electrical troubleshooting on MC

[BLS]

electrical is not my strong subject.

I am starting to work on a MC that I bought three years ago that did not run.

I am getting no spark at the plugs, or accross the points. When I check the voltage at the coil (-) with the ignition switch turned on, I get 2 VDC then it bleeds down to almost nothing (I'm using a digital volt meter). Is this normal?

Thank you.

[shinnery]

I assume a MC is 6 volt positve ground. If so, the negative terminal of the coil should show full battery voltage. The positive terminal should show 0v if points are closed and battery voltage if points are open. The voltage at the negative terminal should come from the ignition switch. Be sure you are checking in reference to a good ground. I hope this is clear and helps.
Bryce

[BLS]

Bryce,

You are correct, (6 volt + ground), I was measureing from the wrong place on the switch. From the ignition switch to (-) on coil i have 6 volts.

I measured from (+) to (-) on coil and I have 1 ohms resistance, but still no spark.

I have tested the ignition switch and it appears to function correctly in each position (I checked with the Volt/ohm meter).

Is there a way to bypass everything (switches, volt meter, etc...) and put power directly to the coil (without burning up my wiring harness)?

Thanks
Brian

[shinnery]

Brian your first sentence tells the tale. On a positive ground system there should be NO voltage difference between the ignition switch and the negative terminal of the coil. And with the switch off 0 ohms resistance. Ohmmeters have a low tolerance for external voltages. At best you can let the smoke out of a fuse (in the meter) very easily. You probably have an open wire somewhere.
You should be able to run a wire directly from the negative terminal of the battery to the negative terminal of the coil no problem if the positive terminal of the coil is wired to the distributor. If you start the engine like this it will be a little harder to stop it is all.
Bryce

[BLS]

Bryce,

I removed the ignition wire from the switch and the coil, checked the wire did not detect any shorts. I applied 6VDC to one end and measured 6 VDC at the other end.
I installed a toggle switch in place of the ignition switch and applied 6VDC. I have the condition you spoke of before (The positive terminal should show 0v if points are closed and battery voltage if points are open). Now that I understand, I think I will get the same indication with the original switch (I will try that tomorrow).
With the condition I have now, I still do not have spark at the points (new points & condenser) I do not get any spark or voltage indication at the end of the distributor wire.
Is there a way to check the coil, things seem to be pointing in that direction?
Thank you,
Brian

[shinnery]

Brian

With all the wires loose from the coil the primary winding should measure a low resistance, probably under 50 ohms, this is between the positive and negative terminals. the secondary should measure somewhat higher maybe 1K or more, this is between the center terminal and the positive terminal. None of the terminals should read to the case, infinite. Your secondary should check to either of the side terminals just one slightly more resistance.
I wouldn't expect a spark at the points if the condenser is doing its job. If the coil is good you should be able to get a spark from the secondary wire to ground just by opening the points with something insulated. I would expect it to jump about an eighth of an inch or more. You may want to be sure the secondary wires are good, coil to dist. and dist. to plugs. On Magneto systems they recommend copper core wires none of this carbon, radio noise suppressing junk. This would be a good idea for 6 volt systems also.
Good Luck
Bryce

[shinnery]

Brian

The basic theory of operation is with voltage at the negative terminal, a current flows thru the coil primary and the points to ground. This current builds up an electromagnetic field within the coil. When the points open the current flow stops and the field collapses. This collapsing field induces a voltage in the much larger secondary winding and it forces a current out the secondary terminal to ground. This voltage and curent is the spark or arc you are looking for. It is called a coil but it is really a step-up transformer. Maybe this will help.

Bryce

[BLS]

Bryce,

The coil checks out just like you said it would. The secondary wires (coil to ground on distributor, and coil to distributor cap) ohm out ok (not open).

I still do not get spark from the secondary wire to ground when I open the points. Any other things I should check?

Brian

[Lavoy]

I think you have to be missing something somewhere. Do you have 6 volts to the coil? You said earlier you did not, you need to address this first.
Second, I think Bryce meant that you need to ground the secondary wire, and see if it sparks. That may be what you have done, but you mentioned the points as well in the sentence, and in this instance, they would not be a factor due to the secondary wire no longer being hooked to the dist.
I would suggest doing this. Make sure that you have 6 volts to the coil. Make sure that the secondary wire to the dist is good, replace it if there is any doubt. With the dist cap and dust cap off, open and close the poiints with your finger and see if you get a spark there. If not, you are not getting power through the coil to the dist, or the points are bad.
Lavoy

[shinnery]

Brian

If you have 6 volts at the negative terminal of the coil, 6 volts at the positive terminal with points open, and no volts at the positive terminal with the points closed. You should be able to hold the secondary wire within 1/8" of ground and open the points and get a spark.
Do you have access to a spare 12 volt coil? If so you could try it, tha output voltage will be a little less (10-15KV) but should still spark.
This long distance troubleshooting is real interesting.

Lavoy, what I meant was the points are in the primary circuit and open them you stop the current flow and should cause a spark voltage and current in the secondary. A good condenser should prevent nearly all arcing and sparking at the points themselves. Of course I put a new bad condenser on a IH Travelall one time and killed the starter before I figured out what the problem was.

Another thing to try is another secondary wire, preferably one with a copper core.

Bryce

[Lavoy]

Okay, other mistake I made was thinking secondary wire as in the wire to the poits, not the coil wire itself. Disregard my previous post, it really does not make sense.
Lavoy

[shinnery]

Lavoy

No problem, amybody with any ideas on how to help Brian is welcome. From my electronics background I consider the low voltage wiring the primary circuit. That is from ground to the positive term. of battery, thru battery to negative term., on thru ignition switch to coil neg. term. thru coil to pos. term. then on to the distributor, the case of which is grounded. If there is current flowing in this circuit, there will be a magnetic field in the coil. Any thing that interrrupts this current such as the points opening, the magnetic field collapses and creates a current in the secondary. This circuit starts from the H.V. terminal of the coil, thru the wire to the center of the dist., then the rotor, out one of the side terminals of the dist. to a spark plug and then ground. This spark current then flows thru half the primary circuit back to the coil. Both these loops have to be complete for the engine to run. I think I am just trying help him find the problem spot.

Bryce

[BLS]

Bryce & Lavoy,

Here is what I got with a 12 V coil installed (same as with the original 6 V coil):

6 v at (+) side of coil, points closed
0 v at (+) side of coil, points open
6 v at (-) side of coil, points closed
0 v at (-) side of coil, points open
6 v at secondary (wire to dist cap), points closed
0 v at secondary (wire to dist cap), points open
6 v at ground on dist, points closed
0 v at ground on dist, points open

With the secondary wire 1/8 inch from ground, still no spark. Am I correct that the timing of the tractor does not matter at this point, the opening and closing of the points should generate the step up voltage in the secondary wire (to the cap) required to get the 1/8 inch spark to ground?

(I have verified # 1 pistion TDC with -spark- in timing window)

This may seem frustrating to some (I know it does to me), as this should be something quite simple.

Thanks for your continued help.

Brian

[Lavoy]

You will not likely get a very good spark running a 12 volt coil on 6 volts. Do you get any spark across the points as you open and close them by hand?
Lavoy

[BLS]

Lavoy

nothing at all.

what am i doing wrong?


Brian