Aug 17, 2016 - 12:41 PM
How do you use the AC filter to do cranking compression with the U Scope?
Connection: The AC Pass Filter connects to the test lead and then to the test point.
Purpose: The AC Pass Filter is used with scopes that do not have a built-in AC input coupling setting. The AC Input Coupling/AES AC Pass Filter blocks the DC portion of the signal and makes it possible to zoom in on the always changing portion of the signal.
Cranking compression is a quick way to get an indication of the general compression of the cylinders. This is accomplished by monitoring the minor voltage drop changes of the battery that occur while the engine is cranking.
To zoom in on the minor voltage drops in the cranking battery voltage waveform, it is necessary to use a low volts per division setting. This is accomplished by using an AC Input Coupling to block the DC voltage. If your scope does not have this feature, an AES AC Pass Filter.
Typical Scope Settings:
Input Coupling: AC
Time per Division: 500ms
Volts per Division: 0.1V
Trigger Level: -0.2V to -0.5V
Trigger Mode: NORM
Trigger Horizontal Position: Left
Trigger Slope: Positive (up)
Install the AES AC Pass Filter to the signal test lead. Connect the test leads to the battery terminals.
Disable the injectors to prevent the engine from starting. Ensure the battery is charged.
If you have a helper cranking the engine, let the screen fill up and press the HOLD button (uScope - top right button marked “A”).
If you are working alone crank for at least 6 seconds. Press the HOLD button so the scope will not retrigger erasing the waveform.
To zoom in on the captured pulses further, with the HOLD activated, you can change the volt and time settings. Note the images above. In the waveform on right, the time per division was changed from 500ms/Div to 100ms/Div to zoom in to further detail of the pulses. Next, if desired, you can scroll through the waveform using the HORZ POS control –
• Press the OK button to open the pop up main menu.
• Use the up/down arrow key to highlight HORIZONAL menu item and press OK.
• Use the up/down arrow key to highlight HORZ POS.
• Use the left arrow key to scroll through the waveform.
The harder it is to turn over the engine, the lower the voltage will drop. The easier it is to turn over the engine the less voltage will drop. Therefore, a cylinder with bad compression will have less voltage drop than a good cylinder.
Overall, analysis is to compare each pulse. The pulses from a good engine will be uniform in pulse width and heights. On the other hand, an engine with a weak cylinder will have a less pronounced pulse that repeats based on the number of cylinders.
Keep in mind that the voltage drop on the battery while cranking can be affected by many things. For example, a bad flywheel gear or other mechanical issues.