Tach signal and wiring specifics - Miata Turbo Forum - Boost cars, acquire cats.

Welcome to Miataturbo.net   Members
 


ECUs and Tuning Discuss Engine Management, Tuning, & Programming

Reply
 
 
 
LinkBack Thread Tools Search this Thread
Old 06-20-2011, 02:24 PM   #1
Free Moustache Rides
Thread Starter
iTrader: (8)
 
matthewdesigns's Avatar
 
Join Date: Jun 2008
Location: Colorado Springs
Posts: 1,656
Total Cats: 49
Default Tach signal and wiring specifics

Posted in this thread last week but perhaps it didn't get noticed as the title is not directly related to my problem, so I'll start a new thread

My tach is currently inop after my ECU/harness swap. Underdog has the same issue on the same setup and he warned me that I'd have to dig around for a solution. For reference, I do still have the stock ecu and igniter connected to the stock harness, but it's no longer part of the ignition system. The Miata and DSM igniters are nearly identical, one exception is the DSMs lack of the IGf pin (Mitsu J702T vs Mitsu J722T). I need to look further into how the DSM tach works, but the ignition is also a wasted spark 1/4, 2/3 setup as in the Miata so it's pretty damn similar I'd suspect.

*Since posting in that other thread I've discovered that the DSM uses a 5V square signal to feed the tach. What does the Miata tach signal actually look like? Is it a 12V signal all the way to the tach, or?? I can't find anything here or m.net about the actual signal voltage. Simply connecting the Miata tach feed at the ignitor harness to the DSM signal output at the DSM igniter does not work. I also found that the tach signal in the DSM setup splits from the igniter and goes to both the tach and ECU, but thus far nobody in DSM land can tell me what that split to the ECU does. I do have engine rpms showing up in my logs, so I'm guessing that it's just a signal feed for the ecu to monitor.

The info from Joe in the above linked thread is very helpful, but another thing I'm needing to know now is about the actual wire to the tach. Based on the wiring diagram shown below for a stock '92, it looks like the signal wire goes directly from the igniter to the tach (1H, yel-blu), without passing through the ecu. Is this correct? There is also a wire dropping off the tach that appears to be a common power source that runs to the meter fuse (2K, blk-yel), but all my other gauges work so I'd guess that the problem does not lie there.

Also, does the signal from the ecu to IGf do anything other than supply the pullup for the tach signal? If yes, what?

Thanks!

matthewdesigns is offline   Reply With Quote
Old 06-20-2011, 02:31 PM   #2
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 72,854
Total Cats: 1,788
Default

the ECU has nothing to do with your tach signal to your dash. However, it does provide the pull-up to power to the tach output wire from your coils (the black/white wire from 2I to the ignitor).

Grab a 1K resistor, jump IG- with B+ in the diagnostics box, and enjoy the tach again.
Braineack is offline   Reply With Quote
Old 06-20-2011, 02:44 PM   #3
Free Moustache Rides
Thread Starter
iTrader: (8)
 
matthewdesigns's Avatar
 
Join Date: Jun 2008
Location: Colorado Springs
Posts: 1,656
Total Cats: 49
Default

Thanks for the quick response, especially verifying that the stock ECU does not intercept the tach signal.

However, I want to be sure that the fact that the stock Miata ignition system is not connected to the motor in any way won't alter your solution. The stock coils are gone, and the igniter while still attached to the harness is no longer being used. The CAS is also now wired into my new harness. This would lead me to believe that there is no rpm input provided anywhere to the stock Miata harness now.

Could it be as simple as jumping my existing tach signal (from my new harness/igniter) to the Miata igniter harness to get that signal back into the Miata engine harness, then doing what you describe above?
matthewdesigns is offline   Reply With Quote
Old 06-20-2011, 02:51 PM   #4
Elite Member
iTrader: (13)
 
Join Date: Dec 2009
Location: Fraser Valley, British Columbia
Posts: 2,502
Total Cats: 13
Default

Just do what brain said. Find a resistor and jump it first. You can do so with just sticking the rsistor into the female holes on the diag connector. If you still have a proble then look into wether your coils are hooked up correctly.
falcon is offline   Reply With Quote
Old 06-20-2011, 02:57 PM   #5
Free Moustache Rides
Thread Starter
iTrader: (8)
 
matthewdesigns's Avatar
 
Join Date: Jun 2008
Location: Colorado Springs
Posts: 1,656
Total Cats: 49
Default

Yeah, I'll give that a shot since it's simple enough. The car is running so I'm pretty sure the coils are hooked up properly. Just to restate, nothing related to the ignition is currently connected to the Miata harness.

Edit: Any particular wattage I need to use? I see that there are 1/4, 1/2, etc watt 1K ohm resistors out there.

Last edited by matthewdesigns; 06-20-2011 at 03:09 PM.
matthewdesigns is offline   Reply With Quote
Old 06-20-2011, 03:21 PM   #6
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 72,854
Total Cats: 1,788
Default

I use 1/4 watt.
Braineack is offline   Reply With Quote
Old 06-20-2011, 03:40 PM   #7
Free Moustache Rides
Thread Starter
iTrader: (8)
 
matthewdesigns's Avatar
 
Join Date: Jun 2008
Location: Colorado Springs
Posts: 1,656
Total Cats: 49
Default

OK thanks. Will report back on result.
matthewdesigns is offline   Reply With Quote
Old 06-20-2011, 04:23 PM   #8
Senior Member
iTrader: (10)
 
Join Date: Aug 2007
Location: Fredericton, NB
Posts: 1,187
Total Cats: 4
Default

Not that this is important for this thread...but with an AEM...that resistor is not needed... there is an option in software to get signal.
Preluding is offline   Reply With Quote
Old 06-20-2011, 04:39 PM   #9
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 72,854
Total Cats: 1,788
Default

I wouldn't need the resistor either had I built the pull-up into my ECU/harness.
Braineack is offline   Reply With Quote
Old 06-20-2011, 11:02 PM   #10
Free Moustache Rides
Thread Starter
iTrader: (8)
 
matthewdesigns's Avatar
 
Join Date: Jun 2008
Location: Colorado Springs
Posts: 1,656
Total Cats: 49
Default

Somebody's gonna get a kiss

Took a break from work, ran to Radio Shack for the resistor, and tried jumping the pins in the diagnostic connector, but that alone did not bring the tach to life. However, after patching the DSM tach signal into the Miata harness via the ignitors' tach connectors it works! I have not verified that it's correct with a log of the ECU, but it sure looks right from the driver's seat.

Thanks Brain!

And for the sake of not being totally ghetto I'll eventually get around to wiring the resistor in behind the diag box, but is it OK just stuck in there until that happens? Maybe a couple weeks?
matthewdesigns is offline   Reply With Quote
Old 08-15-2011, 07:47 PM   #11
Senior Member
iTrader: (1)
 
Join Date: May 2008
Location: south texas
Posts: 1,419
Total Cats: 9
Default

sorry to dig this up, but i have a tach situation also. i have a '95 1.8 with a stock harness and link ecu. i had a problem with the needle jumping around crazy and finally stopped working a long time ago. that turned out to be the ground by the t-body. fixed it and tach was fine again. this time i went to drive one day and no tach. i checked all the grounds and all looked good. then tried a shift light with digital tach readout, first to the diag. box, then even at the ecu (blk./wht. stripe) and still nothing. thinking maybe the electronic shift light/tach wasn't compatible, i tried a basic autometer tach on it, again both locations, still no go. finally i tried tapping the wire at the coil for a signal and still didn't work. originally i thought the ecu was putting out the signal, as the wire goes to both coils, tach, and the diag. box. but is it really the coil that provides the output? is it my coil that isn't providing a signal? can i try the 1k resistor also? if not, is there any other way to "generate" a signal? like the op, my link shows/logs rpm, but that doesn't help while on the track.
spoolin2bars is offline   Reply With Quote
Old 08-15-2011, 09:06 PM   #12
Boost Pope
iTrader: (8)
 
Joe Perez's Avatar
 
Join Date: Sep 2005
Location: Chicago (Over two miles from Wrigley Field. Fuck the Cubs. Fuck them in their smarmy goat-hole.)
Posts: 26,317
Total Cats: 1,915
Default

Quote:
Originally Posted by spoolin2bars View Post
is it really the coil that provides the output?
Depends, as '95 is when they changed it. If your coils each have four wires, then yes, the signal comes from the coils. If they have three wires each, then the tach signal comes from the ECU.
Joe Perez is online now   Reply With Quote
Old 08-16-2011, 06:11 PM   #13
Senior Member
iTrader: (1)
 
Join Date: May 2008
Location: south texas
Posts: 1,419
Total Cats: 9
Default

Quote:
Originally Posted by Joe Perez View Post
Depends, as '95 is when they changed it. If your coils each have four wires, then yes, the signal comes from the coils. If they have three wires each, then the tach signal comes from the ECU.
okay, well i know mine is the 4-wire coils. so my coilpack is bad then? (as far as tach signal) i really don't feel like buying another coilpack. is there any other way to "generate" a tach signal? what do people that go COP do for a tach?
spoolin2bars is offline   Reply With Quote
Old 08-17-2011, 02:13 PM   #14
Boost Pope
iTrader: (8)
 
Joe Perez's Avatar
 
Join Date: Sep 2005
Location: Chicago (Over two miles from Wrigley Field. Fuck the Cubs. Fuck them in their smarmy goat-hole.)
Posts: 26,317
Total Cats: 1,915
Default

Quote:
Originally Posted by spoolin2bars View Post
okay, well i know mine is the 4-wire coils. so my coilpack is bad then?
Maybe. Or your wiring, or your tach, or... ?


Quote:
(as far as tach signal) i really don't feel like buying another coilpack. is there any other way to "generate" a tach signal?
Sure. Splice a couple of 1N4001 diodes into the coil trigger lines, parallel them on the far side, and use them to drive a non-inverting circuit that generates a +12 output whenever it's receiving an input. You can copy the "improved" MS spark output circuit, where there are two NPNs in series. Just build half of it and use +12 through 1k (instead of +5 through 330) for the final pullup.



Quote:
what do people that go COP do for a tach?
The Toyota COPs that we commonly use provide a tach output just like the '94-'95 Miata coils. So we just use a 1k pullup resistor from +12v and parallel two of them together.
Joe Perez is online now   Reply With Quote
Old 08-25-2011, 03:37 PM   #15
Senior Member
iTrader: (1)
 
Join Date: May 2008
Location: south texas
Posts: 1,419
Total Cats: 9
Default

jesus, just tried another coil pack, still no tach. i guess i'll try your circuit build or atleast try and figure out what you're describing. thanks, joe.
spoolin2bars is offline   Reply With Quote
Old 09-20-2011, 10:31 AM   #16
Newb
 
Join Date: Jul 2011
Location: Rehoboth, MA
Posts: 17
Total Cats: 0
Default

i have been talking with matt. i put link in my 91. and i spoke with him and put the resistor into the diag port and hooked up the tach input up to the ignitor....nothing. i tried putting the miata ecu back in the car and powering it up because he still has his miata ecu in there. nothing. so ive come to the conclusion that i fudged something up. what does that resistor actually connect? because if i can figure out what those two wires are supposed to be. i can just manually wire something up. and i saw the post about making that circuit and i couldnt tell you what any of that means.

but what im getting from it is to have a 12v source inline with the tach signal line. with the 1k resistor between them so in between the coils firing it is getting a 12v signal vs a 5v

Last edited by JFontaine; 09-20-2011 at 10:47 AM.
JFontaine is offline   Reply With Quote
Old 09-21-2011, 01:52 AM   #17
Senior Member
iTrader: (1)
 
Join Date: May 2008
Location: south texas
Posts: 1,419
Total Cats: 9
Default

yeah, i'm no electronics technician, so i'll probably have a buddy try and make one for me or something. i gave up for now. I ended up buying one of those cigarette lighter plug-n-play shift lights. it's hard to set the first time, but worked as advertised at the last event. only bad part is it feels really cheap, hope it lasts a while.
spoolin2bars is offline   Reply With Quote
Old 09-21-2011, 12:35 PM   #18
Free Moustache Rides
Thread Starter
iTrader: (8)
 
matthewdesigns's Avatar
 
Join Date: Jun 2008
Location: Colorado Springs
Posts: 1,656
Total Cats: 49
Default

Howdy Jay. Make an intro thread or nobody is going to take you seriously. You've got a cool project with a great swap coming up, I'm sure there are plenty of members here who would like to see it.

No luck with the tach adapter, or have you not tried that yet? It's strange that yours is not working if everything is connected the same way as mine, unless you pulled some critical bit out with the harness removal. Frankly I feel like I got lucky, and I never felt like I got enough info about the DSM ECU/tach signal to figure it out if it had not worked.

And by Link, he means ECMLink, same as what me and Underdog are using, not a Link ECU for Miata. Just in case anybody tries to answer this.

Also should note that his car runs, and he can log RPM, so there's a signal getting to the DSM ECU. This is where I got stumped, but the diag box resistor jumper worked for me.
matthewdesigns is offline   Reply With Quote
Old 09-21-2011, 12:39 PM   #19
Boost Czar
iTrader: (61)
 
Braineack's Avatar
 
Join Date: May 2005
Location: Chantilly, VA
Posts: 72,854
Total Cats: 1,788
Default

the resistor is just a pull up if you have removed it when pulling the factory ECU...
Braineack is offline   Reply With Quote
Old 09-21-2011, 03:03 PM   #20
Free Moustache Rides
Thread Starter
iTrader: (8)
 
matthewdesigns's Avatar
 
Join Date: Jun 2008
Location: Colorado Springs
Posts: 1,656
Total Cats: 49
Default

Brain, are you saying the Miata ecu has nothing to do with the tach operation? That's been my impression after digging around wiring schematics, I just want to be sure as I may be pulling mine out since I'm thinking it's not doing anything but adding weight at this point.
matthewdesigns is offline   Reply With Quote
 
 
Reply

Related Topics
Thread Thread Starter Forum Replies Last Post
My solution for Oiltemp and Oilpressure input into Megasuirt (MS3) Zaphod MEGAsquirt 41 01-24-2016 01:25 PM
Nuke Mars! Joe Perez Current Events, News, Politics 8 09-30-2015 05:41 PM
Low oil pressure after 1.8 swap and new turbo setup JesseTheNoob DIY Turbo Discussion 15 09-30-2015 03:44 PM
Are my coils failing? viriiguy General Miata Chat 5 09-28-2015 08:39 PM
wiring wideband ground to battery terminal btabor ECUs and Tuning 10 09-28-2015 06:33 PM


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -4. The time now is 03:10 PM.