Simplest way to drive the tach with Speeduino or Megasquirt?

[Spaceman7Spiff]

I imagine most of you on this board had to cross this bridge at some point in your journey so I'm very surprised that I'm having such a hard time finding an answer to this question but I am running a Speeduino ECU variant (SpeedyEFI Fishdog UA4C) and I am looking for a simple way to drive my aftermarket VDO tachometer. I was told to use a "pullup resistor" but I haven't been told, and can't find a straight answer on how to do so and the information that I do find looks complicated and risky; involving opening up my ECU and soldering in electrical components. My ignition system is all dub shop stuff: crank trigger kit, IGN-4 dumb coil (wasted spark) and a Bosch "211" style ignition control module.

In my searches, I stumbled on the Compu-Fire 51105-V "Tach Adapter." Would that work?

If there are multiple ways to make my tach work, I would be more interested in the faster/easier approach as long as it isn't wildly expensive. Normally, I'm the type to tinker and figure out the cheapest way to get something done but, right now, I have to get my Baja ready for a week-long dunes trip next month and I don't have time to screw around with delicate electronics that I'm not familiar with.

[Bruce.m]

Old school tachs require a pulse from the points.

There is a simple circuit in this link.

https://speeduino.com/forum/viewtopic.php?t=3235

[Spaceman7Spiff]

Thank you Bruce. That does help to explain the concept of the problem and solution. However, I'm still not sure exactly what I need to purchase.

Would one of these cover my needs?
https://www.bmotorsports.com/shop/produ ... ts_id/5248
https://www.bmotorsports.com/shop/produ ... c3s4l1lv13

[Volky]

Hi,

I made this on my wasted spark setup.

[Clonebug]

I think I used a resistor from 12 volt to the tach signal line to make my tach work. I just pulled that wiring out this spring and tossed the piece in a drawer since I removed the tach and have a digital dash displaying my rpm now.
Some tachs need a little more juice than what a wasted spark coil puts out or the ecu puts out as a tach signal.

[Spaceman7Spiff]

Okay based on what I could find, I have been wiring in a resistor as shown in the photo. I have tried 10k, 5.6k, and 1k. Absolutely no change. What am I not getting?

[Chip Birks]

Hi,

I made this on my wasted spark setup.

This is the simplest way to go. I've seen this work on a friend's car with no issues. No ECU mods. Real easy to do.

[Spaceman7Spiff]

Hi,

I made this on my wasted spark setup.

This is the simplest way to go. I've seen this work on a friend's car with no issues. No ECU mods. Real easy to do.

Okay so I take it the "DICT-ND" portion of the serial numbers doesn't matter? When I google those numbers, most of my results drop that part. I ordered two 200pc kits from amazon that include those mentioned diodes, so hopefully that will work.

[Volky]

Hi,

I made this on my wasted spark setup.

This is the simplest way to go. I've seen this work on a friend's car with no issues. No ECU mods. Real easy to do.

Okay so I take it the "DICT-ND" portion of the serial numbers doesn't matter? When I google those numbers, most of my results drop that part. I ordered two 200pc kits from amazon that include those mentioned diodes, so hopefully that will work.

I just bought the ones in the pictures. I have one inside a MS2. Then I was given the Speeduino and decided to make an external one at the harness with heatshrinks. It never fails.

[Chip Birks]

Hi,

I made this on my wasted spark setup.

This is the simplest way to go. I've seen this work on a friend's car with no issues. No ECU mods. Real easy to do.

Okay so I take it the "DICT-ND" portion of the serial numbers doesn't matter? When I google those numbers, most of my results drop that part. I ordered two 200pc kits from amazon that include those mentioned diodes, so hopefully that will work.

Yeah it's the 1n400x that is important

[Spaceman7Spiff]

A tale of two VDO tachs:

So when I originally created this thread, I was using a pretty old VDO tach (my guess is 70s or 80s) which worked fine 6 months ago when it was running on my conventional ignition system.

But I bought a brand new VDO tach, strictly for cosmetic purposes, as my other VDO gauges are the more modern and common black and white faces with orange needles. And as it turns out, as soon as I fired it up with the new tach, wired in the configuration shown in my photo (10k ohm pull up), all was right with the world.

I did receive my diodes but never ended up using them.

I wanted to follow-up so hopefully this thread will help someone else out in the future. Thanks everyone.