Should I float a scope on a car - Page 1
Forum section: electronics / beginners

author: Seamus16
author_rank: Contributor
timestamp: August 03, 2018, 05:41:21 pm
content: This is a very basic question. I have had a search but cannot find anything relevant.I want to check the K-Line on the OBD2 connector on my car since none of the plugs I am using can make connection.I want to put an analogue scope on it to check if it is working. I have been arguing with myself and now think I should run the scope off an isolation transformer.Can anyone confirm pleaseRegardsDavid

author: JS
author_rank: Frequent Contributor
timestamp: August 03, 2018, 05:53:59 pm
content: The car is floating by itself, while I don't know the best practice you could ground the car to what you want. I have to do a similar thing to a boat but it has a dedicated connection to the water so it gets a bit trickier...Also, breaking the ground connection of the scope should leave the thing floating, this is widely used but not recommended. The best way is to have a differential probe, in some cases you want a isolated probe, like low signals in high level common modes.At the end of the day you will probably want a pair of a bunch of different probes so you end with more money in probes than scopes...JS

author: RoGeorge
author_rank: Super Contributor
timestamp: August 03, 2018, 06:02:38 pm
content: Why floating the scope?Isn't the car at a floating potential already?Are there any other power grid powered devices (like the oscilloscope is power grid powered) that are connected to the car?If either the oscilloscope or the DUT is floating, then there should be no problems.

author: Seamus16
author_rank: Contributor
timestamp: August 03, 2018, 06:30:21 pm
content: Thanks to you all.I thought it was floating but when you attach the scope to car earth I thought it wouldn't hurt to ask.I don't really need any data of it but just want to know that there is a signal there. The voltage does not tie in with what I was led to expect.

author: Benta
author_rank: Super Contributor
timestamp: August 03, 2018, 09:44:39 pm
content: NEVER float test equipment. Float the device under test (DUT). And your car is already floated.

author: Ian.M
author_rank: Super Contributor
timestamp: August 03, 2018, 10:05:12 pm
content: As long as the car is sitting on its own wheels on dry tires, and no mains powered equipment except the scope is connected to it, it should be fine. If in doubt, test the voltage between where you want to put the scope ground clip and the clip itself with a small torch bulb. *DON'T* put the ground clip on anything except a ground or 0V connection.Caution: If you've got the car on a lift by its jacking points, its body is probably grounded through the stand, and as there may be a potential difference between the engine and body grounds due to current in the strap linking them, it would *NOT* be a good idea to connect a scope probe ground directly to any 0V or ground of the ECU or other onboard electronics.

author: cyclin_al
author_rank: Frequent Contributor
timestamp: August 04, 2018, 03:27:28 am
content: I have been planning on connecting a scope to a car also. I just received a DS1054Z a couple of weeks ago; it has been 20 years since I used a scope. My intention is to reverse-engineer the ISO9141 on the K-Line to interface with a microcontroller. I have other posts asking about K-Line and the various protocols.Ian.M makes some points about grounding that I had not thought about. I am going to take his advice and have the car sitting on its own wheels, due to the consideration coming next.However, what reference are you measuring against on the car? If you look at the OBD connector pinout, there is actually a pin for signal ground. That would be the pin to use for the scope ground clip. Then all your scope measurements are made relative to this potential. There is also a body ground pin on the OBD connector, but my research indicates not to use this pin as it is likely to be quite noisy.In theory, you can check with a DMM if there is a signal there. From what I have read, K-Line is either at 0V (relative to that signal ground pin) or at 12V. Therefore a DMM should measure something between 0 and 12 V. I am not sure exactly what since the signal is digital and is not DC nor a known sine wave. Be sure to check the L-Line as well if you will be using this pin (I know it is used for ISO9141, but not sure if it is used for KWP2000 or KWP fast init).Note that this is going to be different from CANbus where the difference between the CAN-Hi and CAN-Lo pins will be 5V.

author: JS
author_rank: Frequent Contributor
timestamp: August 04, 2018, 03:33:05 am
content: Quote from: Benta on August 03, 2018, 09:44:39 pmHard to do so in a boat, which while it's floaring on water it has an electrode tp the water as reference... The best option might be to reference the scope to the boats ground but floar it from the mains reference from ground.JS

author: cs.dk
author_rank: Unknown
timestamp: August 04, 2018, 05:59:06 am
content: Quote from: Seamus16 on August 03, 2018, 05:41:21 pmDo you have +12V on pin 16 in the connector? Missing supply voltage is a fault i've seen often.

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2018, 06:56:30 am
content: Never float a scope, even if you "know" what you are doing .You might kill (or worse, seriously injure) yourself; that's your privilege and it is merely evolution in action.But you might kill (or worse, seriously injure) somebody else, and that's unforgivable.You should use the correct type of probe and/or the correct types of scope. Example fatality fromhttps://groups.io/g/TekScopes/topic/7632418#10795with myemphasis:Quote

author: Ian.M
author_rank: Super Contributor
timestamp: August 04, 2018, 07:17:23 am
content: There's a considerable difference between floating a scope to break a ground loop that would otherwise cause a large circulating current through the probe ground, but with only a volt or two of difference between the ground points in question, and floating a scope to attempt to make a differential measurement with a probe ground clip on a non-ground circuit node. The latter is *ALWAYS* unsafe and if mains or high voltages are involved, is potentially lethal.However using an isolation transformer to power the scope with output Neutral strapped to output Ground, and an adequately sized grounding connector bonding its output ground to the engine block, steel or aluminum hull or sacrificial anode on a boat is just about the only way to avoid the risk of high circulating currents in the ground if the boat doesn't have a permanent shorepower installation with properly bonded ground. Use a RCD on the output of the isolating transformer, connected *AFTER* the Neutral to Ground strap.N.B. the isolating transformer output is *NOT* floating, as it has been re-referenced to ground. Its Line terminal is 'hot' and although its Neutral is grounded, from a safety viewpoint it should also be handled as if it was 'hot'.

author: analogo
author_rank: Regular Contributor
timestamp: August 04, 2018, 08:03:49 am
content: Quote from: Benta on August 03, 2018, 09:44:39 pmI understand the general reasoning, but what about handheld testing equipment that is by its nature is floating?

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2018, 08:22:18 am
content: Quote from: Ian.M on August 04, 2018, 07:17:23 amThe latter is true.The former is trueunlessthe UUT is faulty (which might be the reason you are probing it!), or it is left floating and the next person doesn't notice it.......

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2018, 08:27:01 am
content: Quote from: analogo on August 04, 2018, 08:03:49 amIf it isdesignedto measure high voltages relative to earth, then OK. But if it has, say, a metal case to reduce EMI/RFI then that could be at an "interesting/entertaining" voltage.

author: Seamus16
author_rank: Contributor
timestamp: August 04, 2018, 10:12:28 am
content: Thanks to all who have replied and the warnings are well noted.I suppose to satisfy yourself that you are in a safe environment would be to connect a multimeter between the scope chassis and the car earth.

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2018, 10:33:38 am
content: Quote from: Seamus16 on August 04, 2018, 10:12:28 amUntil something happensafteryou've measured that voltage. The circuit changes, or misplaced probe, etc etc.These problems aren't new, and companies have developed products to address the issuessafely. Use the proper type of probe for the problem; see some references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/There are, I believe, scopes specifically designed for automotive market, but I know nothing about them.

author: Brutte
author_rank: Frequent Contributor
timestamp: August 04, 2018, 10:36:20 am
content: I did the single ended measurement on my car yesterday.The scope was properly earth grounded and the car was floating on its tyres. Then I connected the probe's shield to +12V battery connection making a car chasis float at around -12V below ground.No issues, measurement done (starting current). No smoke escaped.Although I must admit that I stripped all other shielded crocodile clips from other probes because in case one of those touched the chasis then most likely

author: Seamus16
author_rank: Contributor
timestamp: August 04, 2018, 12:16:47 pm
content: Quote from: Brutte on August 04, 2018, 10:36:20 amI think a point should be added that both feet should be inside the car. I have habit of sitting in it with my feet on the ground.

author: Seamus16
author_rank: Contributor
timestamp: August 04, 2018, 12:23:48 pm
content: Quote from: tggzzz on August 04, 2018, 10:33:38 amThat is correct and have several interfaces but none of them are managing to connect, hence I want to check the K Line to see if there is a signal on the line, but it is only giving a basic voltage of 10v which I think implies there is a problem existing somewhere. Thanks anyway I can now forget about any 240v issues and continue scratching my headhttps://www.eevblog.com/forum/Smileys/default/smiley.gif

author: Brutte
author_rank: Frequent Contributor
timestamp: August 06, 2018, 06:53:06 pm
content: Quote from: Seamus16 on August 04, 2018, 12:16:47 pmWhy is that so? The chasis is just -12V (below earth gnd) so no significant currents expected to flow even when you keep one leg outside of the car.It is like touching a battery ( + ) terminal while standing on the ground. It is safe and as probe shield has very low impedance - that touching won't jam the measurements.Although making differential measurement is safer..The starter is wired through copper wire directly to battery ( + ) so it is easy to measure current that way by clipping scope on both ends of that wire. Such measurement is impossible with negative wire as there are several paths of negative current.I would not recommend sensing K-line with serial2USB adapter tied to laptop that is powered from cigarette lighter socket. Been there, done that - it won't work. The laptop's GND jumps like crazy w.r.t. car's GND as laptop draws significant currents.

author: JPortici
author_rank: Super Contributor
timestamp: August 07, 2018, 11:12:36 am
content: All scopes for automotive i've seen were regular scopes, maybe with a different software package.If you can't connect with a k-line analyzer tool maybe your car doesn't have K-Line.Which car is it? (Model, year)Anyway, i suggest you buy an OBD extension cord. Many cars have the port in inconvenient places and it's always easier to work with an extension cord.And maybe search ebay or other sites for OBD male plugs so you can solder wires to make probing easier. All signals there are low speed anyway so don't be too worried about ringing. It won't happen.Re: floating the scope, others have already given very good answers. I just want to add that i love my picoscope / tek TPS for this situations, which is why these are my two main scopes at work. At home i use my bench scopes to measure stuff inside the car with no issues whatsoever. Car is floating.

author: Seamus16
author_rank: Contributor
timestamp: August 07, 2018, 03:10:23 pm
content: Quote from: JPortici on August 07, 2018, 11:12:36 amThanks for all the replies. The points are well notedQuoteIt is a Fiat Seicento 2002. It does have the OBD2 socket fitted. I have tried several interfaces from Ebay and they have not worked. I am waiting for further one to arrive. I am running MultiEcuScan s/w.The reason for wanting to check the socket is that over the winter mice chewed through the loom at the bottom of the A post. This seemed to only effect the power steering (now repaired). So I want to be sure that all outlets are working correctly which they seem to be. The K line is the only one I am not sure what voltage it should be giving (1 to 10v ign off/on).

author: JS
author_rank: Frequent Contributor
timestamp: August 09, 2018, 02:12:02 am
content: You should feed your mice better, mine chewed all my main extensions, live and neutral and still alive... poor bunny, I don't know how it survived...JS