Wiring Modifications By Circuit...

5.0 EFI Wiring Details...

This part is really fairly easy. In addition to the Ford's starting, ignition, and charging wires (seven of them), there are only eighteen wires that need to be spliced to get the EFI system, backup lights, neutral safety switch and gauges operational. First, make sure you have the complete EFI harness, with computer, from the donor vehicle. You'll want to cut off the donor vehicle's chassis wiring harnesses about six inches (car side of connector) from each of the engine connectors mentioned below. This greatly simplifies the hookup, as you'll be making your connections at the five easy to get-to engine connectors listed:
.....Ford C-153 (black) and C-163 (gray), eight pins apiece (lower left front of the engine)
.....Ford C-300 (black) and C304 (Gray), eight pins apiece (lower right front of the engine)
.....Ford C-730 (gray), 3 pins, located near the computer. These connector numbers and locations may differ slightly if a Mustang or other EFI engine is used, but the wire colors should remain the same.

Mount the ignition coil and starter relay on the left side of the engine compartment, as the battery and existing Mazda wiring are already there. In the steps below, 16 gauge wire is adequate for all connections, except those noted.

Step 1...The Ford starter relay needs a cranking + signal to its red/blue wire, so connect this to the black/pink wire ("neutral safety "out") of Ford connector C-163 (gray 8-pin connector, left front of engine). You'll also need to connect this circuit to the TFI ignition module (red/light blue wire exiting C-300, black 8-pin connector, on the right side of the engine). You'll need to run this and a few other new wires in a piece of convoluted tubing, across the top/front of the engine.

Next, connect a wire between the RX-7's thick black/yellow cranking signal wire (in an unhooked Mazda connector, just in front of/below the brake booster) to the white/pink wire of Ford connector C-163 ("Neutral safety "in"), and across the engine to the white/pink wire ("computer cranking in") exiting C-304 (gray 8-pin connector, right front of engine).

Step 2...The Ford ignition coil needs a key-on + signal to the positive side of the coil (red/light green wire). You'll need to connect this to one of the Mazda's thick black and white wires formerly connected to the ignition coils. The EEC power relay and TFI module also need this signal, so connect a 12 gauge wire across the engine to the red/light green wire located in C-300 (black 8-pin connector, on the right side of the engine. The remaining wires exiting C-300 are not needed, so it's OK to cut them back, and tape them up.

Step 3...The Ford ignition coil needs to receive a -- signal (dark green/yellow wire) from the ignition module, mounted on the engine. Connect a wire from the coil - to the dark green/yellow wire exiting C-304 (gray 8-pin connector, right front of engine). This circuit should also be connected to the Mazda's tachometer, via the yellow/green wire found in the bundle formerly going to the Mazda distributor and coils.

While you're working with C-304, this is a good time to hook up the three black/light green engine ground wires (exiting C-304) to a clean and tight engine ground stud.

To finish things up at C-304, you'll want to run a 12 gauge wire from the connector's black/orange wire (B+ to the computer) and yellow wire (B+ to the fuel pump circuit) across the engine, through a 20A fuse holder (one for each wire) and to battery +, which can be conveniently found at the starter relay + cable connection. The remaining wire (light blue/pink) is not needed.

Step 4...We've got one last EFI system wire to connect, a black/light green wire (O2 sensor heater ground) exiting C-153 (black 8-pin connector, left front of engine). You'll simply need to take this wire to a solid engine ground. C-153 also contains the Ford oil pressure and water temperature sender wires (among a few others that are unneeded), but it's probably easier to run new wires to the Mazda senders installed on the engine (covered in step 6).

Step 5...If you want to hear the engine run, it will do so now, if you hook up (temporarily) the Mazda's original fuel pump + (blue/green) and ground wires to the Ford fuel pump. It's a good idea to convert to the Ford fuel pump drive circuit (for safety and durability), and run a larger (12 gauge) wire to the pump before driving the car any distance. Before doing this, you'll want to punch through the firewall, and bring the computer inside the car. Here's how the fuel pump is hooked up:
....At C-730 (the three wire connector near the computer), you'll find three wires: red, yellow, and tan/light green. The red wire supplies switched + to the fuel pump relay (saved from the donor vehicle), The yellow wire supplies battery + to the fuel pump relay. The tan/light green wire should flow through the inertia switch (crash switch), and to the fuel pump relay. The remaining wire that exits the fuel pump relay (pink/black), should be extended with 12 gauge wire to the fuel pump.

Step 6...Now the gauges. You'll want to connect the Mazda's brown/yellow (oil sender) and yellow/white (temperature sender) to the units installed on the Ford engine. These are found within a long/wide (unplugged) Mazda connector just behind the left shock tower. The Mazda's alternator warning light circuit (yellow/light blue, catch it at the choke-check relay mentioned below) needs to be connected to the Ford alternator wiring harness's light green/red wire, near the voltage regulator. For whatever reason, Mazda tied in this circuit with several other warning indicators, so you'll want to disconnect the choke-check relay (two connectors, one with two wires, the other has four), located behind the left shock tower), and the coolant level "sensor unit" (five wires, located just under the instrument panel, left side).

Step 6...The back-up lights. The Mazda's red/white wire (found in an unhooked connector behind that previously went to the transmission) should be connected to the Ford's black/pink wire, found in C-163 (8-pin gray connector, left front side of the engine). The pink/orange wire also exiting C-163 should be routed, through a fuse, to ignition +.

Step 7...Now we're on the home stretch- the alternator output wire and voltage regulator wires. Ford originally ran a pair of black/orange wires from the alternator, through C-262 (a black connector near the alternator), through a fuse link, and to the battery. This was a lousy idea, as the connector tended to melt. Replaced the two wires with a single 8 gauge wire, and run it straight to the Mazda's fuse link "block" connection, (formerly a white red wire) located just in front of the left shock tower. You should also upgrade the "main" fuse link to a 3mm (12 gauge) loop of wire, as the Ford alternator has the ability to smoke the original Mazda link during heavy charging operation.

The remaining alternator/voltage regulator wires are easily connected from C-262, color by color, to the right-side fender-mounted voltage regulator (don't forget to hook up the alternator warning light wire from step 6 at this time).

MAZDA RX-7 Tachometer Re-calibration

Re-calibrating the stock RX-7 tach for V-6 or V-8 use is a fairly straight forward process that involves dis-assembly of the dash, and removal of the tach assembly. The 1st step is to remove the whole dash pod from the car. There are 2 harness connectors to dis-connect from the rear, as well as the speedometer cable (a retention tang must be compressed to dis-connect the cable). After the pod is out, the dash face can be removed, usually attached with 4 phillip's head screws. Then the tach itself can be removed from the front of the main dash circuit board by removing the screws or nuts on the back side that connect it to the circuit board (depending on the model). Try not to leave any fingerprints on the satin black inner part of the dash face, as they are quite hard to remove.

After the tach is removed from the dash, it must be wired up to a signal source, usually done by using 4 long jumper wires w/ alligator clips.

RX-7 tach layout
There are 2 different layouts used for the 1st gen tach, one that is connected to the dash circuit board with wires, the other with studs and nuts to make the connections.

If your tach has wires, the connections are as follows :

trigger" wire is the black w/ yellow stripe.
"ground" wire is the white w/ black stripe.
"12v +" is the black wire (usually connected to keyed ignition)

If your tach has studs, they will be arranged (looking at the back of the tach) with 4 in a row on top, and 2 below that are widely spaced. The 4 along the top are used for out of dash calibration. I refer to them as 1,2, 3, 4, from left to right. 1 being on the extreme upper left, 4 being the extreme upper right. They are as follows :

#1 is the signal, or "trigger" as I call it, usually attached to a coil negative (-).
#2 is the ground.
#3 is the 12v +, usually hooked to a keyed ign. source.
#4 is output for the over-rev buzzer. (not used during re-cal)

The re-cal process
PREFERRED SIGNAL SOURCE... an inexpensive 12v battery charger can be used as a deadly accurate signal source for the re-cal of your tach. This signal source is so steady and accurate that there is no need to compare the reading with another V-8 calibrated tach. Battery chargers typically use a rectifier bridge to convert the AC power source into DC. This turns the 60 hertz sine wave input into a 120 hertz ripple wave DC output. The ripple wave output is read as 3600rpm by the stock rotary calibrated RX-7 tach. For a V-6 application, an RX-7 tach should be adjusted to read this signal as 2400rpm. For a V-8 application, adjust your RX-7 tach to read this ripple signal as 1800rpm. Please note that you must use a 12v battery for a power source, and the battery charger as a signal source only. The ground wires for both the battery and the battery charger should be connected to the ground terminal of the tach.
NOTE: Be sure that the RX-7 tach is about the same orientation as when installed, because the re-calibration may be off if done with the face in a horizontal plane.

Use the test leads to hook-up your RX-7 tach to the signal source and power source as described above.

With your signal source plugged in and your un-modified tach wired as above, it should be putting out a reading of about 3600rpms. To correct your RX-7 tach's reading, the cal-pot must be adjusted. It is a round white adjustable potentiometer located on the end of the circuit board near R3. The adjustment is secured from the factory with a locking compound, so your initial turn of the adjuster will be difficult until the locking compound's seal is broken. Be patient, and don't break anything.

...Early 1st gen tach...

...Late 1st gen tach...

When you get the adjustment free'd up you will be able to observe the effects of adjustment on the RX-7 tach's needle. Try to adjust until the tach reads correctly for your application (2400rpm for V-6, 1800rpm for V-8). If you don't have enough adjustment to get the correct reading, the value of the cal-pot must be altered.

The method we use is to add a 1000 ohm resistor in parallel with the resistor BEFORE the cal-pot (usually marked R3). I usually add this resistor to the backside of the board to make things easier. The 1000 ohm resistor will get the calibration close enough that minor adjustment of the cal-pot is all that is needed.

...Early 1st gen tach...

...Late 1st gen tach...

After adding the resistor, repeat the process. Now, the cal-pot should have plenty of range.

Troubleshooting FAQ...
"My test rig does not operate the tach..."
...Most likely, your battery charger is too good. The more expensive chargers have filters that remove too much of the voltage ripple while converting to DC. The needed cheap battery charger uses only a rectifier bridge to hack the AC into DC, resulting in a distinct ripple instead of a true straight-line DC voltage. You need to use the cheapest of the cheap chargers.

"My tach needle does not rest at zero..."
...The tach needle is a light "press-fit" on it's shaft. Simply forcing the needle against it's internal shaft stops will spin the needle on it's shaft. To re-index the needle, just lightly twist it against the shaft stop in the opposite direction.

"The 1000 ohm resistor does not give my cal-pot enough range for re-calibration..."
...In rare instances, we run into a tach that will not calibrate with the standard 1k resistor. In those cases, we substitute a 4700 ohm resistor in place of the 1000 ohm unit we typically add. This usually does the trick.

If you have any problems with the above process, feel free to contact us, or you can send your tach to us for the modification (we occasionally have re-cal'd tachs in stock and ready to ship on an exchange basis). We charge $55.00, which includes return shipping (inside the US.) Just mail your tach and a check for $55. to:

Granny's Speed Shop
POB 814
Concrete, WA 98237

Next page....


2....Considerations & Requirements....

3....Getting Started....

4....Engine / Transmission Installation....

5....Exhaust / Throttle Cable / Accessory Drive / Pulleys....

6....Cooling / Fuel Systems....

7....RX-7 Wiring Harness Connector ID and Circuit Locations....

8....Electrical System Modifications By Circuit....

9....Start-up / Troubleshooting....