Troubleshooting the DME - no ignition trigger pulses.

wild man · 08-15-2010, 04:44 PM

Wow, that was a pretty exiting PGA championship finale that I just finished watching - won by Kaymer, the young German. Anyways, it looks like I've got a situation where the DME isn't putting out any ignition trigger pulses, causing a no-start situation. The injector pulses do look good though (while cranking), so I know that the DME isn't completely DOA. I was able to trace the lack of trigger pulses through the analog and digital boards, all the way back to the output port of the 8051 microcontroller. I'm doubting that the 8051 port is bad, unless someone knows this to be a common cause of said condition. That pretty much means that the 8051 is not receiving a signal INTO it that is needed in generating the pulses. This is where things start to get foggy, regarding what I know about how the DME operates. Is there anybody out there who would be able to give me some pointers on what to check, or look for?

kdjones2000 · kdjones2000's Garage

Could be your alarm is cutting it off.

wild man · 08-16-2010, 05:20 AM

How would I check that? I have an digitizing scope, and can check the signal/waveform on any pin of the digital motor electronics connector, or on that of any component on either circuit board inside. I really believe it is a problem on one of those 2 boards though. I understand most of what is going on on the digital board, but the analog board is giving me fits. To make matters worse, there are some chips on both boards that have "funny" numbers on them that I can't figure out what they are, with the schematics being no help in that regard, because they only show the outline of the chip, with the numbered pins, showing no indication of what the chip actually is.

From what I seem to be gathering, the no ignition trigger pulses is a very common, if not THE most common, failure mode for the DME. I'm embarrassed to admit it, but I think I may have caused the failure myself, by accidentally installing an incorrect EPROM chip into it. When it wouldn't start, I then put the APE MAF chip that was originally in it, back in, but it still will not start.

Any ideas (short of buying a new DME, of course) on what to do?

ischmitz · ischmitz's Garage

If the 8051 doesn't generate ignition pulses on pin 2 the flywheel decoder IC could be damaged. It doesn't "see" the reference (TDC) signal and hense no spark signals are generated. The 8051 needs to get at least one pulse from the TDC sensor to start generating a train of spark signals based on the signal of the speed sensor. Check pin 12 and 13 on the 8051 for the speed and reference signals while cranking.

The 8051 uses interupt lines 0 and 1 to evaluate the speed and refernce signals. Both need to be triggered at least once after a power-cycle.

I am surprised that a wrong EPROM in itself would cause that sort of damage. More likely it might have been caused by ESD when handling the DME. Early ICs don't have a very sophisticated ESD protection and can be very sensitive to electrostatic discharge.

Ingo

mattinnes · 08-22-2010, 07:25 AM

Have you checked the chip on the KLR? Mine blew this year..I went to the trouble of buying a new DME which came with the KLR too, I swapped my stage two chips over to the new boards still had no fire! when I put the the stock chips back in found the car would run, tracked it down to the chip on the KLR in the end.

zedsn · 08-22-2010, 01:59 PM

Did you put the chip in the wrong direction?

wild man · 05:28 PM

I appreciate any and all suggestions on what to check, but I have literally eliminated the KLR as being a possible culprit, by removing it and jumpering pins 16 and 24 of the connector. At this point, the DME logic board, and all of the components on it, have also been eliminated. That narrows it down to something on the DME I/O board. I hope to be able to spend some time on it tomorrow, and with a little bit of luck, have it narrowed down to the failed component.

I have a thread going in the 911 forum, because I know there are some DME experts there. That's a pretty hot forum - posts move down REAL fast. Anyways, here is the thread:

I think I screwed up my (951) DME, s there anybody here who can help me out/

Victor Askew · 08-23-2010, 10:22 AM

The KLR it self may be good but I would also check the ignition signal input KLR and ignition signal output KLR as these 2 pins on the KLR wire harness connector/ plug were the cause of my very large headache just last week. I had zero spark and it was the only thing left to check and this turned out to be the cause of my no spark issure. Good luck with the troubleshooting.

wild man · 08-28-2010, 03:40 PM

The saga is finally over, but I feel like such an idiot. I've was chasing wild geese the whole time. Thanks tt_944, for informing me that I was jumpering the wrong pins on the KLR connector. After jumpering the correct ones, the car started and ran, so the DME was working fine the whole time. When I pulled the cover off of the KLR, I saw that I had installed the EPROM chip backwards, after removing it for the purpose of burning a backup chip (it is oriented exactly reverse of the one in the DME). The KLR did not work after correctly installing the chip, but when I replaced the chip with the backup one, it worked fine. The irony is that the only thing that was fried was the original KLR chip (it's one that allows 20psi without cutting power to the cycling valve), so it's a good thing that I had a backup. But if I wouldn't have made the backup, the original one would not have fried. It all ended up being nothing more a big waste of time (and a huge aggravation), but thanks none-the-less, to everyone who helped me out.