W
wallometer
Engineering question. (ref. pg. WI-36 in service manual E/G(TB)-01)
Evap. emissions canister got whacked by road junk, 2-pin harness
connector R69 to "drain valve" was smashed, "check engine" light came
on. (One can not buy the connector from Subaru except as part of $700
rear harness!) DTC was P0448, "evap. emiss. contrl. syst. vent
control valve circuit short."
It is likely that the circuit was shorted by the blow.
I disconnected the leads and reset the DTC. DTC code P0447, "evap...
(etc.)..circuit open" appeared. I reset the DTC.
The resistance measured at the pin terminals of the valve is 27 ohms,
drawing about 1/2 amp at 12V at one polarity, and about 5 ohms at the
other polarity, indicating possibly an internal rectifier diode to
snub back-EMF seen by the ECM. I removed the valve from the canister
(you can't buy just the valve either, only the $300+ canister!) and it
cycles upon application/removal of 12 volts DC in the correct
polarity.
I connected a 27-ohm resistor to the harness leads in place of the
valve and DTC P0448 was lodged by the ECM. I increased the resistance
by stages, resetting the DTC each time. I am up to 2200 ohms (6 mA
max) and the ECM still cries "short."
When the DTC is reset and the harness leads are not connected, P0447
("open") is lodged immediately upon starting the engine. However, at
each stage of increasing the resistance, it was only after a few
minutes' driving that the engine check light came on with DTC P0448
("short"), meaning that the ECM does not check for a short on this
circuit at engine startup, but only when it energizes the valve. Just
when or why the valve is operated is not explained in the service
manual. I'd like to know when or why.
The ECM grounds the "low side" of the valve to operate it.
I suppose the question is, is there a failure mode of the ECM in which
it senses a few milliamps and concludes that there is a short in the
circuit? Or is the canister valve a magical device that can operate
on practically no current? (just kidding)
Does anybody have the detailed knowledge to answer this question?
Subaru service dept. is not willing (or able?) to discuss it or to
refer me to somebody who might know. I don't want to buy a canister,
harness and perhaps ECM, which was their proposed solution for
"isolating" the fault. (I was not impressed.)
Evap. emissions canister got whacked by road junk, 2-pin harness
connector R69 to "drain valve" was smashed, "check engine" light came
on. (One can not buy the connector from Subaru except as part of $700
rear harness!) DTC was P0448, "evap. emiss. contrl. syst. vent
control valve circuit short."
It is likely that the circuit was shorted by the blow.
I disconnected the leads and reset the DTC. DTC code P0447, "evap...
(etc.)..circuit open" appeared. I reset the DTC.
The resistance measured at the pin terminals of the valve is 27 ohms,
drawing about 1/2 amp at 12V at one polarity, and about 5 ohms at the
other polarity, indicating possibly an internal rectifier diode to
snub back-EMF seen by the ECM. I removed the valve from the canister
(you can't buy just the valve either, only the $300+ canister!) and it
cycles upon application/removal of 12 volts DC in the correct
polarity.
I connected a 27-ohm resistor to the harness leads in place of the
valve and DTC P0448 was lodged by the ECM. I increased the resistance
by stages, resetting the DTC each time. I am up to 2200 ohms (6 mA
max) and the ECM still cries "short."
When the DTC is reset and the harness leads are not connected, P0447
("open") is lodged immediately upon starting the engine. However, at
each stage of increasing the resistance, it was only after a few
minutes' driving that the engine check light came on with DTC P0448
("short"), meaning that the ECM does not check for a short on this
circuit at engine startup, but only when it energizes the valve. Just
when or why the valve is operated is not explained in the service
manual. I'd like to know when or why.
The ECM grounds the "low side" of the valve to operate it.
I suppose the question is, is there a failure mode of the ECM in which
it senses a few milliamps and concludes that there is a short in the
circuit? Or is the canister valve a magical device that can operate
on practically no current? (just kidding)
Does anybody have the detailed knowledge to answer this question?
Subaru service dept. is not willing (or able?) to discuss it or to
refer me to somebody who might know. I don't want to buy a canister,
harness and perhaps ECM, which was their proposed solution for
"isolating" the fault. (I was not impressed.)