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| Improvised oscilloscope probe for Automotive ignition analysis. |
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| Jim-0000:
A stock car racing buddy has a miss firing engine in his race car. We would like to put an oscilloscope on the high tension leads. At present, the ignition system is the standard, pre-EFI, contact points and capacity, Kettering System. It did have (and will have again) electronic ignition, but this has been replaced with the above in the fault elimination sequence that has been done to try and locate the fault. A lot of stuff on the engine has been swapped and replaced in the search for the miss. The car miss fires most under load; classic HT breakdown behavior. In the attached document from "Electronics Today" 1977, an improvised high ratio probe is described using a common peg and some aluminium foil. This then, allegedly, enables a more common low max voltage oscilloscope to be used with safety. My questions: Is this a safe way to protect the oscilloscope? If so, should it be tested somehow before plugging it into a high tension lead? Or is the probe simply wrapped around the outside of the HT ignition lead? It is hard to tell how this probe is made. If someone reading knows, please advise? I do hope this is the best section of the forum to post this. If not, please advise and re-direct me. Jim. |
| soldar:
The sensing is done capacitively by wrapping a bit of metal around the HV cable. Those ignition systems are fairly straightforward. Things to check are that points and capacitor are good, coil is good (they do fail), coli polarity is connected correctly, HT cables are good and spark plugs are good. All are easy to test and with all things being as they should I can't think of anything else which could go wrong. I guess the distributor but that is less likely or it would be obvious. All cylinders have the same problem? I do not understand something. The electronic ignition has been replaced by the older Delco? How are you going to diagnose the electronic with the Delco? And both are misfiring in the same way? You are not using the same coil are you? Again, try replacing the coil and make sure of the polarity. |
| Microcheap:
On Aliexpress you can find this capacitive probe used to test the secondary of the ignition as is described in the article you linked: https://www.aliexpress.com/item/Hantek-HT25-Automotive-Oscilloscope-Probe-HT25-2-5-meters-Ignition-Capacitive-decay-of-up-to-10000/32822817192.html there is another type of probe but I've never tested one http://www.hantek.com/en/ProductDetail_15_11165.html |
| Jim-0000:
--- Quote from: soldar on January 03, 2019, 10:33:13 pm ---The sensing is done capacitively by wrapping a bit of metal around the HV cable. .. --- End quote --- Is the normal probe just connected to the metal film? --- Quote ---Those ignition systems are fairly straightforward. Things to check are that points and capacitor are good, coil is good (they do fail), coli polarity is connected correctly, HT cables are good and spark plugs are good. All are easy to test and with all things being as they should I can't think of anything else which could go wrong. I guess the distributor but that is less likely or it would be obvious. All cylinders have the same problem? I do not understand something. The electronic ignition has been replaced by the older Delco? How are you going to diagnose the electronic with the Delco? And both are misfiring in the same way? You are not using the same coil are you? Again, try replacing the coil and make sure of the polarity. --- End quote --- The two brothers who are working on it are both career mechanics. They have changed all the above parts more than once, and some, like the spark plugs more times than they can count. They have fabricated a new inlet manifold and fitted a different carburettor. They even changed the camshaft. For a while, the cylinders were behaving differently, with #6 cylinder missing more than others. Then they turned the carb around 180 degrees and the cylinders now (appear) to behave similarly. Coils changed, new capacitor and points fitted to the present Kettering ignition system. etc. etc. This might give you a bit of an idea into the recent history of this Holden Grey motor. Much modified I should say. The engine missfires with both ignition systems in the same way. I have suggested the ignition analysis. And, also, running the engine in the dark of night. Observe for sparks etc. However, it is possible to hold all ignition leads while the engine is running, suggesting all insulation is sound. |
| Jim-0000:
--- Quote from: Microcheap on January 03, 2019, 10:49:05 pm ---On Aliexpress you can find this capacitive probe used to test the secondary of the ignition as is described in the article you linked: https://www.aliexpress.com/item/Hantek-HT25-Automotive-Oscilloscope-Probe-HT25-2-5-meters-Ignition-Capacitive-decay-of-up-to-10000/32822817192.html there is another type of probe but I've never tested one http://www.hantek.com/en/ProductDetail_15_11165.html --- End quote --- Thanks for that link. I will follow it up. |
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