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Re: EVAP /ELCD purge valve operation
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Mon, 17 Jan 2005 15:09:54 Share Post by Email
In Reply to: EVAP /ELCD purge valve, TML [Profile/Gallery] , Sun, 16 Jan 2005 09:10:30
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One side of the ELCD valve is tied to 12 volts through the main relay. The other side goes to the ECU, where it is switched to ground. Yes, indeed it's a 8 hz PWM signal.

The valve is switched on (grounded at ECU) when the engine coolant temp is above 140F (60C) and the intake manifold pressure is below 90kPa - a vacuum. The ECU starts purging by grounding the circuit (allowing current to flow) for very short periods, then gradually increasing the duration. That's becaused the closed loop must be given time to compensate for the additional fuel. The valve operates for 4 1/2 minutes, the stops for 1/2 minute, then operates for 4 1/2 , and so on.

Normally, you should see a signal on the low side (ECU side) of the valve at 12 volts (or so), dropping to 0 volts, and then a very short spike up above 12 volts, because the valve is an inductive load. If you are using an oscilloscope and you're seeing no lower than 7 volts, I'd check your grounding. If you're not using a scope, then you can't believe that 7 volt minimum value - you're seeing an 'artifact' of your meter. Remember, it's not a 8 hz square wave - every 125ms (8 hz) it sends out a pulse - the width of the pulse determines the valve position.

You can check the valve resistance - it should be 26 ohms +/- 3 ohms. If it is truly running all the time (not shutting down for that 30 seconds every 4 1/2 minutes), that would be an ECU issue. I'm surprised you don't have an CEL if the valve isn't right - the ECU reads the voltage on the valve, and if it weren't going all the way down to 0 volts, you'd get a fault.

posted by 12.76.1...
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