I have a Ford 4000. With the switch on, voltage drops in half, 6 volts to 3, across the coil. Is that a sign of a bad coil or does it just indicate the points are closed?
 
With the points CLOSED at voltage at the coil's primary terminal connected to the distributor should be as near to ZERO as possible with reference to chassis ground.

Any significant voltage at that terminal would indicate points not making good contact, a bad connection at the primary connections, or the primary wire itself, or some other issue in the primary circuit.

Or are you measuring the voltage between the primary coil terminal connected to the intention switch and chassis ground?
 
Sounds like the point need to be cleaned and or replaced. As Bob says the voltage with the points should be almost zero
 
I don't follow your description.

"With the switch on, voltage drops in half, 6 volts to 3, across the coil."

Are you saying you have six volts across the coil with the switch OFF and it drops to three volts when you turn the switch ON? That makes no sense.

What do you mean by "across the coil"? Do you put your meter leads on the two primary coil terminals? Or are you measuring from one of the terminals to ground?

Here's what you should see, assuming you have a an unconverted six volt system:

With the ignition ON and the distributor points OPEN:
Coil primary battery terminal to ground: ~6 volts.
Coil primary distributor terminal to ground: ~6 volts.
Coil primary battery terminal to distributor terminal: zero volts

With the ignition ON and the distributor points CLOSED:
Coil primary battery terminal to ground: ~6 volts.
Coil primary distributor terminal to ground: zero volts.
Coil primary battery terminal to distributor terminal: ~6 volts

If, with the points closed you only see about three volts across the coil and about three volts from the coil distributor terminal to ground, then most likely your points are burned.
 
looks like I'm in Bobs camp on this

"6 volts to 3, across the coil"


If theres no ballast resistor, IE just a 12 volt coil on a 12 volt tractor, with the ignition, on Id expect near the full battery voltage of 12 volts to ALLLLLLLLL be dropped ACROSS THE COIL.

With ignition on if you place a voltmeter on the coils small output (to distributor) terminal with respect to ground and the points are closed ID WANT ZERO VOLTS IE the points are good and cleaned and not carboned up and no resistance

If theres a voltage drop across the points they must be burned or pitted or carboned or not closing well

John T
 
It's a 6 volt system. From the coils battery terminal to ground, it reads 6volts. From the coils distributor terminal to ground it reads 3 volts.
Between the 2 coil terminals, it reads 3 volts, which makes no sense to me. Hence, the question.
 
(quoted from post at 20:22:25 03/25/16) It's a 6 volt system. From the coils battery terminal to ground, it reads 6volts. From the coils distributor terminal to ground it reads 3 volts.
Between the 2 coil terminals, it reads 3 volts, which makes no sense to me. Hence, the question.

Draw your self a picture and you will see it, assuming the points are closed:

1) "From the coils distributor terminal to ground = 3 volts". This is actually the voltage across the points, it should be zero.

2) "Between the two coil terminals = 3 volts". Yes, the voltage drops in the system have to equal the battery voltage of 6 volts. The other three volts are across your points. Based on what you have told us, when you clean/replace the points you should see 6 volts between the two coil terminals (across the coil).
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top