March 13, 2005 - I spent several hours in
the shop today following wires for the factory ammeter, and
posted on this page are my findings. However, before I begin,
let me catch you up to date, especially important for those of
you who aren't quite up to speed.
1967-1972 Ford trucks could either have a full-instrumentation
instrument panel (hereafter referred to as FIP) or
standard-instrumentation panel (SIP). The FIP panel includes a
ammeter for monitoring the charging system, instead of the idiot
light setup of the SIP. There have many people asking whether
their SIP-equipped trucks to be upgraded to FIP. I've been one
of the people asking this question, and it's been long reported
to me that the FIP trucks had a wiring harness with a 'shunt',
which powered the ammeter. I've always understood this statement
as being that a shunt was basically a resistor (or resistor
wire/cable) to reduce the voltage to the ammeter. However, my
findings today show differently.
Dirty Harry once said, "A man's got to know his
limitations"....and I know mine. I know almost nothing about
electrical wiring, so for this exercise I had to enlist the help
of my father-in-law Earl, who just recently retired after many
years with the local electrical company, and has probably
forgotten more about electrical theory and implementation
then I'll ever know. After I explained the basics on Ford's
setup, we set out to find out all we could. (And I should
mention, he was very patient with me today, trying to help me
understand a lot of the basics...thanks Earl!) I'm going to to
my best to try to explain what I learned today here, but since
I'm still up up to the 'apprentice automotive electrician' level
yet, some of what I transcribe here might take a little tweaking
in the future for better clarity.
Anyway, here goes...
First of all, I'm not entirely sure whether I was simply
misunderstanding other's descriptions of a shunt or whether it
was a general misconception, but if I WAS understand correctly,
then we're going to need to 'un-learn' what we thought we knew
about the ammeter in Ford pickups. Some of us (or just me?) have
been thinking of a shunt as a noun...that is, something that we
could see and hold in our hands, like a resistor, a magical
piece of something that needed to be wired into the circuit.
However, a shunt is not an object, per se...it's a method of
connection. To "shunt" something means to divert. The Ford
ammeter is not a true ammeter...it's more of simply a different
connection method for a voltmeter. There is no in-line
resistance wire/cable and no inline resistor to the ammeter. All
the Ford factory ammeter does is basically to measure the
difference between the output of the battery vs. the output of
the alternator and reports that difference. In other words, it
measures the direction of current flow.
spread an entire '67 F100 wiring harness out on the floor and
hooked up the FIP and a battery and started taking readings.
First of all we traced the wires coming from the ammeter back to
their source and verified their continuity.
red wire (A) coming from the discharge side of the ammeter goes
through the wiring harness and hooks up to the hot side of the
starter solenoid. The yellow wire (B) coming from the charge
side of the ammeter goes through the wiring harness and ties
into the 10 AWG cable which goes between the hot side of the
starter solenoid and the alternator. The points at which the two
wires connect to the solenoid-to-alternator cable are about
23-1/8" apart. However, this cable does not "meter" power to the
ammeter...it has virtually no resistance. Using a sensitive
ohmmeter, we were only able to detect about .10Ω resistance
between the point where it begins at the starter solenoid and
where the shunt, or power diversion, is connected. Therefore, it
appears that even though the yellow wire is connected in the
middle of the solenoid-to-alternator cable, where the factory
spliced it in, it would be just as effective to hook it up at
the alternator itself at the BAT connection where the cable
itself is attached.
Fig. 1 - Click this thumbnail to open the full-size image
in a new browser window, so you can pop back and forth between
this page and the picture.
Here is a simple schematic I whipped up showing the
factory ammeter wiring.
The distance between connection of the discharge
side of the ammeter and the charge side of the
ammeter on the solenoid-to-battery cable is about
21-3/8"...but the cable between these points is NOT
After hooking up a battery to the wiring harness to check for
power at the ammeter itself, we found we had the EXACT SAME
READINGS at both ammeter terminals! If there was a resistor
inline anywhere in the harness whatsoever, it would have shown
up at the ammeter...and it didn't.
the purposes of this investigation, I had three 1967 wiring
harnesses laying out on the shop floor for comparison...two with
FIP setups and one SIP. The two FIP setups did not have any
fuses, but I've been informed that '68-up harnesses should have
two 4-amp fuses in the circuit...the fuses on the
harnesses are inline immediately after the starter solenoid
connection and after the splice on the wire to the amp gauge.
Also, I've been told that there are five wires coming from the
factory splice in the solenoid-to-alternator cable on '68-up
wiring harnesses. The five wires at the splice go to the 1)
alternator, to the 2) voltage regulator, the 3) switch, the 4)
amp gauge, and to the 5) starter solenoid. However, there are
only three coming from the '67 splice. Two yellow wires (one of
which goes to the ammeter and the other to the voltage regulator) and
one large black wire with a yellow tracer. I ran out of time
this evening before I could trace the large black wire to it's
source. I will do so soon and report back.
is as far as I got on this tutorial this evening. If you have
any input on this, please post a reply to
THIS THREAD on the
FORDification forums, where we're having an ongoing discussion
into the matter.
March 14, 2005 - I sent my father-in-law
the forum thread where we're discussing this, and he
e-mailed back the following, along with a few attachments. It's
some interesting reading:
DASH MOUNTED “AMMETER”
This gauge is a charging indicator. It shows whether the
battery is being charged by the alternator or is being
discharged by an electrical load (lights, engine, radio,
etc.) It measures current flow and strength between the
battery and the alternator. The gauge consists of a
permanent magnet – a pointer – and an armature. The pointer
is held in the upright center position by the magnet when
there is no current flow. When the alternator is providing
power to the battery, the “ammeter” pointer deflects to the
+ charge side and the opposite occurs when the battery drain
is more than the alternator provides. The current is
reversed and the pointer moves to – discharge. If the
“ammeter” were installed in the main circuit between the
alternator and battery, it would work the same, but it would
have to be to handle all of the current that is exchanged in
this circuit. To overcome this drawback, a smaller wire is
tapped into the circuit near the alternator, and then routed
to the “ammeter”. This circuit is continued from the other
terminal post on the “ammeter” and routed to the main
circuit near the battery. This is technically called a
“shunt” or “shunted-circuit”. It simply creates another path
for the electricity to travel without exposing the gauge to
the high currents of the main circuit.
“idiot light” compares the battery voltage to the alternator
voltage. When the battery voltage is below the alternator
output – the light stays off, and when the alternator
voltage falls below the battery – the light comes on.
I have included sections from period MOTOR’S and CHILTON
manuals on the subject.
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