Speedhut CJ (Jeep) Speedometer Cluster - Manual (Page 5)

CJ GPS Speedometer Cluster - Tachometer Connections
(Available on some models)
- 5 -
- negative +
Yellow wire
1 revolution of engine
8 CYL early style single coil example
(dia 1)
1 revolution of engine
8 CYL twin coil packs
(dia 2)
C2
To spark plugs
Yellow wire
C1
2 revolutions of engine
8 CYL coil on plug
(dia 3) Yellow wire
Setup the Tachometer to run
2 pulses per rev when
connecting it to the engines’ ECU.
ECU
Tachometer (Yellow)
ACC (12v)
4.7K -10K Ω 0.25 watt minimum
(dia 4)
Type #1 ignitions
- negative +
Yellow wire
Yellow wire connects to:
negative side of coil.
12 cyl = 6 sparks / rev
10 cyl = 5 sparks / rev
8 cyl = 4 sparks / rev
6 cyl = 3 sparks / rev
4 cyl = 2 sparks / rev
(see step #4)
Yellow wire connects to:
tachometer output terminal
12 cyl = 6 sparks / rev
10 cyl = 5 sparks / rev
8 cyl = 4 sparks / rev
6 cyl = 3 sparks/ rev
4 cyl = 2 sparks / rev
Yellow wire connects to:
negative side of coil (some cars) or
coil control wire (some cars) or
coil trigger wire (some cars).
1 spark / rev. (as a good starting point)
(see step #4)
Yellow wire connects to:
negative side of coil (some cars) or
coil control wire (some cars) or
coil trigger wire (some cars).
1/2 spark / rev. (as a good starting point)
(see step #4)
Diag 5: Tachometer yellow wire connection options
Yellow wire
C1
g ul P n o li o C - 3 # e p yT s kc a P li o C -2 # e p yT
Yellow wire
Aftermarket ignitions / tach output
Yellow wire
Important note: Connecting the tachometer to the wrong wire will NOT damage the tachometer or your ignition.
Your vehicle ignition system will fall under one of these 4 ignition types. The type of ignition system will determine where the yellow tachometer signal
wire (wire #17) is connected and what the number of pulses per revolution the tachometer should be set to.
Type #1 (single coil) - Up until the 1990’s tachometers picked up the signal from the (-) side on a single ignition coil, reading every pulse sent to all the
cylinders. For example, an 8 cylinder (4 stroke) engine fires 4 spark plugs per revolution or all 8 spark plugs in 2 revolutions. Connecting the yellow
wire to the negative side of the single coil on an 8 cylinder results in picking up 4 sparks in 1 revolution (see diag. 1). This type of ignition was used
pre-dominantly until the 1990’s and distributes sparks to each spark plug. In some vehicles during the 90’s the coil and distributer merged into one unit,
but it is the same ignition system - one coil that distributes sparks to all cylinders. When connecting the yellow wire to this style of ignition you will
be picking up all cylinder sparks (see diag. 5).
Type #2 (coil pack) - (diag. 2) is used in the 96 Mustang V8 with twin coil packs. Coil pack #1 (C1) controls the firing of 4 spark plugs and coil pack #2 (C2)
controls the remaining 4 spark plugs. 2 or more separate coils are within each coil pack assembly. In this example each of the 2 coils within each coil pack
sends sparks to 2 cylinders at the same time. When one cylinder is firing in the compression stroke, it’s paired cylinder is “waste” firing in the exhaust
stroke. Each separate coil within the pack is controlled by it’s own trigger wire. In other words, if you hooked up the yellow wire to one coil trigger
wire within one coil pack, it will see only a fraction of the total engine sparks (see diag. 5).
Type #3 (coil on plug) – An individual coil is placed directly on top of each spark plug eliminating the spark plug wires. The yellow wire , when hooked
up to any coil, will pick up only 1 pulse per 2 revolutions or 1/2 pulse per 1 revolution (see dia 3). For this type of ignition the yellow wire from the
tachometer will connect to the trigger wire on one of the coils. Typically there will be 3 or 4 colored wires coming off of each coil. The trigger wire will be
the wire that changes color from one coil to the next. For example, all coils may have red, gray and black wires coming off of them, but the fourth wire will
be blue on one coil and green on the next coil.
Type #4 (tach output from ECU) Some vehicles will have a tachometer output wire coming from the ECU. The yellow wire from our tachometer can
receive signal from the ECU by following diagram 4. 4.7k Ω resistor and shrink tubing are included with gauge.
Note: If you plan to operate the tachometer using OBDII CAN-BUS (J1979), then you do not need to connect the Tachometer signal wire.
FIGURE 9: