install ammeter w/out using the stock wire harness
Oct 7, 2011 4:50:48 GMT -5
Post by sigmfsk on Oct 7, 2011 4:50:48 GMT -5
I found some interesting links, and summarize everything here and hope to answer a few questions I was asked. Rather than prefix every sentence with "I think it is the case that..." I'll just write authoritatively, but please feel free to correct me when I get out of line.
An ammeter could be installed in a car to provide information on things that a voltmeter couldn't, such as
- what is the current draw of the vehicle accessories?
- how much current is the alternator putting out?
But it is not installed this way (at least typically). It is typically installed like this:
from
holdenpaedia.oldholden.com/Ammeter
to show how much current is going in or out of the battery (other than that used to power the starter).
An ammeter showing that the battery is being discharged could indicate that the alternator belt is missing, or that the alternator is working fine, producing maximum current, but the accessories are drawing more than that, and the battery is making up the difference. This information could similarly be obtained (at least mostly) by a voltmeter. There are plenty of places on the web that debate if an ammeter or voltmeter is "better", and I won't attempt to do so here. This is a discussion about ammeters, especially the kind used by a 1974-1977 c-body Monaco.
An ammeter doesn't take much current to move the needle to full-scale deflection:
www.allaboutcircuits.com/vol_1/chpt_8/4.html
So any ammeter that is designed to measure more than a trivial amount of current uses a shunt. The shunt is either internal to the ammeter, or external. If the ammeter electrical connections are designed to take the full current in the circuit, then the ammeter has an internal shunt. If, instead, it is designed to only take a small amount of current, then the small amount of current is provided by an external shunt.
Here's a great post:
www.forabodiesonly.com/mopar/showpost.php?p=788133&postcount=22
That talks about how how Mopars went from internal-shunt in 1970 to external-shunt in 1971 (for all body styles except for the a-body which was internal through 1975 and switched to external in 1976).
And here's an informative page from the 1971 Mopar "Passenger Car Service Highlights" manual that describes the switch to external-shunt:
from
www.hamtramck-historical.com/ServiceHighlights/_1971PassengerCarHighlights-02.shtml?load_img=5
An ammeter can be described as no-shunt (which is really an internal shunt), or shunt (which is as external shunt) ammeter. But either way, there's a shunt that taps off the current from the main circuit and uses just a tad to move a needle. This naming situation explains why the manual describes the new external-shunt ammeter as a "shunt ammeter".
An external-shunt can be either:
a) an actual physical component that attaches to the wiring harness.
Here's a page showing a shunt for a Chevelle:
www.bracketracer.com/nova/shunt/shunt.htm
and this post
www.forabodiesonly.com/mopar/showpost.php?p=788133&postcount=22
talks about how certain Mopar cop cars (with high output alternator) had a distinct shunt
And that same post talks about the other option:
b) the shunt is not a separate physical component, but an artifact of how the wiring harness is designed.
This (b) option is how the 1974-77 c-body Monaco is setup, using the same wiring harness concept as described in the 1971 Mopar Service Highlights page.
Now using the wiring harness itself to operate as a shunt sounds a little scary to me, because if the main portion of the wiring harness failed, then all the current would run through the small gauge wiring to the ammeter, possibly catching things on fire and causing all kinds of havoc. Arguably another reason to use a voltmeter instead of an ammeter, but let's continue with 74-77 c-body Monaco ammeters.
The ammeters come in two sensitivity levels:
74-75 : less sensitive
76-77 : more sensitive
This means that a 74/75 ammeter takes more current to move the needle to full deflection than an 76/77 ammeter. There's no visible different to the driver, though, because the 74/75 wiring harness shunts a higher ratio of the current to the ammeter.
Here's a pic of a 74 ammeter on left and a 76 ammeter on right:
And another pic from the top, showing that the 74 has a smaller induction coil.
And a better pic with the gauge face removed
Being that I won't be using the stock wiring harness, I needed to determine what type of shunt to use with the ammeter. Here's a pic of my setup:
I'm using a 12V power supply, powering three headlights in parallel, with the circuit connected to the ammeter with a 2A / 50mV shunt. With the 1974 ammeter, the circuit needed 6.5A to move the ammeter to full-scale.
So that means it took 6.5A / 2A * 50mV = 163mV. This isn't particularly great news, because:
1) I don't want to overdrive a shunt
2) The highest (at least typical) voltage at full scale for a shunt is 100mV (most shunts are 50mV at full scale, some are 75mV and some are 100mV).
But then I tried a 1976 ammeter!
It took 2.6A for full scale, or 2.6A / 2A * 50mV = 65mV. Now we're talking! It's within the 100mV maximum full-scale range available in shunts.
So it seems that I can use a 1976 ammeter, with a 100A / 100mV shunt with my 80A alternator.
At 65A, the shunt will produce 65mV and the gauge will read at the "maximum" indicator line as shown in the previous pic.
At 80A (max possible for the alternator), the shunt will produce 80mV and the gauge will read just a hair clockwise of that shown here:
As a note of interest, the actual maximum range of the needle movement appears to be around 4.2A / 2A * 50mV = 105mV.
So a 100A / 100mV external shunt is the ticket.
I got this one
from here
www.galco.com/scripts/cgiip.exe/wa/wcat/itemdtl.htm?pnum=FN-100-100-CMI
74-77 c-body ammeters also come in to flavors of "LED alerts"
non LED alert
with LED alert
the LED alert option came as part of a package (that included fender mounted turn signal indicators), and was not a package on the bluesmobile. Here's a pic of an LED ammeter that shows the hole in the gauge face where the LED poked through.
The red & black wires behind and to the right are where the LED circuit board attaches to the instrument cluster. The circuit board mounts on the back of the ammeter, and it has no electrical connections to the ammeter (it just mounts on the studs and the LED pokes through the gauge face).
So for functional use I want a 76-77 ammeter (that is sceen accurate as a 74-75 ammeter), and for screen accuracy I want a non-LED ammeter.
In 1976-1977, most of the Monacos were "fancy" and had the LED package, so it is rare and expensive to find a non-LED 76-77 ammeter.
In 1974-1975, a good number of Monacos were "non-facy" and were found without the LED package. But I can't use a 74/75 ammeter, as the induction coil sensitivity is too low. So I swapped a 74 non-LED face onto a 76 high-sensitivity ammeter.
I used a dremel tool to grind off the head of the rivets attaching the faces to the ammeters (allowing me to take those earlier pictures of the different size induction coils).
And then attached the non-LED 74 face to the 76 ammeter. Here's a pic of the final product:
The screws (and gauge face part number) are not visible in the assembled instrument cluster.
To attach the face I found that these screws were a perfect fit
3-48x5/16"
These screws are just slightly larger OD than the plastic ammeter hole ID, so they actually cut their own threads into the plastic. The ammeter is like its own nylock nut.
Where to get 3-48x5/16" screws?
Not Home Depot. I found that Amazon is a great place for lightweight hardware like this. Ever try to order something and the total is $23, and it says: if you buy $2 more, you can get free shipping. And if you don't buy anything more, shipping is $3? So if you spend $2, you'll actually save $1? Buy some screws and nuts. They count towards free shipping. Here are some for $1.04
www.amazon.com/Stainless-Machine-Fillister-Slotted-Length/dp/B000FN7YH4/ref=sr_1_2?s=hardware&ie=UTF8&qid=1298721834&sr=1-2
I was asked how to tell if an ammeter was internal-shunt or external-shunt. If you have a 74-77 c-body Monaco, you already know the answer. Otherwise, I'd suggest connecting a load (like my three headlights) up to a 12V current limiting power supply. Then verifying that the circuit works and you can adjust max current through the circuit. Then connect in the ammeter (without a shunt), and slowly increase current and see what it takes to put the needle at full-scale. If it takes 2A (and the gauge is designed to measure more than 2A), then you need an external shunt. If it takes 50A (and the gauge is designed to measure 50A) then the ammeter uses an internal shunt and no other hardware is needed.
your friend screen accurate working ammeters,
arthur
from archived info here:
www.bluesbrotherscentral.com/forum/topic/9446-all-about-ammeters/page__view__findpost__p__116800
An ammeter could be installed in a car to provide information on things that a voltmeter couldn't, such as
- what is the current draw of the vehicle accessories?
- how much current is the alternator putting out?
But it is not installed this way (at least typically). It is typically installed like this:
from
holdenpaedia.oldholden.com/Ammeter
to show how much current is going in or out of the battery (other than that used to power the starter).
An ammeter showing that the battery is being discharged could indicate that the alternator belt is missing, or that the alternator is working fine, producing maximum current, but the accessories are drawing more than that, and the battery is making up the difference. This information could similarly be obtained (at least mostly) by a voltmeter. There are plenty of places on the web that debate if an ammeter or voltmeter is "better", and I won't attempt to do so here. This is a discussion about ammeters, especially the kind used by a 1974-1977 c-body Monaco.
An ammeter doesn't take much current to move the needle to full-scale deflection:
www.allaboutcircuits.com/vol_1/chpt_8/4.html
So any ammeter that is designed to measure more than a trivial amount of current uses a shunt. The shunt is either internal to the ammeter, or external. If the ammeter electrical connections are designed to take the full current in the circuit, then the ammeter has an internal shunt. If, instead, it is designed to only take a small amount of current, then the small amount of current is provided by an external shunt.
Here's a great post:
www.forabodiesonly.com/mopar/showpost.php?p=788133&postcount=22
That talks about how how Mopars went from internal-shunt in 1970 to external-shunt in 1971 (for all body styles except for the a-body which was internal through 1975 and switched to external in 1976).
And here's an informative page from the 1971 Mopar "Passenger Car Service Highlights" manual that describes the switch to external-shunt:
from
www.hamtramck-historical.com/ServiceHighlights/_1971PassengerCarHighlights-02.shtml?load_img=5
An ammeter can be described as no-shunt (which is really an internal shunt), or shunt (which is as external shunt) ammeter. But either way, there's a shunt that taps off the current from the main circuit and uses just a tad to move a needle. This naming situation explains why the manual describes the new external-shunt ammeter as a "shunt ammeter".
An external-shunt can be either:
a) an actual physical component that attaches to the wiring harness.
Here's a page showing a shunt for a Chevelle:
www.bracketracer.com/nova/shunt/shunt.htm
and this post
www.forabodiesonly.com/mopar/showpost.php?p=788133&postcount=22
talks about how certain Mopar cop cars (with high output alternator) had a distinct shunt
And that same post talks about the other option:
b) the shunt is not a separate physical component, but an artifact of how the wiring harness is designed.
This (b) option is how the 1974-77 c-body Monaco is setup, using the same wiring harness concept as described in the 1971 Mopar Service Highlights page.
Now using the wiring harness itself to operate as a shunt sounds a little scary to me, because if the main portion of the wiring harness failed, then all the current would run through the small gauge wiring to the ammeter, possibly catching things on fire and causing all kinds of havoc. Arguably another reason to use a voltmeter instead of an ammeter, but let's continue with 74-77 c-body Monaco ammeters.
The ammeters come in two sensitivity levels:
74-75 : less sensitive
76-77 : more sensitive
This means that a 74/75 ammeter takes more current to move the needle to full deflection than an 76/77 ammeter. There's no visible different to the driver, though, because the 74/75 wiring harness shunts a higher ratio of the current to the ammeter.
Here's a pic of a 74 ammeter on left and a 76 ammeter on right:
And another pic from the top, showing that the 74 has a smaller induction coil.
And a better pic with the gauge face removed
Being that I won't be using the stock wiring harness, I needed to determine what type of shunt to use with the ammeter. Here's a pic of my setup:
I'm using a 12V power supply, powering three headlights in parallel, with the circuit connected to the ammeter with a 2A / 50mV shunt. With the 1974 ammeter, the circuit needed 6.5A to move the ammeter to full-scale.
So that means it took 6.5A / 2A * 50mV = 163mV. This isn't particularly great news, because:
1) I don't want to overdrive a shunt
2) The highest (at least typical) voltage at full scale for a shunt is 100mV (most shunts are 50mV at full scale, some are 75mV and some are 100mV).
But then I tried a 1976 ammeter!
It took 2.6A for full scale, or 2.6A / 2A * 50mV = 65mV. Now we're talking! It's within the 100mV maximum full-scale range available in shunts.
So it seems that I can use a 1976 ammeter, with a 100A / 100mV shunt with my 80A alternator.
At 65A, the shunt will produce 65mV and the gauge will read at the "maximum" indicator line as shown in the previous pic.
At 80A (max possible for the alternator), the shunt will produce 80mV and the gauge will read just a hair clockwise of that shown here:
As a note of interest, the actual maximum range of the needle movement appears to be around 4.2A / 2A * 50mV = 105mV.
So a 100A / 100mV external shunt is the ticket.
I got this one
from here
www.galco.com/scripts/cgiip.exe/wa/wcat/itemdtl.htm?pnum=FN-100-100-CMI
74-77 c-body ammeters also come in to flavors of "LED alerts"
non LED alert
with LED alert
the LED alert option came as part of a package (that included fender mounted turn signal indicators), and was not a package on the bluesmobile. Here's a pic of an LED ammeter that shows the hole in the gauge face where the LED poked through.
The red & black wires behind and to the right are where the LED circuit board attaches to the instrument cluster. The circuit board mounts on the back of the ammeter, and it has no electrical connections to the ammeter (it just mounts on the studs and the LED pokes through the gauge face).
So for functional use I want a 76-77 ammeter (that is sceen accurate as a 74-75 ammeter), and for screen accuracy I want a non-LED ammeter.
In 1976-1977, most of the Monacos were "fancy" and had the LED package, so it is rare and expensive to find a non-LED 76-77 ammeter.
In 1974-1975, a good number of Monacos were "non-facy" and were found without the LED package. But I can't use a 74/75 ammeter, as the induction coil sensitivity is too low. So I swapped a 74 non-LED face onto a 76 high-sensitivity ammeter.
I used a dremel tool to grind off the head of the rivets attaching the faces to the ammeters (allowing me to take those earlier pictures of the different size induction coils).
And then attached the non-LED 74 face to the 76 ammeter. Here's a pic of the final product:
The screws (and gauge face part number) are not visible in the assembled instrument cluster.
To attach the face I found that these screws were a perfect fit
3-48x5/16"
These screws are just slightly larger OD than the plastic ammeter hole ID, so they actually cut their own threads into the plastic. The ammeter is like its own nylock nut.
Where to get 3-48x5/16" screws?
Not Home Depot. I found that Amazon is a great place for lightweight hardware like this. Ever try to order something and the total is $23, and it says: if you buy $2 more, you can get free shipping. And if you don't buy anything more, shipping is $3? So if you spend $2, you'll actually save $1? Buy some screws and nuts. They count towards free shipping. Here are some for $1.04
www.amazon.com/Stainless-Machine-Fillister-Slotted-Length/dp/B000FN7YH4/ref=sr_1_2?s=hardware&ie=UTF8&qid=1298721834&sr=1-2
I was asked how to tell if an ammeter was internal-shunt or external-shunt. If you have a 74-77 c-body Monaco, you already know the answer. Otherwise, I'd suggest connecting a load (like my three headlights) up to a 12V current limiting power supply. Then verifying that the circuit works and you can adjust max current through the circuit. Then connect in the ammeter (without a shunt), and slowly increase current and see what it takes to put the needle at full-scale. If it takes 2A (and the gauge is designed to measure more than 2A), then you need an external shunt. If it takes 50A (and the gauge is designed to measure 50A) then the ammeter uses an internal shunt and no other hardware is needed.
your friend screen accurate working ammeters,
arthur
from archived info here:
www.bluesbrotherscentral.com/forum/topic/9446-all-about-ammeters/page__view__findpost__p__116800