Electrical Interface

In addition to the stock 964 DME wiring harness which must be mounted in the SC, a DME relay must be added, and a wiring harness fabricated with the proper connectors to interface with the SC engine wiring. The DME control box is mounted under the drivers seat and the DME harness routed to the engine compartment. This will require obtaining the proper mating connectors, which can be obtained from salvage sources or from the Porsche dealer network. Fabrication of the adapter harness will require availability of the wiring diagrams from both the C2 and SC factory manuals to determine the proper connections from the 964 harness and the 964 engine, to the rear relay panel on the SC and to the dash area. (see footnote 1.) In addition to the DME box to be mounted under the seat, an altitude correction box (pressure sensor) was provided on some models. Apparently this was only with cars without catalytic converter (option M150). There is a connector for the pressure sensor near the DME box connector, but unless the option M150 was supplied with the original engine, the pressure sensor connector simply remains open.

From the DME box under the drivers seat, the routing of the DME harness goes toward the driver's door, back under the carpet, up the side of the rear footwell, back under the side bolster along the level of the rear seat, and through a 2.5" hole (cut with a hole saw) behind the rear seat bolster into the engine compartment. (See photo) Slight modifications must be made to the interior rear trim pieces to accommodate the wiring harness. The harness could be routed from under the seat straight out the back of the car at the floor level, however the harness would have to drop under the torsion bar tube underneath the car and would be exposed to road hazards, then another hole cut to route the harness back into the engine compartment.

Electrical interface connections from the 964 harness 14-pin connector to the SC 14-pin connector in the engine compartment include: starter, oil pressure sensor, oil pressure light, alternator light, backup light switch, +12 volt power, ground, and the blower control for the heater fan. The 12-pin connector from the 964 engine harness includes oil temperature, the fuel injector signals, and engine ground. This connector mates with the 12-pin DME harness connector which has been routed into the engine compartment.

The only modifications to the wiring harness on the 964 engine include adding 5" to the length of the transmission backup light switch wires to reach the 915 backup switch, disconnecting and insulating the connections to the blower motor power resistor and NTC resistor, and one change at the 14-pin connector as noted below. No modifications are required to the 964 DME harness which runs from the DME box to the engine coompartment.

The twin ignition coils were mounted on an adapter plate which was fabricated to allow them to be located in the place of the original CD unit. The two lower bolts which secured the CD unit hold the adapter plate, which in turn holds in place a tinnerman nut to secure the bottom bolt of the mounting plate for the twin coils. The top bolt for the twin coil mounting plate is held by a tinnerman nut mounted in an existing hole on the SC relay plate.

Mods to the SC wiring:

Although the 14-pin connector from the 964 engine harness will physically mate with the 14-pin connector on the SC regulator plate, several pins are not compatible in their electrical function. For this engine swap, the required modifications are summarized below:

On the engine side of the 14-pin connector, move the connector from pin 12 to pin 14.

The other incompatibilities in the 14-pin connector are handled on the other end of the SC harness, that is, at the front of the car. Disconnect the green/black wire at the oil temperature gauge. Disconnect the purple/black wire at the tachometer. Disconnect the fuel pump speed switch behind the dash. Leave the fuel pump relay in place. With the other listed changes, this relay will not operate, but will remain in the circuit to the fuel pump. (Leaving this relay in place will reduce the wiring changes required.) Disconnect the wire to the top of the fuel pump fuse; it will be replaced by a wire in the adapter harness from the DME relay to power the fuel pump. The wire which was disconnected from the fuse will provide an input back to the adapter harness as a switched 12 volt input. A single pin connector under the front fuse panel, which provided power from the fuel pump relay to the 14-pin connector in the SC engine compartment, is disconnected, and the connectors ends insulated. Sketches of the wiring harnesses were made in order to make a diagram of the required adapter harness.

The stock 1978 SC tachometer was found to be compatible with the tachometer signal from the DME computer for the 964 engine.

There are two other electrical functions worth mentioning which can be impemented. These are the "check engine" lamp and the fan belt sensor which indicates a broken belt for the cooling fan. Starting with the '91 C2 engine, the 964 DME has a self check feature which can be carried over with the transplant. With the accelerator pedal depressed, the ignition is turned on, and after 3 seconds, the "check engine" lamp will flash with a code which will indicate certain malfunctions within the engien electronics. These codes may be found in the C2/C4 factroy shop manual. This function may be found on pin 8 of the 14-pin DME harness connector under the driver's seat. To implement this function, connect a 12 volt, 0.9 watt bulb between pin 8 and a switched 12 volt source. The contacts for the fan switch may be found on pins 3 and 14 of this same 14-pin connector under the driver's seat. Since a broken engine cooling belt is not detected by the alternator light for the 964 engine, is suggested that the fan belt sensor switch be utilized.

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