# Circuit simulation for diagnosticians

Dmitriy Analyst Toronto, Ontario Posted   Latest

Let's face it -- electricity is a puzzling phenomenon. We can compare it to flow of water in a pipe or something like that, but, at the end of the day, such interpretations can only help with the simplest of circuits. Faced with multiple components, electrical engineers write down equations for each component to explain overall circuit behavior. Luckily, computers can help with solving these equations. And, with online circuit simulators gaining traction, circuit analysis can now be used by diagnosticians as well.

Here is an example: a bank of 3 injectors is controlled by the PCM. We measured resistance of the injectors and got values 12, 13, 14 Ohms. We are interested in the maximum current that flows through the PCM injector control when injectors are commanded ON. For that we create the following circuit in a free PartSim​.​com simulator:

We can see that there is 12V dropped across each resistor R1-R3, and the total current is 2.78A. We could have used Ohm's law to get this result, but here we get it at a click of a button. It is still important to know, however, what kind of laws are used to produce this result.

Now, what happens if one of the injectors is partially shorted and measures 5 Ohm? After a quick change we get:

The overall current is higher, but the voltage drop across each resistor R1-R3 is still 12V. There will probably be a misfire in cylinder 1, but cylinders 2 and 3 will be OK. But this holds only if connectors/relays/etc. are perfect (in other words, R4 = R6 = 0 Ohm). Let's see what happens if those resistors are 0.5 Ohm instead:

Oh, so now we have less than 9V dropped across resistors R1-R3, and we might get a multiple cylinder misfire. This was a fun experiment and it did not take long at all!

But, wait, there is more! We can see what happens if we add a current limiter into the circuit. Or we can replace resistors with actual inductors and view injector current ramps like in this screenshot:

Possibilities are endless... or may be not. There is a steep learning curve once you go beyond playing with resistors. Which prompts a few questions.

1. As a diagnostician, do you find such circuit simulation useful for: A. Understanding circuits? B. Exploring potential circuit faults?
2. If yes, is anyone providing such training with the focus on automotive circuits? Is there enough audience for it?
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Christopher Technician
San Antonio, Texas
Christopher Default

I'd be surprised if anyone could come up with a reason why an interactive circuit simulator isn't useful in understanding specific circuits (or circuitry in general), as being able to 'swap' components, alter properties or introduce faults while measuring changes instantly at any or all points in a circuit simultaneously is a much more time efficient way of learning electronics. MIT, Georgia

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Chris Diagnostician
Lansdale, Pennsylvania
Chris Default

Chris, There are engine management simulators that are at the level of OEMs, they are used for development/prototyping. One I had the chance to play with through a friend is PCI-Engine for Matlab (incredibly expensive from what I was told, and steep learning curve). Quick google search reveals alot of results, though price is always a factor. The white papers available are also fascinating as

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Christopher Technician
San Antonio, Texas
Christopher Default

Chris, Thank you for that link - I'll definitely be sharing that with some of our techs. It's a bit buggy on mobile devices, but still fun to experiment with.

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Dmitriy Analyst
Toronto, Ontario
Dmitriy Default

I guess I'll play a devil's advocate here: it can be argued that circuit components are too ideal and do not represent "real" components. So a simplistic approach of scattering some components around and connecting them with wires often leads to paradoxical or unrealistic results. For example, Bob P. in a post below mentioned lightbulbs, which are not well described by constant resistors. And to

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Chris Diagnostician
Lansdale, Pennsylvania
Chris Default

Dmitriy, I definitely think there is a place for exactly that kind of education, this forum seems to be a first step towards it. I know I've been looking for just those kinds of classes. As far as the complexity of simulations, I agree. I'm leaning towards this being a great use of ANN (artificial neural networks), with a large training sample of wave forms and actual failure modes…

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Bob Owner/Technician
Bob Default

Dmitriy, That is awesome. The issue of current flow in parallel circuits came up about 6 months ago here and on another forum. This post answers some of those questions. I need to take some time to digest this. In the meantime, here are those other discussion threads for context. members​.​iatn​.​net/forums/search… diag​.​net/msg/m2r5akvesh…

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Bob Owner/Technician
Bob Default

I think one of the take-aways here is that it's one thing to simulate the behavior of current and voltage in simple series/parallel circuits but it's far more complicated to simulate a real circuit made up of many different components, wires, connections and electronics. An example would be trying to simulate a light bulb circuit using a simple resistor. The real world measurement of current

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Dmitriy Analyst
Toronto, Ontario
Dmitriy Default

Hi Bob, can't look up the IATN discussion, but I see what is going on in the DiagNet one. I agree that lightbulbs are no usual resistors, so there will be some fun effects. To me, lightbulbs are a kind of high-power PTC resistors. I will look up whether the simulation engine supports them.

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Bob Owner/Technician
Bob Default

The discussion on iatn had to do with a phenomena that some techs have experienced with bank fire injection systems. Some have experienced a multiple cylinder misfires caused by a shorted injector, where the cylinder with the shorted injector is actually firing but the remaining cylinders aren't. The theory being that the shorted injected robbed the other injectors on the same bank of the

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default

Hi Bob: Electricity is a zero-sum game. In the animal kingdom, which child eats first, the quiet one or the loud one? You'll see a similar result in a truck with multiple batteries. The bad battery (hog) can draw down the good ones, making it appear that they are bad. HTH, Guido

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Dmitriy Analyst
Toronto, Ontario
Dmitriy Default

Bob, I have limited experience with failed windings, but one way windings in motors get shorted is like in the middle of this picture: diag​.​net/file/f794vuru3… ​ The whole thing becomes a shorted-out transformer and the motor stops functioning completely. On the other hand, if parts of the winding melt together without forming loops (like in the picture on the right), the rest

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Bob Owner/Technician
Bob Default

Dmitiry, I like your explanation. That's what I was getting at but you said it better. Thanks

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Chris Diagnostician
Lansdale, Pennsylvania
Chris Default

Dmitriy, This is something I've thought about alot, and thank you for the post. When I was in the Caterpillar program at Pennsylvania College of Technology, we had a couple great electronics/electrical instructors. Part of training was being given fault codes or failure modes, and armed only with a schematic prove where your fault lay, then go to the machine and do your testing (10 minutes with

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Anthony Technical Support Specialist
Kirkwood, Pennsylvania
Anthony Default

Hi Dmitriy: Interesting website. I'm kind of a curious critter. I decided to follow some links to see who these people are. Turns out the parent company has this kind of stuff. aspencore​.​com/media/#ac_subm… I selected the electronics section to take a peek. While I only spent a minute in there, it looks interesting. electronics-tutorials​.​ws/ Thanks, Guido

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Dmitriy Analyst
Toronto, Ontario
Dmitriy Default

Oh, wow, I was not that inquisitive! I sometimes land at electronics-tutorials​.​ws when searching for circuit info, but I did not know it's related to PartSim.

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