Tachometer tech

[zep]

Hey all,

Currently I working on a project to build a replica of the stock Gemini tachometer. They were a pretty rare option so purchasing a cluster with one is getting really expensive.

It looks like this:

My plan is to pull a tachometer unit from another car a bolt it up to a new face that i'm working on. I've looked through pretty much every car at my local pick-a-part and have not found a replacement that has the indicies in the same locations as the Gemini ones. The ones I have found that are very close appear to be "air core" gauges, which, rather than having a postive, negative and signal wire, have sin+, sin-, cos+ and cos-. I assume that these are run from the car's ECU or some kind of microcontroller?

So I guess at this point I have two question:

1. Can I recalibrate another - regular type - 0-8000rpm tacho unit to line up with the indicies in the right place by modifying it somehow?

2. How do you get an air core tacho to work? Can I make my ECU (Link Xtreme G4X) do it with the right calibration? Or, can I somehow make it work with the oldschool coil signal?

The lazy way out is to just make the face to line up with whichever tacho unit I pull from the doner car, but I would really like to have the face looking at OEM as possible.

Anyone know about this stuff?

[h4nd]

This guy did it, (below).

These servos should track 0-full in ~1/2 second (test 'em first) https://www.jaycar.co.nz/arduino-compatible-9g-micro-servo-motor/p/YM2758?pos=1&queryId=fd47b6f1a1e862373396fd71c281927d

https://www.youtube.com/watch?v=eDLsrjLIHxM

 

[kicker]

This thread might have some useful stuff in it 

https://grassrootsmotorsports.com/forum/build-projects-and-project-cars/building-an-analog-race-dash/181498/page1/

[zep]

Cheers bois. will have a geeze at this stuff and no doubt come back with more questions

[Bling]

For the face you could have someone 3D print it exactly how you want it to look. 

[zep]

1 hour ago, Bling said:

For the face you could have someone 3D print it exactly how you want it to look. 

I'm already all over that part. It's making sure that the needle is in the right place that is the difficulty.

[kws]

You could use something like this, https://spiyda.com/smiths-rvi-rvc-conversion-external.html

Its the same guts, just in an external box, that im using in my tacho for the Marina. https://tasteslikepetrol.net/2021/06/project-marina-tacho-conversion/

Calibrate it via frequency audio files and then drive it with the coil.

[kpr]

if drive tach from your link ecu,  you can change the multiplier  in the software  so it reads correct.  

[zep]

9 hours ago, kws said:

You could use something like this, https://spiyda.com/smiths-rvi-rvc-conversion-external.html

Its the same guts, just in an external box, that im using in my tacho for the Marina. https://tasteslikepetrol.net/2021/06/project-marina-tacho-conversion/

Calibrate it via frequency audio files and then drive it with the coil.

Thanks, this looks interesting.

8 hours ago, kpr said:

if drive tach from your link ecu,  you can change the multiplier  in the software  so it reads correct.  

I will be driving it from my Link, but I was hoping to create something that might also work for people who are still running the stock dizzy or a different ECU.

[zep]

A follow on question, how accurate are tachos? Obviously your eyes can't actually see micro variations in rpm and translate that to a number, but how close is close?

I found this EK Civic tacho is is pretty darn close. I'd hazard a guess that if I used this unit it would not be that noticeable. I laid it over the Gemini one.

[mjrstar]

I have the digital output from my ek civic  ecu, tacho is probably within about 300 rpm I reckon. 

[zep]

29 minutes ago, mjrstar said:

I have the digital output from my ek civic  ecu, tacho is probably within about 300 rpm I reckon. 

That sounds ok, unless it's always 300rpm less!

[mjrstar]

Reads a little high on the top end, like passed 9000rpm maybe where 9200 would hand out if it existed, and my data log would show more like 8950.. 

[zep]

Based on that image then, that might work well enough for me.

I wonder if there are any other ways I can calibrate the tacho without using an ECU?

[tortron]

They often have a pot you can calibrate them with. That's how I gutted my mechanical GN125 one and fitted an aliexpress electronic one

[zep]

19 minutes ago, tortron said:

They often have a pot you can calibrate them with. That's how I gutted my mechanical GN125 one and fitted an aliexpress electronic one

The ali one had a pot?

[tortron]

yeah you could vary it by about 1000rpm each way if im remembering correctly

[ajg193]

Bit late, but kp starlet tacho is accurate to as close as I can distinguish, about 50 rpm.

You occasionally find a tacho dash at a reasonable price on trademe or buyee. Perhaps ebay too. They have the right style for what you want

[zep]

This looks like it could help me, especially if the the EG and EK tacho units are the same.

https://honda-tech.com/forums/honda-civic-del-sol-1992-2000-1/eg-civic-tachometer-repair-3336440/

[mjrstar]

Input signal will be the same, I run an eg ecu, in my EK with dc2 engine and an Ep3 instrument cluster. 

It's all electrical Lego.