multimeter generator test while running?

[1930ABONE]

Hi,

I have a digital multimeter where do I hook the leads to check a 6v pos ground generator when running in the car?

I tried hooking it to the battery terminals and got nothing when running?

It's 6.3v when not running.

Thanks for any help.

[Bob Johnson]

I have found that digital equipment can have problems near a running Model A engine. The Model A ignition system puts out lots of RF (radio frequency waves) and can make digital equipment go nuts. Best to use an old fashion analog meter.

Bob

[1930ABONE]

Thanks Bob... I think I have a analog meter in the basement.
Where do you hook the leads when using the analog meter?

[Bob Johnson]

If you are checking the generator there are two ways to do it. The first way is to check the voltage at the generator and check the voltage at the battery. You should see a higher voltage at the generator. The second way is to check the voltage difference from the generator to the "hot" negative post of the battery. You should do these tests a various engine speeds.

Bob

[1930ABONE]

Thanks Bob...I just dug out my old sears analog meter.
I'm still a little confused on exactly where to put my two leads.

[MAG]

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Put the postive (Red) lead on the Battery Postive post. Leave it there.
With the engine off, put negative (Black) on the negative Battery post. The reading will be the static voltage of the battery.
Start the engine, set at fast idle to start the generator charging.
Put the negative lead on the Cut-out BAT terminal. This will be the generator output voltage.
Then put the negative lead on the negative Battery Post. This will be the actual charge voltage going into the battery.
The BAt terminal reading and the reading at the negative Battery Post is the loss due to wire resistance, connectios, etc.

[1930ABONE]

Thanks MAG just what I needed!

[Tom Wesenberg]

As Bob said, best to use an analog meter. The RED + meter lead always goes to the most positve part of the circuit being checked.

1. Lets say you are checking battery voltage, then you touch the red lead to the + battery post and the black lead to the - battery post.

2. If you are checking for voltage drop through wires or connections, you still connect the red lead to the most positive part of the circuit. So, if you are checking for voltage drop in the starter cable, connect the BLACK lead to the - battery post and the RED lead to the terminal on the starter switch. Even though you are working on the negative cable of the starter, the battery is supplying the current and the battery post is the more negative end of the starter cable. You may have to switch the meter function switch to a lower DC voltage setting, because you shouldn't see more than a volt drop from the battery post to the starter switch. THIS TEST IS DONE WITH THE ENGINE OFF, IGNITION SWITCH OFF AND STARTER CRANKING.

3. Now, let's say you are doing a charging circuit check. THIS TEST IS DONE WITH THE ENGINE RUNNING. With the engine running, the generator should be putting out more than battery voltage, so the generator output post is now more NEGATIVE than the - battery post. Connect the BLACK meter lead to the generator output post and connect the red meter lead to the cutout output terminal. This will show you the voltage drop across the cutout. If you are using an electronic voltage regulator, you need to turn the lights on for this test, so you can get current to flow across the cutout contacts. If you don't have an EVR, then the stock generator output is fixed, and turning on the lights won't change the reading. If you want to check for voltage drop across the entire NEGATIVE side of the charging circuit, connect the black multimeter lead to the generator output stud and connect the red lead to the NEGATIVE battery post. Remember the generator should be putting out more than battery voltage, so the generator output stud should be more negative than the negative battery post. If you see too much voltage drop, then just keep moving the red lead from the battery post to each connection, until you get back to the cutout output terminal. This way you can pinpoint the connection with high resistence. High resistence means more voltage drop, but to get voltage drop you need to have current flow.

Remember the ground side of the battery, and the generator case mounting bracket and bolt are also part of the charging circuit and need to be checked for voltage drop.

4. If you are checking the voltage drop across the amp guage and the 2 short wires leading from the amp guage to the terminal box, then connect the red lead to the most positive terminal (should be DRIVER'S side). THIS TEST IS DONE WITH THE ENGINE OFF. Turn the lights on, so you get current flow through the amp meter and 2 short wires. Connect the BLACK multimeter lead to the most negative terminal box post (should be PASSENGER side). This is the post which goes to the starter switch. You may need to remove the terminal box cover to be sure which post this is, because people don't always wire the terminal box exactly the way FORD did. Don't just assume it was wired correctly and the passenger side is the wire which leads to the starter switch and leads to the NEGATIVE battery post.

Remember to always connect the RED multimter lead to the MOST + side of what you are checking. With the engine off and the lights on, the driver's side terminal box post is the most +, but with the lights off and the generator charging, the passenger side becomes the most +, because the generator should be putting out more than battery voltage.

The charging circuit and the lighting circuit share many of the same wires and amp guage, since the power for the lights is taken from the cutout terminal. So to make a check of these parts, I like to have the engine off and the lights on, then check along the line for voltage drop. I just checked for voltage drop across the 2 terminal box studs on my 28 Phaeton. I connected the red lead to the driver's side stud, black lead to the passenger side stud, engine off and lights on, and my meter shows about .05 volts, which means the connections and amp guage are clean and tight.

[1930ABONE]

Thanks for the great write up Tom!
I'm getting 6.9v coming out of the generator(engine running).