Are automotive O-scopes just regular O-scopes? - Page 1
Forum section: electronics / beginners

author: SkipMorrow
author_rank: Contributor
timestamp: August 17, 2023, 06:02:08 pm
content: I've got a rigol DSO that I don't get to use as often as I would like (I'm just a hobbyist). I also recently bought an older car that has some issues, namely misfires. I have gone down the usual troubleshooting paths (plugs, coils, injectors, etc) and none of them solved the problem. Thinking it might be something electrical I started watching some YouTube videos and I saw them using different automotive osciloscopes. I immediately started wondering whether or not I could use my scope. The only probes I have are "normal/regular" O-scope probes. I don't have any clamp probes, and those certainly seem useful. Are there any cheap ones the community here can recommend? What else might be useful when it comes to using regular o-scopes for automotive troubleshooting?

author: CaptDon
author_rank: Super Contributor
timestamp: August 18, 2023, 01:31:04 pm
content: In most cases they have special features like triggering modes and the ability to not be damaged by 50KV spark voltage on some inputs. Measuring both spark voltage and spark current can provide useful information beyond what either alone can provide so the scope may have both a spark current clamp probe input and a specialized adapter for spark voltage. A conventional scope can be used with proper caution but you also need to know what normal and abnormal traces would look like. As for your misfire let me relate two weird ones I ran into. 70 Plymouth Road Runner 383 magnum engine, ran rough as hell off idle up to mid-throttle but was a solid screamer at full throttle. Problem was bad plug wires and I have to assume some sort of leakage current path? We measured the 'bad' wires and couldn't find any real defect. They were resistance core and all measured around 8,000 ohms end to end. 89 Dodge full size V8 van, misfires like hell and OBD light comes on when running below a half full fuel tank. Problem was a bad in-tank fuel pump. The replacement pump failed again 6 months later!!! One other comes to mind, 74 Chevy K5 blazer with a '68 vintage 327 300HP dropped in engine. Ran great but backfired through the carb and exhaust violently above 4000 rpm. Standard single point kettering ignition, problem...bad condensor!!! Some MOPAR's that had a condensor on the battery side of the coil would just simply 'act weird' at any rpm and removing the battery side condensor would solve the issue....perhaps internal arcing from spike voltage / current inside the condensor?The old 'SunScope' coupled to a Sun Distributor Analyzer allowed for a lot of 'Wins' at the race track!!!

author: CaptDon
author_rank: Super Contributor
timestamp: August 18, 2023, 01:42:28 pm
content: As for your misfires, don't forget vacuum leaks and if it is carburetted a vacuum leak around a worn throttle shaft. Vacuum leak misfires are most noticed at idle and low speed operation. If you spray some WD-40 at a suspected vacuum leak and see an rpm increase you may have solved a mystery but BE CAREFUL....I have seen WD-40 burst into a flash fire if it hits the exhaust manifold!!

author: G-son
author_rank: Regular Contributor
timestamp: August 20, 2023, 10:05:23 am
content: Automotive scopes can have functions dedicated for automotive use (say, an automatic battery condition test for example) and can often survive higher voltages on the inputs.The dedicated functions may be helpful, but if you know what to look for to tell a good battery from a bad the automatic test is just a convenience (and good for a quick printout to show your customer he needs a new battery). Same for a relative compression test, you can see for yourself that the starter current draw (or the resulting battery voltage drop) is far lower for one cylinder so it will be obvious one cylinder is down on compression, you don't need any guided tests or fancy displays giving you percentages, those mostly help you when you want to print it out to show someone else how f***ed their engine is.As for the voltages, using attenuators you can reduce just about any voltage to something any scope can handle. You just need to know when to do it, as you can burn the scope otherwise. When in doubt, attenuate. Ignition secondary voltages can EASILY damage any scope, so that may not be the first thing you go for - especially as it's harder to get to it on modern cars with coil on plug.So, in short, basically any DSO can be used for automotive use, a signal is just a signal, but you want to know a bit more about how to interpret what you see instead of when you use guided tests on an automotive scope where you can basically paint by numbers in some cases. So in a way you may be better off, you need to learn exactly what you are doing and understand the results you get, not just plug things in and read good/bad off the screen.