Testing a charging system

[Kern Dog]

My Brother in Law's 72 Duster has been at my place since 2007.
He recently said he wants to use it as a fair weather Daily Driver at his place in Phoenix AZ.
He likes these cars but has always had me around to guide everything and advise him. Once he takes the car, he will be on his own.
I'm trying to periodically when time permits, work on this car to ensure that it will be safe and reliable. Today I decided to check the charging system. The gauges are not working, that is on the list of things to do. I suspected that the alternator might not be charging so with the engine running, I removed the negative battery cable. The engine shut down. I should have checked the voltage with my meter but I did not, I just grabbed another alternator from the shed, a nice clean one with a rebuilders tag on it. Once it was up and running, It showed a 13.9 volt reading at the alternator and the battery was at 12.8. I pulled the negative cable and it shut down again. There is a ground cable from the block to the firewall. Why would the engine still stall if the alternator was putting out the right voltage? The battery was just charged. The wiring is all old but seems to work. I have a few Voltmeter gauges I could install.

 

[kursplat]

start here post #6

 

[Mattax]

My Brother in Law's 72 Duster has been at my place since 2007.
He recently said he wants to use it as a fair weather Daily Driver at his place in Phoenix AZ.
He likes these cars but has always had me around to guide everything and advise him. Once he takes the car, he will be on his own.
I'm trying to periodically when time permits, work on this car to ensure that it will be safe and reliable. Today I decided to check the charging system. The gauges are not working, that is on the list of things to do. I suspected that the alternator might not be charging so with the engine running, I removed the negative battery cable. The engine shut down. I should have checked the voltage with my meter but I did not, I just grabbed another alternator from the shed, a nice clean one with a rebuilders tag on it. Once it was up and running, It showed a 13.9 volt reading at the alternator and the battery was at 12.8. I pulled the negative cable and it shut down again. There is a ground cable from the block to the firewall. Why would the engine still stall if the alternator was putting out the right voltage? The battery was just charged. The wiring is all old but seems to work. I have a few Voltmeter gauges I could install.

Because there wasn't enough power generated for field current, plus, ignition, plus charging.
Don't do that again, @67Dart273 has posted the reasons yanking the wire is a bad way to check.
The fact the alternator output stud is above battery voltage indicates its working.
The fact there is a voltage difference in the main circuit indicates (a) current is flowing (b) there is resistance to that flow between those to points.

Every place marked V should be the same voltage.
If the Alternator out is at 13.8 Volts, then all of the other locations should be 13.8 Volts.
A voltage drop of a couple tenths is to be expected. More than that indicates a poor connection along the flow path.
The illustration is a positive controlled regulating system, but the concept is the same with the ground control system on a '72.
Voltage to the coil could less than V when the key is in run position because it runs through the resistor (when the points are closed).

Use your hand held voltmeter and the ammeter to determine whether the battery is charging.
That's what the ammeter is for - it tells us if the battery is charging or discharging.

 

[Kern Dog]

Thanks. The ammeter is not moving at all. I'm pretty sure I bypassed the ammeter. The gauges are not working.
I'll dig into it again soon. Thank you again.

 

[Mattax]

Ugh. I'd say hook it up unless there was a problem.
You can run a parallel output line through a grommet to the ammeter's alternator side if running a 60 amp or larger alternator.

Regardless, the rest of the gages and oil pressure light (if its not a gage) are not related. They're on their own circuits which get power only when the key is in Run.

 

[Mattax]

This is the '73 Dodge 60 amp alternator wiring in case you're interested.
Not sure if they kept the standard alternator feed through the bulkhead connector. Works about the same whether its was retained or not. Considering they went to an 8 gage feed, my guess is they didn't keep the one through the bulkhead.
Not shown is some of the additional equipment added at the ammeter stud such as rear window defroster. That's one reason they used 8 gage feed.

Intro to the other gages and some trouble shooting:
Thermal-Electric Gauges (Session 227) from the Master Technician's Service Conference