like a fuse or fusible link, though.



Also, just for the purpose of comparison, here's a picture taken comparing two '67 shunt splices. Both are definitely '67....but the bottom one looks amateurish, even though it definitely had what appeared to be factory harness wrap around it...not that it really matters, I guess. It'll function just fine.



But it does illustrate how easy it will be to wire in the ammeter now. I think I'm actually fairly close to getting this figured out, and I can see now that it won't be nearly as hard as we'd originally thought. It'll definitely be a LOT easier than finding and swapping in a shunt-equipped harness.

[68F250]

Yes they are cartridge type fuses. That's one of the problems, over time resistance developes in the fuse connections and the ammeter slowly dies. If I remove the fuses and clean the ends along with the cartridge connections, the ammeter comes back to life.

I'm still wondering about that article I sent you about changing the Mustang idiot light over to a gauge. They left the existing harness in place and just connected up the ammeter, one side to the alternator output lug and the other side to the starter solenoid lug. Is that all it takes? If so, then all it's doing is measuring the small voltage drop between the alternator and the solenoid. It's gotta be in the millivolts. If that's the case, the ammeter is really a millivolt meter that when centered reads zero.

[bluef250]

My ‘68 truck does not have the field relay as a separate device. I wonder if this was an assembly line change with the relay incorporated into the alternator. The 4 amp fuses in the '68 harness are cartridge type, not fusible links. When checking to see why the amp gauge did not work, I removed both and checked them. I am planning on changing them to spade ATC type when I install a larger alternator. After all the discussion on the shunt, I am thinking of increasing the wire size to the gauge to see if that will make it more responsive and to move the connection point from the splice to the alternator. The resistance in the gauge wire will decrease, increasing the current through the gauge. I think Ford may have had other reasons for creating the splice. The splice is awkward and a potential trouble spot although their reason may have been as simple as this connection was the least costly method - save a penny or 2. After the discussions, I think that Ford was not overly concerned with the movement of the amp gauge. Also I plan to either run an additional (power) wire from the larger alternator for accessories (an electric fan and for the lights) or run a wire parallel to the existing to handle the additional power needs.

[68F250]

The separate relay was only used with the high power alternator (65 amps was high power then).

[FORDification]

...I'm still wondering about that article I sent you about changing the Mustang idiot light over to a gauge. They left the existing harness in place and just connected up the ammeter, one side to the alternator output lug and the other side to the starter solenoid lug.

That's exactly what I'm thinking is what needs to be done to use the full-instrumentation panel on ours as well. There are only a few things I want to check on first before stating for certain that it's possible:

- verify that the size of the alternator-to-solenoid cable on an idiot-light truck is the same as the full-instrumentation trucks....and if they're different sizes or lengths, find out what the difference is resistance would be. The small differences in length or size probably wouldn't be enough to matter, but I just want to make sure.
- get the AWG size of the ammeter leads (they're in the neighborhood of 18-20 gauge, I believe) and the factory length, and figure out what the overall resistance would be for the complete circuit, from the solenoid through the harness to the gauge and back to the alternator.

So does anyone have access to or know where I can find online a chart showing the resistance factors of various wire gauges of similar lengths? It would also seem to be true, with things as I understand them now, and as 'bluef250' suggests, that the factory ammeter could be 'adjusted' to be more responsive by using slightly larger ammeter leads, which would decrease resistance. It would be kinda tricky to figure out the best wire size and length to supply the ammeter with a little bit keep from frying the gauge. I guess what we'd have to do is find out what the factory amp rating for the gauge was. If anyone here had access to a 'load pile' (a.k.a. a variable ammeter, amp tester) and could hook up a factory ammeter to it and spin the rheostat to check for needle response, it would really really help out.

[bluef250]

Do a search for wire resistivity. Here is a list I got from a site. Each wire manufacturer will publish different results. As wire sizes increase resistivity increases by about 60%.
AWG No. Ohms / ft Ohms / 1000 ft
18 ------- 0.006385 - 6.3850
16 ------- 0.004016 - 4.0160
14 ------- 0.002525 - 2.5250
12 ------- 0.001588 - 1.5880
10 ------- 0.000999 - 0.9990
8 -------- 0.000628 - 0.6280
6 -------- 0.000395 - 0.3950
4 -------- 0.000249 - 0.2490
2 -------- 0.000156 - 0.1560
1/0 ----- 0.000098 - 0.0980
2/0 ----- 0.000078 - 0.0780
I just found this site that has a "calculator"
http://www.stealth316.com/2-wire-resistance.htm

[bluef250]

What is the stock alternator size when you have gauges? The wiring diagram for the amp gauge shows a 65 amp alternator. The alternator size for the "Charge indicator lamp" is not labled.

[FORDification]

bluef250: thanks for the info and links....that'll make it much easier to get this figured out.

[EB]

I'm still wondering about that article I sent you about changing the Mustang idiot light over to a gauge. They left the existing harness in place and just connected up the ammeter, one side to the alternator output lug and the other side to the starter solenoid lug. Is that all it takes? If so, then all it's doing is measuring the small voltage drop between the alternator and the solenoid. It's gotta be in the millivolts. If that's the case, the ammeter is really a millivolt meter that when centered reads zero.

Just a quick response to 68F250.
Don't be confused by the discussion of resistance and voltage drop as it relates to the effect on current. In simple user-friendly terms, the "ammeter" measures current and indicates which direction it is flowing.
When the alternator puts out more electricity than the system is using, the needle will point to charge + because the electrons are flowing from the alternator to the system (battery & loads). When the alternator can not keep up with the system demands, the current reverses and flows from the battery to the loads and the needle will swing to discharge -. Auto makers resorted to tapping off the main circuit with a smaller circuit so that the gauge did not have to carry the full current load. Any time you let either air - water - or electricity take an alternate path to it's destination, a fraction of the main load will try to take the alternate path depending on the size of that path. This is where engineers use wire size and length (or pipe size) to figure out exactly what the voltage drop is (resistance to flow) so that they can calculate exactly how much current will flow in the smaller path. The trick is to match the current flow in the alternate path with the rating of the "ammeter". If the "ammeter" is rated at 30 amps full scale and the alternate path only allows 5 amps - the "ammeter" will under-perform. My guess is that Ford installed gauges that were seriously over-rated for the amperage that they were exposed to. This was very safe - but not particularly useful for the driver. Like Keith suggested, someone needs to hook a gauge up to a load-tester and see what range of amperage it performs at. Once this is known, it will be fairly simple (for an engineer) to calculate suitable circuits based on alternator sizes.

[68F250]

Thanks EB. I understand what you said about ammeters but I was wondering if maybe the Ford 'ammeter' is really a voltmeter with an ammeter scale painted on. The shunt is just the resistance of that section of 10 gauge wire that feeds the battery and the ammeter may not be really carrying any current per se (yes, there is current going thru it but very small). It's measuring the voltage drop which is relative to how much current is passing thru that section of 10 gauge wire, more current, more voltage drop. The second to the last pic in the link posted earlier on ammeters shows what I’m thinking. To prove this out, somebody's gonna have to test their ammeter without letting the smoke out.

But maybe it's really measuring current, diverting a fraction of the total current across the shunt (10 gauge wire section) Since the inline fuses are rated at 4 amps, maybe it really is diverting current! Otherwise, why wouldn’t they have used smaller values? Ok, I don’t know what it is without some more measurements.

I believe there is only one harness for the ammeter except for the 65 amp and I've only seen one gauge part #. Standard alternator was 38 amp and optional were 42, 55 & 65 amp. More alternator output would only deflect the gauge more and these hardly move anyway.