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Old 05-27-2014, 12:33 PM   #1
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Default VSS and tach question, definitely totally NOT for a turbo timer.

Okay I lied, it's a turbo timer. BUT, in my defense, it lets you look at top speed etc. after a track day session / autocross run without having to get out a laptop , and i'm just rice like that. I'm well aware of the synthetic oil making it "not necessary", "stop being in a hurry and wait 30 seconds if you drift into your parking space bouncing off the limiter" etc.

SO. On topic. After doing some research, it seems as though the VSS and tach signasl are 12V VR signals, and are converted to a 12V digital square waves at the cluster. So, would I simply add a 1k resistor between the points on the back of the cluster and the turbo timer signal wires, and be good to go? And if so, this is the same process for both tach and vss signals?

EDIT: 2000 NB. Sorry, left that out. and for searchability sake in case someone else comes along later looking with the same question: HKS Type 1

Last edited by Schuyler; 05-27-2014 at 12:58 PM.
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Old 05-27-2014, 12:38 PM   #2
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it seems as though the VSS and tach signasl are 12V VR signals, and are converted to a 12V digital square waves at the cluster.
Since you didn't tell us the year of the car, I have to do some guessing here.

Tach is always a squarewave signal on all Miatas. It comes out of the igniter ('90-'93), ignition coils ('94-'95.5), or ECU ('95.5+) in this form.

VSS is an AC sawtooth wave between the VR sensor on the transmission and the speedo on the '99+ cars, which is translated at the speedo into a squarewave and sent to the ECU. On the '90-'97 cars, VSS is generated locally at the speedo with a mechanical reed switch, and is therefore inherently a squarewave signal.



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So, would I simply add a 1k resistor between the points on the back of the cluster and the turbo timer signal wires, and be good to go?
That would depend entirely on what the inputs of your turbo timer require. Assuming they are configured to take a 12v squarewave, then yes.


Also, you don't need a turbo timer.
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Old 05-27-2014, 12:43 PM   #3
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Sorry, tried to ninja edit but was about 45 seconds too late. So basically what I had concluded with for a 99-00 should work. The turbotimer wants a 5V digital (square) signal, so I would just need a 1K resistor between the tach wire headed to the ecu and TT, and VSS signal coming FROM the reed switch on the cluster to TT, and that should get the signal down to a safe voltage for the timer to input. (I had found a thread on here at one point in which I believe braineack was recommending this method).

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Since you didn't tell us the year of the car, I have to do some guessing here.

Tach is always a squarewave signal on all Miatas. It comes out of the igniter ('90-'93), ignition coils ('94-'95.5), or ECU ('95.5+) in this form.

VSS is an AC sawtooth wave between the VR sensor on the transmission and the speedo on the '99+ cars, which is translated at the speedo into a squarewave and sent to the ECU. On the '90-'97 cars, VSS is generated locally at the speedo with a mechanical reed switch, and is therefore inherently a squarewave signal.



That would depend entirely on what the inputs of your turbo timer require. Assuming they are configured to take a 12v squarewave, then yes.


Also, you don't need a turbo timer.
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Old 05-27-2014, 01:03 PM   #4
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The turbotimer wants a 5V digital (square) signal, so I would just need a 1K resistor between the tach wire headed to the ecu and TT, and VSS signal coming FROM the reed switch on the cluster to TT, and that should get the signal down to a safe voltage for the timer to input.
All by itself, putting a single resistor in series with a signal will not* decrease the voltage of that signal.

It strikes me as slightly odd that a device intended for use in a car cannot accept 12v input signals, but if this is actually true then you'd need a resistive voltage divider:



Make R1 = 1.5k and R2 = 1k.


* = I am assuming a high-impedance input on the load side, which is true in vast majority of modern applications.
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Old 05-27-2014, 01:21 PM   #5
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add turbo timer and gets the ban.
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Old 05-27-2014, 01:24 PM   #6
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lol kind of a shame cause he started off so solid.
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Old 05-27-2014, 02:06 PM   #7
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Okay guyz truth is I have this friend who is turbo charging his honda (b18 JDM **** bro) and needs a turbo timer because the turbo he uses get like over 9000 degrees rankine, cause his rally car spec anti-lag keeps that thing spooled 25/7.

Anywho 18psi, don't worry, I won't let you down. I just got a sick turbo off of ebay that some guy let go for dirt cheap, like $75 bucks. What an idiot. It even has God in the manufacturer name. It's so shiny and nice. Brand NIB! **** is gunna spool so fast. It'll go great with the TurboXS Type RFL I just bought. (all be it as sarcastic as this is, I know you had a decent experience with your godspeed, so it isn't quite as funny as it could have otherwise been)

Anywho Joe, the manual says nothing about the speed signal having a voltage limit, just the tach line. I know you hooked your oscilliscope up to the VSS lines and check the wave and voltage, as I was reading through a few threads you had posted that in. You said the tach wave is square from the beginning, do you happen to know if it's 12V or 5? Given that the NBs have it going to the ecu, I guess I'd just assumed it was 5. I will check when I get off work this evening otherwise, just figured I'd ask in case you knew. Haven't seen anything definitive yet, as most posts where it's brought up just turn into a war of turbo timer necessities. *clears throat*



Disclaimer: Unless added to build thread, no aforementioned purchases were actually made.
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Old 05-29-2014, 11:07 PM   #8
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Update: Hooked up and everything is working. I'll do a formal write up for anyone else that comes along later looking for info. Its reading speed, rpm, and hand brake status perfectly.
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Old 05-30-2014, 09:49 AM   #9
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Actually, pretty busy at work today, and have a lot to do this weekend. Soooo if someone wants the info, a search will inevitably lead them here anyway. Here is the wiring guide I made before going out to install it. Hopefully it helps someone.

(Turbo Timer wires depicted are for an HKS Type-1.
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