Electrical mystery? Spark tester won't work

JDEM

Well-known Member
I am kind of stumped. Note I worked for many years as a mechanic and as an electrician. I don't say that to pretend I know everything. I mention it to indicate I am not totally clueless.

I am teaching a home-school group soon an intro course on engine mechanics (mostly small engines). 10 kids. I wanted to get some cheap diagnostic tools for each kid. $9 for each student and stuff direct from China $9 buys a digital multimeter and a spark-tester. The multimeters work fine. I tested them all with various voltage ad resistance settings. The spark-testers do not work and it is hard to believe. Not much there to go wrong. I have never owned a factory-made tester before. I always just made one from a piece of copper-core plug wire and an old spark plug with the ground-electrode removed. Those have always worked consistently well. With 10 kids coming, I figured it was easier and cheaper to just get these things from China. $3.50 each to my door. I have a 1936 outboard motor here I tried a tester out on. No spark to see. Tried my home-made tester and get a bright blue spark. Note the Chinese tester is adjusted with the same gap to jump. I then went to Autozone and spent $8 on one of their el-cheapo testers that looks much like these Chinese testers. It also shows a bright blue spark. How can this be? Note the Chinese tester has a long wire-lead instead of a ground-clamp. So I removed that wire and clamped the tester directly to engine-ground and it still does not work. I also checked resistance and continuity on all relevant parts and all checks fine. ??? I am almost to the point I am just going to buy 10 new D16 spark-plugs and just make 10 testers. But this seems to defy reason. Note I have also tested in a dark room and see no spark escaping anywhere. The Chinese testers DO work on a high-energy ignition source but not on most of the small engines I have with flywheel magnetos or self-contained magnetos. Battery-coil ignition - yes. I posted a photo of the Chinese tester.
a246433.jpg
 
I have an el jipo spark tester laying around at home somewhere. It is a clear plastic style. I think I tried it once on a small engine and had no luck with it. Went back to the old way of holding wire near cyl. head bolt.
Maybe it has the same defect as yours on small engines. If you figure it out let us know.
 
Just a couple thoughts: Did you check the resistance of the brass rod from end-to-end? Is the black threaded rod metal or some kind of hi-resistance carbon-impregnated plastic?
 
Hello JDEM,

Many ways to make a spark tester. I made three. I Think the problem is either in the jumper wire or the screw. Is it possible that the screw has a coated finish that is insulating the spark? Check the jumper on both ends. Your ground on the tester is the screw end, That part is the one you ground it. If that is what you did I would check the continuity between the adjusting screw and the tip. The spark plug end looks good, but at this point you can check that as well for continuity.

Guido.
a246443.jpg
 
Yes checked resistance on all metal parts. Black part is some sort of plastic.
 
It would be interesting to connect a "megger" across it and see what the insulation resistance is.
 
I'll bet the problem is the high resistance of the black threaded rod. Good enough to work as you said with high voltage modern auto ignitions but not on small engines. Replace that rod with a metal one & you'll be in business.
 
I bought DIS test set years ago and it came with the identical looking spark tester.
The first time I tried it I had similar problems.
Cut the wire off where it connects to the tester, strip it back and solder it onto the brass by the thumbwheel.
Has worked fine ever since.
 
The black screw checked out with low resistance, but I wire-brushed it until it was shiny and it still did not work.
 
Hello JDEM,

Put one lead of your ohm meter on the clamp of the tested, then check for continuity at the eyelet side. They may have crimped the eyelet right over the wire insulation. You should read zero ohms or very little resistance. With the wire tight you should have continuity through the screw and the tip. I m betting one of those points is the problem,

Guido.
 
assuming the wire path is good and the screw gap is a connected conductor in the test circuit



resistance is too LOW...

when a coil discharges, the lower the resistance, the lower the voltage... ((resistor carbon wires and resistor plugs actually raise the voltage to a higher level).

When a capacitor discharges, the opposite is true, the lower the resistance, the higher the voltage.
 
Guido,
I've used things like you invented. Sometimes when working on mowers, I'll just remove spark plug and lay it against the engine.

Sometimes I discovered a spark plug may work just fine for a few minutes and then either it quits completely or engine sputters. Then the only real good test is substitution. Install a new plug and problem goes away. I keep my used plugs that still work so I can grab one for substitution purposes.

Have a happy new year.
geo
 
Hello Geo-TH, In,

Yep! Pressure makes the difference. If you look real close one of the plugs is marked test. Not my first rodeo,


Guido.
 
(quoted from post at 17:42:13 12/28/16) Guido,
I've used things like you invented. Sometimes when working on mowers, I'll just remove spark plug and lay it against the engine.

Sometimes I discovered a spark plug may work just fine for a few minutes and then either it quits completely or engine sputters. Then the only real good test is substitution. Install a new plug and problem goes away. I keep my used plugs that still work so I can grab one for substitution purposes.

Have a happy new year.
geo

Guido pix is deceiving to the un-trained eye.

Lets answer a question its the number one issue you need to resolve.

"if your plug you just pulled out sparks is the spark good/healthy"
 
(quoted from post at 08:56:04 12/29/16)
(quoted from post at 17:42:13 12/28/16) Guido,
I've used things like you invented. Sometimes when working on mowers, I'll just remove spark plug and lay it against the engine.

Sometimes I discovered a spark plug may work just fine for a few minutes and then either it quits completely or engine sputters. Then the only real good test is substitution. Install a new plug and problem goes away. I keep my used plugs that still work so I can grab one for substitution purposes.

Have a happy new year.
geo


I've been told that sometimes a spark plug may produce a spark lay out where you can watch/see it but not work because of the pressure of the cylinder.

Dusty
Guido pix is deceiving to the un-trained eye.

Lets answer a question its the number one issue you need to resolve.

"if your plug you just pulled out sparks is the spark good/healthy"
 

We sell tractor parts! We have the parts you need to repair your tractor - the right parts. Our low prices and years of research make us your best choice when you need parts. Shop Online Today.

Back
Top