On the subject of distributors: I was quite badly surprised when I realized that my distributor is
totally incompatible with the PM6 ECU I decided to use after the swap. In the Honda distributors one
can find a lot of things: The HV coil, the electronic switch that pumps the current through the coil, and
some sensors: 2 ( for DPFI) or 3 ( for MPFI). Each sensor consists of a Ferrite sprocket with a few
teeth and a pickup coil next to it. When a tooth passes by the coil, current is induced in the coil and
the ECU then bases some decisions on those signals. The Cylinder Position Sensor (CYP) looks like
a snail shell and produces 1 impulse per camshaft rotation. In my Si distributor it was located in the
bottom of the distributor: the place closest to the camshaft. This sensor is only present in the MPFI
distributors and if it's missing, the ECU will throw an error code 9. You CAN run your car without that
sensor, so in some cases, it is possible to make the swap without an Si distributor. Do that only until
you get a real Si distributor.... this is a temporary solution. As far as I understand, this sensor is
mostly used to synchronize the ECU and tell if when cylinder 1 is at TDC.
Next on the sensor list is the Top Dead Center (TDC). In my Si distributor, this was the middle
sensor, but in the DPFI (DX) distributor, it was located on the side of the side of the distributor shaft
that is furthest from the engine. It is a sprocket with only 4 teeth and therefore produces 4 impulses
per camshaft revolution. The ECU uses it to know the times when there is a piston at
top-dead-center. The ECU doesn't know which cylinder this is based on this sensor alone. It
combines the information with the signal from the CYP, to find out exactly which piston is at TDC.
This sensor is found in both DPFI and MPFI distributors and looks the same in both.
The last sensor, and certainly the one I had most trouble with is the Crank Angle Sensor (CAS). On
the MPFI distributors, it is a sprocket with 16 evenly spaced teeth. As I found out, on my 90-91 DPFI
distributor, it had 20 teeth: 4 groups of 3 closely spaced and 2 widely spaced ones. The signal from
this sensor is believed to help the ECU decide when to ignite the mixture i.e. the spark advance. If
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the ECU doesn't have that sensor, or doesn't like the signal from it, it throws a code 4, and goes into
"Limp" mode to prevent any damage to the engine, but to still give you the chance of reaching the
closest repair shop. You can easily identify limp mode by the fact that the ECU cuts your fuel at
about 3200-3300 RPM. It's ugly, but the car runs... One thing I realized while trying to work with an
MPFI ECU and a DPFI distributor is that if the ECU doesn't find and signal from the CAS, it
immediately throws a code 4, goes into limp mode and stops checking for other sensors. In my case,
the ECU didn't send a code 9 even though i didn't have a CYP sensor. When I connected my DPFI
CAS to the ECU and reset it to clear the errors, the ECU first threw a code 9 ( it was seeing some
CAS impulses) and after a second or two, it threw a code 4 and went into limp mode. The difference
was that firstly the ECU threw a code 9 and after some time, it realized that the 20 CAS impulses
were not what it was expecting. At the time, I wasn't able to find an Si distributor, so I carefully
removed 4 teeth from my CAS sensor. Remove the middle tooth in all the four 3-teeth-groups. This
will leave you with 16 evenly spaced teeth.
Worked like a charm!!! Now, remember this. The removal of the teeth is only a temporary solution.
The car will run really well, but the engine will not have a CYP sensor and will not know the order in
which to pulse the injectors. This proved not to be a big issue, but hey, you want a perfect install,
right?