Carpokes

Is it pin 3 of the 8051?

Tom wrote: Wed Sep 03, 2025 7:51 pm Is it pin 3 of the 8051?

No, the ADC (marked S703 on the board)

It's common for the idle routine to use ignition timing to make quick adjustments to idle speed. The ISV works slower. That's why the Idle Timing map uses relatively low values at idle- if needed, the code can add timing to bring idle speed up. If idle timing was already near max, then adding timing would have no effect. This is a common strategy in every ECU/DME I've tuned.

The timing and ISV values usually go through a PID routine to determine how much of a change is needed, it'll look at how far off the current RPM is from target RPM. In modern DME/ECU's there will be a PID to see how quickly the RPM is moving away from or toward target speed.

It's possible the 'compare to 49' section is a min/max check. Perhaps if the load at idle is higher than the table value, it may exit the idle routine and assume it's on the part throttle map, or limp mode? Or maybe it won't go into idle mode until Load drops below the map value? Just throwing out ideas.

Actually I made a mistake about ADC channel 5, I only connected power and ground to the DME. I should have had pin 28 grounded so I'll have to take another look tomorrow...either way it's definitely a fixed value.

If it's a fixed value, I wonder if Bosch used the same code on different hardware, and if there is something on that channel on the European DME's? Or maybe for future use that never got used....

Dave -- thanks for that info on the idle timing, makes perfect sense...

OK here's what I found out. I tested a DME on the table again and this time I took care to ground pin 28 of the main connector as shown in the schematics. Then I see ~0v on ADC channel 5 (pin 3). In the code, the only check that's done on this is to check if it's < 128 decimal (i.e. 2.5v) which it should be. In that case, a flag is set that causes the car to use the map that Dave W's car is using.

So @Tom I don't know what's going on - if you happen to have your DME removed at some point it would be interesting to know if pin 28 of the main harness is grounded. If it's not, then that pin on the ADC is pulled high and you would get the map that your car is using.

Looking at the code paths that this flag controls, they're mostly the same and it really looks like the alternate one is just something that was used for testing or some kind of region coding that was abandoned. I'd be curious to know if your idle speed changes with or without AC?

One thing I don't see is where your extra 40rpm is coming from. There's a check done for the AC switch and a minimum value is substituted for the map value if AC is on. But in the normal code path it's 840 which is already the lowest value, so it has no effect. And in the code path that uses your map, the AC min value appears to be set to 0 so it still wouldn't have any effect. But it's totally plausible that there are more minimum checks elsewhere that clamp it to 840 so it's not a big deal.

As far as the map Rogue referred to in the XDF, it's definitely very close to what Dave W and I were thinking. It's a max load map for idle, which depends on engine temp. If the load value for the current temp is reached, then the ISV won't add any more air even if the rpm is below target. The spark advance routine later checks the current rpm against a single stored value (which is set to 14h, i.e. 800rpm) and adds timing if it's below that target.

Awesome work, thank you! I have a theory that I'll test tomorrow. The DME harness does ground pin 28, I've confirmed that in the past. However, when Vitesse Racing first designed his MAF conversion, he repurposed pin 28 to accept a MAP sensor signal. He would have you would cut the harness wire going to pin 28 and send the MAP sensor signal into that DME via the cut wire. When Rogue Tuning introduced its MAF, it did the same thing. I have a Focus 9 DME in my car right now from @FTECH9, who worked closely with Rogue when developing the Ftech9 DME. I suspect the grounded harness wire on pin 28 is not connected inside the Ftech DME, so that that ADC input could be used to connect a MAP sensor. Just a more elegant way to hook up the map sensor without cutting harness wires. If so, it would explain why that signal is pulled high on my car, and low on Dave's...

From a Rennlist post by Ftech9 while developing this DME...

MAP Sensor Support
By convention, the unused analog input on pin 28 (ADV28) of the DME system connector is used as a MAP sensor input. I will follow this convention and default this input for that purpose.

I might have missed it in the details but would love to see the map details/parameters for idle you’ve come with. I’m still working on perfecting my idle using a standalone.

Great information! Thank you.

944m3 wrote: Fri Sep 05, 2025 9:25 am I might have missed it in the details but would love to see the map details/parameters for idle you’ve come with. I’m still working on perfecting my idle using a standalone.

Great information! Thank you.

The most recent discussion is about finding the idle RPM targets in the stock DME. Not sure how useful that would be for a stand-alone? That said, I've been working on my own idle (via the DME) and can confirm there are a lot of factors/parameters that go into a nice idle -- and I'm sure many I'm not even aware of... I'm working on a DME tuning guide, and some of that may be useful regardless of ECU...

That would be fantastic. I’m sure I’d be able to use some if not all of that info.