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1G Tach Question

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90GSXBlue

Probationary Member
15
6
Nov 27, 2019
Littleton, Colorado
I'm doing some wiring clean up and was wondering if i can just run a wire from my factory tach to the ecu pin 109 to get the signal without any issues? its a 1GA 90 with a rywire harness

Edit
im seeing RPM in link
 
I'm doing some wiring clean up and was wondering if i can just run a wire from my factory tach to the ecu pin 109 to get the signal without any issues? its a 1GA 90 with a rywire harness

Edit
im seeing RPM in link
I am not so sure what exactly you want to do. Do you want to make the stock tachometer work? If so, and I assume that you are receiving the RPM signal properly at ECU, then yes you should be able to do that without any issues since originally the ECU pin #109, PTU pin #4 (the white wire) and the tachometer are connected.
 
I am not so sure what exactly you want to do. Do you want to make the stock tachometer work? If so, and I assume that you are receiving the RPM signal properly at ECU, then yes you should be able to do that without any issues since originally the ECU pin #109, PTU pin #4 (the white wire) and the tachometer are connected.

Sorry i shouldve explained better. can i use the ECU pin 109 and Directly wire that to the back of the factory tach to make it work (using that as the signal). i see rpm in link so can i rob that signal to use it for my factory tach without any issues. or is it better to steal it off the PTU?
 
On a 1990 ONLY, there is a Tach Filter that must be used or the factory tach will not function (or of you figure out how to make it work and NOT use the filter, PLEASE POST IT UP).
Here is the Tach Filter....
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Sorry i shouldve explained better. can i use the ECU pin 109 and Directly wire that to the back of the factory tach to make it work (using that as the signal). i see rpm in link so can i rob that signal to use it for my factory tach without any issues. or is it better to steal it off the PTU?
ECU is taking the signal from the PTU, so you are reading the RPM signal in ECMLink means you have ECU pin #109 already connected to PTU pin #4.
And you are asking if it's OK to take the RPM signal (run a wire) from ECU pin #109 (between ECU #109 and PTU #4) to the tachometer, that's actually how the factory wiring goes. So you would be able to do that without any issues.
 
I want to see this work.
Hell, if that just plain works, lets write it up so all the 90 owners can have that info. :thumb:
Those little filters are hard to find and nobody knows whats inside, LOL.
 
ECU is taking the signal from the PTU, so you are reading the RPM signal in ECMLink means you have ECU pin #109 already connected to PTU pin #4.
And you are asking if it's OK to take the RPM signal (run a wire) from ECU pin #109 (between ECU #109 and PTU #4) to the tachometer, that's actually how the factory wiring goes. So you would be able to do that without any issues.
Sorry, forgot to mention that if you still have a '90 coil pack it's from tach interface (a small square box on the coil pack that has 3 wires). If you are reading the RPM signal properly in ECMLink then it's connected properly. Anyway either case with '90 or 91-94, you shouldn't have any issue with taking the signal from the ECU pin #109.
 
On a 1990 ONLY, there is a Tach Filter that must be used or the factory tach will not function (or of you figure out how to make it work and NOT use the filter, PLEASE POST IT UP).
Here is the Tach Filter....
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I want to see this work.
Hell, if that just plain works, lets write it up so all the 90 owners can have that info. :thumb:
Those little filters are hard to find and nobody knows whats inside, LOL.
Marty, I am interested, too. Actually I was wondering about that long time ago but forgot its existence. Because when I bought my 90 gsx the tachometer wasn't working and didn't have that filter. So I was doubting that's the cause, but somehow I fixed it without it. The tachometer had been working since... I really don't remember well what exactly I did or what was the cause... What I at least remember is it had the RPM signal at ECU since SAFC or some pig tail could read the RPM signal through ECU pin #109, so I was using it as RPM gauge by putting it on the cluster. And I tried two 91-94 cluster but they didn't work.

**UPDATE**

The what i believe to be a 90 Tach does not work

The what i believe to be a 91 tach does but reads about 300RPM low.
So do you have a 91-94 coil pack and PTU? Can you provide us a bit more details of your car?
 
yes running 91-94 coil pack and PTU. Car is a 90 shell with Rywire engine harness... this harness uses the 91-94 configuration for everything else.. the chassis harness is a 90 however and im still trying to figure out all the dash wiring and accessories.

also when I had it set up to see if it worked the indicator wasn't on correctly, now it reads what the ECU reads for RPM
 
yes running 91-94 coil pack and PTU. Car is a 90 shell with Rywire engine harness... this harness uses the 91-94 configuration for everything else.. the chassis harness is a 90 however and im still trying to figure out all the dash wiring and accessories.

also when I had it set up to see if it worked the indicator wasn't on correctly, now it reads what the ECU reads for RPM
So it reads 300rpm lower than ECU but it's reading. Do you have a diagram? and did you make sure all ground wires that are related is properly grounded?
 
Its reading the tach signal so I say yes and I hope Hiroshi will give his .02¢ on it also.

Marty thanks for posting all those pics of the little gizmo and its connector. I don't think I've ever seen mine and didn't know where to look.
Seriously you just about need that many pics to see the thing itself, and where it's located, and to get a good look into the connector.
That was a stock 1990 GSX engine that I saved from a totaled out shell that I bought so I say that is where you need to look. I have a few other 90s and need to see if it is in the same exact spot on them.

Edit: I will see if I can add a picture to the thread showing where it plugs into the harness. I have it plugged into the Red 90 Talon harness and no motor to clutter up the picture for that too.
 
Thanks again for the help guys.

Next time i have the tach out ill take pictures of both units and post them to here, maybe someone much smarter than me can figure out how to make the 90's work.

Sorry to hijack the thread, but is pin 109 the wire you would tap for a shift-light? Thanks.
No worries the more info about the tach on this thread the better
 
Sorry to hijack the thread, but is pin 109 the wire you would tap for a shift-light? Thanks.
Its reading the tach signal so I say yes and I hope Hiroshi will give his .02¢ on it also.
Technically yes. and it depends on the device and its setup. But if you still have the stock tachometer, sometimes more than 2 devices can't share the tach signal very well like I mentioned in the link below. In that case, better to take the signal from somewhere else. Some devices can also accept the injector or coil to read RPM.
https://www.dsmtuners.com/threads/autometer-tachometer-install.530091/#post-153772478
 
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I will see if I can add a picture to the thread showing where it plugs into the harness. I have it plugged into the Red 90 Talon harness and no motor to clutter up the picture for that too.

That would be good!
Something I wonder about the tach noise filter is if the way it is attached is actually part of the circuit. I mean, it's in a metal case, screwed down to metal in the engine bay - it's grounded. It would make sense for that ground to be a necessary part of the circuit if this thing is a low-pass filter.
A low pass filter makes sense. It allows low frequencies to go through, and it attenuates higher frequencies (noise). The lower frequencies would be the "fundamental" of the signal we want.

In this diagram, our little 284L02901 would be the capacitor. The "in" and the "out" would be the 2 blades in the connector.
The leg of the capacitor that goes out to ground would just be the metal case of 284L02901 screwed down to metal. The resistor, well, other stuff farther away in the circuit would have some resistance.

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You have a resistor and cap in that picture. (It reminds me of the bass blockers I used to put on speakers)
I wonder whats inside the filter too as it does have 2 wires so I assune in and out. I will go out to the engine bay and get a picture of where it connects today. :)
 
Sorry to hijack the thread, but is pin 109 the wire you would tap for a shift-light? Thanks.

Do you still have stock dash? If so, you’ll see a screw marked tach, ground and power on the backside. You can run a shift light to those
 
That is where mine is tapped in also. (and a light in my dash) :shhh:

Here is the Tach Filter location in the harness on a 1990.
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As noted the output from the PTU or Tach Gate on the 90 feeds both the ECU and the gauge cluster. On a 90 the noise filter connects the Tach Gate signal to the harness so without it nothing reaches the cluster with out it.

There is a thread around here about replacing the noise filter with a simple bypass and it worked for that person.

There are also many threads on the problems of using 91+ PTU and ECUs with 90 gauge clusters. The short answer is that doesn't work.

Lastly, What DSMLink or a datalogger uses of it's RPM signal is the CAS not the PTU. The PTU/Tach Gate signal is used to verify that it pulled the coil low when the ECU requested it.
 
As noted the output from the PTU or Tach Gate on the 90 feeds both the ECU and the gauge cluster. On a 90 the noise filter connects the Tach Gate signal to the harness so without it nothing reaches the cluster or ECU.

There is a thread around here about replacing the noise filter with a simple bypass and it worked for that person.

There are also many threads on the problems of using 91+ PTU and ECUs with 90 gauge clusters. The short answer is that doesn't work.

Lastly, What DSMLink or a datalogger uses of it's RPM signal is the CAS not the PTU. The PTU/Tach Gate signal is used to verify that it pulled the coil low when the ECU requested it.
Steve, thank you for the correction! I memorized something improperly. And as for the noise filter, I don't really remember well what exactly I did to make it work without the noise filter since it's a long time ago. But your input reminded me of something little bit more. As much as I remember I tried many things, I ran a wire directly to the tachometer from somewhere and I cut some wire (I believe the one from the tach interface to ECU), and also I think I jumped something, too. I don't know which one actually made it work. Do you think actually I just bypassed the filter made it work? or Do you believe there is another way to make it work without the filter?
 
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