MC Waste Gate Frequency Valve operational info

Wallace White wallace at
Wed Jul 25 08:44:49 EDT 2001

Bernard -

I've been out of town, so excuse me for being very late on this reply.
The ECU uses a pulse-width modulated (PWM) signal to the WGFV of varying
duty cycle. There is no analog voltage involved in the control of the
WGFV, at least not outside of the ECU box. 

If you have a duty cycle or dwell function on your DMM (Sunpro makes a
cheap automotive DMM that I got for $15 that has dwell), you can measure
the duty cycle at the WGFV. Or you can hook up a test LED across the
terminals, as Scott Mockry's site describes. The duty cycle ranges from
0 (no LED) to 50% (LED flashing) at WOT, depending on other variables.
The frequency is ~5 Hz.

Once you do that and verify that the electrical is in order, you can
check whether the WGFV is working. You should be able to do so with a
vacuum pump (Mityvac) and the ECU's diagnostic mode (bridging the fuel
pump relay's fuse socket).

Or maybe you've already fixed it by now!

- Wallace
  '87 5kcstq 175k

Bernard Littau wrote:
> Hi List,
> I am debugging a low boost problem, and I am down to the distinct
> possibility that the waste gate frequency valve, or its input, are not quite
> correct.
> I get roughly the same max boost in all three of these cases: 1) no upper
> hose on Waste Gate, 2) upper hose OK with stock ECU, and 3) upper hose OK
> with 2.0 bar ECU.  When I remove the lower hose of the waste gate, or pipe
> the manifold hose directly into the top of the waste gate bypassing the
> solenoid, I get serious boost!  Everything else checks out, too.
> Do any of you MC'ers out there know how the Waste Gate Frequency Valve is
> supposed to operate in terms of inputs and outputs?
> I can see the output is a duty cycle between pressure and vent duration that
> allows the ECU to vary the pressure added to the top of the waste gate in
> order to compel the waste gate to produce more boost by staying closed
> beyond the waste gate spring pressure.
> I haven't gotten out to the car to examine the input side -- I was hoping
> some kind soul had already done this and could save me some steps.  I only
> have a DMM, no scope, so I am a bit limited in my ability to track faster
> signals.
> My fantasy is that the input to the Waste Gate Frequency Valve is a variable
> voltage from the ECU that is translated to a duty cycle frequency of on-off
> signals to the Waste Gate Solenoid.  I would further fantasy that more volts
> would mean give me more boost, please. (ECU's are very polite :-)
> It would make sense in my case that the input is reduced in voltage, as the
> duty cycle seems to be not aggressive enough to generate the ECU requested
> boost.  Old wiring would certainly account for a voltage drop.  Hey, I can
> hope I understand all this stuff ;-)
> Does anyone have the SAE paper mentioned on Scott's web site describing the
> operation of the MC boost control?  I am thinking the info I might wish for
> is in that paper.
> Thanks,
> Bernard Littau
> Woodinville, WA
> '88 5ktq

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