89 200TQ, O2 Sensor, Decel Valve, Long post

[scott.john.mockry@bangate1.TEK.COM]

Igor,

I assume that your 89 200TQ has the early MC engine with single knock
sensor. If that is the case, here is what I know about the decel valve, O2
sensor....

Igor asked:
>On the Decceleration cut-off valve:
>1. A voltmeter is hooked up in parallel to the Cut-off Valve connector,
>which remains pluged in.
>2. The RPMs are risen above 2000 after which the throttle is released,
>slamming shut.
>3. The meter reads 90mv (~0v as far as this test goes), although should
>read 12v.
>4. Idle switch checks OK. Some text deleted

The 1986 New Model Information that describes the MC turbo
engine outlines the operation of the deceleration/idle stabilizer system. 
It indicates that the decel valve will only work with the engine above 1400 RPM, 
the engine temp above 122 degrees F and the throttle switch closed as per
your test procedure above. 

The decel valve provides an alternate path for the air flowing through 
the CIS air flow metering plate to allow the plate to fall and shut 
off the fuel during deceleration. The schematic indicates that the Idle
 stabilizer/decel control unit provides the ground for this decel valve
with the above mentioned conditions. The valve gets the +12 V 
from the fuel pump relay. You should verify that one side of the decel
valve is getting +12V when the engine is running.

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. I have also found mis-adjusted
idle switches when someone inadvertently tried to adjust the idle speed using
the throttle stop and then readjusted the switch incorrectly. 

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.

>On the OXS sensor:
>1. My Wavetek multimeter is connected to the OXS test connector, i.e.
>"common"(black lead of the meter) to brown and "V/f/%"(red lead of the
>meter) to double Blk/Blu.
>Some text deleted >3. The meter should read 40% duty cycle steady 
>regardless of the engine speed.
>4. In reality the meter reads 60% steady regardless of the engine speed.
>Is it possible that I am reading the same duty cycle, only my meter's
>aquisition occures on the rising front, whereas the VAG factory test
>device uses the falling front or vise versa (60%=3D100%-40%). =
>I hooked up my Techtronix scope, expecting to see a nicely shaped
>rectangular wave - no such luck. The graph looks rather funky, 
>>with sharp spikes superimposing the
>rectangles (perhaps some strobing impulses).

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. 

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. 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. If you partially remove your dipstick and watch
the duty cycle, you should see the duty cycle go up to 80-90 % as the
ECU trys to compensate for the lean mixture. If you have a two channel
scope you can watch both signals, a delayed time base works even better
if your scope is so equipped.... The O2 sensor should be viewed at 0.5 sec/div
setting and the frequency valve should be viewed at around 2ms/div.

I replaced the O2 sensor on my 89 200TQ after it had 80K miles on it
 and I found that the slits in the tip were partially plugged up with 
carbon which made the sensor respond very slowly. The idle mixture
was girating up and down which made the exhaust CO% swing between
 0.2% and 0.5%. It was taking the O2 sensor over two seconds to swing
high and low at idle. After I replaced the sensor, the O2 would swing
high/low in just under a second and the idle mixture, i.e. exhaust
CO% was  steady at 0.1%. The surging I was experiencing during partial
cruise when the car first warmed up was eliminated as well.

Hope that helps
Scott M.