I've been chasing down an electrical conundrum that I figure I'll share here in the event others have run across a similar situation. In trying to diagnose an issue with a running condition, I started working through the ignition system with my digital multimeter. It's a cheaper $80 unit, but it's always read accurately compared against a friend's automotive Fluke. 327 SBC, new stock single points distributor, dwell set at 28, stock coil with 1.5 ohms cold resistance across the terminals, 1.5 ohms cold across the external ballast resistor for a total of 3 ohms which is pretty ideal for points. I've verified with the multimeter that the engine, body, and battery all have good grounds. With the engine off, I get steady VDC readings across the battery terminals, at the fuse panel, ignition switch, input and output of the ballast resistor, and at the coil with the points open. Here's where things get weird. When I tried to read volts at the ballast resistor output and positive terminal of the coil with the engine running, my multimeter was jumping all over the place from 0 to 14. I started chasing back and got the same wild readings at the ignition post of the ignition switch, the battery post of the switch, and even at the main feed to the fuse panels coming off the starter post. I then checked volts across the battery terminals and got similar wild readings between 3 and 14.7 volts. As soon as the engine is off, the readings go back steady. I figured I had a ground issue. Already long story shortened as best I can, I ended up isolated the ignition system to bare bones. I have one battery at 12.8V with the positive jumped directly to the starter post and a remote pushbutton starter jumping from that post to the starter solenoid post. The negative from this battery goes straight to the engine block with a confirmed good ground. I then have another 12.8V battery with a jumper from the positive terminal to the ballast resistor and a jumper from the negative terminal to the engine block with a confirmed good ground. So the entire car's wiring harness and chassis is bypassed. I've confirmed a good ground from the distributor housing to the intake manifold and block. Volts at all connections and across battery terminals all read steady with the engine off. Once I fire the engine, I remove the positive cable from the starter battery thus removing this circuit from the equassion. With the engine running, the meter readings go all over the place at the ballast resistor, positive coil terminal, and across the battery terminals. I figured I have an issue in either the ballast resistor, coil, or distributor. I have spare parts and swapped out the ballast resistor. Same issue. Swapped out the coil. Same issue. Swapped out the distributor. Same issue. With the engine running, I pulled one plug wire at a time from the cap and continued to get the wild readings, so the plug wires and plugs aren't the issue. I've taken these reading with another cheaper multimeter as well with the same results. I don't have an analog meter to compare. Am I chasing a non-issue here, as in does a points ignition system do funky things to digital multimeter readings unless using a high-end automotive meter that is dampened from such fluctuation or something? The oddest thing to me is that I'm getting these wild readings across the battery terminals. Anyone else run into a similar issue?
It is possible for ignition noise to affect digital meters, especially cheaper ones. Borrow and try an analog one.
Electrical issues can be weird. My guess (because it happened to me) is that the distributor doesn't have a good ground. Yes, I know that the distributor is clamped to the block and the distributor is enclosed in the metal engine block so how could it not have a good ground? The distributor get corroded and the engine block get a oil residue in the hole the distributor goes in. Pull the distributor, polish the housing and the distributor clamp. Clean the hole in the block with something that will remove baked on grease (I use Berryman's carb cleaner on a rag).
Figure this; all a digital volt meter is doing is periodically taking a current reading through a shunt, via a fixed value resistor. If at that point in time there is a floating transient (noise) that'll cause that fraction-in-time measurement to be thrown off. An analog meter will give you an average due to the dampening of these transients and as such will be easier to read. However, that is a band-aid covering an underlying issue. Your real question here is "what's causing that to happen?". 99% of that will, somewhere along the way be a poor ground connection, forcing current via a path it should not take, but will because it is the "easiest" to follow. Check all your ground connections as people have suggested. Phil
This. When the engine is running - the points opening and closing vary the voltage at the ballast resister and coil - a digital meter gets confused - it is the wrong tool for the job - you need an analog meter for this.
put an oscilloscope on the positive coil terminal, you'll see some wild stuff. long story short...always use an analog meter when trying to diagnose the voltage on the primary side of an ignition system with points....or you'll get highly confused.
Depends on the meter. Fluctuations yes. Wild swings not so much. My results with my cheapo digital multimeter, for reference to show it is possible to get a semblance of sanity from one.
While I appreciate all the responses, I've already clarified all the grounds are good. In the current state of diagnostics, there are two grounds: The ground from the distributor to the block via the clamp and the block to the negative terminal on the battery. I've already cleaned these contact points to bare metal, and both show 0 ohms resistance when testing for ground. The only other ground would be through the motor mounts to the chassis, which ignition current in my diagnostic setup wouldn't follow since the battery is not attached to the chassis in any way. The path of least resistance is through the three-foot cable I have attached from the block to the negative terminal on the battery. I'm following what some are saying about using an analog meter to read voltage at the coil due to interference, but I am still suspicious why a digital meter would be having such wild swings reading voltage across the battery terminals. I've never had that happen before outside of a bad ground.
This why I kept my Simpson 260 when I left work. Try setting a 3 brush generator with digital meter... doh! You said you set the dwell at 28; my old dwell meters had a voltage meter included. If yours does use it. The digital does work as a voltmeter used on the battery and when checking milli-amps between the ground cable to the ground post.
I agree with the others regarding digital voltmeters. Use an analog meter, which won't react to those instantaneous spikes in the electrical system, but will give you a good average reading. I have both a Simpson 260 and a Triplett 630 that are my go-to meters. If I need a real accurate resistance reading on a component I use the Fluke digital.
Along the same lines, I was using a very good digital autorangeing meter working on my trailer wiring When testing the turn signals, it showed about 6.something on the trailer, steady reading when the lights on the truck were flashing. A test light flashed normally. It was apparently sampling the input and showing the average, or something like that.
The volt function on my unit for setting dwell gave up the ghost a while back. Dwell still seems to work, though. I'll order an analog meter this week and retest. I have a feeling that the meter is the issue since the engine runs fine, but I've just never before seen this kind of voltage swing at the battery, fuse panel, switch, etc. using a digital meter. That being said, it's been a good while since I had a points setup, so maybe a digital ignition system is less prone to the interference. It sounds from the multiple posts that others have seen this issue, so here's hoping it's just the digital meter's inaccuracy. I'll update once I receive the analog meter. Thanks for the input everyone.
I have a cheapo [as in HF] digital and it reads pretty steady, +/- .2v tops no matter where I check in the Ign circut. Just thinking that what the OP is finding is not just because of digital/analog meters. Maybe point lobe is bouncing around do to bad dist bearings something loose in the advance weights; you know something weird, out of the box.
If you understand how a point ignition system works, it shouldn't surprise you that a digital voltmeter with a fast sampling rate, will show some wild things when connected to a running engine. There is a lot of electrical noise generated by the coil, the primary side swings around 300 volts every time the points open. Spikes like that do strange things.
Highly unlikely it's the components. I checked with one brand new points distributor, one freshly rebuilt points distributor, and one used but working dual point. Also checked with three different new coils (oem, 35K volts, and 40K volts), and three new ballast resistors to get the correct resistance for each coil. Checked with two different good batteries. The odds of these components in their varying configurations all malfunctioning to create the exact same meter reading outcome is slim.
A digital meter that is auto ranging can't react fast enough on a circuit that has a constantly changing path to ground (the points with the engine running). Your meter may have a way to lock out the auto range feature, most do. You may get a steady reading with the auto range off. Also, a better way to check your distributor ground is to use your volt meter. With the engine not running, ignition on, and the points closed, connect your voltmeter negative lead to the battery post. Connect your positive lead to the point base (ground side). Your voltmeter should not read over 0.1 volt. If it does, you have a bad ground. It's called a voltage drop test and much better than using an ohmmeter because you are loading the circuit.
Thanks. I'll give that a try next time. Sure enough, an analog meter cleared up the confusion. With the engine running, I'm getting a steady 14.5 across the battery, 13 at the ignition input on the ballast resistor, 10 at the coil positive, and 9 at the coil negative. Everything is where it should be, and I've learned something new. Thanks for the help.
You are... chasing a non-issue that is. Digital meters 'sample' whatever they're reading and display a steady value within whatever parameters the meter designer decided on. Big fluctuations (like the voltage change from points opening and closing) will exceed those parameters, so the meter tries to display but can't 'settle' on a value. You won't find a digital hand-held meter that will read that accurately. You might try increasing motor rpm, you may get a somewhat better reading as the interval between the coil being 'off' and 'on' gets smaller. A analog meter will read an 'average' value, in this case you should see about 9 volts off the output of the ballast resistor with the motor running.
Steve said it before I did, you are creating an issue where there isn't an issue. Basic first day of class in ignition tells you that when the points are closed the circuit is grounded and the magnetic field builds in the coil. when the points open that ground connection is broken and the the magnetic field in the coil collapses sending the high voltage to the plugs. The condenser's job is to absorb the current going to the points so the points won't arc when they open.
Same problem with my stock model a,the digital multimeter.put a volt gauge on and problem solved,after I spent some money on unneeded parts
I read the whole post. What exactly was the initial problem that caused you to chase this non issue? Sorry if I'm a slow learner. Lippy
There isn't really enough accuracy to measure the tiny fractions of ohm resistance effectively in a low voltage, high current situation. Just a small (a few hundredths of an ohm) extra resistance is enough to hork things up but good. And you'd have to already know what the normal value is anyway, and add to that consumer grade equipment has a +/- calibration tolerance. So voltage drop test while the circuit is energized is the only method used, which infers the amount of resistance, going back to the earliest days.
^^^Excellent but you forgot my favorite...paint and powder coat. I had a buddy base and clear the block, powder coated the hold-down and the distributor and in the process of looking pretty...lost all of the ground for the distributor.
You're telling me a digital multimeter with low batteries will read steady for tens of minutes on end with the engine off, coincidentally start getting erratic readings only with the engine running, just to coincidentally go back to steady readings with the engine off? Low batteries might be someone else's issue with his multimeter, but it was the difference between a digital and analog in my case.
The Jalopy Journal
