Turbo control in a GM ECM

[avos at cochlear.com.au]

     The solenoid sits in the line between the manifold (or possibly before 
     the throttle butterfly/intercooler) and the wastegate. When the 
     solenoid is open minimum boost is run as the wastegate is opened due 
     to the boost getting to it. When the solenoid is closed, no boost gets 
     to the wastegate, and it stays closed, so maximum boost is run. The 
     question is - Is the solenoid normally open or normally closed. My 
     guess is normally open, and therefore is something goes wrong boost 
     should be minimum. The ECM probably had two aims in controlling boost, 
     the first is obvious, which is to maintain the set boost level as 
     accurately as possible, the second is to maximise boost at low revs. 
     The method of maximising boost at low revs is to leave the wastegate 
     closed until the last possible moment when it needs to open to control 
     boost. This is probably why the output is grounded most of the time. 
     This way the wastegate can't open and boost is maximised. This output 
     will probably be high % grounded until about 75% of full boost is 
     reached, as this will close the wastegate until close to full boost is 
     reached therefore maximising the boost at low revs. If this wasn't 
     done, the pressure to the wastegate would open it well before max 
     boost is reached which is a waste. Look up some of the posts on 
     electronic boost controller for the theory behind this.
     
     Adrian Vos
     avos at cochlear.com.au


______________________________ Reply Separator _________________________________
Subject: Re: Turbo control in a GM ECM
Author:  diy_efi at coulomb.eng.ohio-state.edu at INTERNET
Date:    06/10/97 7:55


Shannen Durphey <shannen at mcn.net> wrote: 
>What did this ECM come out of?
     
The version with the extra (wastegate?) PWM output is from a 1985 Pontiac 
Fiero with the 2.8 MPFI V6.
     
>The turbocharged diesels with efi (94-current 6.5) use a pwm control to 
>a solenoid in the vacuum line to the wastegate.
     
Vacuum?  Vacuum?  During turbo boost?  :-)
     
>Boost is measured by a map sensor.
     
Yup.  And it also "runs" the engine.
     
>No ground for the solenoid results in vacuum vented to atmosphere and no 
>boost.  Any trouble and the ECM pulls the plug on boost.
     
This seems to match the logic in the function.
     
>I don't know why the solenoid would
>ever be fully grounded, the pickup never goes over 90% on time.
     
It looks like the function will ground the output 99.9% when there is no 
boost.  I haven't figured out the logic enough to say what range of computed 
values it will produce.
     
                   unsigned long BinToBCD(unsigned long i) {unsigned long t;
Ludis Langens         return i ? (t = BinToBCD(i >> 1), (t << 1) + (i & 1) + 
ludis at netcom.com                 (t + 858993459 >> 2 & 572662306) * 3) : 0;}