rags
Senior Member
2 c's. both ammeters working. ain't gonna wear any stinking mask either.
Scratch one GM product
- Thread starter mindshark13
- Start date
69 300 vert
Well-Known Member
I think a "real" ammeter would be wired with 10 gauge wire. A "shunt" style like in our former '67 and current '68 Fury are much smaller. From memory 16ga? Mebbe 14ga? Too small for a real ammeter.
As a former mechanic I would like to have an ammeter and voltmeter. If I have to pick one, I choose the ammeter. I can know immediately if it's not charging as opposed to after the fact with voltage being low. 2¢
As a former mechanic I would like to have an ammeter and voltmeter. If I have to pick one, I choose the ammeter. I can know immediately if it's not charging as opposed to after the fact with voltage being low. 2¢
Yea, I'm not sure when they actually did the change. In 1971, they started mentioning it in the FSM and I actually noted the "Nacho" type bypass wire with fusible link when I was researching a 72 or 73 Dodge wiring problem for someone. I know my '65 Barracuda has a #12 wire running to the ammeter, but that's a real simple wiring system.
mindshark13
Active Member
69 300 vert
Well-Known Member
I "think" the shunt magic happens under the hood somewhere, probably in the alternator charge wire. I don't think there's a "shunt" ammeter proper, just keeping the full current flow away from the ammeter. I haven't studied the diagrams to see how it's done, all I know is from mechanic talk back in the day. I'm relying on the reduced wire gauges as evidence this information is correct.
70_NPORT
Well-Known Member
Old decrepid wiring insulation becomes brittle over time from repeated hot/cold cycles...cracks happen, then fraying may occur.....bare wire touches metal, causes sparks.
Fire red Oldsmobile.
Fire red Oldsmobile.
3175375
Senior Member
From Wikipedia:
An ammeter shunt allows the measurement of current values too large to be directly measured by a particular ammeter. In this case, a separate shunt, a resistor of very low but accurately known resistance, is placed in parallel with a voltmeter, so that virtually all of the current to be measured will flow through the shunt (provided the internal resistance of the voltmeter takes such a low portion of the current that it is neglible). The resistance is chosen so that the resultant voltage drop is measurable but low enough not to disrupt the circuit. The voltage across the shunt is proportional to the current flowing through it, and so the measured voltage can be scaled to directly display the current value.[3][4]
Shunts are rated by the maximum current and voltage drop at that current. For example, a 500 A, 75 mV shunt would have a resistance of 150 microohm, a maximum allowable current of 500 amps and at that current the voltage drop would be 75 millivolts. By convention, most shunts are designed to drop 50 mV, 75 mV or 100 mV when operating at their full rated current and most ammeters consist of a shunt and a voltmeter with full-scale deflections of 50, 75, or 100 mV. All shunts have a derating factor for continuous (more than 2 minutes) use, 66% being the most common, so the example shunt should not be operated above 330 A (and 50 mV drop) longer than that.
This limitation is due to thermal limits at which a shunt will no longer operate correctly. For manganin, a common shunt material, at 80 °C thermal drift begins to occur, at 120 °C thermal drift is a significant problem where error, depending on the design of the shunt, can be several percents and at 140 °C the manganin alloy becomes permanently damaged due to annealing resulting in the resistance value drifting up or down.[citation needed]
If the current being measured is also at a high voltage potential this voltage will be present in the connecting leads too and in the reading instrument itself.[3] Sometimes, the shunt is inserted in the return leg (grounded side) to avoid this problem. Some alternatives to shunts can provide isolation from the high voltage by not directly connecting the meter to the high voltage circuit. Examples of devices that can provide this isolation are Hall effect current sensors and current transformers (see clamp meters). Current shunts are considered more accurate and cheaper than Hall effect devices. Common accuracy specifications of such devices are ±0.1%, ±0.25% or ±0.5%.
The Thomas-type double manganin walled shunt and MI type (improved Thomas-type design) were used by NIST and other standards laboratories as the legal reference of an ohm until superseded in 1990 by the quantum Hall effect. Thomas-type shunts are still used as secondary standards to take very accurate current measurements, as using the quantum Hall effect is a time-consuming process. The accuracy of these types of shunts is measured in the ppm and sub-ppm scale of drift per year of set resistance.[5]
Where the circuit is grounded (earthed) on one side, a current measuring shunt can be inserted either in the ungrounded conductor or in the grounded conductor. A shunt in the ungrounded conductor must be insulated for the full circuit voltage to the ground; the measuring instrument must be inherently isolated from the ground or must include a resistive voltage divider or an isolation amplifier between the relatively high common-mode voltage and lower voltages inside the instrument. A shunt in the grounded conductor may not detect leakage current that bypasses the shunt, but it will not experience high common-mode voltage to ground. The load is removed from a direct path to ground, which may create problems for control circuitry, result in unwanted emissions, or both. Devices to be used in current sensing include:INA240, INA293, and INA180. Several other style devices can found here.
Low-side insertion can eliminate common-mode voltage, but not without drawbacks.
High-side insertion resolves low-side drawbacks but guarantees common-mode voltage.
Isolated amplifiers resolve all the difficulties and limitations with high- or low-side current shunt measurements.
Not that the advice given here on the ammeter is not valid but this is a 72 based on the tail lights. According to the owners manual, 72s did NOT have an ammeter, only a generator light so unless it was aftermarket, the speculation of the ammeter being the cause is likely wrong.
69 300 vert
Well-Known Member
Your eyes are better than mine. I couldn't discern if it was a '70, '71 or '72.
Found more info on the HAMB
https://www.jalopyjournal.com/forum/threads/installing-an-amp-guage.114424/
"If you want an ammeter and are concerned about fire hazard, use a shunt ammeter. Only 50-100 milliamps through the meter."
That is exactly what caused the wiring fire in a '64 Malibu SS harness I had. The shunt broke! That left the entire load of the car's electrical system going thru the gauge which was designed for the milliamp current. Sure was exciting until I got the battery unhooked!
The shunt looked like an old style ballast resistor. Chevrolet changed the design in 1965.
Found more info on the HAMB
https://www.jalopyjournal.com/forum/threads/installing-an-amp-guage.114424/
"If you want an ammeter and are concerned about fire hazard, use a shunt ammeter. Only 50-100 milliamps through the meter."
That is exactly what caused the wiring fire in a '64 Malibu SS harness I had. The shunt broke! That left the entire load of the car's electrical system going thru the gauge which was designed for the milliamp current. Sure was exciting until I got the battery unhooked!
The shunt looked like an old style ballast resistor. Chevrolet changed the design in 1965.
3175375
Senior Member
Shunts are common. I used them in DC current measurements in test aircraft when I worked for Hughes Aircraft / Raytheon.
That's really true and I agree 100% that bad wiring can cause a fire. I've seen some fires waiting to happen on this forum. Why they don't is sometimes beyond me. There's been a couple times where I wanted to say, "just set the car on fire now and get it over with".
The real killer is when the resistance goes up in bad wiring and loose connections. Resistance = Heat and so forth. Even using new wiring that's undersized... Same thing.
But, how do you know that his wiring was bad?
FWIW... From 1971 Plymouth FSM
Edit: larger pic added
Last edited:
This popped up a few minutes ago.
Wildaugust
Senior Member
I hate to see a classic car destroyed too. I have to wonder though, how much of a load he may have been running. I also wonder if he may have been using a Chinesium aftermarket ammeter. My first car was a '63 Chevy and it just had a light. It had an alternator, not a generator. I was helping a friend install an ammeter into his '68 Impala, because it just had a light. We followed those instructions and diagrams to the letter. As soon as he started the engine, damned if it didn't blow the voltage regulator. 
My '71 seems to have the old style charging circuit. I don't have a radio in my dash. I can reach through behind my ammeter while I'm driving. I can feel the connectors and wires and they are cold, even after a couple of hours. My truck ('69 Ford) has a shunt type ammeter in it. It's factory. There's a 10 guage wire from the battery terminal of the starter relay to the alternator. There are two light guage wires going to the ammeter. Each wire has a 4 amp fuse. The only thing is that being shunted, the needle moves such a small amount, it's almost impossible to notice it.
I don't intend to run a bypass in my Newport unless I significantly increase the load on the charging circuit. I wonder if the owner of that poor Olds was running an electric fan, and/or maybe EFI, and/or maybe a high power stereo amplifier, etc. I think that a lot of people make the mistake of thinking that if they run the high amperage stuff on relays, they will be okay. Not so. Any added accessories will increase the load on the charging circuit, whether they are using relays or not. Even if they take the power right at the battery, the alternator still has to recharge the battery. All of that current is going through the wire between the alternator and battery. If the original ammeter is in that circuit, it's carrying the load. Things can get really hot really quick.
My '71 seems to have the old style charging circuit. I don't have a radio in my dash. I can reach through behind my ammeter while I'm driving. I can feel the connectors and wires and they are cold, even after a couple of hours. My truck ('69 Ford) has a shunt type ammeter in it. It's factory. There's a 10 guage wire from the battery terminal of the starter relay to the alternator. There are two light guage wires going to the ammeter. Each wire has a 4 amp fuse. The only thing is that being shunted, the needle moves such a small amount, it's almost impossible to notice it.
I don't intend to run a bypass in my Newport unless I significantly increase the load on the charging circuit. I wonder if the owner of that poor Olds was running an electric fan, and/or maybe EFI, and/or maybe a high power stereo amplifier, etc. I think that a lot of people make the mistake of thinking that if they run the high amperage stuff on relays, they will be okay. Not so. Any added accessories will increase the load on the charging circuit, whether they are using relays or not. Even if they take the power right at the battery, the alternator still has to recharge the battery. All of that current is going through the wire between the alternator and battery. If the original ammeter is in that circuit, it's carrying the load. Things can get really hot really quick.
mindshark13
Active Member
What about the trendy one wire alternator? Seems to be popular these days to add one to an older vehicle.
Joseph James
Senior Member
I use an insulation resistance tester (megger) on any questionable wiring. I should probably do it on all circuits.
70_NPORT
Well-Known Member
I don't know for sure....BUT age of car equals a strong and distinct possibility. Just bc it's spit shined and pretty on the outside doesn't mean the same on the inside. After five or six decades if it's all original I bet there's stiff insulation that will crumble and/or crack in your hands with any manipulation behind that dashboard.
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