Thread: RPM signal to autometer gauge fine, but car stalls if i hook it up to the dash

So im trying to get a rpm signal to the dash. My ecu i believe isnt sending the signal out. So i put in a huge autometer tach and tapped green/yellow wire coming from the coil and its always worked great.
SO i figured lets get rid of this horrible autometer tach that takes up too much space and directly wire the rpm signal from the coil to the blue/black wire in the dash going to the rpm tach.
The car wont start with it connected, i thought it was other things so i decided to just cut the wire and bam car starts right up. If i touch the wire to the dash wire the car stalls out but really quick i did see a signal on the dash
What could this be? Is there anyway to bypass the ecu and have a signal go to the dash tach? I even cut the wire from the ecu to the dash and the car woudlnt start that way

Ok so no one has a idea? Is there another spot to tap into to get a rpm signal?

Let me know when you figure this out, same issue I have.

I was hoping for some help or at least another solution. It looks like I'll be sending my ecu to JWT to get the ecu to send the rpm signal to the dash.
Your the first one to reply on both sr20 forums.

Hmmm, Let's see if this helps (if I can explain myself correctly):

*The Autometer tach works connected to the Coil Ground.

*I took my ECU to another car and the stock tach does NOT work. ECU is bad... No voltage from rpm pin coming out from ECU

*I took my dash to the other car and it reads rpm correctly. Dash is good...

*I tried the other ECU on my car and it does NOT trigger the tach in either of the dashes. But hey, that ECU is not bad because it works fine on the other car and it still works perfect after this experiment when back on the other car.

***My conclusion is that my ECU went BAD because of something that is BAD on my harness and will eventually damage any ECU until I fix that. I tried B14 dist with / without resistor and VE dist with / without resistor but all the same the dash does not display rpm. I left the VE without resistor.

***This problem also leads to my VVL switch not working correctly, only picking up the rpm as a two cylinder which means it reads twice as much rpm, and thus is limited to 7950 off for VVL (15900 rpm as the switch understands it). Any other cylinder setting does not work properly on the Granatelli Window Switch.

bump for help

Get the ECU fixed, or get another ECU and swap the board into it.

They're your two solutions for fixing the problem.

thats a bummer

has it ever worked originally? wat year is the car? originally ser, xe, gxe, e, se? tried bypassing straight from ecu to gauge with your own wire? all grounds ok on cluster/car?

The two signals are very different. The factory tachometer runs on a clean 50% duty cycle square wave with an amplitude of 9 volts. The signal that comes from the green/yellow wire at the coil is very messy, peaking at about 160 volts every time a spark is made. Also the frequency corresponding to the RPM of the engine is different in each signal. The factory tach isn't designed to get a reading from the messy coil signal.

I'm guessing the car dies when you connect the coil wire to the factory tach because it essentially shorts the wire to ground, not leaving enough power to the coil to make spark. Even if your car kept running, the tach could be damaged at worst or not show an accurate reading at best.

tl;dr - the signals are different.