It looks like some GM EGR valves have the pintle position controlled by a PWM waveform (square wave with variable duty cycle) applied to the position solenoid, not a variable DC voltage. This is possible because both A and E go back to the computer, neither line is fused power or ground.

George

 

Well let me know if you find anyone that can measure their voltages to compare to mine. I just know every test I've found online says best to test with egr connected and key in on position. And it should be 12v on that darn red wire while connected. I have run across a couple posts where people had the same issue....12v unplugged, 0 volts plugged in, but nobody ever answered them. Tomorrow I'll check resistance on all that you mentioned.

 

I’ll keep working on it. For most mechanics you command the valve with a scanner and watch the output in live data. Next best thing is force both modes with a drive test and watch the reported position in live data. As soon as we confirm the pintle position method on those two outer wires we can finish this up, unless you find that the valve is shorted to ground.


George

 

Problem is I'm not a mechanic lol. I don't have that expensive scan equipment to command anything, or read live data as far as I know. Just the Bluetooth obd2 and torque lite on my phone.

 

I guess I could just go buy a new egr valve and compare the test. Return if nothing changes. I know parts stores hate it when people buy parts for testing, lol. Maybe I'll Amazon prime one. They accept all returns 😂

 

Another helpful member on this forum - Christine, is scanning and sending me the G2 on the EGR valve in her 1999. She has already confirmed that the 12v control on the two outer wires is with a pwm duty cycle so it’s essentially a low frequency AC waveform. This means that dc voltage measurements are of little value. The computer may not start the ac control waveform until the coil is plugged in. I’ll get back as soon as I receive and evaluate her information. Thank you Christine.

George

 

I have reviewed the excellent information that Christine sent me from her 1999 shop manual. Here is what we now know:

P0404 is set when the reported pintle position is more than 10% different from the requested pintle position for more than 10 seconds. This can be caused by:

A faulty EGR valve
Faulty wiring/connectors
Poor 5v ref high or low
Carbon deposits preventing full range of pintle motion
Excessive exhaust back pressure
Internal VCM fault with a short to power or ground between the EGR position control circuit wires and other VCM circuits
Faulty MAP sensor circuit (used for testing)


Testing the 5V reference high and low is straight forward as it is std issue dc voltage. I believe that we have done this

After you clean the carbon deposits you need to clear the DTC's to force the system to relearn the pintle positions vs commands.

Most testing starts with using a bidirectional scan tool to command various pintle positions. Most DIY'ers do not have this capability. Further complications are that the control of the pintle position via the two outer wires on the EGR connector is achieved by using a pwm control signal which is a low frequency square wave of varying duty cycle, not a straight variable DC voltage. You would need a scope to monitor this control signal. Applying various voltages to these wires will probably move the pintle but that's not how it is controlled by the VCM. This probably explains your 4.5v vs 4.9v dilemma as the duty cycle of the control signal probably never presents the equivalent of a full 12 or zero volts.

Testing for shorts to power or ground internal to the VCM can be done with a standard digital meter and a test light but is a little tedious and involved. I would be glad to provide that testing info (charts) and/or walk you through this but it is a process.

Let me know if and how you want to proceed. Thank you to Christine for this excellent assist, you rock!

George

 

I'm curious about the map sensor and exhaust back pressure. Is there a way to check/test those before digging into the pcm testing? Just trying to do the easiest things first lol.

 

But it is interesting that bad egr valve is at the top of the list. I wonder if they're in "most likely to occur" order.

 

Oops one more possibility I read somewhere but can't remember where or exact instructions. Something about the correct procedure for clearing the code so pcm is forced to relearn. Also that this needs to be done after a new egr is installed (if that's what's wrong). It was something about the timing of clearing the code, like right after starting the car, clear code and then shut off immediately? Like even though code gets cleared, it's still in the pcm memory? Don't know if that's legit...do you know if there is a particular way it must be cleared/reset?