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JDM to USDM tach communication problems.

IncorpoRatedX

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May 28, 2003
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The problem: My tachometer acts like a boost gauge with a laggy turbo, sits at zero at idle, then as my rpm's increase the tach "spools up" as the rpm's increase the rate the gauge sweeps increases as well, so around 4000 rpms, im at 8000rpms on the gauge.

I've got a 1990 Galant GSX Chassis, remember that part as it might be important, as in it may not relate to 91/92 galant electronics.

It has a full 87-89 JDM galant engine harness, cyclone engine, and dual board Eprom ECU, cant be logged. md128618 ECU to be exact.

I ordered a Keydiver jumper because the previous owner stated it was needed to correct the problem. After getting it and reading the paperwork i realized it was for using a 91+ ecu in a 90 car, not exactly my problem. I tried it anyhow, didnt help.

Is anyone familiar with the situation im in? Am I stuck needed a JDM 87-89 gauge cluster or needing to retrofit a 91+ spark transistor?


here's some dual board eprom porn.


 

What you need to do is determine exactly what tach parts are involved here:
1. Where is the tach signal coming from on the motor? From a 7-pin transistor, or from a "tach driver" module on the coilpack?
2. What tach pulses was the ECU designed to work with? I'm going to *assume* that the dual-board ECU's only work properly with the "tach driver" module, which puts out positive tach pulses.
3. What year is the dash tach, and what tach pulses was it designed to work with? If its an original 1990 GSX tach, the Galant Service Manual shows it as having the newer style 7-pin transistor with the tach driver integrated into it, which puts out ground pulses (collector driven):


But, the Galant manual shows the 1989 cars as having the older "tach driver" module built on to the coilpack, with positive pulses:

img.php


I can't say for sure which design was used on your build car. Maybe some other 1990 GSX owners can help.
 

IncorpoRatedX

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Thanks Jeff, I'll look into those items. the car is build date of october 89, dash cluster is original. As for the rest... to be continued.
 

yeah jeff I asked about this exact topic and am still having issues. from this thread >>> Tach Wiring.

although I don't think I have the same ECU, my car is a late 89, 90 model galant gsx like Alpha Male. I've tried running directly from the so called tach driver from the coil, maybe the gauge cluster is looking for the other kind of pulse signal that you mentioned. how do I determine which gauge cluster I have?
 

IncorpoRatedX

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looking at the original engine that was in my ggsx, it looks like it used a 91+ (in dsm linage) TPS and ISC and coil pack ignitor but a 90 CAS, So essentially its like using a 90 wiring and ecu with a 91+ gauge cluster. I will re-verify everything later. But right now that's what im thinking. Since the Galant came from japan i could see it getting updated quicker than USDM vehicles.. makes sense to me anyhow.
 

I thought this problem sounded familiar. If you go "by-the-book", it would appear that a 1990 GSX has the newer 1991+ style DSM tach, so it will not work well along with the older 1990 DSM "tach driver" module. That module put out positive pulses, but the newer tachs are expecting ground pulses. You either have to change the transistor to a 1991+ DSM style, or swap the tach out to an older 1990 DSM tach.
My tach adapter circuit would actually work also, because it is designed to do the same thing for the ECU, change the 1990 positive tach pulses into ground pulses instead. You would just have to figure out a way to bias the output wire to 12 volts, like with a 10K resistor, to make it work.
 

IncorpoRatedX

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Arizona
I thought so.

I have never needed to retrofit 90 electronics to a 91+ before, is swapping to a 91+ transistor an ordeal?
 

I actually have an adapter harness I made years ago around here someplace. I used an old 1990 transistor to remove the socket, so its plug and play. You just need to also tap into the harness for +12 volt power to the 7-pin 1991+ transistor. You just need to grab +12 volts from the coilpack wiring. That +12 volt input and the tach output are the 2 extra wires that the newer transistor has.
 

Scientist

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Jan 18, 2007
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530
Location
Roseau/Dominica
Guess i will have a similar circumstance

I have an 89 JDM galant..but have a 91 EPROM Ecu that I need to wire in
 

thats probably the reason why my two instrument clusters didn't work for me, I don't think they're original to the car.
 

IncorpoRatedX

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Joined
May 28, 2003
Messages
5,593
Location
Arizona
Quoting Scientist:
Guess i will have a similar circumstance

I have an 89 JDM galant..but have a 91 EPROM Ecu that I need to wire in



That's actually an easy fix, if the entire car is an 89 JDM galant. All you'll need is the Keydiver jumper. I'll gladly sell you the one i bought and did not need.
 
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