Weird DME failure? 06 C2S 71k
09-30-2012, 09:22 PM
#16
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That's what I suggested to the dealership when I took it in: replacing the oil pressure sending unit.
They agreed that it had to be that and told me it would cost "$100 for the part and 2 1/2 hours labor for all the stuff [they] have to take off to get to it". I have since been told that it is a $60 part and that an experienced tech can do it in 20 minutes by removing a heat shield and using a crow's foot with an extension. That is partly the cause of my suspicion when they later tell me that they ruled that out, and were trying to see if it was something in then wiring from the aftermarket radar unit they installed or the aftermarket sat/nav system installed a year ago by someone else. THEN, they tell me they ruled that out and found that it was the DME by "trying out" a replacement and it worked. I think that BS.
I don't think that a DME even gets a signal from the oil pressure sending unit because there is no reason for it to monitor oil pressure. Those cars that have an ECU monitoring oil pressure do so so because, if the pump fails, the ECU can shut the car down. That is the only reason.
I believe Porsche has a dedicated mechanical pump that works as a function of RPM's , the higher the revs the greater the oil pressure. Period. If the DME were involved, it would shut down the engine in case of oil pressure failure. It does not. You get a warning light that you better pay attention to and shut the engine off because the DME DOESN'T KNOW. I believe that the oil pressure sending unit goes straight to the INSTRUMENT CLUSTER. If the DME were involved, it would need to know to regulate the oil pressure in the car as a function of changes in other measurements. It doesn't because there is no need to. Oil pressure comes from the pump and automatically increases, the higher the revs. In a straight line graph. Nothing complex about it.
What cause problems in the DME is electrical surges, like jumpstarting the car--or trying to track down wiring problems without disconnecting the battery to avoid surges--which is what I believe happened here.
Can anyone explain to me how I am wrong about this?
They agreed that it had to be that and told me it would cost "$100 for the part and 2 1/2 hours labor for all the stuff [they] have to take off to get to it". I have since been told that it is a $60 part and that an experienced tech can do it in 20 minutes by removing a heat shield and using a crow's foot with an extension. That is partly the cause of my suspicion when they later tell me that they ruled that out, and were trying to see if it was something in then wiring from the aftermarket radar unit they installed or the aftermarket sat/nav system installed a year ago by someone else. THEN, they tell me they ruled that out and found that it was the DME by "trying out" a replacement and it worked. I think that BS.
I don't think that a DME even gets a signal from the oil pressure sending unit because there is no reason for it to monitor oil pressure. Those cars that have an ECU monitoring oil pressure do so so because, if the pump fails, the ECU can shut the car down. That is the only reason.
I believe Porsche has a dedicated mechanical pump that works as a function of RPM's , the higher the revs the greater the oil pressure. Period. If the DME were involved, it would shut down the engine in case of oil pressure failure. It does not. You get a warning light that you better pay attention to and shut the engine off because the DME DOESN'T KNOW. I believe that the oil pressure sending unit goes straight to the INSTRUMENT CLUSTER. If the DME were involved, it would need to know to regulate the oil pressure in the car as a function of changes in other measurements. It doesn't because there is no need to. Oil pressure comes from the pump and automatically increases, the higher the revs. In a straight line graph. Nothing complex about it.
What cause problems in the DME is electrical surges, like jumpstarting the car--or trying to track down wiring problems without disconnecting the battery to avoid surges--which is what I believe happened here.
Can anyone explain to me how I am wrong about this?
09-30-2012, 10:35 PM
#17
Nordschleife Master
Without going into too much detail the failure mode, the behavior, is not that far out.
What can be open for discussion, maybe, is the source/cause of the behavior.
Briefly, the oil pressure sensor input to the DME is a voltage signal that is converted into a digital value by an analog to digital converter peripheral (AD/C) in the DME then this digital data is sent to the instrument cluster where it is converted back into an analog signal by a digital to analog converter (DA/C) peripheral in the instrument cluster controller and this is a signal then sent to the oil pressure gage which causes the needle of the oil pressure gage to reflect the oil pressure.
Any interruption of the voltage signal from the sensor to the DME's AD/C input pin can result in an erratic oil pressure reading. But one hopes this cause has been eliminated.
At the DME the AD/C on the other end of that input pin can fail. There are many of these in the DME and they are rather isolated from each other and the failure of one doesn't necessarily affect the performance of the others nor does it affect the performance/behavior of the DME in areas not directly related to this one input signal.
There is the path and subsequent processing the digital oil pressure data takes and receives from the DME to the instrument cluster controller and from this controller to the oil pressure gage.
This path is probably over a bus that is constantly checked directly or indirectly for proper operation.
The instrument cluster controller does a reverse operation on the digital data using one of its DA/C's conerting the data back into analog which is what makes the gage needle move about.
The instrument cluster controller DA/C peripheral is like the AD/C peripheral in the DME in that it is relatively isolated and generally it either works right or not at all.
Thus the external path from the instrument cluster controller to the gage and even inside the gage itself is suspect.
But the dealer has said "DME".
I guess it boils down to whether you believe/trust the dealer or not.
If you do not then unless someone else comes forth with info that is directly helpful and leads you to a solution that doesn't involve a new DME you are left with the dealer's diagnosis.
You can of course obtain a second opinion.
Now you are being told something rather expensive is wrong and this diagnosis is based on trial/error testing.
Given the amount of money you are looking at spending you are entitled to a bit more detail a bit more explanation as to the how and why this conclusion was reached.
No need to be argumentative or confrontational -- which I'm sure you would not be.
I have found when the few times I've been given a conclusion as to what is wrong, expensively wrong (to the tune of around $3K in one case), when I've asked for some background, some further/deeper explanation as to what led to the conclusion the techs have been quite agreeable to take the time to talk to me, to go over in some detail and to answe my questions.
I didn't expect to receive a full understanding of OBD diagnostics or a full understanding in whatever is involved.
But you have concerns that given the dealer's admission trial and error testing led it to the diagnosis that has you facing a large repair bill you want to be sure the techs are sure that when you have this repair done it is the right thing to do and will address the symptom.
Sincerely,
Macster.
What can be open for discussion, maybe, is the source/cause of the behavior.
Briefly, the oil pressure sensor input to the DME is a voltage signal that is converted into a digital value by an analog to digital converter peripheral (AD/C) in the DME then this digital data is sent to the instrument cluster where it is converted back into an analog signal by a digital to analog converter (DA/C) peripheral in the instrument cluster controller and this is a signal then sent to the oil pressure gage which causes the needle of the oil pressure gage to reflect the oil pressure.
Any interruption of the voltage signal from the sensor to the DME's AD/C input pin can result in an erratic oil pressure reading. But one hopes this cause has been eliminated.
At the DME the AD/C on the other end of that input pin can fail. There are many of these in the DME and they are rather isolated from each other and the failure of one doesn't necessarily affect the performance of the others nor does it affect the performance/behavior of the DME in areas not directly related to this one input signal.
There is the path and subsequent processing the digital oil pressure data takes and receives from the DME to the instrument cluster controller and from this controller to the oil pressure gage.
This path is probably over a bus that is constantly checked directly or indirectly for proper operation.
The instrument cluster controller does a reverse operation on the digital data using one of its DA/C's conerting the data back into analog which is what makes the gage needle move about.
The instrument cluster controller DA/C peripheral is like the AD/C peripheral in the DME in that it is relatively isolated and generally it either works right or not at all.
Thus the external path from the instrument cluster controller to the gage and even inside the gage itself is suspect.
But the dealer has said "DME".
I guess it boils down to whether you believe/trust the dealer or not.
If you do not then unless someone else comes forth with info that is directly helpful and leads you to a solution that doesn't involve a new DME you are left with the dealer's diagnosis.
You can of course obtain a second opinion.
Now you are being told something rather expensive is wrong and this diagnosis is based on trial/error testing.
Given the amount of money you are looking at spending you are entitled to a bit more detail a bit more explanation as to the how and why this conclusion was reached.
No need to be argumentative or confrontational -- which I'm sure you would not be.
I have found when the few times I've been given a conclusion as to what is wrong, expensively wrong (to the tune of around $3K in one case), when I've asked for some background, some further/deeper explanation as to what led to the conclusion the techs have been quite agreeable to take the time to talk to me, to go over in some detail and to answe my questions.
I didn't expect to receive a full understanding of OBD diagnostics or a full understanding in whatever is involved.
But you have concerns that given the dealer's admission trial and error testing led it to the diagnosis that has you facing a large repair bill you want to be sure the techs are sure that when you have this repair done it is the right thing to do and will address the symptom.
Sincerely,
Macster.
As always Macster, thanks for your insight and contributions. Learn something every time. But if that's "brief" and skimping on detail, what does a detailed comment cover?
10-01-2012, 12:59 AM
#19
Drifting
Paul, there are several postings on renntech re failed sending units and their associated causes... some noted that a bad connector on the sending unit would cause erratic OP even though they had replaced their sending units. Some simply cleaned the connectors on the sending unit. Do a search for sending unit on renntech and you'll find 2 pages of postings. This one was particularly interesting as it explains how the sending unit and DME interact indirectly... Should be similar to our 997 even though the write up was for a 996. http://www.renntech.org/forums/topic...ending+%2Bunit
10-01-2012, 03:00 AM
#20
Race Director
You don't wnat to know.
My engineering manager says to put my conclusions on the 1st page, preferrably in the 1st sentence, of the 1st page of an email and throw the other 50 pages away.
Of course I do what he says but my Outlook draft folder is huge....
Sincerely,
Macster.
My engineering manager says to put my conclusions on the 1st page, preferrably in the 1st sentence, of the 1st page of an email and throw the other 50 pages away.
Of course I do what he says but my Outlook draft folder is huge....
Sincerely,
Macster.
10-01-2012, 03:09 AM
#21
Race Director
Uh, no. Nothing against Apple products but it just has worked out I've never owned any.
(I'm sorely tempted to buy an iPad -- a sales rep gave me a demo of his some weeks back and it s an impressive device -- but I just can't bring myself to spend the money.)
Macster is Mac for a good (though sadly deceased) friend of mine and 'ster' is just something I guess that came from Boxster, my first Porsche.
I could have used my real name of course but I was concerned about doing this and my concerns later proved to be without any merit, but it is too late to change now.
Sincerely,
Macster.
(I'm sorely tempted to buy an iPad -- a sales rep gave me a demo of his some weeks back and it s an impressive device -- but I just can't bring myself to spend the money.)
Macster is Mac for a good (though sadly deceased) friend of mine and 'ster' is just something I guess that came from Boxster, my first Porsche.
I could have used my real name of course but I was concerned about doing this and my concerns later proved to be without any merit, but it is too late to change now.
Sincerely,
Macster.
10-01-2012, 10:32 AM
#23
Nordschleife Master
I'm just guessing here,but I'm curious what would happen if you'd be able to disconnect the sender unit and put the key on,then measure the voltage at the plug. There should be a certain voltage that is sent by the DME(if the DME controls it). I guess you'd be able to find out the correct voltage that is sent to the sender unit somewhere in a repair manual.
I am aware that the sender unit might be hard to reach to do this,but probably possible...
If the voltage sent by the DME to the unit is correct,that would rule the DME out,wouldn't it?
I am aware that the sender unit might be hard to reach to do this,but probably possible...
If the voltage sent by the DME to the unit is correct,that would rule the DME out,wouldn't it?
10-01-2012, 11:26 AM
#24
Drifting
This is how the 996 works and I believe it to be the same for the 997s.
The sender has two terminals. The larger terminal sends a pressure signal to the oil pressure gauge within the instrument cluster(1-5 bars) and the smaller terminals send a signal to the low oil pressure "idiot" light.
The DME taps into the idiot light to determines whether you have enough oil pressure or not... if you have enough oil in the engine per the oil level e-guage without the idiot light when the oil pressure gauge is fluctuating then you should: Have the car serviced as it's likely an issue with the sender or it's associated circuitry so a non-critical condition. If, however, the idiot light comes on and you see no/low oil levels on the oil e-gauge then you should: Stop, turn off the engine, check the oil level(reminisce about ye ole dipstick), add oil if you don't see a significant oil leak... this is now a potentially critical condition.
The sender has two terminals. The larger terminal sends a pressure signal to the oil pressure gauge within the instrument cluster(1-5 bars) and the smaller terminals send a signal to the low oil pressure "idiot" light.
The DME taps into the idiot light to determines whether you have enough oil pressure or not... if you have enough oil in the engine per the oil level e-guage without the idiot light when the oil pressure gauge is fluctuating then you should: Have the car serviced as it's likely an issue with the sender or it's associated circuitry so a non-critical condition. If, however, the idiot light comes on and you see no/low oil levels on the oil e-gauge then you should: Stop, turn off the engine, check the oil level(reminisce about ye ole dipstick), add oil if you don't see a significant oil leak... this is now a potentially critical condition.
10-02-2012, 10:54 PM
#25
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UPDATE. I paid the dealership for their time in diagnosing the problem and picked the car up. On a test drive, I found that (sometimes) a hard turn would make the oil pressure gauge react ( by coming down from max to the appropriate reading for the RPMs. On a hunch, I drove over a speed bump faster than I normally would. The gauge began to operate normally! After awhile, it maxed out again. Another speed bump and normal operation for about 20 minutes and then maximum reading again.
I believe they are right and have ruled out everything but the DME. It operates properly with a replacement and sometimes with the defective one (like driving over a bump).
Does this sound like a "loose connection" like a defective solder joint or something?
Is anyone familiar with SYSTEM CONSULTING ( www.systemsc.com)?
Who would anyone suggest I contact for repair?
And thanks again for the replies especially Macster for enlightening me to the possibility!
I believe they are right and have ruled out everything but the DME. It operates properly with a replacement and sometimes with the defective one (like driving over a bump).
Does this sound like a "loose connection" like a defective solder joint or something?
Is anyone familiar with SYSTEM CONSULTING ( www.systemsc.com)?
Who would anyone suggest I contact for repair?
And thanks again for the replies especially Macster for enlightening me to the possibility!
