Little Nugget from Steve Wong on ECUS, early Tachos and 3.2 Swaps

[tdw28210]

Originally Posted by ischmitz

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

Ingo, so I for one, could care less about the shift light. (That's why we have ears and a tachometer in the first place). 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?

[mysocal911]

Originally Posted by tdw28210

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?

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.

[ischmitz]

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.

[tdw28210]

Originally Posted by mysocal911

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

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.

Either of you gents have a picture of said resistor, for those of us who have never peeked inside an ECU?

[mysocal911]

Here's the resistor (R786), which was changed from 10 ohms to 100 ohms.

[wazzz]

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?

[mysocal911]

Originally Posted by wazzz

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?

Yes! Read post #22. As preventive measure, change it to 100 ohms.

[wazzz]

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).

[mysocal911]

Originally Posted by wazzz

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).

It's not a current sense resistor. The output is a pulse signal to the tach for the upshift indicator.
The original value was 10 ohms, which causes a failure of the 0127 when wiring problems occur.

[wazzz]

Sorry, my bad. I meant R409, which is indeed the current sense resistor that goes to pins 14 and 15 (injector drive line). This resistor value is shown as R10, while it is in fact 0.1 ohms. So it should be labelled 0R10.

As for resistor R786 that goes to pin 11 (tach upshift indicator), its value is currently labeled 0R and I will relabel its value to 10R.

As I said I have also swapped signals ADV12 and ADV13 and changed their arrow head directions accordingly.

If you know of other errors on these schematics, please let me know. Thanks.

[ant7]

Just curious, was the upshift light only on US models ?
I ask as I am not aware of that function on any UK cars.
Ant.

[wazzz]

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.

[mysocal911]

Originally Posted by wazzz

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.

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

[wazzz]

One more customized IC that is no longer available?

[ant7]

Cheers Gilles.
Ant.

Originally Posted by wazzz

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.

[ant7]

Interesting!
Any idea why it was limited to certain markets ?
Ant.

Originally Posted by mysocal911

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

[wazzz]

Originally Posted by ant7

Cheers Gilles.
Ant.

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.

[zuch]

I believe the up-shift light was added to the US market cars in an attempt to improve fuel efficiency. The idea being if you shift to a higher gear sooner you will burn less fuel.

[mysocal911]

Originally Posted by wazzz

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.

Datasheet - https://www.alldatasheet.com/datasheet-pdf/pdf/619872/TI1/ADC0808CCN.html

Vcc (+5V) is pin 11. Various semiconductor suppliers produced this part.

[3rd_gear_Ted]

Side question:
For us folks that are high level Object Oriented

Is there any kind of Entity Relationship Drawing (ERD) that shows the flow of the logic thru the fundamental logic blocks for a typical Motronic ECU???