Vintage Mustang Forums banner

1 - 9 of 9 Posts

·
Registered
Joined
·
1,055 Posts
Discussion Starter #1 (Edited)
66 289, to confirm, I should be getting 6 or 7 volts on the red/green wire going to the coil with the ignition on (pic below with arrow). I am gettng 12.3 volts, ignition turned to the right, car not running.


745520
 

·
Premium Member
Joined
·
13,170 Posts
Do you still have the Pink wire in place? The engine should be running to see the reduced voltage. It's one of those strange things about electricity that we non-EEs don't understand.
 

·
Registered
Joined
·
21,330 Posts
You can read either 6-7 volts or 12 volts. It depends on wether the points are closed which means you're reading the actual volt drop on the coil or if the points are open in which case you're putting the meter in series with the coil. In a series circuit what has the most resistance, drops the most voltage. The volt meter has a very high resistance, so much so, the resistance of the coil and resistor wire is basically non existent.
 

·
Registered
Joined
·
37,647 Posts
You can read either 6-7 volts or 12 volts. It depends on whether the points are closed which means you're reading the actual volt drop on the coil or if the points are open in which case you're putting the meter in series with the coil. In a series circuit what has the most resistance, drops the most voltage. The volt meter has a very high resistance, so much so, the resistance of the coil and resistor wire is basically non existent.
^^^ Spot on.
 

·
Registered
Joined
·
1,055 Posts
Discussion Starter #5
Huskinhano & Woodchuck,

I am a bit confused, I pulled the red green wire off the coil, and measured the voltage of the red/green wire only with the ignition on, which was 12.3 volts. The points are closed when doing this if it makes a differance.

Awhtz,

The pink wire is still in place. Are you saying, I need to start the car and pull the red/green wire off and then measure it? Or just start the car, pull the red/green wire up on the stud a bit and measure it?


For what ever reason, I believe I have a weak ignition (see spark plugs below), all 8 plugs look like this. Car runs and idles, but has no power and flutters when driving when pushing the pedal to the floor. Points set at 17, spark plug gap at 35 if I remember. I do not believe that it is a fuel/rich condition (Autolite 4100 carb just rebuilt by Gottafish, one that I sent him that was in outstanding condition to start with), mixture screws are set, 2 turns out and no black smoke coming out of the exhaust.

Trying to isolate the problem rather than parts changing. I did remove/replace the cap with a spare I had, no change in engine performance.


745538
 

·
Registered
Joined
·
5,623 Posts
also a DVM might not pick up the 6-7 volts

i find it best to use an old fashioned needle or analog meter

you can still buy em at any hardware store
 

·
Registered
Joined
·
21,330 Posts
@Fly2xs Ohm's series law states that everything has the same amperage, what ever has the most resistance drops the most voltage and that the voltage off all devices adds up to the source voltage. In a parallel circuit it's just the opposite. So getting back to your reading. When you disconnect the green wire from the coil and put the volt meter on the green wire and the terminals on the coil where the green wire connected to, negative, you will read the full battery voltage despite the coil and pink wire resistance. The volt meter has so much resistance, actually it's impeadence, the pink wire and coil resistance in relationship are non existent compared to the meter. That's why you're seeing battery voltage. To measure the voltage of the coil, the green wire needs to be on the coil and the points closed to complete the circuit. Now you have to put the red lead on the + side of the coil and the black wire on the green wire, - side of the coil. That will be the voltage the coil is seeing. Subtract that voltage from the actual battery voltage. That number will be the voltage drop of the resistor wire.

The reason I mentioned parallel circuit is if the volt meter had a low resistance. It would change the dynamics of the circuit you're trying to diagnose. With a high resistance, it greatly reduced this risk. It's called "meter loading".
 

·
Registered
Joined
·
29 Posts
The manual states 6.9 volts or less to the coil. If I remember right, the reduced voltage is mainly to keep the points lasting longer.
I don't believe the 12v at the coil could be the problem with it having loss of power. Unless it has been like that for quite a while and has
already effected the points. You may have to get a new set of points now. The plugs look like they're not getting enough spark.
I would check and clean your points, cap, rotor, and plugs. You may end up needing a new set of points. But definitely fix the resister wire.
 

·
Registered
Joined
·
383 Posts
You need to learn about voltage drops. Everywhere there is resistance in a circuit there is a voltage drop. All of those voltage drops will add up to your total source voltage. Remember they must all add up to the source voltage. Depending on where you place the voltmeters leads you can be taking the overall drop in voltage in the entire circuit which will always equal the source voltage or you can be measuring the voltage drop anywhere along the circuit between wherever you place your two leads. With the circuit energized If you put the meters leads across both sides of just an individual wire or a component then you are measuring just the voltage drop across that wire or component. That is how we find unwanted resistance in a circuit, bad wire, switch, a part or a bad connection. if you can understand voltage drops you can diagnose just about any electrical problem. There are not that many actual mechanics that know how to do this.

The way this works is just like a water pipe. If you had a water pipe with no restrictions there would be no drop in water pressure as the water flowed through the pipe. The same pressure going in one end of the pipe would be the same pressure coming out the other end. If you had a restriction somewhere in the pipe there would be a pressure drop at the restriction but you could only measure the amount of that drop while the water "was flowing" otherwise pressure would equalize on both sides of the restriction when the water was not running. If you think of your voltmeter as another path for the water to flow around the restriction, some of that water will still go through the pipe and whatever is backed up behind the restriction will try to flow around the restriction and through your meter. Your meter would measure how much pressure is trying to flow into your meter. That would be measuring the water pressure drop at the restriction. Electricity works "exactly" the same way as flowing water. Always think of electricity as flowing water and not some magical invisible force. Voltage is the pressure, amperage is the amount of electricity that is actually flowing and resistance is just a another word a restriction.

If you started off with 12.6V (12.6 lbs of pressure) and you measured everywhere there was a restriction all of those restrictions would need to add up to 12.6V. If they don't you have not found all of the restrictions. Remember the system has to be flowing to check for these restrictions. There could be an unwanted restriction or an engineered in restriction in the primary circuit of the ignition that drops your voltage (pressure) to the coil. That would be a restriction in the wire from the positive terminal on the battery to the ignition switch, the ignition switch itself, the resistor wire, the primary winding in the coil, the negative lead from the coil to the points, the points, the ground path from the points back to the negative battery post and any connection between these components. Any unwanted restriction would reduce the actual amount of flowing electrons (amperage). Normally you do not want to decrease the amperage flow in a circuit but the wire used in the primary winding inside the coil is so small that if too many electrons tried to flow through it it would overheat and melt. To reduce that flow of elctrons we need to install a restriction, that would be your pink resistor wire, an external ballist resistor like Chrysler used or a resistor built into the inside of the coil.

You cannot test for voltage in the primary ignition circuit with the engine running because of the opening and closing of the points. They act as a voltage averaging device and will drop your voltage reading. You cannot check for the available voltage is some one is turning on and off the pressure. The points need to remain closed. Pull off your cap and rotate the engine until the points close and turn on the ignition switch. Take your meter and touch one lead to the positive terminal of the coil and the other lead to the "positive" terminal of the battery, not the negative post. Now your measuring for any pressure drop between the batteries positive terminal and the positive terminal of the coil. We know there is resistance in that part of circuit because of the resistor wire. Say you get a reading of 5.6V. That means there is 7.0V of pressure still getting past that resister wire which is what we want. We want less pressure by the time we get to that small primary winding inside the coil.

If you were to take your meter's leads and place one on the ground end of the resistor wire, that would be the end going to the coil and the other lead to the "negative" battery terminal you would read that missing 7.0V. That is the drop in pressure between the resistor wire and the battery ground terminal. That means if we have no more resistance in the circuit after the coil we have 7.0V of pressure at the coil. If we put our leads across the positive and negative leads of the coil it should read 7.0V. What if we only read 2.0V. We are missing 5V? It has to be another restriction (resistance) after the coil. Bad connection at the point's contacts, bad wire or a bad ground. We need to find that missing 5V. Stick your meter leads on both sides of the point's contacts. Say you get a reading of 5.0V, we know we are only allowed a .2V voltage drop across a switch and a ignition points are just a switch. The points are burned so bad that its creating resistance. That my friends is how you narrow down the problem in an electrical circuit, you look for that unwanted voltage drop. Find it and you have found the problem.

There is only a drop in pressure where there is resistance. This is important to remember, there is no pressure (voltage) left after the last spot of resistance in the circuit. Wires should have no resistance. Connections maybe a 1/10 of a volt (pressure drop). Switches I think were 2/10th of a voltage drop.

Homework lesson. Take your meter and place one lead on the positive battery terminal post and one lead on the battery cable end connected to the that same post end and turn on the parking lights. If you have a good battery post connection you should have a zero voltage reading between the post and the cable. Now turn off the parking lights. Go back and remove the battery cable from the post and touch the meter's leads to battery post and the removed battery cable end and turn on the headlights. Now you should have a 12.6V drop because you have a tremendous amount of resistance between the post and the cable. No battery pressure can get past that opening so the entire 12.6V is being blocked from moving on through the circuit. The entire pressure is trying to flow around the opening through your meter. Now turn the headlights off. You should have a zero voltage reading because no current is trying flowing through that opening. Now just loosely sit the battery cable on the post and turn the parking lights back on. Now touch your leads from the battery post to the loosely fitting battery cable end. I bet you get a low voltage reading because you have enough of a connection for some of the pressure to get through the connection but some of the pressure is going to try and take a detour around the poor connection and go through your meter.

I say this over and over. if you want to play with cars you need a digital voltmeter. Nothing great, but buy one with enough "impedance" that it can be used on computer controlled circuits on modern cars without damaging the computer. Pretty much impossible to damage anything on a old car using a voltmeter. Go to YouTube and watch some voltage drop teaching videos. The go learn about "Ohm's Law" so you will understand how resistance, voltage and amperage effect each other. You can go waste months of your time, five day's a week taking college classes in AC and DC voltage theory and lab classes like I did or you can learn about 90% of what they taught in just a few hour's at home.
 
1 - 9 of 9 Posts
Top