Higher voltage on Aux 9/10 supply

[KarlB]

I'd like to get more torque out of my e-throttle motor by installing a 24V boost converter on the "+14V Aux 9/10" pin. I know the MC33886 H-bridge driver in my G4+ Xtreme is rated to 40V on the input.

 

Is this supply rail isolated internally or is it tied to some other protection circuitry that would be unhappy at elevated voltages? For example: the Aux 9/10 rail might be diode isolated to the main rail for load dump protection.

Are there filter caps on this rail? If yes, are they rated to that higher voltage?

What is the "+14V Aux 9/10" voltage sense circuit scaled to? Will I rail the ADC if I put 24V on that net? If so, is the ADC input externally overvoltage protected or are you relying on diodes internal to the micro to provide that protection?

 

I'm trying to understand if there are any technical reasons why this won't work. I understand that there are thermal limits in both the driver and the motor windings that may preclude this from being an effective solution.

[Adamw]

I will have to ask engineering to confirm some of this.  However I first want to understand why you need to do this.  What are you trying to control that needs more current?

[KarlB]

I have a set of S54 throttle bodies driven by a modified S85 actuator. The motor requires ~70% DC before there is any actual motion in the system. The system oscillates wildly without a very low P-value, which instead leads to integral windup and large overshoots. The lack of a feedforward table means I'm basically dead-ended without making hardware modifications.

I'm in the process of removing some of the return springs (which I'm reluctant to do in the first place). I'm also not convinced this will solve the problem, so I'm trying to explore all my options.

[Adamw]

The problem with the S54 throttle is the throttle shaft is not centred in the bores, so if you let the throttle close too far the vacuum behind the plates pulls it shut very hard and it takes a lot of force to overcome that.  On the few I have set up you need to run very retarded idle timing and have the idle valve closed to its minimum which then means the throttle plates need to be open quite a bit more just to achieve a normal idle RPM - the vacuum effect on them is then much less difficult overcome.  I have done 2 cars locally and didnt need to remove any springs.

 

[KarlB]

Bumping this to provide an update. The car is back together now (after some cylinder head woes). We removed four of the six throttle return springs and re set all the throttle plates. It idles nicely with fully closed throttles at 850RPM with 15 degrees of timing. The e-throttle response is much better. It's still not great, but seems to be acceptable. I did some testing with retarded ignition timing before pulling the springs and I wasn't happy with the resultant exhaust gas temperature.

The idle base DC table is set to 3D and uses AP(main) as the secondary axis. This allows tip-in to be smoothed out by feeding in air via the idle motor at low pedal angles.