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How to Wire an Electronic Tachometer as Easy as 1-2-3

Out For A Good Time

John Gilbert Feb 17, 2015
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It’s a good bet most anyone reading this article is quite familiar with the bright yellow 1957 Chevy 2-door coupe that’s known as Project X. This article on how to install an electric tachometer in a jiffy doesn’t really have anything to do with Project X other than that ’57 is the granddaddy to Super Chevy’s 1967 SS396 Chevelle known as the AMD Chevelle.

In the vein of Project X, the AMD Chevelle started with an extensive ground up restoration, with the overall goal to make it a long-term test project car. With the original 396 long gone, we built and installed a .060-over 402 (413) with a solid flat-tappet cam and Weiand Street Warrior intake manifold (breathing through stock manifolds), and a TCI 4L80-E electronic overdrive automatic transmission. Next up was a set of Edelbrock aluminum cylinder heads. Problem was the car had never been to the strip and we wanted to see what we were starting with.

We drove the AMD Chevelle 50 miles from our office to Auto Club Dragway in Fontana, California, to make a few baseline passes down the quarter-mile. One of the instruments our ’67 didn’t have after the restoration was a tachometer, so the day before we threw in an Auto Meter tach we had in the garage. Without having a tachometer we wouldn’t be able know where the ’67 was shifting, or how many revs it was turning going through the traps—not to mention what it was revving at freeway speeds. Because we have future plans for installing a complete custom gauge cluster into the ’67, the decision was to toss in a quick solution. Although the Auto Meter tach was installed temporally the wiring we did was done to last for the long run.

After we got our baseline runs in, we drove the ’67 over to Westech Performance in Mira Loma, California to make a few pulls on their Superflow chassis dyno.

The AMD Chevelle with its slightly heated-up 413-inch ran 13.534 at 102.46 mph on its first pass, which would turn out to be its best as the rain was headed in and the density altitude climbed to nearly 2,500-ft. At Westech, our trusty SS396 Chevelle produced 318.9 (corrected) rear-wheel horsepower. Thanks to our temporarily mounted Auto Meter tach when the day was done we jumped on the freeway and knew our engine never revved over 2000 rpm at highway speeds, despite a set of 3.73 gears.

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1. After disconnecting the battery the next step was to locate an existing hole in the firewall to route the wiring through from the distributor to the tachometer.

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2. Then a scratch awl was used to punch a hole through the firewall insulation mat for the wiring to pass through.

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3. To prevent electrical damage always look before stabbing: Viewing from the inside of the firewall notice the sharp end of the scratch awl protruded in an area where it would not damage existing wires.

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4. Here is everything that was needed to install the Auto Meter tachometer including the tools that were necessary.

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5. Additional wire need to supplement the length of the Auto Meter tach harness came from Painless. The wires were cut leaving extra length, stripped and then joined permanently with butt connectors.

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6. The engine side was the starting point to run the wires from. Electrical tape was used to wrap and keep (hold) the three wires together as they were pushed through the firewall.

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7. We used the existing original equipment black plastic sheath along the firewall to conceal the red, black and white tach wires from sight.

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8. The black ground wire (in plug) that plugs into the back of the tachometer was grounded to the firewall.

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9. To provide a clean look we covered the tachometer wiring harness with Painless Power Braid. To prevent creating straggling strands sharp using a sharp pair of cutters, or industrial scissors works best.

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10. The pink lead from the tachometer connects to the (+) positive post (terminal) of the coil. The pink lead supplies12-volts to the tachometer when the ignition is turned on.

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11. The white wire from the tachometer connects to the (—) negative post this supplies the ignition impulse to the tachometer.

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12. With the engine compartment side (coil) connected the length of the dashboard side of the wiring harness was ready to cut to length.

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13. The plug end of the tachometer wiring harness was spliced into the Painless wires with butt connectors.

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14. Although installing the Auto Meter tach was a temporary measure the wiring we installed is a permanent solution that will stay in place.

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15. Our .060-over 402 has 10:1 compression, mildly reworked smog heads with a 3-angle valve job, and a solid flat-tappet cam with .550-inch lift intake/.570-inch exhaust, 238/248 duration at .050 and an LSA of 1112. It breathes through a bone-stock exhaust system.

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16. The starting line at Auto Club Dragway had teeth. In spite of the fact that we were running on smallish 225/70R15 BFG Silverstone radials from Coker Tire, tire spin was never an issue. We powerbraked it slightly, mashed the throttle and let the 413 do its thing. The TCI trans shifted hard at 5,700 rpm.

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17. Horsepower has gone up since our last dyno test. When we tested the car on our old DynoJet in Tampa, it made 301 rwhp. On the Superflow unit at Westech Performance (with an additional 1,800 miles on the engine, we were now up to 318 rwhp and 439 hp. Air/fuel was a tad fat (12.4:1), but we held off on the super tune until after we swap on our new cylinder heads.

Sources

Auto Meter
Sycamore, IL 60178
866-248-6356
www.autometer.com
Painless Performance Products
Ft. Worth, TX 76105
817-244-6212
www.painlessperformance.com
Eastwood Company
Pottstown, PA 19464
800-343-9353
http://www.eastwood.com

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