5 Volts at the "+" side of ignition coil ?

[McL0V3N1121]

1966 2.5L 4 cylinder Mercruiser. At the ignition coil I have ground on the "-" side post when the key is turned to the off position. When I turn the key to the on position I loose ground. I am getting 12Volts on the "+" side post. I verified this by taking my DMM red lead attached to the "+" post and the black lead attached to the negative post on the battery. If I hook the black lead of the DMM to my battery and the red lead to the "+" post on the coil with the key in the on position I get 12 Volts like I'm supposed to, but when I take the black lead from the DMM and attach it to the "-" side post of the ignition coil, no ground. But then I turn the key to the off position and I have ground at the "-" side post of the ignition coil. Where could I be losing ground to the coil when the key is turned to the on position ?

 

[stonyloam]

OK so if I understand correctly with the ignition on, you read 5v with the red lead on the coil + and the black lead on the coil - correct? What you are reading is the voltage drop across the coil resistance. BTW if your points are open, you will have no ground. With the ignition off, on ohm function, read between the coil - and ground. If the points are closed, should read 0 ( dead short) if open will read infinity.

 

[McL0V3N1121]

If I have the red lead on the "+" side of the coil and the black lead on the "-" side of the coil I read 0.00 with the DMM set to 12V. But, if I take the black lead off of the "-" side of the coil and put it to the "-" post on my battery ( a known ground ) with the key on I read 11.77 on the DMM. So that means I have no ground when the key is turned over, right ?

 

[McL0V3N1121]

Also, if I put the red lead of the DMM to the "+" side of my battery and the black lead of the DMM to the "-" side of the coil, with the key off I read 12.07 volts, with the key turned "on" I read 0.24 volts. Seems to me I'm losing my ground when the key is in the on position.

 

[stonyloam]

Just edited my first post, your points could be open.

 

[stonyloam]

If I have the red lead on the "+" side of the coil and the black lead on the "-" side of the coil I read 0.00 with the DMM set to 12V. But, if I take the black lead off of the "-" side of the coil and put it to the "-" post on my battery ( a known ground ) with the key on I read 11.77 on the DMM. So that means I have no ground when the key is turned over, right ?

That would indicate your points are open.

 

[stonyloam]

Pop off your distributor cap and make sure your points are closed.

 

[McL0V3N1121]

Ok that sounds like what could be going on here. I will test it now and thanks a lot. I bet this will be the problem.

 

[McL0V3N1121]

OK so if I understand correctly with the ignition on, you read 5v with the red lead on the coil + and the black lead on the coil - correct? What you are reading is the voltage drop across the coil resistance. BTW if your points are open, you will have no ground. With the ignition off, on ohm function, read between the coil - and ground. If the points are closed, should read 0 ( dead short) if open will read infinity.

What setting should the ohms be set to ? 200, 2K, 20K ?

 

[stonyloam]

Also, if I put the red lead of the DMM to the "+" side of my battery and the black lead of the DMM to the "-" side of the coil, with the key off I read 12.07 volts, with the key turned "on" I read 0.24 volts. Seems to me I'm losing my ground when the key is in the on position.

This is getting kind of confusing, but it seems you do have ground with the first reading, but the second reading looks like you are just reading the voltage voltage drop across the coil. Sounds like you may have a very poor ground somewhere. Could be the points are glazed over or you have a poor ground connection at the distributor or coil. With the ignition off check with the ohms function of your multimeter. Between the coil - and bat - should read 0.00 ohms. Try cleaning the connections.

 

[McL0V3N1121]

I put the black lead of the DMM to the "-" post on the coil and the red lead to the "-" side of my battery and with the DMM set on ohms on the 200 setting I read 04.8. Switched the ohms setting to 2K and I read .004. Switched the ohms setting to 20K and read 0.00. This is with the wires still hooked up to the coil. Should I test it without any wires hooked up to the coil ?

 

[McL0V3N1121]

I am not reading 5Volts. I just noticed the title. It was supposed to say 0 Volts. Sorry about the confusion. I am not reading 5 volts anytime.

 

[stonyloam]

I put the black lead of the DMM to the "-" post on the coil and the red lead to the "-" side of my battery and with the DMM set on ohms on the 200 setting I read 04.8. Switched the ohms setting to 2K and I read .004. Switched the ohms setting to 20K and read 0.00. This is with the wires still hooked up to the coil. Should I test it without any wires hooked up to the coil ?

On 200 should read 0.00. You are reading 4.8 ohms. Test the ohm meter by touching the red to the black lead on 200 range, should read 0.00. If your points are properly grounded you should read 0.00 with them closed. Start tracing down the high resistance. Take the cap off, and check resistance between the coil - and the black wire in the distributor , and across the points.

 

[McL0V3N1121]

I took the cap off and tested the "-" post of the coil to the black wire inside the distributor and got 0.00, how can I test across the points ?

 

[Bt Doctur]

what are you trying to test?you need power at the + side of the coil. remove the wire and test the wire for power.On the neg side that wire has your tach feed and your points feed. points are closed with a good connection you will read 0 ohms, with the points open ,you will read infinity

 

[McL0V3N1121]

I am trying to figure out why I do not have ground on the negative post of my ignition coil. I have my DMM connected to the positive and the negative post of the ignition coil with it set to 12V. When I have the key turned off I get 0.00 and when I turn the key on I still get 0.00. But if I take the negative cable of the DMM off the negative post on the ignition coil and put it to the negative post on my battery, leaving the red lead on the positive side of the coil I show 12V. Ultimately I'm not getting spark at all and I'm trying to figure out why. I was told that it may be losing ground through the distributor. I'm totally lost and would really like to figure this out lol. It's really confusing me today.

 

[McL0V3N1121]

Am I supposed to have ground on the "-" side of the ignition coil ? Here's what I know, if I take my DMM and place the red lead on the "+" side of the coil and place the black lead on the "-" side of the coil and turn my key to the on position I show 0 on my DMM set on 12V. From my understanding I am supposed to show 12V when the key is turned to the on position at the coil.

 

[funk6294]

Check out this vid o, it's a little long winded but really covers the basics on testing the ignition coil and explaining the wiring.

 

[funk6294]

As far as having ground on the negative, it is a ground but that ground is switched on and off via the ignition points so if the points are open then you would not get a ground reading. For you to test that you need to disconnect the negative wire on the coil and use the ohm setting on your multimeter to test between that wire and the ground (engine block or negative wire). If it shows zero ohms then it's grounded, if it shows infinite (or the 1 if it's a dmm) then the ground is open.

Your tests for voltage are good. You have 12 volts on the positive, the voltage readings your seeing in the negative side may not be that unexpected as the coil is essentially a large resistor so the way you were testing you simply measured how much the voltage dropped due to the resistance. The question to answer now is how much resistance the coil is supposed to have. i posted a video link in my previous post that will walk you through the process of testing the coil.

 

[stonyloam]

OK this is how you can test your coil: take off the cap. Place a piece of insulating material between the points (any pieces of plastic or paper will do). Take the center wire out of the cap and place it near bare metal of the engine. Make a little jumper wire by stripping the insulation off the ends of a piece of wire. Turn on the ignition. Briefly jumper across the points by touching and quickly removing the wire. If your coil is good you should get a nice blue spark out of the coil wire every time you REMOVE the wire. if you do you have a good coil and ground. If the coil is Good, remove the insulating material and have someone hit the start key and look for the spark. If you do not get a spark, take a voltage measurement between the coil + and the battery - (or good ground) while the engine is cranking. If you see battery voltage ( probably less than 12 v) while the starter is turning then your points are bad.