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Need Some Help From The Electricals Genii Here


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#1 Code-Red

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Posted 15 July 2008 - 08:20 AM

Hey guys,

I'm doing an very difficult engine swap on a car here, and I need to convert not only the voltage range but sign wave from the new engine's sensor to work with the stock tach. I need to know what kind of circuit, or piece of electrical equipment (resistor, transistor, etc) I'd need in order to convert this. It needs to be precise, because it's for a tachometer.


Here's the tach sensor output from the swapped engine. It runs in a 7-8v range:

IPB Image Idle Speed
IPB Image Engine Speed 2500

I need to turn it into this voltage/wave, so that the original tachometer will work with the new sensor. The tach reads in a 6-7v range, like so:

IPB Image Idle Speed
IPB Image Engine Speed 2500

The only way around this, is to use an aftermarket tach, which I'd REALLY prefer not to. This just makes it cleaner and cheaper for customers cars. I figured I'd ask here because XS is full of think'ers and do'ers, some very bright minds here, especially when it comes to electronics.... so thanks for any and all help, it's really appreciated!!

P.S. If you could guide me to a better place to ask this question, that would be great as well.

#2 twistedsymphony

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Posted 15 July 2008 - 02:11 PM

that's not a sine wave thats a square wave... big difference and the difference is that a square wave is MUCH MUCH easier to work with biggrin.gif

it seems like you've got two problems here
1. cleaning up the signal
2. changing the signal to the proper number of pulses

if you want to take the easy way out Dakota Digital makes a universal converter for $80 that will convert just about anything to just about anything in terms of tach/speedo signal.

if you want to do this yourself, that's a bit more tricky... just cleaning up the signal you could probably do with a 555 timer in Astable mode using the incoming signal as the trigger. to get the voltages right you could use a pull up on the input (just so you don't overload the 555) and a transistor on the output (which will let you use whatever voltage you want for the output so you'd need to tie it into a 7V source or whatever)

that still wont solve your problem of timing... your tach will get a clean signal but it will be reading wrong.


personally I think using an MCU with an A/D converter is your best bet for getting it done RIGHT. you'll be able to tweak it however you like and the circuit will be quite small. but then you'll need to learn how to program an MCU.

Edited by twistedsymphony, 15 July 2008 - 02:21 PM.


#3 Code-Red

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Posted 16 July 2008 - 12:09 AM

Well, damn. I knew XS was the place to put this...haha twisted, you answered my question before anyone on 4 car forums....and they have yet to respond! Thank you very much for the information, I think I'll try using the Dakota Digital SGI-8 Signal Interface Unit. It's cheap enough, looks good, and it's compact. I'm going to cross my fingers that it will appropriately transfer the 6 Cylinder ECU tach output to the 4 cylinder tach input.

Thank again! You're a big help smile.gif.

#4 twistedsymphony

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Posted 16 July 2008 - 04:55 AM

Glad I could help...

Just about everyone I know who has done a radical engine swap uses the Dakota Digital adapter and I've never really heard anyone complain about anything but the price laugh.gif

Sometimes if a car is designed to work with a 6 or 8 cyl there might be an adjustment POT or jumper on the back of the gauge cluster to work with one setup or the other, but it's pretty rare.

anyway... good luck with your swap pop.gif

#5 StrictPuppet

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Posted 23 July 2008 - 04:41 AM

I made something similar in the past. I was going to swap a 4cyl Swift GT instrument cluster into a 3cyl Turbo sprint.... (don't ask...turbo sprints are sweet). I got all prepped to do the swap, and built a circuit before I had my new cluster.

IPB Image

I am sure there were much simpler ways to do what I was trying to do, but part of the reason I built it was to see if I could. The brain of the unit was a pic that was programmed with the necessary instructions. The input side of the circuit was taken from the primary of the ignition coil, so the normal input voltage was around 14v, but as the magnetic field in the coil collapses, voltages can reach over 100v, hence the need for the zener clamped opto isolator. The PIC was programmed to do the pulse counting and control the MOSFET for the output signal.

As it turned out when I got the cluster, there was a huge margin for adjustment, via two pots on the back that let me set it however I wanted, negating the need for my circuit. So I didn't end up using my project, but it did seem to work when connected to the scope.

Best of luck to you, just give your tach a good look over as it may have provisions for calibration.

#6 Code-Red

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Posted 23 July 2008 - 03:22 PM

QUOTE(StrictPuppet @ Jul 23 2008, 12:17 AM) View Post

I made something similar in the past. I was going to swap a 4cyl Swift GT instrument cluster into a 3cyl Turbo sprint.... (don't ask...turbo sprints are sweet). I got all prepped to do the swap, and built a circuit before I had my new cluster.


You won't hear any griping with me wink.gif. I'm a fan of anything with four wheels. I'd assume not only that the 1.0T is great on gas when out of boost, but can haul when in boost. Thumbs up in my books! We've actually got a Cappucino coming to the shop next week, it'll be fun playing around a bit with that 660cc turbo.

QUOTE
As it turned out when I got the cluster, there was a huge margin for adjustment, via two pots on the back that let me set it however I wanted, negating the need for my circuit. So I didn't end up using my project, but it did seem to work when connected to the scope.

Best of luck to you, just give your tach a good look over as it may have provisions for calibration.


Well, a little background here would probably help: I'm swapping a Nissan 3.5L transversely mounted V6 from an Altima/Maxima into a Nissan Sentra SE-R Spec V. I'm going to be doing this for customers once I work out all the bugs, but unfortunately I doubt there is any room for play with the instrument cluster. The 02/03 models might have some, I have to check yet, but the 04-06 models use a CANBus system, so I will need something for that. I'm going to have to use a converter for those model years.

Thanks for the good wishes!




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