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GM Flex Fuel Sensor
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gizmodo wrote:
Well I found a super old oscilloscope but I have no idea if it works. I'll try playing around with it tonight to see what I can come up with. Any pointers on using this thing would be helpful!


I built one of those kits myself, many years ago. Smile It'll be useful to show any signal activity and waveshapes, but don't count on it being accurate about measuring voltages and times/frequencies. Unless you can put some known voltages and frequencies to it to see how they look... and I bet you could tell your RCP to generate a pulse/PWM output at a known frequency, and it'd be 0-5V levels. So you could use it to "calibrate" your scope.


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I figured I wouldn't be able to get real accurate results with it, but I was at the very least hoping I could see some type of signal other than a straight line. I may play around with it a bit more this weekend and take the scope apart and see if there's anything that looks bad inside it.

I'm afraid I'm going to have to abandon this though and just buy a Zeitronix ECA.

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I spent some time with this over the weekend. I took the scope apart because, why not? Smile I didn't see anything blatantly obvious wrong inside. I did blow it out with compressed air. After putting everything back together and fiddling around some more I appear to have been able to get it to show something. In the linked video, the video starts with me holding the probe to the RPM input on the RCP. At about 4 seconds I switched the scope's probe to the flex fuel input. I tried to include all of the knobs so people that know what they're looking at might understand what's happening. It appears as though there is a signal from the sensor.

https://youtu.be/dRVX7QwjfCk

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Should I just start trying various settings in the RCP to see if anything changes?

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Sorry, I just got back online after a week vacation, and my office blocks youtube. Looks like your vertical settings are in the millivolts range? Change up the left-side knob to about 2V/cm, and adjust Y1 (vertical) position so the flatline (nothing connected) is at the bottom of the screen. That'll let your entire signal show up on the scope - 0-12v signal levels would be about 6 divisions tall.

You can check how accurate the scope's vertical calibration is by connecting a known DC level (9V battery, or the car battery after you use a handheld meter to verify charge level, somewhere between 11-14V). Also the RCP VREF should be 5.0VDC, pretty accurately. You'd change the vertical Volts/cm knob to 1v for the 0-5V signals.


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gizmodo wrote:
I fiddled around with this one I have and on the calibration post I can't get it to show a square wave. It is a Heathkit IO-4550

The cal signal is 1Vpp square, right? You'll need to turn down the trigger level (red knob in the trigger area) to a lower level - start just a little clockwise of center. Each time the input signal rises above the level you set with the knob, it triggers one horizontal sweep onscreen.
Your setting (full clockwise) is probably higher than the RPM signal, so it never triggers, so it doesn't sweep and you get a vertical bar. It also looks like your RPM signal is mostly less than zero volts - does that make sense? Does RPM indicate properly in the car?


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I'm guessing I'm just wasting time with this scope. The RCP is showing the correct values for RPM.

Anything greater than 20mv and the line didn't move vertically. I did play with the dials a bit more. Here are a couple more videos.

https://youtu.be/tVfeMhLF_6U
https://youtu.be/BDBy6Zm18ds

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It seems as though the blending of e85 in my area has become very inconsistent so I would like to revisit this. My first question is will this work with the RCP? Right now I have it hooked up to one of the Pulse inputs. Here is some documentation on the sensor and the requirements.


According to the RCP documentation "Each input has a built-in 10K pullup resistor connected to 5V." Is this the source of my problem? Before I again try to track down an oscilliscope I'd just like to know if the sensor's input requirements even jive with how the RCP is built.

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I am pretty sure I am just missing a setting in the RCP to make this work. I hooked up an Arduino with a 22k resistor on the input, wrote a little bit of code and I am able to get the ethanol content from the Arduino. For whatever reason, no matter what settings I try, I can't get a value other than 0 on the dashboard for the RCP.

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MK2 / MK3 / Apex now have floating inputs - no pullup resistors on the RPM inputs. Have you tried adding an external pullup resistor on your MK2 input to see if that makes a difference?

Also, if you can point where you found that in our documentation, I'll get it fixed / clarified.


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I have not tried that. I've screwed around with this enough and now that I have a working solution I'm inclined to just leave well enough alone. As for where I saw that, it is here: https://wiki.autosportlabs.com/RaceCapturePro2_Hardware_Install#Frequency.2FPulse_Input_1_-_3

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Got it - and we fixed the documentation. Thanks!


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