Modifying your Tachometer for a V-8 Conversion

By Mike Stanford

When you pull your 6-cyl and drop in that V-8, the dash mounted tach will need to be modified. I finally got around to doing mine so I documented what I did. It is a simple procedure, just requires disassembling the tach and adding a resistor or two to re-calibrate. The whole thing takes less than an hour and costs about 25 cents.

Step 1: Remove and disassemble the tachometer.
Press the face of the tach in towards the dash firmly and rotate the entire unit counter-clockwise to release the retaining pins. Withdraw the unit and disconnect the power, lamp and signal wires. On a clean work surface, using a small standard screwdriver (1/8" blade) pry the fascia ring loose around the rim of the tach. This is best accomplished by inserting the blade of the screwdriver into the grove behind the fascia and turning the blade while working around the ring. You are actually bending the flange back straight to release the front from the tach. Take care not to damage the surface of the ring that is visible in the car. Once the fascia is removed, remove the three small screws and the large aluminum slotted nut from the back. Slide the movement out of the housing.

Step 2: Install resistor(s) to reduce speed indication.
Locate the two black wires that are soldered from the circuit board to the meter movement. Carefully cut one of these wires leaving at least 1/8" of the wire protruding through the circuit board. Using needle-nose pliers, remove the insulation from the section still attached to the circuit board. With a razor knife, strip the insulation back on the other section about 1/8". Obtain a 220 ohm 1/4 watt and a 47 ohm 1/4 watt resistor. These are available at any Radio shack or electronics parts house for a few cents. With a small soldering iron, tin the section of wire protruding through the circuit board and the stripped end of the wire coming from the movement. Cut all but 1/8" of the lead from both resistors and solder them in series with each-other. Tin the ends of the resistor assembly and solder one end to the wire protruding from the circuit board and the other end to the meter movement wire. It might help to arrange the resistors in a 90 deg bend. Apply a dab of silicone, hot glue or other non-conductive adhesive to the resistors and circuit board to prevent vibration.

Step 3: Reassemble the tach.
Locate the two rubber tube sections around the indicator lamp lenses as they were when removed. Position the tach movement in the housing and replace the screws and aluminum retainer (actually a grounding nut). Carefully position the fascia on the front of the housing and place the unit face down on a firm surface covered with a soft rag. Using a blunt object or punch, carefully, BY HAND, bend the retaining lip back into position, working your way around the unit. A permanent black marker will mask any scratches in the fascia ring.

Re-install the unit and enjoy a correct reading.

Please note: the XJ-S and some earlier XJ-6 tachometers will require a 330 ohm 1/4 watt resistor instead of the 267 ohm combo mentioned above. A prudent test would be to use an inductive tach to measure engine speed and verify that your modification is correct.

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