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Location: Where nothing ever grows. No rain or rivers flow, Texas
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Quote:
Originally Posted by Zea mays
The P0444 code usually refers to an electrical problem, not a mechanical problem with the EVAP purge valve itself. A constant battery voltage is applied to one terminal of the purge solenoid. The wire from the other terminal goes back to the ECU where it is grounded through a switching transistor. When the switch is open, the ECU expects to see battery voltage at the input to the switching transistor (because no current is flowing); when the switch is closed, the ECU expects to see near zero voltage at the input to the switching transistor (because current is flowing and the battery voltage is dropped across the solenoid coil). If what the ECU sees and what it expects to see differ, a DTC is set. One of the DTCs is P0444. It think it means the solenoid circuit is open.
I'd be curious to learn how this is resolved. Keep us informed.
I follow what you're saying. There should be a mechanical relay that supplies the 'constant' voltage to the purge valve circuit. I wonder if that relay was stuck and keeping the purge valve open. And maybe since this is the first time I disconnected and then reconnected the plug while the engine is running, that some jolt loosened up the relay switch. Sounds far fetched but I've had plenty of experience with sticky relays becoming useful again :\ When I was testing the valve I got a considerably stronger than normal contact spark
BTW the SES came out as soon as I replaced the battery and started the car a couple of months ago
I follow what you're saying. There should be a mechanical relay that supplies the 'constant' voltage to the purge valve circuit.
In a Honda Accord the purge valve solenoid is always supplied with battery voltage. There is no mechanical relay. The EVAC purge solenoid circuit as depicted in the Honda shop manual looks like:
The "LOAD" is the 12 V (approx.) solenoid. The NPN transitor switch is on the ECU circuit board. The transistor is a short to ground when the base (at B) is high; current then flows through the solenoid and transistor to ground. The transistor is open (and no current flows through the solenoid to ground) when no base current flows at point B. The base current at B and whether the canister is opened or closed to the intake manifold vacuum depends upon a computer algorithm in the ECU.
The switching circuit above acts as a mechanical relay. Manufactures choose transistor switches (relays) over mechanical relays where ever they can.
I don't think this helps much with your problem; I just wanted to address the mechanical relay issue.
Location: Where nothing ever grows. No rain or rivers flow, Texas
1,085 posts, read 1,582,067 times
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I think that picture is just a simplified schematic diagram. Wiring diagrams dont use those symbols. I have a degree in ECE, not that I'm good, just saying I drew a lot of those diagrams years ago
http://www.garage411.com/images/Mazda/EnginePerformance/SelfDiagnostics/Fig.56.gif (broken link)
If you're saying that the power to the engine goes from battery to ignition key to engine component ie purge valve, then I don't believe that. It has to have a relay to separate the different circuits.
All the switches within the ECU must be solidstate (transistors), I agree.
There is always a main relay, which remains closed while the ignition is on.
What I am saying is that the ECM terminal 3H in your circuit is not tied directly to ground. Terminal 3H is tied to the collector of an NPN switching transistor, which is on-off modulated by the ECM which applies a signal to the transistor base. When the voltage at the switching transistor collector is not in sync with the ECU applied signal at the transistor's base, a DTC is set.
Quote:
I think that picture is just a simplified schematic diagram. Wiring diagrams dont use those symbols.
Well apparently Honda does, because the symbol is take directly from Honda's shop manual for my Accord. I could give you the page number.
Quote:
I have a degree in ECE.
Now don't go waving degrees at me — I could wave some back too.
Location: Where nothing ever grows. No rain or rivers flow, Texas
1,085 posts, read 1,582,067 times
Reputation: 468
Quote:
Originally Posted by Zea mays
There is always a main relay, which remains closed while the ignition is on.
What I am saying is that the ECM terminal 3H in your circuit is not tied directly to ground. Terminal 3H is tied to the collector of an NPN switching transistor, which is on-off modulated by the ECM which applies a signal to the transistor base. When the voltage at the switching transistor collector is not in sync with the ECU applied signal at the transistor's base, a DTC is set.
Well apparently Honda does, because the symbol is take directly from Honda's shop manual for my Accord. I could give you the page number.
Now don't go waving degrees at me — I could wave some back too.
I dont know man. Mechanical relays are still used in many if not all cars today. On the X5, there are 8 types
I think the diagram you posted is more for diagnosing problems on the ECU. It's just the sensor circuitry (3H itself), which sense the state of the valve as you've said
it doesn't make sense that a purge valve has constant power or else it would always be open when the engine starts. if that was required then it would have been actuated by vaccum. and if its opened and closed by ECU signals then it will have to employ relays.
In some cases the power to the purge valve can be coming from the same relay for the o2 sensor heater, which only needs to run a few seconds on cold starts.
This tiny pic shows a purge valve relay for the BMW E30
it doesn't make sense that a purge valve has constant power or else it would always be open when the engine starts.
Let's try it one last time.
Battery voltage is always present at one terminal of the purge valve solenoid whenever ignition is ON.
No current flows through the purge valve solenoid at start-up because the second terminal of the purge valve solenoid is not grounded through the switching transistor on the ECU board.
The canister purge valve is CLOSED if no current is passing through the purge valve solenoid.
Combining statements (1) and (2) above, the canister purge valve is CLOSED at engine start-up.
When the engine becomes warm and a few other conditions are met, the ECU grounds the second terminal of the purge valve solenoid through the switching transistor on the ECU board.
Because the purge valve solenoid is now grounded by the switching transistor, current flows from the always present battery voltage, through the solenoid, and through the switching transistor to ground.
The purge valve OPENS.
Gasoline fumes from the canister pass through the canister purge valve and into the intake manifold.
Location: Where nothing ever grows. No rain or rivers flow, Texas
1,085 posts, read 1,582,067 times
Reputation: 468
In this day and age, there is ABSOLUTELY no way ANY TINY TRANSISTOR on the $1000 ECU board to be in direct contact electronically with any cheaper engine component that is subjected to the elements, extreme temperatures, contact sparks, vibration, malfunction, human error, etc. That is like handling corrosive chemicals without gloves.
Just imagine daytime running lights. do you think the ECU will supply ground and complete the headlight circuit when it detects darkness? I guess that might work if you have a tiny LED bulb as DRL
Hi, I have a 2001 X5 with a 4.4. Problem is it stalls after running a short time. It won't restart until cooled. So it is just a one way driver for now. I suspect the fuel pump or relay may be going. Has any body had this problem and how do I check the items? Thanks
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