Hyundai Excel Ambiguous Codes Read

Some thoughts: Your one second pulses on the scope -- are you counting these when the signal is high (12v)?

Is there any difference in timespans of any of the low signals (0v)?

Are these rises and falls sharp like square waves, i.e. transistor switched?

Another possibility is that a continuous code one is simply a 'system pass' code. You didn't mention whether the MIL is on or not.

Keep in mind that a (long) one flash followed by a (short) one flash is code eleven, O2 sensor. A dead O2 sensor would be a likely cause of the dark exaust smoke you are seeing. Scope the O2 sensor wire with a backprobe in the connector, engine hot, just for giggles.

Before replying, I would suggest you try probing the '8' and 'E' terminals. That's what my info shows for the Excel. All other models show the '8' and '7' terminals as correct. It won't hurt or take much time to try. If my info source is wrong, it wouldn't surprise me, especially for makes such as Hyundai.

BTW, if I were in your situation (without an analog VOM) I would grab a low current LED testlight first, rather than breaking out the scope just to watch code flashes.

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Thanks for the quick reply. I thought I'd reply and let readers and you know that I plan to test all your suggestions within the coming week. The vehicle has two O2 sensors and I'll test them both?

RR

On a '91 Hyundai Excel? That doesn't sound right... What emissions package does it have (Federal, Canada, California, etc...)?

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Sorry for the delay in replying.

I was only going from the Haynes manual (which I may have misunderstood) I had only found one at the time of writing (front of the engine on the exhaust manifold) and was going to find the other later. The setup is Canada. Thanks again. Tests this weekend.

I am hesitating to use a led for reading the flashes for two reasons;

- I can measure the signal more precisely with a scope or Labjack D/A converter sampling every 0.05 s

- I think 10 V would overpower the LEDS I have handy

Would a led make a difference?

I analysed the signal from both pins 7 and E and it appears the same, a continuous train of square waves with an (-10V?) x 300 mSec average pulse or depending on how a threshhold is set for distinguishing between short and long pulses, the codes 72 71 43 41 6 2 . Although repeatable, this may be an artifact of the measurement process. In any case they don't correspond to the codes in the Haynes manual.

I have disconnected the negative terminal of the battery for a minute to reset and the results are the same.

Is this an indicator that the ECM is damaged? Or is this what a normal "no trouble" signal looks like?

O2 Sensor I will measure the O2 sensor response next week, however, I had not done this already because disconnecting the sensor has no effect on the original problem (rough idle and rich condition) and so I judged it not to have a bearing on the problem? Could I be wrong here?

Thanks for all help.

I can see and process a light flashing once every couple of seconds better than following a scrolling screen of square waves.

There's most likely B+ available at the diagnostic terminal. If you read 10v with a 10Mohm input impedence scope, then you probably have a weak battery. The signal *is* probably run through a resistor inside the ECM so it can be pulled down easily, but that shouldn't affect it's no-load voltage reading.

I don't know enough about how the system works to be sure if the impedence of a typical automotive LED test light (or high impedence meter) would be adequate to initiate communication, but it wouldn't hurt to try, and they're cheap. They're great to have in your tool box, too. My guess *still* is that there is a constant pulsed grounding of this B+ output at all times regardless of any load on the circuit. That's why I suggested the low current test light, and feel that the scope, while unnecessarily clumsy for this app, should have worked as well.

I really don't know. I would be tempted to borrow an analog VOM at this point and follow the directions to the letter.

The ECM is likely OK, as they aren't major failure candidates. Disregard that statement if this is a Mitsubishi ECM as they are trouble prone. I don't know what ECM is in your 91 Excel.

A normally operating system will see very low voltage (0.0 to 0.4) from a lazy sensor and run rich for a while until the ECM determines this as a problem and sets a code. Then it will run in 'backup' mode where it doesn't rely on the O2 sensor input. Disconnecting the O2 sensor (0.0 volts constant) will expedite this process.

You've never mentioned anything about the MIL (check engine light) being on. Does the MIL come on with the key on, engine off? Does it stay on with engine running or does it go out? If it is working properly and goes out engine running, then you have no current codes stored. OTOH, if the light is inop, and you have a slurry of nonsensical codes stored, I would be looking for a bad EFI ground.

If all of this is basically academic because you just want to learn how to read codes on a car that has none, I would suggest that you disconnect the coolant temp sensor and wait til the MIL comes on, then check for code 21 (I think its 21).

Toyota MDT in MO