Checking for spark - part two: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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June 1, 2011 11:45 a.m. Klayfish Reader

I posted a while back asking how to check for spark on my '76 Fiat Spider. I wound up laying a plug on the block and figured out I wasn't getting any spark. That's where I stand right now. The folks on the Spider forums are really helpful, but traffic can be a bit slow. I wanted to throw this out here too.

I bought a digital multimeter at Autozone. As I joked in the Fiat forum, prior to this my only electrical experience is hanging ceiling fans and stringing Christmas lights. With my father in laws help, I did some poking around. There are 12V getting to the ignition coil. That power is making it to the distributor. But it's not getting from the dizzy to the plugs. The points seemed to be opening and closing when we turned the engine by hand. I need to buy a feeler gauge to make sure the gap is right.

How do I figure out what the issue is? What will the points look like if they're bad? Bonus points given for a simple answer that a novice mechanic can understand.
June 1, 2011 12:25 p.m. porschenut New Reader

Putting the plug against the block doesn't always work. Have your father in law hold the plug. If there is spark he will curse. The louder he curses the better the spark is.
June 1, 2011 12:29 p.m. ncjay Reader

If there's power at the points and the coil, check the condenser after you check the point gap. Double check and make sure the coil isn't wired backwards as well. If none of that helps, I'd guess something is wrong inside the coil itself. Making sure that the distributor/ engine has a good ground also helps.
June 1, 2011 12:54 p.m. alfadriver SuperDork

Seems like a dumb question, but are the points opening? If they don't, there will not be a spark. You should be able to "bench time" the set up by noting when the gaps open vs. crank angle. it's not perfect, but close enough to get the car running.

AFAIK, you can't really trace the spark plug wires from the coil to the plug- mainly due to the gap in the distributor (the rotor and cap don't actually touch- it would wear out too quickly if it did).

Anyway, it's the sudden loss of 12 v due to the points opening that causes the spark. The wonders of transformers and electromagitism.....
June 1, 2011 2:08 p.m. AngryCorvair SuperDork
there's an old test light trick involving a couple of wires with alligator clips on them. since you're looking at the primary (ie 12v) side of the system, you probably won't blow up.

all of the following are done with the dist cap off.
1. mark the location of the dist. housing relative to engine.
2. loosen distributor clamp
3. rotate dist. housing until moving side of points assy is about to be moved by points cam on dist. shaft
4. connect one test light lead to + side of coil
5. connect the other test light lead to dist housing.
6. put key in ON position. if points are closed, test light should light up.
7. rotate dist housing until points cam opens points, which breaks ground and turns test light off.
if light doesn't go off after points cam goes past, you know your points aren't opening.

if light doesn't come on, you know your points aren't closing. well, unless your test light is broken or your battery is dead, LOL.
June 1, 2011 3:15 p.m. Klayfish Reader

Thanks guys. We did see the points open and close when we rotated the engine by hand. I'll check the gap and then the condensor, since they're cheap parts.

porschenut wrote:

Putting the plug against the block doesn't always work. Have your father in law hold the plug. If there is spark he will curse. The louder he curses the better the spark is.

He's an old "salt of the earth" plumbers crack kind of guy. He don't need no stinkin' spark to make him curse....
June 1, 2011 4:21 p.m. DILYSI Dave SuperDork

My favorite way to test for spark - spray starter fluid in the intake. If it runs, you've got spark.