Understanding the Voltage coming into an Evap Purge Solenoid

Question

I am trying to understand how the evap purge solenoid is controlled by the ECM. In my case we are talking a 2011 Wrangler JK, but I believe the system works similarly in many other vehicles.

I understand the the opening and closing of the solenoid is controlled by the ECM sending a ground signal.

I have a few specific questions related to this:

I have heard people say that the solenoid received "Constant Voltage." I am sure that the ground signal is not working between my ECM and the solenoid, but when I test the positive and "ground" coming from the ECM into the solenoid with a multimeter, I get around 12+ volts.

If there was no ground signal being sent, wouldn't it show zero?

If I replace the wire coming from the ECM to the solenoid with a wire connected to the negative post on the battery or any other ground wire, the solenoid works. I am able to command it to open using a scan tool and I can feel it, so I know it is working properly. My guess is, the solenoid is now going from always closed to always open, but I am not 100% sure. My guess is this is causing a P0443 because the valve is now always open compared to before when it was always closed.

If the positive cable from the ECM and the ground cable from the ECM are connected to the solenoid, it doesn't work even though testing these two cables with a multimeter shows around 12v to 13v. In this case, I am thinking the solenoid is always closed because the ground signal is never being sent properly from the ECM.

So, I am trying to understand why these wires from the ECM to the solenoid would be showing 12v to 13v if there was not a ground signal instead of zero.

Additionally, I am then trying to understand exactly what "switching ground" means. Is the system just sending power via the negative cable and then it switches it over to ground when it wants it open? Is the system otherwise sending voltage via both the positive AND negative cables until it switches to ground and that is why I get the 12v to 13v?

Answer 1

From what you are describing, it sounds like you are testing the voltage to the solenoid while the solenoid is unplugged.

If so, that is not a valid test, you need to have a “load” connected to the output of the ECU.

When you disconnect the solenoid, and connect your voltmeter across the wires to the ECU, there is very little “load” on the output of the ECU due to the high resistance of the voltmeter. Then, since the output of the ECU will be controlled by transistors and probably has some circuitry to detect a faulty solenoid which adds a low current voltage to this output, the wires will “float” at 12v.

The solenoid has a relatively low resistance and draws a lot larger current than the voltmeter, so it will drag the fault detecting voltage down to zero which make the ECU happy.

When the ECU turns the solenoid on, it will provide a larger current down those wires which will be enough to drive the solenoid.

In other words, you need the solenoid connected, or at least a resistor connected in parallel with the voltmeter to simulate the solenoid, while you measure the voltage.