Voltage drop on reader, but not guage cluster

[The Director]

So, I was running my 88 Fifth Avenue, and I have one of those FM transmitters through the cigarette lighter. Its reading a voltage at 10.5 or so while running, but the guage on the dash is reading perfectly center, just as always.

I don't understand, and I don't know what to trust. I'm inclined to trust the guage cluster because I had to crank the car for a little while (must've flooded the carb by accident a bit), and I saw it go down, and then returned to center within a bit of driving.

Update: Ran the multimeter on the battery... while idling, it eventually got to 12.45 or so. When I hit the gas, it actually went downnin voltage. I'm going to bench test the alternator, as I only replaced it 2 years ago, but the car sat for a long time. If that's good, then I'll try new battery cables. Battery and voltage regulator were recently replaced, and the problem started just a little bit before I replaced both of those components.

 

[Aspen500]

The guage is an amp meter and is basically useless. At the center it means the amp load and alternator amp output to maintain the battery are equal. Left of center, the load amps are higher than alternator output amps. Right of center, alt. output is higher than load amps.

Some aren't aware, the vehicle runs off the battery and the alternator only maintains the battery state of charge. That's why a weak battery will prematurely cause alternator failure since it has to work much harder.

Reading 10.5 at the cigar lighter could be from voltage drop through the circuit but just to be sure, check it at the battery with a volt meter.

 

[The Director]

I never knew the guage amp worked like that, well at least I can rule that equipment out as basically decor then.

I did the checks with the multimeter, and it was reading the 10.5 and slowly went back up in charge, but stopped at the 12.45 area. The cigar lighter reader followed suit with the voltage movements, so it's accurate.

I even unplugged the voltage regulator, let it run on battery a second or two, watched it drop in voltage, plugged it back in, and same thing, it would top out at 12.45 or so while idling. Revving made it drop to 11.something, maybe .4 of a volt or so. In quick succession, it would drop it to low 11s if I revved it several times, but would eventually return to 12.45. I'm assuming a weak charge by the alternator at this point.

Thanks!

 

[Mikes5thAve]

After making sure the cables are clean and tight try changing the voltage regulator. They're cheaper then an alternator. Try charging the battery first.

If battery and charging system are good the voltage should be in the 14V range at the battery with the engine running.

The guage on the dash is useless. They usually barely move even if there is a significant load.

 

[The Director]

I'll charge the battery up externally and see where it holds once the car starts running. I replaced the voltage regulator, unfortunately didn't solve the issue.

I'm also noticing what appears to be soot on the alternator, as if it burnt or something. Maybe it's just me, but I've had one burn out on me before. Just doesn't make sense when I still can get some charge out of it, and it being only two years old.

I may even install new cables while I'm at it. Haven't had issues before, but they're probably hitting their age, and better off replaced. I wonder if the belt spinning the alternator has anything to do with it? I'm hearing some squeaking, and I tried tightening it a bit, but to no effect. Unless, it's time to change that belt? (Seems to feel fine, and spin everything fine though.)

 

[AMC Diplomat]

I'll charge the battery up externally and see where it holds once the car starts running. I replaced the voltage regulator, unfortunately didn't solve the issue.

I'm also noticing what appears to be soot on the alternator, as if it burnt or something. Maybe it's just me, but I've had one burn out on me before. Just doesn't make sense when I still can get some charge out of it, and it being only two years old.

I may even install new cables while I'm at it. Haven't had issues before, but they're probably hitting their age, and better off replaced. I wonder if the belt spinning the alternator has anything to do with it? I'm hearing some squeaking, and I tried tightening it a bit, but to no effect. Unless, it's time to change that belt? (Seems to feel fine, and spin everything fine though.)

As someone who almost lost a Diplomat to an electrical fire, replace everything. Replace the starter too. Just dump it all. And make sure you route everything right so those positive electrons don't get any funny ideas about grounding

 

[Mikes5thAve]

Some rebuilds are questionable or poor quality so that could very well be the issue.

If the cables don't look bad they're not worth replacing unless you know there's an issue with them. Same for the starter. If it's working don't bother. You could end up like the alternator that you've replaced it for no reason with one that'll fail in 2 years.

 

[Ash_Whole]

best to bypass the amp meter all together. the charge wire for it has to move from the alternator, through the firewall bulk head connector, to the dash, through the gauge, back through the buckhead connector and then to the battery. so many connections to go bad, lose power and more likely melt.

just run the alternator wire to the battery and skip the dash, your alternator, wiring, dash and battery will thank you

 

[Mikes5thAve]

best to bypass the amp meter all together. the charge wire for it has to move from the alternator, through the firewall bulk head connector, to the dash, through the gauge, back through the buckhead connector and then to the battery. so many connections to go bad, lose power and more likely melt.

just run the alternator wire to the battery and skip the dash, your alternator, wiring, dash and battery will thank you

These cars aren't wired that way. The guage is a shunt and very little power goes though it.

 

[anjdevil2]

"These cars aren't wired that way. The guage is a shunt and very little power goes though it."

Don't kid yourself on that - I replaced my 50 year old harness in my 70 road runner. I had charging and electrical issues that culminated in a plastic burning smell. My brandy new bulkhead connector is melted right at the connection for the ammeter. I am in the process of taking the ammeter out of the loop and doing a direct wiring job. I have a voltmeter that is much more accurate.
Go here for more information madelectrical.com
I am seriously considering doing the same to my 1980 Diplomat Coupe. My road runner is one of 179 and the Diplomat is one of less than 2600. I'd rather have a safe electrical system than a burned up vehicle.

 

[AMC Diplomat]

"These cars aren't wired that way. The guage is a shunt and very little power goes though it."

Don't kid yourself on that - I replaced my 50 year old harness in my 70 road runner. I had charging and electrical issues that culminated in a plastic burning smell. My brandy new bulkhead connector is melted right at the connection for the ammeter. I am in the process of taking the ammeter out of the loop and doing a direct wiring job. I have a voltmeter that is much more accurate.
Go here for more information madelectrical.com
I am seriously considering doing the same to my 1980 Diplomat Coupe. My road runner is one of 179 and the Diplomat is one of less than 2600. I'd rather have a safe electrical system than a burned up vehicle.

No, @Mikes5thAve is right, late M bodies are not wired this way. I confirmed this on my own 87 when I traced all the wiring after it almost burned down.

Earlier Mopars are wired the way you said. Not the M bodies.

 

[Mikes5thAve]

"These cars aren't wired that way. The guage is a shunt and very little power goes though it."

Don't kid yourself on that - I replaced my 50 year old harness in my 70 road runner. I had charging and electrical issues that culminated in a plastic burning smell. My brandy new bulkhead connector is melted right at the connection for the ammeter. I am in the process of taking the ammeter out of the loop and doing a direct wiring job. I have a voltmeter that is much more accurate.
Go here for more information madelectrical.com
I am seriously considering doing the same to my 1980 Diplomat Coupe. My road runner is one of 179 and the Diplomat is one of less than 2600. I'd rather have a safe electrical system than a burned up vehicle.

Heh, whos kidding who? The 70 is wired that way but not your 79 or 80. Have a look under the hood and grab a copy of the service manual. The ammeter only has 16ga wires going to it through a push in connection.

 

[Camtron]

"These cars aren't wired that way. The guage is a shunt and very little power goes though it."

Don't kid yourself on that - I replaced my 50 year old harness in my 70 road runner. I had charging and electrical issues that culminated in a plastic burning smell. My brandy new bulkhead connector is melted right at the connection for the ammeter. I am in the process of taking the ammeter out of the loop and doing a direct wiring job. I have a voltmeter that is much more accurate.
Go here for more information madelectrical.com
I am seriously considering doing the same to my 1980 Diplomat Coupe. My road runner is one of 179 and the Diplomat is one of less than 2600. I'd rather have a safe electrical system than a burned up vehicle.

Fortunately our cars aren’t wired like a ‘70 RR. Not an issue, though the voltmeter will be much more functional to have in dash.

 

[Aspen500]

F body has a shunt also. Unsure if '76 had a shunt though (disclaimer)

 

[81ImpNC]

I wouldn't trust a reading off the cigarette lighter for any number of reasons. When I test voltage it is at the battery or the battery post on the alternator. I have an FM transmitter on my Imperial and I love it. It also has a voltage reading and it migrates around but I don't pay attention to it. The Imps got the 114A police alternator so I have plenty of juice whenever. I rarely use the headlights or the A/C so the load draw is minimal. My alternator is OE and I check it regularly but she still puts out like a beast. 975CA Group 24 battery. I have a factory CB radio (which I am keeping because it is a conversation piece at car shows) but replaced the factory amp and speakers with quality units. Despite not having a digital tuner (slide bar) the FM Bluetooth to phone arrangement works well and is exceptional for driving tunes. I have Sirius/XM from my daily and it works great off the phone app.

 

[JohnRogers]

These cars aren't wired that way. The guage is a shunt and very little power goes though it.

"These cars aren't wired that way. The guage is a shunt and very little power goes though it."

Don't kid yourself on that - I replaced my 50 year old harness in my 70 road runner. I had charging and electrical issues that culminated in a plastic burning smell. My brandy new bulkhead connector is melted right at the connection for the ammeter.

No, @Mikes5thAve is right, late M bodies are not wired this way. I confirmed this on my own 87 when I traced all the wiring after it almost burned down.

Earlier Mopars are wired the way you said. Not the M bodies.

This is why I scanned this thread. I've been exposed to amp meter horror stories this week. Bulkhead connectors melting, some causing fires. Some dude's Challenger on fire in an Instagram post this morning. When did Mopar start using these shunt amp meters? What does that mean 'the gauge is a shunt'?

F body has a shunt also. Unsure if '76 had a shunt though (disclaimer)

 

[Aspen500]

Basically, on the older systems all the current between the battery and alternator went through the ammeter. Depending on which direction it was flowing, the needle would show charge or discharge. Since a car runs off the battery and the alternator is intended only to maintain the battery charge, only the amount of amps being used to run the car will be flowing to the battery, and through the alternator. If the battery is weak, the alternator will need to output more amps to maintain voltage, which means more amps through the ammeter and bulkhead connector. Moral of the story is, never run with an old weak battery.

With a shunt, you more or less bypass the ammeter and it only has a very small current flow. Still the same voltage going one direction or the other but with minor amperage. The path between the battery and alternator goes through a shunt wire. All the vehicle loads attach at a splice with the shunt, one the alternator side of the ammeter, and also the main cable to the alternator. This eliminates high current at the bulkhead and ammeter.
In other words, the red and black wires to the ammeter are hooked together by the shunt before going to the ammeter. Instead of 10 gauge ammeter wires, they're 16 gauge and don't get attached to the guage with studs and nuts like the old fire prone setup. Alot of problems came when the cable nuts loosened over time, which creates a poor connection which of course causes heat.

The shunt deal is a little more involved than all this but, you get the general idea.

In the end though, an ammeter is pretty much useless (JMO).

 

[JohnRogers]

Basically, on the older systems all the current between the battery and alternator went through the ammeter. Depending on which direction it was flowing, the needle would show charge or discharge. Since a car runs off the battery and the alternator is intended only to maintain the battery charge, only the amount of amps being used to run the car will be flowing to the battery, and through the alternator. If the battery is weak, the alternator will need to output more amps to maintain voltage, which means more amps through the ammeter and bulkhead connector. Moral of the story is, never run with an old weak battery.

With a shunt, you more or less bypass the ammeter and it only has a very small current flow. Still the same voltage going one direction or the other but with minor amperage. The path between the battery and alternator goes through a shunt wire. All the vehicle loads attach at a splice with the shunt, one the alternator side of the ammeter, and also the main cable to the alternator. This eliminates high current at the bulkhead and ammeter.
In other words, the red and black wires to the ammeter are hooked together by the shunt before going to the ammeter. Instead of 10 gauge ammeter wires, they're 16 gauge and don't get attached to the guage with studs and nuts like the old fire prone setup. Alot of problems came when the cable nuts loosened over time, which creates a poor connection which of course causes heat.

The shunt deal is a little more involved than all this but, you get the general idea.

In the end though, an ammeter is pretty much useless (JMO).

Thanks for the quick, well put reply.

 

[The Director]

Figures, because I ended up changing out my alternator for the problem I originally described. I also have a '75 Imperial, so knowing that bit about wiring on older mopars may come in handy. I once had a faulty voltage regulator put through close to 18 volts and burn out my headlights within a day a couple of times. My battery even exploded under the hood when I was driving it! Luckily I fixed that before more went wrong...

 

[Aspen500]

I always wondered why they used to have ammeters on vehicles instead of volt gauges like now. Ammeters don't really tell you much of anything useful. Voltage is way more useful. Of course then you have those like my '08 Mustang that have neither, only a red battery light. All that tells you is either it's not charging OR it's overcharging, and in some alternator failures the light never comes on at all. You know it's not charging when the battery goes dead. Of course the light can come on even if the charging sytem is working fine and only fault is the warning light circuit, but I digress.