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Recently I needed to check my dwell and can never remember where to hook up the meter, since I only do this about once a year. I did a search here and found nothing. Checked other sites I have known and still no result. The specs are easy to find, but where does the meter hook up? Finally found a Porsche site that instructed to hook to coil neg and ground. This is one of those stupid things that a lot of poeple take for granted until they can't remember. Can we post it in a tech forum for all?

Devin

One lead to the Coil (-) or at the distributor, the other to ground.
Fred

A dwell meter is nothing but a voltmeter with a different scale calibration. It reads an average of the voltage at the distributor side of the coil with the points open (battery voltage), and the points closed, (zero volts) For instance, a V-8 engine has a possible "points closed" dwell of 45 degrees of distributor roatation, and a desired setting of 66% dwell, or 30 degrees. If an analog voltmeter (the one with a needle, not a digital), reads 4 volts with the engine running, there is a dwell reading (points closed) of 30 degrees. (Assuming a battery voltage of 12 volts)

a 6-cylinder engine has 60 degrees distributor rotation between cam lobes, with a 66% reading of 40 degrees. Once again, an engine-running reading of 4 volts on a 12-volt system, 2V on a 6-volt, should be pretty close to the right point setting. Try it sometime!
Jerry

Jerry,

I hate to reveal my ignorance by asking a ridiculous question, but . . .

Why does one check the dwell? There isn't any way to "change" the dwell, is there? OK, I know, setting the points is setting the dwell, but that's always been "setting the point-gap", not "setting the dwell". I assume it's just to confirm that the coil and condenser are functioning properly, confirm the points are gapped properly, and that the ignition primary isn't getting sloppy?

-Michael

Mike,
Back in the day when I ran points , I used my dwell meter basically as a conformation that I had the point gap set correctly , Setting points with a feeler gauge is like most anything else ..what I think the proper drag on the gauge an what someone else feels is proper can be different & make a marked difference in performance , Dwellin your points just lets ya know you are dead on the $.

Given the choice of setting point gap or dwell, I'll choose dwell every time. It's much more accurate than the guesswork involved in using a feeler gauge, and if there's any roughness on the surface of the points, a gap setting is totally inaccurate. Also, if you have access to an oscilloscope, dwell variation from cylinder to cylinder can be monitored, indicating distributor bushing wear, or cam irregularities.
Jerry

this all started because my truck was poppin and farting on me intermittently, but would run ok if I hit the ign with some spray dry. I wanted to check the dwell before I did any further inspection. Once I got the cap off it was obvious that the points were worn, but the reason I started this was to ask if there was a place the simple info could be easily accessed for the next guy who can't remember where to hook his meter.

A dwell meter serves two purposes: confirming that points have been set properly, and permitting dwell variation measurement at a fast idle speed, when the breaker plate moves with vacuum advance. Dwell variation beyond a few degrees indicates wear in either the breaker plate pivot bushing or in the distributor shaft bushings. The older Delco distributors with the fixed breaker plate, where the whole distributor moves with vacuum advance, rarely showed dwell variation. Also, most dwell meters have a "Distributor Resistance" bar or range on one of the scales. The width of that bar/range represents .2 volt drop in voltage across the distributor, primarily the points and breaker plate connections, but including the primary wire between the coil (-) terminal (the usual point of connection) and the distributor. As Hotrod Lincoln pointed out, dwell meters measure voltage; if the coil ground circuit (the distributor) has resistance beyond an acceptable level, the dwell reading will be inaccurate. So the "Distributor Resistance" indication on the dwell meter also confirms that the coil ground circuit is satisfactory. The distributor resistance check with a dwell meter is done with the key on, points closed, engine stopped. Voltmeter check is done under the same conditions, but voltage should not exceed .2 volt. High distributor resistance will reduce primary current, and if high enough, will lead to misfires at higher engine speeds, or ignition system failure altogether.

Maybe this is better left unsaid, but ignition dwell meters actually measure the "open angle" of the points during each ignition cycle- thats when the meter has system voltage applied to it. The dwell meter internal circuitry handles the measurement so the meter scale represents the "closed angle" or dwell angle of the points. Example: an 8-cylinder distributor has 45° between cam lobes (the lobe angle), points are adjusted to a gap that provides closure for 30° (the "closed angle" or dwell angle, by definition), the reamining 15° of the lobe angle is the "open angle" that applies voltage to a dwell meter- but the meter scale represents the "closed angle"- the dwell angle of the distributor. Its internal trickery, so to speak, but it works.

Dwell meters were also used to measure the operation of mixture control solenoids in the days of computer-controlled carburetors, before electronic fuel injection took over.

Harvester

All right - you guys have gone and made me feel inadequate . . . I'm going out and buying a dwell meter!

Jerry, when you say "setting dwell every time over setting points", I just want to be sure I understand: They ARE one and the same, correct? The difference is what you're measuring to determine the proper point-gap, yes? So I'm wondering how you do this with the dwell meter, ie: do you adjust the points the best you can with feelers or dial indicator, close it up, then hook up dwell meter to see if you got it right?

See what happens when you encourage me?!

-Michael

Michael,

You have it correctly; you want to set the point gap with the feeler guage and then check with the dwell meter. While this process is usually easy with a new set of points, with a broken in set that you are trying to adjust setting with the feeler guage can be quite inadequate as there is usually some amount of material that has transferred across the point gap that can make them rough and hard to set via the feeler gauge.

Thus the dwell meter is an easy check on point gap.

Ditto! An engine will run with just about any point gap, even a couple of thousandths, it just won't run right. I sort of eyeball the point gap, or do a quick check with a feeler gauge, enough to get the engine running, then do the final check and adjustment with the dwell meter.

Since I've still got a Sun distributor tester in working shape, all the point-type distributors I work on get set up in the tester, not on the engine. As well as the dwell, I check the point spring tension, point alignment, centrifugal and vacuum advance, and the advance curve. That's the way I learned to do it back in the 1950's, and old habits are hard to break! When every other shop in town was getting $5.00 labor for an engine tuneup, the family shop was getting $7.50 for "Tune engine and calibrate distributor". 50% more labor charge, for maybe 15 minutes more work, pulling the distributor and running it across the tester, instead of standing on their heads over the fender, installing points! Trying to charge that much more for a conventional tuneup would have made the customers scream, but giving them a premium-quality job, even at a higher price, kept them coming back. Quality outsells price every time!
Jerry

Exactly right, Jerry.

Harvester

Geeze!!! You guys are KILLING me! Now I gotta get a SUN distributor tester TOO?!!! I thought I was ahead of the game with my Biomatic Parts Cleaner and my Compression Tester!!!

I agree that it's certainly easier to check/adjust the distributor by removing it first. I just gotta figure out where the SUN machine's gonna fit in my garage . . .

As always, thanks to you guys for helpin' "edge-u-muh-katin'" us amateurs! Good to know a little bit more about what I don't know.

-Michael

Ok, Michael, after the distributor tester comes an engine tester- with an oscilloscope. After that comes a battery/starter tester, and a generator/alternator/voltage regulator tester. Or one of the combination units that tests batteries, starting and charging systems. And then...

But before all that, get yourself a good vacuum gauge, timing light, and multimeter if you don't already have those.

Harvester

Harvester,

I'm seeing the creation of an online class here: "Truck Restoration 101". What a blast THAT would be! With our antique fire engine club, I've taken the opportunity with whatever my most recent tear-down is, to host "show-n-tell" at one of our meetings. The 216 block in "Rosie" the '44 fire engine proved to be a great session for those who knew even less than me. I don't think people often get the chance to actually lay eyes on a crank shaft, open block, pistons, etc. It all starts to make a lot more sense when you actually can see it.

So . . . I love my trusty Fluke Multi-meter, but am asking for my birthday the inductive amp meter attachment, so I can test loads on the generators, bat.s, etc.

Plenty of timing lights around the clubhouse to borrow, recently did that for the first time on my Dad's '66 Dodge.

I'm hoping when the 216 is done, to actually permanently install (discreetly) a vacuum gauge - it's such a useful indicator of engine condition and health. But yeah, need a test gauge as well. And don't tempt me on the engine tester - I've wanted one ever since the article in Auto Restorer told about one.

I often wonder what people without hobbies do to pass the time at their "real" jobs . . .

-Michael