Electronics > Projects, Designs, and Technical Stuff
Improvised oscilloscope probe for Automotive ignition analysis.
floobydust:
I've had similar weak spark problems on my old musclecar and none were in the textbook.
You can look at the HT waveforms but you've replaced everything there, so the problem is likely elsewhere.
Some things to check:
1. It was a defective manufacturing batch of Champion spark plugs, the internal resistors were pooched/missing and the arc was inside the porcelain making up for that.
2. Problem was the tachometer, its input circuit was loading the coil primary.
3. The ballast resistor - it was nichrome wire in the harness, partially broken so a high ohms value.
4. A set of points with a weak spring, the contacts bounced at anything much above idle RPM's.
5. I would look at dwell and see if the distributor is stable - one had loose bearings and the shaft wobble changed dwell at speed and it was very hard to track down.
I would check the 12V feed to the ignition system is not sagging, and you are (now) using a ballast resistor/condenser with points?... so as to not damage the coil. Transistorized ignition does not need the ballast resistor, and move the condenser to the 12V power input.
Most digital scopes can't handle automotive ignition systems. The EMI will make the scope add false signal spikes and noise to the displayed waveform. So be prepared to go on a goose chase looking for a "blip" on the waveform that actually isn't there.
Jim-0000:
--- Quote from: floobydust on January 04, 2019, 03:42:59 am ---I've had similar weak spark problems on my old musclecar and none were in the textbook.
You can look at the HT waveforms but you've replaced everything there, so the problem is likely elsewhere.............................................................................
4. A set of points with a weak spring, the contacts bounced at anything much above idle RPM's.
5. I would look at dwell and see if the distributor is stable - one had loose bearings and the shaft wobble changed dwell at speed and it was very hard to track down............
--- End quote ---
Point #4 is not likely, as they are new points.
But............
Point #5 might be the problem! They are using an old Holden grey motor distributor. (however, come to think of it, they did have a Falcon distributor in it with the electronic hall sensor type ignition.........same problem exhibited).
--- Quote ---I would check the 12V feed to the ignition system is not sagging, ......
--- End quote ---
That is what one of the mechanic (brothers) is going to check first opportunity on Sunday. He will power the ignition system only with a separate battery and cables. The car battery is in the back of this Stock car, so there is a long wiring loom. Potential there for voltage drop as you mention and/or one or two poor connections. Also, there is a "kill" safety switch on the dashboard!
--- Quote ---and you are (now) using a ballast resistor/condenser with points?... ...............................................
--- End quote ---
No ballast resistor during the last iteration of the ignition system. In other words, the last time we worked on it. Might have had one previously.
Thanks for the detailed reply.
Jim.
joeqsmith:
Coax is RG400/U. I have a trimmer cap up near the clamp to trim the voltage level. The large braid goes to the block, the clamp goes around the plug wire. As mentioned, this forms a capacitive divider. I feed this into a 50 ohm termination (built into the scope). I test the ignition parts off the engine.
tautech:
Things to double check:
Battery to chassis and engine block grounds.
Coil ground.
Points plate ground.
Broken cable conductors, typically old cabling where bending forces have broken the internal strands.
Cracked dizzy cap or rotor.
Dicky carbon and/or spring in the dizzy cap.
vk6zgo:
--- Quote from: tautech on January 04, 2019, 07:09:32 am ---Things to double check:
Battery to chassis and engine block grounds.
Coil ground.
Points plate ground.
Broken cable conductors, typically old cabling where bending forces have broken the internal strands.
Cracked dizzy cap or rotor.
Dicky carbon and/or spring in the dizzy cap.
--- End quote ---
All of the above!
Plus, if you are using suppressor leads, check that they are around 15 to 20 k resistance.
Back in the day, when the carbon core looked pretty much like burnt string, it was not unusual to find one' or more leads had drifted up to 5 megohms or so.
Sparkplugs are not all equal, even when they have the same part number & brand.
I fitted a set of German made Bosch spark plugs to my HQ Holden 6 back in the day----should have been good, eh?
Nope, the insulators cooked up, went brown, & started leaking badly.
The same number but made by Bosch in Australia were noticeably different, in that the insulation was thicker where it went into the metal body of the plug.
It turned out that there were two plugs using that part number, one which was designed for aluminium heads, & the other for cast iron ones, with their worse heat dissipation.
The "aluminium head" ones were never meant to be sold in Oz.
Holdens fitted AC Delco Spark Plugs as standard, & crappy things they were, too!
My HQ had just run over the time for the last warranty service, so I fitted (Oz made) Bosch plugs, thinking that as I would have to pay for spark plugs anyway, they might as well be the ones I like.
I ran into an acquaintance who worked at the Holden dealer, who said "You are only a "bee's whisker" over, bring it in!"
Well, I thought I had a win, till the next week I was travelling down to a country town about 200km away from home, when suddenly, there was an almighty "bang" followed by "chuffing" noises.
The entire insulator & "guts" of one spark plug had been blown out, never to be seen again
On investigation, I found the dealer had removed all my beaut new Bosch plugs , fitting AC "crappo" ones instead!
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version