Ammeter Resistance and Voltage drop

[Stihl011]

I'm troubleshooting an issue with an aftermarket radio for my 1968 Barracuda. The radio is a retro sound radio that requires an always-on 12V and 12V from accessory.

Often upon applying the brakes and/or the blinker, the radio will cut out. I've just verified today with a oscilloscope that the voltage on both the "Always on" supply and the "accessory" supply drop down to 8-9V momentarily (maybe 50 milliseconds, then rises back up to a slightly lower voltage than where I initially started) which I believe is causing the issue (load of the lamps dragging down the voltage). Radio is only specified to work down to 11V.

Both the always on and the accessory supply are attached to the alternator side of the ammeter. I am seeing a resistance between the positive battery cable and the alternator be about 5.3ohms, and a nominal voltage drop of about 1.5V-2V between the battery voltage and the alternator voltage when the car is off. Other than cables, there is only a fusable link and ammeter between the battery and the alternator.

This seems high to me but I don't really know what is normal. I believe my ammeter works, since I can watch it move ever so slightly when the blinkers are on.

I am also seeing what I believe is WAY too much resistance from the starter switch between the alternator side of the ammeter and the accessory output of that switch. I believe this to be my primary issue, because when in accessory or run, I can watch the resistance change between 5 and 50 ohms just by moving the key around. If this accessory output voltage is drooping more due to this high resistance, then maybe that voltage input turning is triggering the turn off. The only thing that makes me question this is I can watch both input voltages with the o-scope, and I don't see one significantly different than the other, the accessory is maybe 0.5V lower at all times,

In short, can anyone tell me what is anormal value for voltage drop between the battery and the alternator when the car is off, and/or what resistance I should be measuring between the battery and alternator? I may have both problems here.


In summary, with car off
Battery Voltage: ~12.5V
Alternator Voltage ~10.5V
blinker or brake lights drop alternator voltage to ~8V temporarily.

Thanks.

 

[340sFastback]

My battery and alternator voltages match when engine is off in my '69 A Body. You might want to check the bulk head connectors. Recently I had to clean mine to get my voltage drops down. The fusible link also has a terminal connection in the harness which should be checked cleaned. Check volt drop between battery and starter relay too. I also recently put a new ignition switch in too.

 

[Mattax]

Both the always on and the accessory supply are attached to the alternator side of the ammeter. I am seeing a resistance between the positive battery cable and the alternator be about 5.3ohms,

Are there two wires attached to the alternator side of the ammeter?
On a standard wiring harness there should be one. A second wire there would be some sort of option.

There should be no measureable resistance between the battery cable positive erminal and the alternator's BAT terminal.

and a nominal voltage drop of about 1.5V-2V between the battery voltage and the alternator voltage when the car is off.

There should be no voltage drop when the car is off.
Something had to be running. Was the door open or your foot on the brakes or ?

I am also seeing what I believe is WAY too much resistance from the starter switch between the alternator side of the ammeter and the accessory output of that switch. I believe this to be my primary issue, because when in accessory or run, I can watch the resistance change between 5 and 50 ohms just by moving the key around. If this accessory output voltage is drooping more due to this high resistance, then maybe that voltage input turning is triggering the turn off. The only thing that makes me question this is I can watch both input voltages with the o-scope, and I don't see one significantly different than the other, the accessory is maybe 0.5V lower at all times,

Not sure how/where your measuring voltages but agree there should be only small resistance in the key switch. 5 to 50 ohms is huge.

In short, can anyone tell me what is anormal value for voltage drop between the battery and the alternator when the car is off, and/or what resistance I should be measuring between the battery and alternator? I may have both problems here.

The first can only be answered when the currrent is known.
If the ammeter shows 10 amps, then there may be a bit of voltage drop. If it shows 0 amps the voltage drop must be zero. V=IxR

I'd like to see zero resistance using standard meter.
Note that meters use small voltage difference to measure resistance. (most modern meters have a battery)
So can't measure resistance if there is currents flowing in the system

In summary, with car off
Battery Voltage: ~12.5V
Alternator Voltage ~10.5V
blinker or brake lights drop alternator voltage to ~8V temporarily.

System voltage should not drop to 8 volts unless the battery is badly discharged.

The main power circuits for a standard 1968 Barracuda

With the engine off, the ammeter will indicate current flowing to anything turned on. ie Battery Discharge
With the engine running, the ammeter will indicate current to recharge battery. ie. Battery Charge
But the ammeter will not show current flowing from the alternator to any other equipment.

 

[Stihl011]

Car door probably was open, drawing current. I’m pretty sure I have both an ammeter problem and a key switch problem. I can measure across the ammeter and it measures 4 ohms, which seems pretty high. I disconnected the battery, and traced the resistance though the fusable link, bulkhead connection. 4 ohms across the ammeter means I would get 2v drip with only 500mA. I’m surprised the ammeter works at all but I can see it move just a bit which is kinda throwing me off. In any case, I have another ammeter and switch on the way. I’m pretty sure that will fix the problem. And yes, I plan on measuring the resistance of the ammeter when I get it.

 

[Mattax]

Agree it should be zero.
I measured three magnetic deflection type ammeters here
A nice one and some really messed up connectors and ammeters here

 

[67Dart273]

There are three big "points" in the system that are suspect........the bulkhead terminals for both the RED and BLACK ammeter wires, and the ammeter connections (wires and terminals) and the ammeter itself.

The RED and BLACK terminals right at the ammeter can fail, and the INTERNALS of the ammeter can fail, here is how.

The "guts" of an ammeter is just a brass strip that forms a shunt of sorts and generates the magnetic field that causes the needle to deflect. At the ammeter terminals, they are a sandwitch, of the bolts, washers, etc, all compressed together UNTIL (on some models) that high current can loosen the insulating washers, and on plastic dashes, soften the dash housing, as well as the usual corrosion. Part of the fix, other than eliminating the ammeter, is to pull it apart and solder or silver braze the "shunt" to the studs.

I would make more checks if you can. See if you can clip onto the red (battery) side of the ammeter and compare that reading, then clip into the RED and BLACK terminals at the engine bay side of the bulkhead connector and compare those readings.

I LIKE ammeters, but many of us have just thrown in the towel and gone to voltmeter. There is an old thread about doing that

Ammeter to Voltmeter...who does it?

Im interested in converting my stock ammeter to a voltmeter and was wondering which companies can do this, and approx how much they charged? I did a search and saw that redline does it for about 140, but I'm trying to see if anyone has any experience with other vendors? I thought about...

www.forabodiesonly.com

Also if you have not, read the MAD article on the how and why of ammeter / bulkhead connector failure.

Catalog

Most of us here do not do it that way. "Fix" the bulkhead, bolt the ammeter connectors together, and then run a fused/ breakered BIG wire from battery + direct to alternator output.

Crackedback, on here, does or did make these

 

[67Dart273]

It may be "something else." The VR should try to keep system voltage up regardless of a little bit of drop in the charging circuit. ANY chance the belt is loose? Or bad VR

 

[BillGrissom]

I measured that long ago in my Mopars. The voltage drop across the ammeter at full-scale is ~0.7 V. Since you are seeing much more drop, some component may be getting hot. The ammeter bolted connections are a common problem and can melt from corrosion, as can the connectors in the bulkhead terminal (often found melted). The later is not usually an issue in 1963 and 65 cars since they use robust buss-bars to feed the high ALT and BAT wires thru the bulkhead. I like that so much that I installed a 1965 bulkhead (Fury) in my 1964 Valiant.

 

[Mattax]

Did you write down the load? The load is everything with a voltage drop.

From '67 FSM
For the tests from Battery to alternator, they are looking for less than .3 Volts when a 10 Amp load is applied.
To have the circuit from the battery to alt output (or reverse) drop under .3 V with 10 amps of current, the voltage drop across the ammeter alone needs to be well under a .1 Volt. The drops occur mostly in the connections as you've noted.
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Side note:
The voltage drop test from Battery Positive to Field side of regulator with ignition ON, engine off, should not exceed .55 Volts. (disconnect leads at ballast for this test).
This is mostly a test for maximum field current through the key switch, regulator and all connections and wires back to the battery with an original alternator, regulator (points), and ignition (points). Alternators that draw more current may exceed .55 volts even though the resistance is within factory spec. Another reason to do test field current draw of any replacement or rebuilt alternator. This FSM does not provide a voltage drop spec to the ignition side of the regulator. It's possible that might be in '70 up FSM or bulletin. I probably looked at one time but don't remember