Hello,

I'm a former electrician and for fun I work on older cars and motorcycles as a hobby. I have very limited electronics experience.
Some of the vehicles come with misfiring engines and those misfires mostly happen while driving/riding the vehicle and I'd like to have something that I can put on the dash or handlebars that indicate which cylinders are firing a spark and which aren't. If the misfires were happening at idle, I could easily use my timing light and move it from cylinder to cylinder.

I've seen some posts on here for DIY timing lights and they could work. I don't need to time the ignition, just verify a spark. Maybe I could build a box with one power lead, 4 plug wire leads, and four LEDs. Even better would be something that recognizes a missed firing that happens only occasionally and can't be seen by the naked eye (some motorcycle engines rev way up into the teens of 1k rpms) and would indicate that a misfire had happened.

Any thoughts on this? Is it possible to read from the 12V primary side of the coil to trigger the lights? As an electrician, I utilized current transformers to trigger relays for dust collectors and lighting. Can they be utilized? Again, I have very limited electronics knowledge.

Thanks,
Ed

 

Such an indicator could indicate that spark voltage is present, but not necessarily that the voltage is too low for a good spark, so it could still misfire.
Is that okay?

A simple, old-school technique was to wrap a few turns of wire around the spark plug cable, and connect it to a small neon bulb, which will blink from capacitive coupling of the spark voltage to the wrapped wire.

 

I made a monitor moons ago, but it was for POC ignition so had to monitor the POC coil for amps. The assumption was, if there was POC amps than it was assumed a spark happened.

To do this was to inline a precision air resistor (current sense OAR) with the primary voltage line to each POC, then use an op-amp to amplify that diff on the OAR and feed that to a counter IC. The counter ckt would count up, say to 8 for 8cyl, then it would reset back to zero if all 8 were counted, if at the end of counting it was not 8 then I would get a latched on for an LED telling me I just had a miss.

You can use an IC that has 4 or 8 op-amps in it, and just allow them to drive a fet or bjt to drive a LED, but as rpm's go up it's hard to see if a LED does not light up. Another option is just to count each drive pulse and display that count via 4 or 8 displays (one for each cyl), they should all count up the same, if one starts to miss then it's count will dwindle as the others keep counting up.

Again, I was monitoring components and not actual spark event.

There are sophisticated ways to know if that spark current actually happened, there are electrical characteristics that vary on the coil when the spark current (coil secondary) happens or does not happen (primary current but little to no secondary current in the coil). This is as close to spark monitor as you get, because a spark can happen but perhaps does not happen through the spark plug.

From what I know of it, mondern day vehicles can detect an actual cyl non-fire by monitoring engine force related to crank and cam sensors. At certain crank/cam positions the ECU expects to sense the force from that specific cyl firing off.

 

It doesn't work that simply.

If You are dealing with a Points-type Ignition,
the best solution is to get rid of the Points and replace them with an Electronic-Ignition-System.

EVERY-SINGLE-PART in a Points-type Ignition-System breaks-down gradually over time.

Misfires are normally caused by a "weak" spark, not a "missing" Spark.
The Ignition fired, but there wasn't enough Voltage to keep the Spark-Plug clean, or,
the Spark jumped to somewhere else besides the Spark-Plug.
This causes a "dwindling-spiral" of worse and worse Plug-Fouling,
every time the PLUG doesn't successfully fire the mixture, the more Plug-Fouling occurs.

Things that can cause misfires ...................

Fouled Spark-Plugs, caused by Oil-Burning, or excessively-Rich-Fuel-Mixtures / Dirty Air-Filter.
Incorrect Spark-Plug-Heat-Range
Incorrect Spark-Plug-part-number.
Incorrect Spark-Plug-Gap.
The Spark-Plugs must be BRAND NEW, NOT "CLEANED".
The old-Spark-Plugs should be carefully inspected with a Magnifying-Glass,
they should all be in the exact same condition as each other,
and the Porcelain-Insulator should be a light-Tan-color,
if it's not, then replace ALL of them.

Worn or mis-adjusted Breaker-Points,

Bad Condenser,

Damaged Ignition-Coil, ( Magneto-Coil on Motorcycles or Lawn-Mowers ),

Rotten or damaged Spark-Plug-Wire-Insulation,
or Resistor-Spark-Plug-Wires that are older than roughly ~5-years-old.

Distributor-Cap, or Distributor-Rotor, that is not in perfect "like-new" condition, and which is absolutely DRY.

Missing, or damaged, or incorrect Temperature-range, Engine-Coolant-Thermostat.

In rare instances, there could be an issue with the Carburetor.

There is no "device" that will tell You which one of these numerous problems are causing a misfire.

It's always a bad idea to replace just one single part.
REPLACE ALL PARTS listed above, then drive to see if the misfire went away.

I haven't seen a misfire problem since the mid-'80's.

Modern-Engines detect and record even a single misfire by precision-monitoring of the Engine's RPM,
but they will normally go for ~100,000-miles before that happens.
This is mandated by the government.
.
.
.

 

Another form of misfire is due to low compression in the cylinder. Had a friend with a truck that wouldn't pass emissions due to misfiring issues at idle. A compression test showed low compression. Likely worn rings or scored cylinder(s).

 

Usually, low-Compression in a single-Cylinder is caused by a burned-Exhaust-Valve.
This usually results in the affected Cylinder being completely dead at low-Engine-RPMs,
and intermittently firing at higher-RPMs.
AKA = a Boat-Anchor.
.
.
.

 

The truck I mentioned was equipped with a K&N air filter, one that was supposed to be oiled to trap the fine particles. It was never oiled. Hence, the engine ingested a lot of dust. A sure fire way to scour a cylinder wall or two. Or all of them. The misfire was reported by the computer as "Random misfire". In other cases I've seen misfire in cylinder #X. It can't be denied that a burned valve could cause the problem. I only offered my comment about low compression to aid the TS in understanding that detecting a true misfire can be from other sources than just electrical failure to fire.

 

Such an indicator could indicate that spark voltage is present, but not necessarily that the voltage is too low for a good spark, so it could still misfire.
Is that okay?

A simple, old-school technique was to wrap a few turns of wire around the spark plug cable, and connect it to a small neon bulb, which will blink from capacitive coupling of the spark voltage to the wrapped wire.

Yes and thank you for replying. I really just need to know if the coil is getting the "signal" to fire from the crank triggers so I'd like to monitor the primary side. The wire wrap and neon would be on the secondary side but that's still useful once I confirm that the coils are getting the message to fire.

 

I made a monitor moons ago, but it was for POC ignition so had to monitor the POC coil for amps. The assumption was, if there was POC amps than it was assumed a spark happened.

To do this was to inline a precision air resistor (current sense OAR) with the primary voltage line to each POC, then use an op-amp to amplify that diff on the OAR and feed that to a counter IC. The counter ckt would count up, say to 8 for 8cyl, then it would reset back to zero if all 8 were counted, if at the end of counting it was not 8 then I would get a latched on for an LED telling me I just had a miss.

You can use an IC that has 4 or 8 op-amps in it, and just allow them to drive a fet or bjt to drive a LED, but as rpm's go up it's hard to see if a LED does not light up. Another option is just to count each drive pulse and display that count via 4 or 8 displays (one for each cyl), they should all count up the same, if one starts to miss then it's count will dwindle as the others keep counting up.

I like both ideas. Are they easy schematics to find?
If the misfire is steady, then I should be able to see an LED not lighting up. The counter would be great for fleeting misfire diagnostics.

 

One way to tell if a cylinder is not firing is to squirt some water on the hot exhaust pipes. Where the cylinder is not firing water won't boil off quickly as opposed to the cylinders that are firing. They will be much hotter. Especially if you have headers. A factory exhaust manifold might tend to transmit heat from hotter spots to colder spots. But it might work.

 

The neon bulb indicator described in post #2 is valid in sensing the occasional misfire as a function of plug voltage. A neon bulb like an "NE2" across the coil PRIMARY will also respond differently to abnormal conditions. AND while the supply is is only 12 volts or less, the inductive spike can be much higher voltage.
For those who disagree, I suggest grabbing the "points" side ignition coil terminal while the engine is running, while in good contact with the engine "common" side.

 

I may be off base with this but I think the TS wants an indication when a misfire happens. Not have to watch a flashing bulb and pay close attention to when there is a missing flash. It's much easier to get an indication when there's the random occasional misfire as opposed to watching four, six or eight neon lamps firing off 750 times each per minute per cylinder (12 1/2 times per second per cylinder).

Even better would be something that recognizes a missed firing that happens only occasionally and can't be seen by the naked eye (some motorcycle engines rev way up into the teens of 1k rpms) and would indicate that a misfire had happened.

 

Thanks for the replies.
I found this YT video:

That would be cheap and easy to make and install. I can quickly narrow down to which cylinder to focus on.

 

Just today, going thru some ancient archives, I came across the complete circuit for a "percentage of misfire" electronic counter. Terribly complex, really.

 

People do tend towards making things more complex than they need to be,
and it usually winds-up costing ~2 or ~3 times what ordinary standard maintenance would have cost,
or even more if You calculate-in the Labor-Cost.
.
.
.

 

Going back to post #1, any of the misfires would happen while the engine is delivering power, not idling. So suddenly it gets a lot more complicated to even sense. It is not clear that spark plug impedance would be an adequate variable to check, especially for a multi cylinder engine. Probably monitoring a vibration of the cylinder head will be the way to go. So an accelerometer on each head should see a different signature when there is no explosion.

 

Or ........ You could just take out the Passenger-Seat and install one
of those old "Sun" Engine-Analyzers with the built-in TV-Screen-Oscilloscope,
it'll show any and all misfires in real-time.

But, of course, You'll have to have an Inverter big enough to power it.
.
.
.

 

Is it really possible to detect a low-compression misfire due to a valve leakage through the spark plug signal profile?? Or due to an injector failure??

 

The Temperature, and Pressure, in the Cylinder,
change the Voltage required to jump the Spark-Plug-Gap.

It shows-up plain as day on a Sun-Engine-Analyser,
it even shows You what Cylinder-number is having a problem,
even in the higher-RPM-ranges where there's a lot going on on the Display-Screen.
And, it even comes with a Book
telling You how to interpret changes, or various waveforms, that You can observe on the Screen.

Sun has been doing this since the ~'50's,
and plenty of then still work, and are becoming serious Collector's-Items,
going for ~$500 to ~$5000.oo depending on the Model, current-condition, etc..
.
.
.

 

Those SUN instruments are indeed good, BUT neither small nor especially portable. Especially not suitable for use on a motorcycle.
So possibly a fast processor examining the spark current sensed by a clamp-on current pickup is the way to go. The current pickups are often used to trigger timing lights and so they are quite available. at least in some parts of the world.