A
Anonymous
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I got an extra Yellow grommet D2 from the boneyard for my little MPG project - cost me $5. I could have it installed by now if I didn't have to spend the weekend putting a new A-arm in the wagon.. 
I'll add a pix of the sensor, when I figure out how to post it here.
Here's how it works: the Yellow grommet D2 module has an extra 3-pin connector coming out of it. If you connect nothing to this connector, it acts just like the black or blue grommet versions. BUT, if you add the vacuum sensor onto the extra connector, it adds 2 degrees of advance. If you connect manifold vacuum onto the sensor, it adds 6-10 degrees of advance (depending on which sensor you have).
Now, here's the good part: the system retards all of the advances while the starter is engaged - this helps starting, since less=more where starting advance is involved. Then, when the manifold vacuum comes up, it adds 8-12 degrees of spark advance. Now, when you pull a heavy load with the engine (low manifold vacuum), the advance drops back off to the 2-degree advance setting to prevent ping. So, since my original advance should be 10 degrees ( this is an EGR engine - that's 8 degrees for many non-EGR 200s), this gives me 22 degrees at idle and down to about 5" Hg of vacuum, becoming 12 degrees when I'm standing on it, 10 degrees when it's being started.
Overall, this extra advance adds lots of efficiency to the engine. The idle, for instance, must be turned 'way down so that it idles around 550-650 RPM (depending on your cam), which becomes about 350 RPM if you unplug the sensor from the D2 unit.
This system is found in many LTDs and T-Birds of the late 1970s and early 1980s. It's easy to recognize in the boneyards; the D2 has 3 connectors and the sensor is usually located in the front driver's side fender, near the headlight. Under the greasy stuff, the D2 grommets are both yellow. The sensor has a real long vacuum line that goes to the manifold vacuum at the back of the block. This long vacuum line prevents a too-sudden switching action when you stand on the throttle. Be sure to make your line long, too - 4 feet is about right.
Technical details: there are 3 wires going to the sensor connector and 2 wires to the sensor itself. There is a 5100 ohm resistor connected inside, between the BROWN and BLACK/WHITE wire. This signals the D2 that the sensor is connected and makes the D2 switch into ECONOMY mode, immediately adding 2 degrees of spark lead (this 2 degrees goes away during starting mode). There is a vacuum switch connected between the BLACK/WHITE wire and the YELLOW wire. This switch is closed (shorted) until the vacuum applied is over 5" Hg. High vacuum (open switch) causes the D2 to switch to the long spark lead.
I'll add a pix of the sensor, when I figure out how to post it here.
Here's how it works: the Yellow grommet D2 module has an extra 3-pin connector coming out of it. If you connect nothing to this connector, it acts just like the black or blue grommet versions. BUT, if you add the vacuum sensor onto the extra connector, it adds 2 degrees of advance. If you connect manifold vacuum onto the sensor, it adds 6-10 degrees of advance (depending on which sensor you have).
Now, here's the good part: the system retards all of the advances while the starter is engaged - this helps starting, since less=more where starting advance is involved. Then, when the manifold vacuum comes up, it adds 8-12 degrees of spark advance. Now, when you pull a heavy load with the engine (low manifold vacuum), the advance drops back off to the 2-degree advance setting to prevent ping. So, since my original advance should be 10 degrees ( this is an EGR engine - that's 8 degrees for many non-EGR 200s), this gives me 22 degrees at idle and down to about 5" Hg of vacuum, becoming 12 degrees when I'm standing on it, 10 degrees when it's being started.
Overall, this extra advance adds lots of efficiency to the engine. The idle, for instance, must be turned 'way down so that it idles around 550-650 RPM (depending on your cam), which becomes about 350 RPM if you unplug the sensor from the D2 unit.
This system is found in many LTDs and T-Birds of the late 1970s and early 1980s. It's easy to recognize in the boneyards; the D2 has 3 connectors and the sensor is usually located in the front driver's side fender, near the headlight. Under the greasy stuff, the D2 grommets are both yellow. The sensor has a real long vacuum line that goes to the manifold vacuum at the back of the block. This long vacuum line prevents a too-sudden switching action when you stand on the throttle. Be sure to make your line long, too - 4 feet is about right.
Technical details: there are 3 wires going to the sensor connector and 2 wires to the sensor itself. There is a 5100 ohm resistor connected inside, between the BROWN and BLACK/WHITE wire. This signals the D2 that the sensor is connected and makes the D2 switch into ECONOMY mode, immediately adding 2 degrees of spark lead (this 2 degrees goes away during starting mode). There is a vacuum switch connected between the BLACK/WHITE wire and the YELLOW wire. This switch is closed (shorted) until the vacuum applied is over 5" Hg. High vacuum (open switch) causes the D2 to switch to the long spark lead.
