[William Miller]

The one thing I know for sure is that I herd you comming and going. The sound when you guy's left was sweeeeeet!

I read the Bosch book a little this morning to get a better understanding of the dwell meter readings we saw.
First some quick background data:

At 50% duty cycle the dwell meter should read 45 deg (1/2 of 90 deg)
9 deg dwell= 10% duty cycle.
45 deg dwell = 50% duty cycle
90 deg dwell = 100% duty cycle
There is a good diagram in th book that helps you convert the dwell reading to duty cycle at a blink of an eye. (I might post that later.)

There are a few basic readings that should be checked.
Starting at the bottom of the scale:

10% duty = closed loop lean stop (This indicates a very rich mixture and is the limit of the control unit trying to lean out the mixture. Test: get a 2 volt battery, ground the - side to the engine, touch the O2 sensor wire to the plus side of the battery. (This replicates the voltage comming from the now disconnected O2 sensor when it is reading at it's lean limit.)

45%-55% duty = normal tempature closed loop operation. The needle should move consistant with the control units attempt to maintain 14.7-1 AFR.

50% duty = open loop warm up. This is what you should read after the tempeature is warm enough to close the contacts on the thermo switch, but the O2 sensor is not hot enough to provide the correct signal. 50% is also the default if the O2 sensor is disconnected. The book also mentions that 50% is the full enrichment output on some cars that have an acceleration switch on the throttle. (My guess is that is what the SC does.)

60% duty = open loop cold. This is what you should read before the tempeature reaches the setpoint on the thermoswitch and it's contacts are open. (It mentions that this switch setpoint is normally arround room tempeature.) I'll look it up later.

95% duty = closed loop rich stop. This indicates a very lean condition and is the limit of the control unit trying to richen the mixture with 0 voltage from the O2 sensor. The test is to ground the lead that goes to the O2 sensor.

Remember O2 sensors are like a generator fed by Oxigen the more O2 in the exhaust gas the more voltage it put's out or sends to the control unit.

I think the accelerator switch can be tested by watching the dwell meter while you close the switch. The meter should go from fluctuating from 45-55% duty to a steady 50% duty.

The 2 warm up, open loop modes can be tested by first jumping the 2 wires of the thermo switch together (in effect closing the switch) and or disconnecting them (opening the switch) This needs to be done after the car has cooled for a while (O2 sensor has to be cold.) You can also watch the dwell in different modes as the engine tempature ans the O2 sensor warms up.
These make sense on a car where everything is working properly.
It also helps diagnose symptoms.

Last night when we first hooked up the dwell meter we couldn't understand why the reading was so high. Unplugging the O2 sensor brought it to where it should default 50% in open loop.

Now after reading a little this indicates to me that the wire going to the O2 sensor is grounded out some where along the way or the O2 sensor is bad or dirty. This condition told the control unit that it was running very lean and the control unit was attempting to richen the mixture. As Charlie said it was eye burning rich so it couldn't have just been the mixture adjustment being off. With the sensor now disconnected the frequency valve is running at 50% duty cycle and I think Charlie got it pretty close to the correct mixture at idle.

Ken, I think think you may be missing one feature running in open loop.
This is probably debateable. The system is running in open look and probably adjusted somewhat rich. What you are missing is the acceleration enrichment function because you are already running at 50%.
I don't know, maybe porsche goes as far as the 60% duty cycle when the throttle switch is closed.
Charlie, take it from here. My fingers are tired... for now.
I never did learn how to type the right way.































Open loop = 50% duty/45deg on the dwell meter.
This is what we saw with the sensor unplugged it is also what