With the installation of the new engine, this is a good time to convert older positive earth cars to a negative earth system. As this has been covered in many other articles, I will only briefly outline some the changes to accomplish this conversion.
Two changes are required to convert a positive tach to a negative one.
The tachometer's impulse-loop leads, the ones connecting the ignition switch to the coil (usually white), are cut and reversed. These connections should be soldered and covered in heatshrink-tubing.
The tach is disassembled and the power and ground wire are unsoldered and reversed. The power lead is identified by the male spade lug connecting to the green power wire. Just next to this is the earth connection. Unsolder the resistor and wire going to these two points and reverse them. Solder and de-flux the joints. Reassemble the tach.
Nothing needs to change in the wiper motor or its connections. It will rotate in the proper direction as before.
Use the stock Toyota starter. When relocating the battery to the boot, as described later, the solenoid lead connects to the main starter stud by a 10g wire. This will make the solenoid engage when ever the starter is energized.
These should all be O.K. Remove any polarity reversing circuits, such as might be used for a radio.
In some installations, the standard Toyota alternator may prove to be physically too large to clear the left shock damper. In this case, a smaller Mazda (small Ford truck) or Suzuki (GEO) unit may be adapted. All that is required is building a mounting bracket and adding a matching voltage regulator.
The voltage regulator is mounted on either the firewall or footwell and connected to the alternator as per the donor vehicle schematic. Always use a voltage regulator matched to the alternator you are installing. Although sometimes voltage regulators and alternators can be mixed-and-matched, this may lead to problems and is best avoided.
The new alternator is connected to the old wiring loom at three points. The first is the power output connection, and this goes directly to the battery. It goes through a fusible-link for protection and should not be switched. The second is the ignition line. It goes to the switched side of the ignition switch. The white lead from the ignition switch to the fuse box is an good place to make this connection. The last wire is to the "IGN." indicator light in the tachometer. This now functions as a "ALT" indicator.
The proper points to pick up these connections from the alternator loom can be read from the wiring diagram of the vehicle donating the alternator and voltage regulator.
The battery most likely will have to move the boot. The best place is tucked in on top of the frame rail behind the left wheelhouse opposite the fuel filler neck.
The battery box may be folded from 18g sheetmetal or be a generic box purchased from any number of hotrodder's supply houses. Fix the box firmly to the frame rail and to the bumper mounting point on the boot floor.
Having the starter cable live all the time could be dangerous should it develop a short to earth. This situation is eliminated by installing a relay between the positive terminal of the battery and the cable before it begins to make its way forward to the engine. The relay is mounted on the side of the battery box and is controlled by a wire from the starter terminal of the ignition switch. The wire connected from the ignition switch to original starter solenoid is removed. The solenoid on the top of the Toyota starter is then connected to the main starter cable. This forces the solenoid to engage the starter clutch whenever the starter is energized.
If you live in a part of the country where the services of a heater are required, it will have to modified to fit the new constraints of the heater shelf. The approach I took was to keep the Smith's heater core while throwing out the original heater box. I fashioned a new heater enclosure that not only contained the core but also a small, squirrel-cage blower from a Japanese import. This yielded a very compact unit that also now had a multi-speed blower with integral resistor pack.
The water control valve used is a universal in-line type available from any automotive parts house. It is a cable-operated type that is coupled to a heater temperature control knob from a MGB and is mounted on the dashboard.
A general-purpose multi-position fan switch is connected between the blower resistor pack and the existing MG heater power supply lead.
No modification is necessary to the speedometer other than calibration on a rolling road at a speedo shop after installation of the speedo cable.
If it is desired to route the speedo cable inside the transmission tunnel rather than down and along the outside of the floorboard, a right-angle drive coupling is necessary on the transmission to redirect the cable up and over the gearbox mount.
A new, longer cable is fabricated to mate to the metric Toyota threads on one end and the UNC of the MG speedo on the other.
Nothing is done to the dual oil pressure / water temperature gauge. As luck would have it, Toyota for some totally inexplicable reason, chose to use British Standard Pipe thread on all plumbing fittings. So, the oil pressure sense pipe and the water temperature sensor bulb simply screw into the proper holes on the Japanese engine.
This is a 1/2" BSP thread on the sensor bulb retaining ring and screws right into the hole in the thermostat housing normally holding the electrical water temperature sender.
This is a 1/4" BSP fitting that threads into the hole on the right side of the engine block reserved for the oil pressure warning switch. If one wants to retain the low oil pressure warning light, a T- or Y-piece will be tightened into this hole with the switch and oil pipe thus fitted to it.