Wayne Dresser Vista 1 Series, Service Manual

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Part No. 920160 Rev A February 1997
3.7. PULSER ASSEMBLY
The pulser assembly consists of two parts:
• Photocoupler
• Pulser Disc
3.7.1. Photocoupler
The photocoupler (see Figure 3-7. and Figure 3-8.) consists of two infrared
LEDs and two phototransistors, one for each LED. These four parts work
together to provide two separate pulse return lines to the computer. These pulse
return lines are labeled “pulse A” and “Pulse B”. The computer must receive the
pulses in an alternating sequence (i.e., Pulse A, Pulse B, A, B, A, B…). In this
way the computer can determine correct operation. If the computer detects too
many A or B pulses in a row it will assume that the photocoupler is defective,
shut down the sale and generate a fault code (see Table 3-1.). If the computer
senses that the meter is turning in the wrong direction, it will also shut down the
sale and generate a fault code. Figure 3-7. shows how the photocoupler is wired
for non-blending dispensers.
Blending dispensers combine two products to attain a mix with the desired
octane. This requires two photocouplers, one for each grade, to operate at the
same time. In these dispensers the LEDs in each photocoupler are wired in
series. Because of this, a problem in one of the photocouplers can cause symp-
toms to appear on the other. For instance if the LED in the LO product photo-
coupler was to become an open circuit, no voltage would be applied across the
LED in the HI product photocoupler. This would cause symptoms to appear on
the HI product as well as on the LO product photocouplers. Figure 3-8. shows
how the photocoupler is wired for blending dispensers.
3.7.2. Pulser Disc
The pulser disc is plastic with square holes cut around its outer edge. It is
connected to a shaft which is turned by the meter. The holes in the edge of the
disc pass between the LED’s and the phototransistors. This alternately blocks
and reveals the infrared beam to the phototransistors creating the pulses.