[Jazzbass]

So here's the inside of the ML3.1 DME that came with my 1985 911 Carrera. This is a picture I took during the 8k ROM upgrade I did in March 2008. Using the schematics I downloaded from FR Wilk, I've labeled and identified some of the main components.



1. S700 - The Intel 8051 Microcontroller that controls the DME. This is the brain the runs everything inside. More Info.

2. S703 - The ADC0809 8 bit A/D converter. This is how the 8051 reads analog inputs like AFM, Air temp, cylinder head temp (or whatever you water cooled guys use). It even reads the wide open throttle (WOT) and closed throttle switches, despite them being binary.

3. S100 - The mystery chip: the F12438. According to what I've read, this is the only custom part in the DME* and is the only part you couldn't buy for less than $5 from DigiKey and replace. It's apparently a custom chip designed by Bosch to drive/read the reference and speed sensors. If this goes, time for a new DME.

4. S701 - This empty socket is when the EPROM goes. If you look close enough, you can see the difference in color between the first 4 pins of the socket and the other 24 pins. This is because my DME was originally a 24pin, 2k ROM unit that I had just upgraded to a 28pin, 8k ROM unit. Instead of putting in a whole new socket, I just added the missing 4 pins.

5. X700 - The Fuel Quality Switch. Changing this changes the fuel mixture/ignition timing based on some charts I'll find later and post. What this switch does is different for each DME type.

6. Indicates the "digital" part of the DME

7. Indicates the "analog" part of the DME.

8. T560 and T561 - There are two transistors labeled "8". These are ON588 NPN Darlington driver transistors that control the Idle Control Valve. When your ICV shorts out, these are the transistors it takes with it.

9. T504 - The Ignition driver transistor. Labeled 1012 in the schematic and on the part itself, it is thought to be similar to a ST BU941 (Bill in OKC on Pelican made this connection and it seems to be the case looking at the data sheet for the BU941).

10. D1 - Big-ass OF622 diode that's part of the injector driver circuit.

11. T404 - RBDT93 NPN Transistor that's part of the injector driver circuit.

12. T402 - RBDT65A NPN Darlington driver that's also part of the ignition circuit.

13. S300 - L4705 5V regulator. Supplies Vcc to the digital board.

14. S800 - CA139 Quad OpAmp. Two of these drive the O2 sensor circuit. The other two are part of the coil/tachometer circuit.

15. T480 - ON588 NPN Darlington driver that controls the fuel pump part of the DME relay. More info.

16. S400 - Labeled simple 0127, this chip has something to do with the injector driver circuit. What? I have no clue. I suck at analog electronics.


* OK, it's true that the 8051 in these units is a custom mask-programmed 8051 that also cannot be purchased. However, since you can convert the ML3.1 DMEs to use an 8k external EPROM that contains all of the program code, this point is pretty much moot. Once you make the 8k conversion, the 8051 basically becomes a 8031 and all the special internal program code is ignored.

[racer]

Wow.. I may never need to understand any of what you wrote.. so I will just have to be impressed by the fact that someone knows what all that stuff is. Impressive

[Lupin..the..3rd]

I learned something while wiring up my G2X. Everyone says to tie the tach signal wire to Pin 6 of the DME. But what if you don't want to mess with the DME harness and you want to tie in to the backside of the tach instead?

Wiring diagram in the Bentley manual was not helpful in identifying which wire on the back of the tach corresponds to DME Pin 6, so I had to test them and figure it out myself. Turns out DME Pin 6 is running to Tach Pin 2. This is true on '87 Carrera at least, don't know about other MY's.

I just released Pin 2 from the connector on the back of the tach, opened up the little crimp, stuck in the G2X signal wire, then crimped & soldered and slid it back into the connector. Easy peasy.