Originally Posted by ischmitz (Post 10741427)

Let's do away with the myth here. An early tachometer is not "sending a signal back" or does anything bad that damages a 3.2 DME. The issue is something else:

A 3.2 DME has two outputs that sometimes get confused with each other. Pin 21 is the engine speed output and it is a buffered signal from T500. The BC546 transistor can easily drive any tachometer out there. More so, it requires the tachometer having an internal pull-up resistor to VBat. R500 limits the current through the transistor to less than 30mA even with a hard short to Vbat at 14.4V. This doesn't damage the DME at all as that transistor can handle 100mA continuous current.

The second output of the DME on pin 11 is a signal required for the shift light in the 3.2 tachometer. It comes directly from the Bosch 0127 IC and contains engine load information. And therein lies the problem. Once you connect pin 11 to anything but the shift light input of a 3.2 tachometer you are in trouble. The tachometer will initially work since the fuel pulses have the same base frequency as the ignition pulses. However, the 0127 IC gets really upset in the process (read: it destroys itself). This is due to the internal pullup resistor of the tachometer biasing pin 3 of that IC to Vbat. And within 10 - 15 minutes the DME will fail with no fuel signals. The higher the RPM the more stress it put on the IC and this affects how soon it fails.

I have repaired multiple 3.2 DME that were affected by this phenomenon. And trust me, a simple re-soldering will not fix that kind of damage.

Ingo

Originally Posted by tdw28210 (Post 10743655)

Wouldn't the sensible thing to do be to modify/block Pin 11 so it can't be tapped/used in anyway? Maybe snip it off a bit?

Originally Posted by mysocal911 (Post 10744279)

A simple solution is just to remove the 10 ohm resistor going to pin 11 on the PCB.
Bosch should have used a 100 ohm resistor instead of 10 ohms, that would have
prevented damage to the I.C.

Originally Posted by ischmitz (Post 10744369)

yes - remove one side of the resistor and that'll be perfect protection against this unfortunate failure mode. And in the unlikely event you ever need/want to restore the DME to stock you can just solder the resistor leg down again.

Originally Posted by wazzz (Post 12491003)

Reviving this thread to clarify one thing.
How come this resistor R786 is labelled 0R on the schematic? Until I read the posts above I always thought this was merely a 0-ohm resistor, or some sort of jumper to go over PCB traces.

Is the schematic wrong and is this resistor indeed a 10-ohm resistor?

Originally Posted by wazzz (Post 12491094)

Dave, thank you, I had noted this recommended change to 100 ohms. So you confirm the schematic is wrong about this resistor value and it should read 10R, not 0R?

Asking this because I'm currently editing my own copy of the Motronic schematics with annotations for ease of understanding, so I might take the opportunity to also fix any error. I have already fixed the swap between signals ADV12 and ADV13 and changed the R10 value to 0R10 for the current sense resistor (R786).

Originally Posted by wazzz (Post 12491786)

Same for us in France, never seen a shift light.

So, as I understand it, this indicator lights up when the injector pulse width reaches a certain value AND the engine speed is above a certain RPM threshold. The logic for both these conditions is inside the tach.

Originally Posted by wazzz (Post 12491786)

Same for us in France, never seen a shift light.

So, as I understand it, this indicator lights up when the injector pulse width reaches a certain value AND the engine speed is above a certain RPM threshold. The logic for both these conditions is inside the tach.

Originally Posted by mysocal911 (Post 12491792)

Correct, it's a special IC (~ 24 pins) in the tach doing the calculations.

Originally Posted by ant7 (Post 12491837)

Cheers Gilles. :)
Ant.

Originally Posted by wazzz (Post 12491856)

Cheers Ant,
Currently trying to decipher and understand as much as I can from the Motronic schematics. My circuit analysis ability has gone long ago and those schematics are not easy to follow. Trying to annotate them for reference.

Does anybody know why the ADC appears not powered? VCC is connected to ground via a 4.7k resistor. Or is it powered via VREF+ and VREF-?

I may open a dedicated thread later to ask other questions, hoping that guys like Dave, Ingo, Sal and others will chime in. :D