Your experience and one other source would suggest that two transistors per side are involved in throttle control. Could the ON4239 be a PNP high side switch while the ON823 is PWM-ing the low side? That would provide some redundancy if either transistor were to fail with a C-E short.
Exactly. I just found this:
FunctionThe M70 used in the E32 and later in the E31 is the first BMW engine to use an electronic throttle
control instead of a mechanical connection between the accelerator pedal and the throttle valve. The
German term is Elektronische-Motor-Leistungsregelung (EML). The EML consists of a pedal position
sensor, i.e. a potentiometer connected to the accelerator pedal, an EML control unit and electric
actuators on the throttle valves. The DK are held in the closed position by a spring and can be opened
up to an angle of 88°.
The DK motors move the butterfly valves via reduction gears. They are supplied with a pulsed direct
current whose cycle length determines the torque output and thus the opening angle. The motors can be
controlled in both directions, i.e. opening and closing.
The motor of DK I receives its operating voltage
via contacts 17 and 35 of the EML control unit. For the DK II, contacts 16 and 34 are used. To open the
DK, positive voltage is applied to contacts 17 and 16, whereas contacts 35 and 34 are grounded. This is
reversed to close the DK.
The shafts of the buterfly valves are connected to a potentiometer which reports the opening angle back
to the EML control unit. In addition, a throttle position switch with a series resistor opens at 17°. A
circuit diagram is shown in Figure 1. The values of the resistors can vary from unit to unit [4][5]. The
accelerator pedal is also connected to a position switch which closes at 9°. Both do not influence the
EML’s function, they only serve logic monitoring purposes. In normal operations, both switches should
not be open at the same time. However, if this is the case, the EML control unit reduces the engine
power by closing the throttle valves.
I would start by metering the good transistors and determining whether they test like BJTs or MOSFETs or Darlingtons.
Is there some easy way to figure out if the part is a BJT or MOSFET or Darlington?
I have ordered a Peak Atlas DCA Pro DCA75, but it hasn't arrived yet.