Key-on Circuit Onboard Diagnostics

Andrew Technician Commack, New York Posted   Latest  

Sometimes a code will clear and not return until the car is driven - the component is only tested while it is doing its job. I think this is typical of actuators. Other times a code will reset at key on immediately, which makes sense for a sensor whose value can be rationalized the moment the circuit is powered up.

And yet I have seen in comparatively rare cases an actuator circuit is pinged real quick at key on to check for an open or short or generally incorrect resistance. You can only see this with a scope as it occurs in milliseconds.

I am wondering if anyone else has seen this before?

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James Educator
Olathe, KS, Kansas
James
 

Yes sir on a BMW X5, clear codes and nothing. Drive and several K-CAN codes show up as history after a road test.

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Caleb Technician
Mishawaka, Indiana
Caleb
 

Guess this is an open ended question?? Most airbag sensor circuits I've ever scoped do a quick few millisecond self test on the circuit at initial key on. And this may or may not apply but I was scoping the vent an purge valve control circuit on a 07 Hyundia (for other reasons). Anyway the PCM was always pulsing the purge an vent circuits when scan data showed purge 0% an vent off.

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Dave Mobile Technician
Ironton, Ohio
Dave
 

Transmission solenoid circuits on many vehicles will self test at key on. On Chrysler vehicles it will test all solenoids in milliseconds and does require a scope to see it happen.

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Brian Owner
Parma, Ohio
Brian
 

Here is a Chysler OD ckt that I was testing to verify repair.diag​.​net/file/fvoojvjgx… 08 avenger 4 speed ( old school LOL) note that KOEO it would pulse the ckt for all trans (all solenoids are tested in a staggered pattern. ) every 20 seconds, KOER it was every 10 seconds just as the description in SI. This is the testing of the OD and another solenoid (I think…

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Dave Mobile Technician
Ironton, Ohio
Dave
 

With a current probe on the common feed you will see all of the solenoid cycled. BTW I enjoy your videos.

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