Old School Tachometer without reverse engineering

For those who are unfamiliar, you can connect a tachometer gauge for the ICE by tapping one of the wires in the glove box connectors. A ScanGauge II will give you more readouts, and I have one, but I want to have an actual tachometer clearly visible on my dash.

Why, you ask? It gives a better understanding of when the ICE is running, and how hard it's working. On the highway the ICE will be running continuously. Also, I find the physical dial easier to read than the ScanGauge's digital display, which I have to focus my eyes on to read. It's definitely not a necessary modification, but it's an extra data point on how the car is functioning.

Most modern tachs can apparently tap the tach signal, be connected to the 12v accessory circuit, and work fine. Here's a thread with a successful install of a new-style tachometer: Tachometer installation option | PriusChat

However, some tachs of an older design expect a much more powerful signal directly from the ignition coil. Hobbit details some impressive work on adjusting his older tachometer to work with the weaker Prius tach signal: http://techno-fandom.org/~hobbit/cars/tachack/ (the information on tapping the signal can be found here)

There are a lot of tachometers available out there, but I wanted one that represented the RPM range of the Prius (0-5000 RPM). This was tough to find, as a lot of tachs like this were specifically for motorcycle engines. I finally found one that I liked, the "Auto Meter 2306 Autogage Mini Tachometer". This had a max of 6000 RPM, so the position of the needle would have a better representation of the engine's RPM range.

When I hooked this up originally, the tach didn't seem to work, and I eventually figured out that it was the older style of tach that required ignition coil level signals. Reverse engineering the tach was a little outside my time budget, so I researched other alternatives.

It turns out that there are tachometer signal amplifiers out there. But, they cost $60 and, sometimes more than $100. I managed to find a product on eBay called "TACHOMETER SIGNAL BOOSTER, 3~16V Input -> 48~63V Output, compare to MSD 8920" made by "Widget Man". It was only $30 after shipping, and just hooks up to 12V power and sits in between the tach signal wire-tap and the tachometer. (it probably cost them $3 to make it but hey I'm not complaining)

I just got it hooked up and tested with alligator clips, and I compared the RPM to the scangauge, and it seemed to be accurate! Pics will be coming later once I get it properly installed

 

I've had this in my car for a week and I'm happy with it. I've found it's very easy to redline the car when I'm not intending to, and it's helped me drive the car more smoothly or be more aware when I'm at the limit of acceleration (merging passing etc), or how hard I'm pushing the car up a hill. I also like how it looks on the dash! I never really used the ScanGauge much for this because I found it too distracting to read and drive (especially because, when you're doing something that's boosting RPMs there's a good chance you will also want to pay attention to your driving)

I finally remembered to get a picture it at night, so here's the pics:

Tapping the tach cable, using a T-tap. I finally got a combination wire stripper/crimper that made working with these connectors much easier; I had been using a pliers before to crimp them and it wasn't pretty. The smaller red, green, and black wires that merge into the gray wire insulation are from ccdisce's ECT spoofer.



Tapping the 12v accessory power from the under-dash lighter socket, which will power both the tachometer and "Widget Man" amplifier. The one with the black cable is ground; actually it's not necessary to tap the ground wire as you can hook a tach (and amplifier if needed) up to the dash ground, I just remembered. I didn't get a picture of it but it's near the steering wheel assembly and pretty obvious.


Here you can see the red and black power cables from the above pic which I used a coat hanger to pull through the nav, as well as the green tach signal wire. The wire with clear insulation is for the EV button, and ccdisce's ECT spoofer is the small circuit board wrapped in translucent plastic, zip tied to the bunch of cables.


Tapping the light signal for use in the tachometer. I tried measuring different cables on the dimmer and was getting all sorts of crazy readings from operating it. I wasn't able to find a voltage I could use that would both change with the dimmer knob and stay on when the knob was at max, (at least without shorting dimmer knob wires or messing around with diodes maybe.) Right now it's connected to +12v that's on when the headlights are on, but I'm actually fine with it being on at full brightness at all times; I don't always want my LCD lighting super bright (I guess that's for in bright sunlight?)

The 2005 EV button I have installed is to the right. I finally figured out the leads to connect to get it to light up, so that was a bonus. This job (and the ECT spoofer) were done before I discovered T-taps. Also, the 2005 EV button (i think 04/05 and i can't remember if 06 has it) is different from the 07/08/09 EV button, and has a different pinout if I recall correctly. I figured it out via trial and error (the switch contacts were easy with a multimeter continuity test, then you have 2 contacts for lighting, where one is 12v and one is ground)


So the light's now hooked up properly. You can also see the Widget Man tach amplifier, which is just zip tied to a cluster of wires, with the tach signal in (from the glove box), tach signal out, and power cables attached. The tach and Widget Man are in parallel with the accessory circuit, I should probably fuse the power cables independently. The tach cables go behind the corner of the dash plastic with the right driver vent (more on that to come)



So my right drivers side vent plastic had previously snapped when I was pulling it out about 4 inches from the top. I epoxied it back together and it wasn't perfect, but instead of getting a new piece from a junkyard, why not reinforce the structure and not have to feel bad about screwing my tach into it, since the piece is already junk?

So I did my best to rig something up that would glue the piece back in place. During this process, the clips that sit the top of the vent plastic in the dash broke off completely and the plastic they attached to pretty much disintegrated, making them impossible to reconnect. I epoxied in some ah recycled material and left it overnight:



Definitely not perfect, but fine by me. I'll need to fill in the gap with some more epoxy to smooth out the appearance. The excess epoxy came off pretty easily (not sure if that's a good sign). The tach naturally sits above zero when not connected to power.



Here it is installed. I'm pretty happy with the position after having it in my car for a week. It's easy to check/monitor the RPM, but it's not in the way:



One part I neglected to mention is that my reinforcement hack job didn't fit in the dash clip, so I ended up taking chunks out of it with a mini hacksaw until it sat well in the dash. Despite it not having clips or being secured at the top, I haven't noticed any vibration.



At night, in person the light is not blindingly bright and looks pretty good, although the lighting is slightly uneven from the top. This is probably the best shot of how it looks while driving; I angled it so that it would be pointed directly at me:


So pardon the busted up appearance of the dash piece (still have to improve this); if you install a tach in this location I would recommend maybe attaching it with an adhesive pad, and be very careful removing and installing this piece of dash trim once the tach is attached, taking care to support the top (I broke mine a few months ago just putting it back in the dash, without a tach on top of it). I'm not sure if you'll be able to get the wires through without drilling a hole, or cutting out a corner (as my corner was already conveniently chipped off). You may want to figure out a way to structurally reinforce this plastic, but the fit is fairly tight so it takes some effort to figure out the right places to put material. But, I do think this is a great spot for a small tach while driving at least.

I may just pick up another one of these pieces at a junkyard and go the adhesive pad route myself, but I'm not in a hurry