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Re : '89 200TQ Decceleration cut-off valve, OXS sensor
I would like to thank Dave Lawson and Scott Mockry for quick and
detailed replies to my questions.
I have not started implementing the advice yet. Weather permitting, I'll
address the issue this coming weekend.
> Re: idle switch ok?: On my previous 5000T and on my 89 200TQ, I
> found that the idle switch had some cracked internal solder joints which made the
> switch work ok one minute and then go open circuit or have high resistance
> the next minute. I found this to be the case on another list members 87 5000TQ
> as well. If you have never replaced this switch or have never taken it apart then it
> very well could be the source of the problem. Taking the switch apart is a bit of a
> pain as the plastic cover is solvent welded on.
Yeah, I've done it a year ago and the procedure was a subject of one of
my first posts with this news group.
> The schematic also shows a coolant temp sensor that connects to pin "T" on the idle/decel
> control unit and that it should have a resistance of aprox 130 ohms at 176 deg F. You
> may want to verify this temp sensor is working ok.
I will do that.
> The signal driving the frequency valve (OXS test connector)
> that controls the mixture is around 70 Hz and is a
> Pulse Width Modulated (PWM) signal (resembles a square wave) It
> is the Negative Duty Cycle that you need to be concerned with. In other
> words it is the time when the signal is low (pulled down to ground) when
> the Frequency valve is turned on. I am not that familar with the WaveTek
> DMM's but in some cases you can select whether you are measuring the
> negative duty cycle or the positive duty cycle. Sometimes those obscure
> details are buried in the owners manual.
Aha! That explains it! The Wavetek measures Duty C-le as a ratio of a
"hill"/period. If it's a negative one, then it's a "valley"/period. I
get 60%, Bently sez 40%, so we're in the same boat here. That's a
> When you were viewing this frequency valve waveform using the oscilloscope,
> the sharp spikes you were seeing on the square wave were caused by
> the inductive voltage "kick" that occurs when the solenoid is turned
> off and is considered normal.
Audi could've invested $0.20 in a reversed biased 1N4002 across the
solenoid coil on this $40k+ car. Slubs.
> With the O2 sensor connected,
> engine and sensor warmed up etc. you should be able to view the
> change in the Pulse width as the ECU responds to the voltage swing
> from the O2 sensor. O2 sensor voltage goes high (~0.85 V, rich) and then
> the OXS frequency valve signal pulse width should be reduced, i.e. Duty cycle is
> reduced, then O2 sensor voltage swings low (~0.15V, lean) and then
> the duty cycle is increased slightly and so it goes cycling back and forth to
> tweak the mixture slightly.
> On my cyberdyne a/f meter, when rpms are above 1200 and throttle
> is closed, the o2 sensor reports no fuel, ie no bars are lit on the gauge.
I built my own version, that also listens to the OXS sensor and shows
the results on a colour LED bargraph (I did not like the Cyberdine's
enclosure from Radio Shlock and did not like the Hal Meter's zig-zaging
light travel). The light bar swings exactly like you are describing.
Incidently, before the Deccel Valve had stopped working, I did indeed
see the light disappear completely, when cruising downhill in a low gear
with my both feet clear of the pedals. No fuel was being delievered to
Right now the light bar happily swings left to right even on the
downhill. I am essentially feeding fuel to the the engine while using it
to slow down the car. Rather dumb. Need to address it immediately. Right
now the trip computer shows 12mi/gal. Ouch! And it ain't no Watkins Glen