HELP !!! DME defect

ML952 · 12-31-2003, 12:02 AM

Hi guys,

I tried to read out the contents of my tuned DME-chip that was in my Carrera 3,2, 930-20-spec, MY89 and now my motor doesn´t start anymore !

I just removed the EPROM-chip, read it out with a EPROM-reader, installed it back again (in the right direction) and now the motor doesn´t start anymore.

Everything is connected, I double-checked my work, read the EPROM out again, got the same results.

What´s obvious is that when I only turn the key to IGN, not to start, some relais click several times and I got a spark for several seconds in the distributor (you can hear it and I made it visible with a spark plug connected to the IGN coil)

HELP !!! I´m so unhappy now !

Greetings from Germany
Marco

Dutchie · 12-31-2003, 01:12 AM

Maybe Steve Wong can help you out. Bump it around 8 pm our time to get on top of the list most pelicans are asleep now. I woulden,t put it back in to be on the sure side a new chip is alot cheaper then a new dme.

Good luck

Lorenfb · 12-31-2003, 04:28 AM

You may have installed the chip incorrectly in the DME unit. Make sure
that the chip is oriented correctly in the socket. As you view the connector
from the front, the chip notch should be to the right on a 3.2 DME.
Hopefully, you haven't damaged the DME circuit board by an incorrect
chip installation.

Good Luck
Loren
'88 3.2

Thomas Owen · 12-31-2003, 04:45 AM

The DME's can experience failure due to poor or cracked solder connections. It is possible that in the course of removing and re-installing the chip you have disturbed a faulty or weak solder connection on one of the boards. My suggestion would be to remove the boards and check all solder connections with a magnifying glass, resolder where necessary. If you can still read the chip it is ok. Installing it backwards will not hurt the DME.
Good luck,

Lorenfb · 12-31-2003, 07:32 AM

When the EPROM chip is installed backwards, the VCC (normal +5 volts) and
the VSS (normal GRD) get forward-biased which results in a large substrate
current on the chip. This can in some cases damage the chip, e.g. some of
its inputs or outputs. It's fair to assume that if it can be read by an EPROM
reader, then it can be read by the 8051 in the DME.

The problem which occurs, though, is that when the substrate is forward
biased, the other chips connected to the EPROM are subjected to possible
heavy loads on their inputs and outputs. This may or may not damage the
other chips, e.g. the 8051 or the A/D converter. It's just not a good idea
to have the EPROM in backwards.

If it's a cracked circuit board or a bad solder connection, it's not easy to
find the problems without checking all the signals using a scope. A lot
of connections may look bad but are usually good and vice versa.

Good Luck
Loren
'88 3.2

mjshira · 12-31-2003, 07:38 AM

Marco

Loren is the guy to go to with DME issues. Send your DME to him or get one of his rebuilt ones. I spent about 2K on other parts before I heard about him and got a rebuilt from him. Now my car runs great!

James

cbsmallz · 12-31-2003, 08:48 AM

Hi Marco,

Were you grounded to the car (via a wrist strap) when fiddlin with the DME ? Were you grounded to the PROM reader (again via a wrist strap) when fiddlin with the PROM reader? Though a rarity, electostatic discharge is a well known destroyer of IC's.

How did you conclude the read was successful? Did you perform a checksum check of the data? You mention you got the same results the second time around. Is this with respect to the read or to the engine not starting? If it is with respect to the read, just prior to reading out the PROM the second time, did you clear and verify the clear of the memory used by the PROM reader to store the read data? I ask this because I've seen many a case where the PROM is not read by the PROM reader, for whatever the reason, and the PROM reader memory has the data from the prior read (in this case perhaps your 1st read). In this situation, the second read would, when compared byte by byte, appear as correct with the first read.

Check each and every pin of the PROM. Ensure that there is no crap that may have somehow found it's way onto the pins and ensure that the pins are not out of alignment or bent. Depending upon the connector used in the DME, a PROM reader connector typically (but not always) makes electrical connection with the PROM in a different place on the pin(s).

Check the DME connector very, very closely. Not only with respect to solder joints (as posted by others) but also with respect to the pin recepticles. It's possible that one of the recepticle pins is not making an adequate electrical connection with the PROM pin(s).

Regards,
Carlton

Por_sha911 · Por_sha911's Garage

Simple troubleshoot: Did you double check to see if you bent a pin when putting the chip back in?

ML952 · 12-31-2003, 11:51 PM

Hi,

thanks to all,

I just found a weak solder connection

Greetings,
Marco

Dutchie · 01-02-2004, 03:28 AM

So you fixed the problem ? Any pics of what you had to repair maybe ? A Porsche buddy of mine had something like you when his car was in an accident a few months ago. He had to buy a new dme in the end. Maybe you can help out others by sharing how you fixed it.

Cheers,

Ed

ML952 · 01-02-2004, 07:51 AM

The solder point at the IGN transistor (left side of DME, big transistor on heatsink)