Help identify DME specs and damaged resistor

[917_Langheck]

The Car: 1984 Carrera


The Issue: Replacement DME of unknown specifications.


The subissue: Burned out R402 on the PCB. No known symptoms resulting.

Known to me was that the DME was replaced by a shop hired by a previous owner. I suspect that the shop was "fixing" some issue by emptying the PO's wallet as there was a receipt for this replacement DME, as well as receipts for chasing a "lean condition" by other solutions. Nevertheless, the car runs excellent today and has passed two CA smog checks during my ownership. I had been curious to know what was inside the DME supplied by Vertex Auto as there was only the 082 scribed on the harness receiver. After opening the box to identify whether it was a 24 or 28 pin box I found this tag. It was located on the "chip" at one point. I don't know what GEGD means. Anyone care to decipher what +2.1* gegd indicates?





And another pic without the tag.



I have read through as many of the prior DME threads that I could (e.g., https://forums.pelicanparts.com/porsche-911-technical-forum/398595-calling-motronic-experts-ic-information.html; https://forums.pelicanparts.com/porsche-911-technical-forum/110038-84-89-dme-chart.html, etc.) and noted that the 082 scribed on the outside of the box doesn't necessarily mesh with the idea that the eprom has been flashed with the Club Sport settings. Anyone have a suggestion as to what the tag may actually represent?

Also, while I was in there I noticed this black soot. I didn't see any corresponding color change on the opposite side of the PCB, nor discolored or broken solder joints - though I didn't inspect the joints with a loupe, so there could by one or more...but that is for another time as I want to deal with the source of the soot first.





On the opposite PCB I found this little lovely item. It is located at R402, which research indicates is part of the injector pathway. Interestingly, as I mentioned above, the car runs great. But clearly that resistor has been smoked so how is this still working? Or rather, would things be even better with an unburned resistor in its place? Again, based on research here and elsewhere, this resistor appears to be a 15 Ohm, 1 watt, at +/-1% and 50ppm/K type. My plan is a simple desolder and replace. Anyone have additional information I should know before doing that?




For reference here is a schematic that outlines the circuit as I gathered from the other discussions. R402 is located at top center. I am no EE/ET, but have access to one. I take it that this is taking 12 volts and regulating that current to the S400 IC. If that is the case would an earlier overvoltage from a faulty Voltage Regulator pop this resistor? I had such an event about a year ago - I replaced the VR with an NOS Valeo 14.4 VR (documented output on a DMM), and installed a new battery at the same time - I know it got cooked too, as the H2S indicated.

From ischmitz's post I found this pic that identifies R402 - which based on the color codes I read Brown Green Black Gold Brown Red - which I think was confirmed in text in the post liked above as the aforementioned 15 Ohm.... Anyone have a different reading or take based on these data?





Cheers, and thanks for any help or suggestions. Again, car runs fine, and hasn't evidenced any problems recently, but I don't need to go poking fate if I don't have to.

[FrankM_]

Hi,

I believe the 082 is an indication of the Bosch sticker that would have been on there (box looks painted over) with the part number : 0.261.200.082 (87-89, M30.25 model, 27C64 EPROM - and that fits with your pictures).

The eprom looks to be specific for a Clubsport CS (which would have the number 1267355330) ? And you can only guess what the +2.1 degrees in timing would be and how they 'adapted' the parameters. I would be very interested in looking at the contents of that eprom (HEX file).
No clue on what the 9E9D would mean.

Apparently the resistor still works then (have you measured it with an Ohm meter ?), but I would look at D400 (the ZY27 Zener) to check on it and also C406, R419 for correct operation before replacing the resistor.
As far as I understand, it is a current limiting resistor and supplies power to the all-important peak&hold chip S400 (pin 14) and if you follow the path to the pin 2 and pin 4.

Regards,
Frank

[917_Langheck]

Thank you Frank. 9E9d makes much more sense (I should have seen that as that same 9 is right above that code) as that could be Vertex's hex code, possibly related to the 2.1° note. Allegedly the 330 EPROM raised the electronic rev limiter to 6850+/- rpm (please correct me if I am wrong) and that would explain why I don't hear any limiting when I breach the nominal red line indicated on the tach. I doubt Vertex (or anyone else) improved the rod bolts at the same time, so I'll keep to the 6250 red line.

I haven't tested the operational parameters of any of the components at this point - I wasn't expecting any issues when I opened the box and wasn't prepared for such an investigative effort at that time - but I will do so soon - I shouldn't say "shortly."

Cheers,
Richard

[FrankM_]

When revisiting this article, I suddenly see the red wire running past the large diode to the transistor (or S300, the 5V regulator). That is certainly not standard.
That points to some other repairs done on the analog board ? Where is that wire coming from?

Looks to be 12V to the input side of the L4705CV (pin 1), check the underside of the board for any damage/changes in the PCB paths ?

And apparently SteveW did the analysis of the chip a long time ago (std chip for CS) : U.S. Carrera ClubSport demystified - DME chip analysis

[917_Langheck]

Hi Frank,

Thank you for revisiting. I did notice that wire, but thought nothing of it at the time. It appears to jump between T300(?) and pin 35 along the short side of the bus. You can see that someone opened the gap between the two upper pins, 17 and 18, if I am reading the pin-out drawing correctly.



Detail:



Thank you for the link. I will inspect the other side of that board next time I'm in there - I didn't have the trick in disconnecting the board from the baseplate in my pocket at the time and I wasn't interested in breaking that PCB... yikes.

[917_Langheck]

For future seekers of information related to this issue I found a schematic for a 1988 DME linked here: http://www.914world.com/bbs2/index.php?act=Attach&type=post&id=732070



If my interpretation of the jumper Frank pointed out is correct then Vertex Auto, or maybe someone else, was working on the Injector side of a problem. I wonder if the trace on the other side of the PCB shorted, and that is why they ran a jumper?



This is becoming curiouser and curiouser.

[FrankM_]

Pin 18 and 35 are both just 12V power supply pins for the ECU. S300 is a voltage regulator that makes a stable 5V mainly for the digital board.

The injector part is through pin 14 and 15 that provide a path to ground for the injector banks. The other side of the injectors is connected to 12V.

So the red cable does not ‘touch’ your injector circuit per se. However, your toasted resistor is part of the peak and hold driver for the injectors.


Sent from my iPhone using Tapatalk

[917_Langheck]

Originally Posted by FrankM_

Pin 18 and 35 are both just 12V power supply pins for the ECU. S300 is a voltage regulator that makes a stable 5V mainly for the digital board.

The injector part is through pin 14 and 15 that provide a path to ground for the injector banks. The other side of the injectors is connected to 12V.

So the red cable does not ‘touch’ your injector circuit per se. However, your toasted resistor is part of the peak and hold driver for the injectors.

Thanks Frank,

From this I understand that the red wire is merely acting as a power jumper to the T300 regulator. Is that correct? I will have to look at the backside of the PCB to see what is going on that would require such a connector.

Cheers

[MBAtarga]

Could 9E9D just be the checksum for the EEPROM?

[mysocal911]

The residue from the resistor fire needs to be cleaned, i.e. A/D key inputs area.

[917_Langheck]

Thanks Dave, yes indeed it does, if for nothing else to see if any of those parts also suffered in the incident. Unfortunately, I wasn't at a point to do that last week.

I don't see anything obvious at this visual scale, but the resistor second from the top in the center looks to have taken on a thick coating of soot. I will get this cleaned up asap.




Cheers

[917_Langheck]

An update:

Managed to decarbonize both PCBs. Inspected all traces and solder joints; the only ones slightly suspect were the two related to the burned resistor. The resistor still worked, but at 37 Ohms! Desoldered the old one and installed the new 15 Ohm resistor.

Left it unboxed but secured to the base plate and installed the DME in the car. Gave it a start and the car fired right up and idled at 1100 rpm sweetly. Let it run a bit with no issues. Removed the DME to reinspect the new resistor for any problems. None found, so put the cover on and the DME back in the car, then set off for a drive. I am sure it drives even better than before, not power-wise but just smoother and more sweetly. Of course this is just based on the assometer and is most likely a false perception. Nevertheless, all is fine thus far. Can't wait to really put my newly installed Karsten spring to the test. Thanks for the help everyone!


Still a little carbon embedded on one of the green block capacitors but everything else cleaned up and appeared to be in spec.



It's not green like the original, but it seems to work... 15 Ohms at 1W +/-1%

I also didn't remove the jumper wire as I still don't know why it was installed, but it all works so it's good for now.

[ischmitz]

The only possible way for R402 heating up like this is when C407 or D400 shorts to GND. What likely happened is that D400 shorted and eventually failed open. The 0127 IC itself will never be able to sink that amount of current into its supply and not self-destruct in the process.

Overvoltage is also unlikely as it would have destroyed other more sensitive components in the process. You're really lucky that the 0127 survived this ordeal since these custom Bosch parts are extremely hard to come by. Count yourself lucky that the DME still works and maybe test/replace D400. It is a 27V Zener diode that protects the 0127 against spikes (read pulses from leaking ignition wires, etc.) on the input. With that diode open/missing from the circuit you are leaving the 0127 vulnerable.