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Sample ECU signals without altering

I have an older car with a simple ECU. It only reads a few signals, coolant temp, O2 sensor voltage, tach, maybe a couple others. I want to record the signals without altering them. I have no problem constructing the recorder, but am unsure about sampling the signals at the ECU without altering them. Do I just need a high impedance op-amp?
 
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Diver300

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I have an older car with a simple ECU. It only reads a few signals, coolant temp, O2 sensor voltage, tach, maybe a couple others. I want to record the signals without altering them. I have no problem constructing the recorder, but am unsure about sampling the signals at the ECU without altering them. Do I just need a high impedance op-amp?
If the signals are analogue, that is what you need to do.

It's unlikely that you need a very high impedance, so any op-amp will do. Car electronics try to avoid very high impedance signals, or damp connectors would cause all sorts of problems.

A bigger problem can be working out which ground the sensor is referenced to. It's common for ECUs to provide a local ground for their sensors, and that should be what you connect the reference of your recorder to.

The local ground is often just connected to ground within the ECU, but having that connection means that there is minimal current in the ground wire to the sensor, so minimal voltage drop, and the ECU's signal ground is the same as the sensors ground. If the ECU takes significant current, it's power ground may be a few hundred mV above the bodyshell or engine ground, which would affect the readings if the sensor were grounded to the engine. If the ECU has to work when the engine is cranking, the ground voltages can be very different and unpredicatable.
 

KeepItSimpleStupid

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Generally, many sensors are ratiometric and 0-5V meaning, the references are Vcc and 1/2Vcc whatever it happens to be. they don't go to zero volts and they don't go to 5V. They miss 0 and 5V by a little bit.

There may be switches and resistance sensors (temperature),

A MAP sensor might have 0, Vout and +5, You can use these as a reference, just don't draw significant current from ground.

The oxygen sensor might be problematic. You might have to buy device that converts an OBD bus signal to a voltage.

tach might be problematic too.
 
My system is pre-OBD. Actually, they call it OBD-I, but it was before there was any standard. What concerns me is not altering the signal in any way.

The aftermarket DIY ECU called Megasquirt uses this for sensor inputs. I just didn't know if this was appropriate for me, since I'm not replacing the ECU.

6-4-2020 12-21-59 PM.png
 

crutschow

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I suspect if you keep the measurement impedances no lower than 100k ohm, it won't affect any of the signals.
 

KeepItSimpleStupid

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That type of interface to your device is not a bad idea. It protects the inputs in two ways. It limits the current if Vcc happens to go to zero and it limits the input to your device to 5.1 V, both are good ideas. It also provides a filter.

Disturbing depends on the input impeadance of your voltage acquisition system.

That's not bad tach interface. You know how many coils you have or it you have a "wasted spark" ignition.

Read up on Oxygen sensors. I believe there are two general types. This https://www.reddit.com/r/AskElectronics/comments/4p67f7 was an interesting thread combined with the reference material within the thread.
 

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