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Old 12-02-2008, 04:08 AM
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Join Date: Sep 2005
Posts: 15
Purge Valve Diagnostics

I have a 1994 c280 with a PO441 code. I ran the car at operating temp and noticed the purge valve was not cycling on/off. I went ahead and borrowed my friends scanner which has activations and tried to manually activate the purge valve, but still no clicking. Unplugged the 2 prong plug at the purge valve to check for voltage and ground. I am always getting 12v on one side, but not ground on the other. My understanding is that the control unit cycles the ground on/off to the ground side of the purge valve. While activating with scanner, I never get switched ground from the ecu. Do I have a bad computer? I'm going to check for a ground signal right at the ecu to make sure tomorrow.

Also, I tried manually giving ground to the purge valve and nothing happens, no click either. I will try taking the valve itself out of the car and jumping it with a battery, but i'm starting to think that both the valve and the ecu is bad. Anything else I can check to verify before I buy both? I notice the car still has the original engine harness, anyone experience a bad harness causing this problem?

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Old 12-02-2008, 08:02 AM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Most likely just a bad valve, to test the wiring you need to connect a 10-20 watt consumer (light bulb) that draws about the same current as the valve.

IF the ECU see a open OR shorted circuit it will STOP sending the ground.
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Old 12-02-2008, 08:27 PM
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1983 300SD, 4 speeed
Join Date: Jul 2006
Location: Raleighish, NC
Posts: 664
You can remove the valve from the engine and clean it out with carb cleaner spray. It seems to me I had to do this twice before it stopped throwing a code. It has been working fine for ~9,000 miles or so now. Just stick the spray tube inside and spray, shake it up, drain it out and spray again. Keep doing this until the cleaner runs out clean. Good Luck.
If it ain't broke take it apart and find out why.

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1994 C280
1987 300TD wagon
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Old 12-03-2008, 12:29 AM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,804
If you want to check the electrical specs of the purge, I would just Ohm the windings . The ECU spec for a good purge is a current draw of .2-.3 Amps, so that equates to a purge valve coil resistance value of 40-60 ohms. That would be the simplest way to check it.
Most purge faults are dirt and crud from the canister and simply need a cleaning, but if the problem is electrical, the first test I would do is OHMS verification at the Valve with plug removed.
The purge is a frequency based signal and is dependant on engine load factors determined by ecu sensor inputs..the freq is from 5 to 20 Htz, so the R value of the purge has to be in spec for ECU sig to exist.
A Dalton
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Old 12-03-2008, 09:00 AM
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Join Date: Jan 2008
Posts: 92
Disconnecting Purge Valve

What would be the effect on the system if purge valve is disconnect and input at intake is blocked? Can the pipe from charcoal filter be left open?

I am planning to do this temporarily due to bad purge valve.

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