vw 20v PWM idle, help please!

[Denisuk]

Hello everybody. So i've swapped an 1.8 20v engine (engine code AGN, from mk4 golf) in my 1988 vw scirocco.
I'm using v3.0 board, stock temp sensors, vr sensor, stock coilpack, stock throttle body with TPS and PWM idle regulator.
Everything works just fine, except idle control. (As for hardware, all is ok, i upgraded pn2222 transistor to TIP120, using msextra manual schematics)
This is oscilloscope diagram from autodata 3.24 for stock ECU pwm idle control (2v/2ms):

So if T=2ms, then valve frequency is 1/0.002=500Hz
10000/500=20, so this is the idle valve frequency value i need to set in megatune:

next step, i need to calculate true dc (diagram above shows idle speed on hot engine, about 800 rpm)
pulse time is 0.5 ms, period is 2 ms, so true dc must be 25% (100x0.5/2). In megasquirt values it's 5 (5x100/20=25% true dc)
Setting this to idle duty at upper temp.
Setting idle duty at lower temp to 10 (true dc=50%) for higher rpm when engine is cold.
Starting a cold engine approximately at 10 degrees of celsium (true dc is about 43%), and having 3000 rpm, slowly raising to 4000 rpm. Warming up to about 50-60 degrees - no difference, may be little lower rpm (3500 or about it).
80 degrees - a little slower rpm, about 3000 rpm.
Fully warmed engine (radiator fan working) - rpm drops to 1100-1200, trying to raise rpm a little by gas pedal - getting about 2500 rpm, then very slowly falling to 1100.
So i'm thinking that overall dc is too high, so setting it lower:
lower temp dc is 6
upper temp dc is 3
Starting a cold engine at 10 degrees - and it seems there is no idle control at all. With no difference at cold or hot engine.
So, where am i wrong?
BTW, what is minimum dc step in my case? is it 1dc, or is it 0.05dc? May be i should try lower frequency?
My laptop is dead, so i need to take megasquirt home to my PC every time to set different values, and take it back to the car just to test settings
Maybe someone with 1.8T engine (same pwm idle control) have sold this problem?
P.S. Sorry for my bad english, i'm russian

[jsmcortina]

Enter the duty cycle, not a calculated number.
i.e. if you want 25%, enter 25.

I would also suggest that you will need to run the valve more slowly to get any reasonable precision. Otherwise the step size is too large.

James

[Denisuk]

Ok, so with the 500 Hz valve frequency the step size is 1 dc, wich is large. I'll try to lower it to 100-200 Hz next time.
But what is true dc? What does this value mean? Is that a real valve duty cycle? I tried to set higher dc values, such as 25. In that case i have 4000 rpm constantly. I calculated true dc for this value (25х100/20=125%), so the valve seems opened all the time. Am i wrong?

[vlopsahl]

Hi
Any news about this. I also measured the idle control to 500 Hz. I was thinking about using this http://www.jbperf.com/pwm_converter/index.html to be able to use the high frequency the oem ecu use. I do not know if it will work anyway.

Thanks
Vegard

[prof315]

Most people running 20v motors with MS ditch the throttle body with the integrated IAC and replace it with an OBD1 (93-95) ABA throttle body and IAC. The throttle body is a direct bolt on and the 2 wire seperate IAC is much easier to control. It does require some plumbing changes but as far as I know nobody has had any great success with the integrated IAC

[vlopsahl]

Yes, I know. I have done this myself on one install, with a Bosch PWM-valve. It would be interesting to make the OEM work though. It can’t be impossible.

Vegard

[colossusuk]

i can confirm i have used the pwm convertor at 500hz and many other values and couldnt get it to work. it either opened all the way or partly, but then you had to open the throttle manually and it would stick there. i didnt have any control over it tho.

it would be nice to get this working, but i'll have to look into plumbing in a seprate 2/3 wire idle valve at some point.

cheers,

james.

[vlopsahl]

I did a little research on my OEM Audi A4 B5 today. I scoped the two wires driving the DC-motor in the throttle housing. At steady idle it is as you can see to the right, stable with a frequency of 500 Hz and a period of about 0,4 ms period. I gave it some throttle and let go. When the ecu senses the rpm going down it inverts the signal to the motor. Then when the idle is stable again it switches back and looks like the picture to the right. I am not sure how this actually influences on the throttle plate but this seems to be a complicated system…

Vegard