R/R Test Reveals Reverse Flow??

[reddirtrider]

Sheeesh! Now I'm getting confused. I've been trying to follow various threads which discuss measuring the voltage drop in the coil primary circuit.

My DMM manual indicates that when measuring for voltage drop I should put the RED lead on the battery (+) terminal and the BLACK lead on the positive (+) terminal of the coil.

Additionally, it is my understanding that in order to measure a voltage drop, current must be flowing in the circuit. How does current flow in the circuit with the KOEO (Key On Engine Off) if the ignitior pulls the primary circuit to ground momentarily, only when the signal generator is spinning? And if one trys to read the voltage drop with the KOER (Key On Engine Running), the meter is going to be bouncing around between open circuit voltage and loaded voltage.

Could one of the electrical gurus give us a refresher course to be sure we are all using the correct method in determining the coil primary circuit voltage drop?

I'll try to explain the coil circuit.

So, here's what's going on. When the ignitor fires for a specific set of coils, it grounds the low side allowing current to flow through the coil primary. This current is coupled to the secondary side and there is a large voltage gain defined by the inductance relationship between the primary and secondary side set by the turns ratio. The voltage developed on the secondary side is applied to the spark plug and when it reaches a level high enough to bridge this gap with an arc, the plug fires.

The current is pulsed, not constant. Thus, the voltage drop is a time derivative of the current since we are dealing with inductors. What this means is that the current is going to be very much smaller that what you would expect if you divided the voltage applied by the dc resistance on the primary side. Specifically v=L*di/dt or i=integral of v/L

The voltage drop leading to each coil is a pulse defined by the current. Depending on the meter it's going to read an average of the voltage (a guess - I'm not a meter expert and I don't have time too look it up now, I'm working on a presentation for Monday). So, yes your bike should be running.

 

[pdqford]

So, here's what's going on. When the ignitor fires for a specific set of coils, it grounds the low side allowing current to flow through the coil primary.

So far, so good. That's when the coil primary circuit begins to flow current and that is when I want to measure the voltage drop, right?

The current is pulsed, not constant.

Right. The ignitor turns the current on by pulling the low side of the coil to ground long enough to saturate the primary windings and then turns the current off by releasing the ground (which induces a high voltage in the secondary).

The voltage drop leading to each coil is a pulse defined by the current. Depending on the meter it's going to read an average of the voltage (a guess - I'm not a meter expert ....

Yes, as I understand it some meters read the average voltage, and an RMS meter reads the Root Mean Square of the voltage.

So, yes your bike should be running.

Okay, so we should put the red lead of the DMM on the battery (+) terminal and the black lead on the primary (+) terminal of the coil, and use the min/max feature of the DMM to capture the lowest voltage drop during the current on (pulse) periods? (That's not a statement, its meant as a question.

 

[pdqford]

When I tested mine the first time, I disconnected my coils and tested the orange/white (IIRC) leads. But mine were aftermarket with spade connectors and easy to do this.- JC

See, that's what is confusing me. If the coil primary is disconnected there is no current flowing through the coil primary feed harness and the meter would just read OCV (Open Circuit Voltage).

So leave the engine off, key on (some drop in battery voltage can normally be expected), and test the battery across the terminals and compare it to the volts being supplied to each coil.

Can you elaborate on which meter lead was connected to where when you tested your coil feed voltage drop?

 

[crash-harris]

Nice pics, thanks! PO (previous owner) liked red, eh?

Are you sure the relay placement will clear the tank? You might want to do a test fit before you get too far just to be sure.

That's a decent result for the coil relay mod, but you should be getting less of a drop. Be sure you are checking the battery voltage across the terminals with the key turned on and use that for comparison to the voltage at the coils.

There's almost nothing in the circuit to the coils, so it should be very close to the same as your battery.

Looks like you have quite a project on your hands. All the better when it's running!

- JC

I was reading the voltage with the key on, I think I may need to look at my coils ground connection since I left the stock wiring there for the ground. As for the relay, I don't think it will clear the tank, just putting it there until I get the tank so I can figure out a more permanant home for the relay.

Both my brother and the guy he bought it from rattle canned the bike and got over spray EVERYWHERE 1 random coat of black primer and 3 random coats of crap red...

 

[reddirtrider]

Okay, so we should put the red lead of the DMM on the battery (+) terminal and the black lead on the primary (+) terminal of the coil, and use the min/max feature of the DMM to capture the lowest voltage drop during the current on (pulse) periods? (That's not a statement, its meant as a question.

That should give you the voltage drop across the wiring to the coils.

 

[pdqford]

That should give you the voltage drop across the wiring to the coils.

That's the voltage drop we are trying to minimize to get maximum spark performance out of the coil, correct?

Considering that the current to the coils (at least on my 1980 GS750ET) travels through the positive battery cable, the main fuse contacts, the ignition switch contacts, the ignition fuse contacts, the engine kill switch contacts, eight connectors, and several yards of wires, what would an acceptable voltage drop on this circuit be?

Likewise, the ground side of the coil primary travels through three connectors, the ignitor unit, several feet of wire, the bike frame, and the battery ground cable. Any voltage drop in this circuit is going to take voltage away from the coil also. What kind of a voltage drop would be acceptable on the ground side?

 

[reddirtrider]

That's the voltage drop we are trying to minimize to get maximum spark performance out of the coil, correct?

Considering that the current to the coils (at least on my 1980 GS750ET) travels through the positive battery cable, the main fuse contacts, the ignition switch contacts, the ignition fuse contacts, the engine kill switch contacts, eight connectors, and several yards of wires, what would an acceptable voltage drop on this circuit be?

Likewise, the ground side of the coil primary travels through three connectors, the ignitor unit, several feet of wire, the bike frame, and the battery ground cable. Any voltage drop in this circuit is going to take voltage away from the coil also. What kind of a voltage drop would be acceptable on the ground side?

To tell you the truth, I have no idea what voltage drop would be acceptable. All I can say is the obvious - you want to minimize this drop. Sorry, I just don't have any data.