DC power enters the AFRecorder Model 4800P through the smaller of the three keyed
circular plastic connectors (CPCs) on the rear panel. The input voltage must be between 11
VDC and 16 VDC. A 10 A Slow Blow fuse for the DC power is located below the
connector. Only the supplied 12 VDC power cord should be used. One end of the cable
mates with the smaller CPC on the rear panel and the other end should be connected directly
to a 12 VDC automotive battery so as to supply the cleanest possible power to the
AFRecorder. The red clip is positive and the black is negative. Use cable ties to keep the
cable away from hot or moving objects and ignition wires. For the case of a vehicle with two
12 VDC batteries connected in series, the ground of the AFRecorder power cable (black clip)
must be connected to the negative terminal of the battery that is connected to the vehicle’s
chassis. The red clip of the cable must be connected to the positive terminal of the same
battery.
A terminal block is provided to electrically connect the AFRecorder’s case to the vehicle’s
chassis, the engine block, the negative terminal of the battery, or a “ground mecca”. This
may provide improved noise rejection in some environments. A braided cable should be
used for this purpose. The AFRecorder Model 4800R joins the AFR sensor cable shields to
the case outside the instrument using a removable jumper. This gives the Model 4800R the
maximum flexibility for integration into specific installations. The AFRecorder Model
4800P joins the AFR sensor cable shields to the case inside the instrument.
Air-to-Fuel Ratio Hook-up
The AFR sensors are mounted in the engine's exhaust by threading them into M18X1.5 mm
bosses that are welded or brazed to the engine's exhaust pipe. This thread size is identical to
that of the exhaust oxygen sensors used in current production automobiles with 3-way
exhaust catalysts. Mounting bosses can be easily made by threading a length of 1¼"
diameter steel bar with a M18X1.5 mm tap and cutting off 5/16" wide pieces as they are
needed.
To mount a boss, first drill a ¾" diameter hole in the desired AFR sensing location. If an
individual cylinder's AFR is to be measured, drill approximately 12" from that cylinder's
exhaust valve. If two or more cylinders' AFR is to be measured using one sensor, drill
approximately 12" from where the cylinders' exhaust pipes join. These recommended
locations are based on trade-offs between sensor response time, sensor temperature, exhaust
mixing, and typical engine/exhaust packaging.
Long distances between the engine and the AFR sensors should be avoided because such
installations result in increased condensed water being sprayed on the sensors during engine
start-up. Liquid water striking an AFR sensor thermally shocks it and can lead to sensor
failure.
Many engines have air pumped into the exhaust as part of their emission control strategy.
The AFR sensors must be mounted upstream of where the air enters the exhaust system or
else the measured AFR value will be leaner than actual.
Before drilling, take into account the length of the sensor, any engine or chassis movement,
the routing of the cable, and avoiding the collection of liquid fuel or condensed water on the
sensor. After drilling the hole, clamp the boss over the hole and weld or braze it to the
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