The Stovebolt Site Forums The Shop Area Engine Shop Generic Ignition-testing question

OK, this question comes from working on a Seagrave V-12 engine, but I think it may still apply to Chevys, so I'm throwing it out there. AND, it's driving me nuts trying to figure out.

Setting the timing on this V-12. HUGE distributor with TWO rotors in it, 2 sets of points, 2 condensors, and 2 coils. Set up like 2 six-cylinder engines running side-by-side.

Anyway, my chosen method of setting static timing is to use my continuity tester tone on my MultiMeter. I hook one side up to the hot wire, either at the coil or the distributor, and the other to ground. Then, turning the distributor, I find the point when the tone just goes out, indicating points just opening.

At least, that's how I THOUGHT I did it. This time, I got tone even when the points were CLEARLY open. The ignition switch was definately off. This being a fire truck, there are 2 ignition sets, so I had 4 sets of points to try this on. It was unanimous.

Took one of the distributors out of the truck, and set the points in the vice, with the continuity tester - no problem. So HOW is the coil primary grounded with the ignition off? Are the primary and secondary windings in the coil by necessity connected, at least enough to get continuity? Or am I missing something? I thought it was the breaking of the primary current that triggered the secondary, so how is it triggered if there's always continuity?

Matter of resistance?! Somebody educated me . . .

-Michael

Let me suggest an alternate method. Instead of powering the test light with battery voltage, ground the lead from the test light and turn the ignition switch to the "run" position. Touch the point of the tester to the insulated point, or the wire going to it. As long as the points are closed, the light will not burn because the points are providing a low-resistance ground. Turn the distributor housing opposite the direction of rotation until the points just break- - - - -the light will come on as soon as the circuit across the points is broken.

If you have two sets of points in parallel, one will probably break slightly before the other to increase the coil saturation without setting an excessively narrow gap. The "leading" point will initiate the current flow through the coil primary, and the "lagging" point will be the last one to open, creating the spark. For instance, the old Corvette 283 dual-point distributor had to be set with a dwell of about 24-25 degrees for each point, but the combined dwell of the two together was around 35 degrees. A single-point dizzy couldn't be set above 32 degrees effectively.
Jerry

Jerry,

One reason I like using the continuity tester is that I don't have to turn on the ignition! I work almost entirely on old fire trucks, and they all have electric fuel pumps (along with mechanical, usually) that come on with the ignition. I don't like the fuel pump to be working at rest.

As to the points in parallel, it's a little different here, since one set drives the left bank, and the other set drives the right bank. Dwell on both is identical, and they fire 15 degrees apart. Further, each set is driven by its own coil (yes, 4 coils in total!).

That being said, the positive wire to the coil is jumpered from one coil to the other - so if one set of points was closed, maybe it affects the other set?!

I guess I should test this on my Chevy to see if it's the same. I just don't understand a ground loop on the primary side with the points open. Am I wrong about that?

-Michael

The primary and secondary windings are connected, but the secondary has such a high resistance (thousands of ohms) that it's hard to imagine the continuity tester getting enough of a circuit to buzz. You might try disconnecting the wire from the coil while you're using the continuity tester to prevent any possibility of a stray circuit triggering the tester buzzer. The ground through the points will be the same whether or not the coil is connected.
Jerry

Yep, that's how we figured out it was grounding out through the coil, by disconnecting the wire.

I guess one thing I'm worrying about is whether it may indicate failing insulation in the coils or something, leading to poor running. However, it COULDN'T be all FOUR coils going at once. OK . . . it could, but I sure doubt it. Might be worth throwing in one new coil though, just for giggles . . . hmmm

-Michael

Anything else that's powered by the ignition switch (like that electric fuel pump, for instance) might have a low enough resistance to be fooling your tester. Electricity will find the path of least resistance, just like water. Anything and everything that's powered up when the switch is in the "run" position will parallel itself with the coil(s) and give a low resistance reading to keep the continuity tester indicating even with the points open.
Jerry

Best thing to do is remove the wire from the points to the coil and put you're tester one lead to a ground and the other on the wire going to the points. The condiction that you're having is called in the electrical trade as "Backfeed" it's when there are more than one circuits on a hot lead,when the circuit is off you're reading the ground thru the other OFF circuits thru the ignition switch connection.

Pete

Yeah, that makes sense, I guess. All those lights and things are confusing the issue!!! Dumb firetrucks - shoulda started collecting model trains!

Thanks for the help, as usual!

-Michael