Where Will You Hide When the Revolutions Come? A Proletariat’s Guide to Tachometer Installation

Photo courtesy of Duncan Millar

Cowering behind the steering wheel, you wonder if with all of the abuse you’ve reaped on your engine, will you be held to account when the guillotine (or ‘clutch’ in this metaphor) finally drops?

Beyond the faded paint, and rusted door corners. Beyond the hibernated, storage-unit affair of its interior.

Underneath the makeshift sunshade, used to protect only the most highly sensitive electronic equipment…

…lays a digital tachometer.

Photo courtesy of Duncan Millar

A purveyor of the most vital information besides water temperature, engine speed is an essential diagnostic measurement for maintenance and reference for track driving. Our preference is for a digital, as opposed to analog, readout of that information. We found a home-grown LED counter and sweeping shift light combination called the “SL10” on eBay from a French seller (and designer) named rgtracetech883 with a tonne of functionality for it’s size and price. But you can use any you prefer as long as it can read a 5 volt or higher pulse signal and be powered from a 12 volt system.

Even if you only have a tentative understanding of electricity, this really isn’t that hard! We’ll try not to drone on about theory, but getting you to understand your car (whether it’s an E30 or not) is what we’re here to do, so no promises. In this case, we’re going to try not to bore you with the installation of this simple power/ground/signal tachometer.

Step 1: Unfuck the Cluster

If the first word you think of after “cluster” is “fuck” then you’ve come to the right blog. We pulled this entire dashboard out for what was ostensibly no reason once, and in subsequent efforts, just the cluster at least a dozen times, so a few steps might be missing since we’ve left a few things out to streamline the process. As you’ll see in the picture, the plastic trim panel that hides the manufacturing nether-regions below the cluster has already been removed (and misplaced). After you’ve navigated to a better resource and figured out how to remove the outer-most panel, grab a PH1 screwdriver (or bit, extension, and impact gun) and remove the four screws imprisoning the bezel. Once that’s removed, there would normally be two more screws holding the cluster itself to the ceiling of the dashboard. Pop those out and get to wigglin’.

It’s in there pretty tight. This is normally where we’d say “that’s what she said” but our adjacent German-language erotic Scott Pilgrim fan-fiction blog only comes out once a month and we don’t want to spoil anything for you by going on a tangent. We found that if you pull up on the bottom, and swing it in towards the front of the car, you can use the space inside the dash to point the face at the floor and eject it like a DVD of Paul Blart: Mall Cop.

Here’s the part where we could just walk you through the details to make it easy for you, but knowledge is power, and power is something you’ll need when you have a 238,000-mile old M42-powered BMW.

Step 2.0: The Basics

The M42 tachometer is aesthetically analog, but is technically digital. It receives a digital signal that gets translated and represented by a sweeping needle across a handsome 6200 RPM redline face. That signal comes in a pulse directly from the computer. That pulse is a beat of 5 volts so-many times per second (where in the off-beat it sends nothing, like turning a light bulb on and off). That beat of so-many pulses-per-second is the hertz frequency or just hertz. So if the computer is sending 5v pulses 20 times per-second, that’s a 20 hz frequency. In the case of the M42 owner specifically, those 20 count, 5v pulses-per-second, represent 800-900 revolutions of the engine per minute.

Step 2.1: All the Right Signal

Now that you’re a little more knowledgeable, it’s time to test that theory on some of the wiring behind the cluster to confirm which wire you’re going to circumcise for the good of your new tachometer signal input. Remove the blue connector and disassemble it as shown in the pictures. Since we’ve already installed ours, you should be able to tell that the black wire is going to be the one sending that 5v, 20hz signal from the computer. Start your engine so you have a frequency to read and place the setting dial of your multimeter in the “Hz” position. With your black wire still attached to the blue connector, you should be able to stab the red lead of your multimeter down into the top of the harness and hopefully contact the bare metal of the wire, and complete the circuit by attaching the black lead to any metal that is attached to the frame of the car. Grounding circuits out to the frame is a concept we won’t go into here because we have to get back to the Planet of the Apes marathon and there simply isn’t time. If at idle, you see “.02 khz” (move the decimal over a few spaces and you’ve got 20 hz) come across the screen then by Jove, you’ve done it! Splice into the black wire and lead your new wire away from the cluster to a location that will let you hook it up to your new tach. Our old tach no longer worked, so we opted to cut it out completely before re-routing it to the top corner of the dashboard on the driver’s side. Turn the engine off for these next steps or you’ll risk turning components of your new tachometer into tiny mushroom clouds.

Photo courtesy of Duncan Millar

Step 2.2: POWER!

With the hard part over, and depending on the state of things in your car, you are now tasked with the trial of finding a way to power the damned thing. With most of our HVAC control and radio missing, we had a treasure trove of wires to choose from to give life to the tachometer. Specifically, one that would gain power when we turned the ignition to ON and would invariably lose power when we turned the car off. We determined that by testing the voltages of wires as we turned the ignition ON and OFF. A tedious process, but an effective one. An easier option would have been to defer to a repair manual with an updated wiring diagram, finding a circuit that met our needs, and confirming with our multimeter. Run that power line from its source to your digital tachometer and move on to the next step.

Step 2.3: Don’t forget the little people – Staying Grounded

Remember when we said we didn’t have time to go into series circuitry? We still don’t have the time. Battle for the Planet of the Apes just started and we’ve already missed the crash landing of the spaceship into the Forbidden Zone and we don’t want to miss Taylor’s first meeting with Nova. Oh, Nova. But we will tell you that it’s not like a battery. You don’t have to route the ground cable back to the battery, it can simply be grounded to a nearby bolt, as long as that bolt has an all-metal path back to the frame.

We’ve learned to always anticipate that things will be removed for maintenance or upgrade, so instead of crimping or soldering the tachometer directly to our newfound power, ground, and signal wires, we snapped them together using parts from the same connector kit we used to adapt the oxygen sensor in our exhaust manifold blog entry from last week.

If you haven’t already, go ahead and plug everything in and give it a test! If you see numbers that make more sense than the timeline of the five original Planet of the Apes movies, than you’re now basically an electrician. Go out into the world and remember that it’s not the voltage that’ll kill you, it’s the amperage. Luckily for us, our Hitachi Magic Wand only pulls about 1.5 amps at the most. Don’t ask us how we found that out.

Now get out there and use that new digital tachometer for good by racing your E30!