slvr98camaro

iTrader: (1)

Hi all, I want my stock tach to work with my SBC carb swap.

I searched and searched and I'm just coming up with the white wire in the c230 connector but it looks like it is an output.

I bought a Dakota sg converter so I can hook my coil up to the input of the box but I need to know what wire is the input to the gauge itself is and where I can find it. I was thinking maybe hooking it up to the old crank position sensor?

Thanks

slvr98camaro

iTrader: (1)

Anyone? This was a v6 car and I'm thinking I may be able to install my box to a wire from the icm where i believe the tach gets its signal.

My phone crashed and I lost all of my diagrams and info I had.

Thanks

WhiteBird00

iTrader: (4)

There is no output from the tachometer, unlike the speedometer. The tachometer runs off a solid state logic circuit in the instrument cluster that is fed by class 2 serial data. The tach needle deflection is controlled by a PWM (pulse width modulation) signal from the solid state circuit through a trace in the cluster circuit panel. I suppose you could pull the tach and determine the modulation rate through experimentation but there is no connection external to the cluster that you could just tap into.

slvr98camaro

iTrader: (1)

Thanks. I don't know a whole lot about how the instrument cluster is set up but the logic circuit/serial data has to be getting the info from somewhere, right?

I was thinking it got it from the crank position sensor or the ignition control module, out is that not how it works at all? I have seen the wiring diagrams buy they don't explain how exactly everything you are talking about works out where they get their info

WhiteBird00

iTrader: (4)

No, the factory tachometer doesn't use a pulse signal like aftermarket ones do. The PCM provides class 2 serial data to the instrument cluster logic circuit which in turn produces a PWM signal to make the needle move. Basically, you need the PCM in order to get the tach to work unless you want to come up with some way to bypass the logic circuit and determine the correct pulse rates to feed the tach directly.

Spartan7

iTrader: (7)

WhiteBird, isn't the 98 cluster different from the 99-02 ones? I thought I had read that they use analog signals rather than serial for everything.

WhiteBird00

iTrader: (4)

I had to look that up. I knew the '98 cluster was different but I hadn't considered that the tach input signal would be different. Turns out it is and it isn't... the solid state circuit in the '98 cluster still gets its data from the PCM but as electrical pulses rather than class 2 serial data. The circuit processes those pulses and outputs an electrical current to the tach to move the needle in much the same way as the VSS manages the speedometer.

There is no documentation on what it does to process the pulses so we're still left with no sure way to substitute another source but it looks like the needle would deflect based on voltage so it's possible some testing with a potentiometer might determine a conversion value for using an alternate signal source.

slvr98camaro

iTrader: (1)

Thanks. I am keeping the pcm which is why I thought it may be possible to do this.

I don't believe my converter can put out a pwm signal only a ppm signal. I remember reading somewhere that you can hook up a scanner to the car and give it some type of signal (to test the tach) and the tach will read around 1500 rpm but I believe it was with a gm tech ii scanner

slvr98camaro

iTrader: (1)

OK I just found the wiring diagram. Im not an expert on wiring diagrams but it looks like the solid state logic gets its info from the white wire coming off of the ignition control module. I'm not understanding why I can't hook up the negative of the coil to my box for the engine rpm, then the output of this box to the icm connector and keep adjusting the box until the rpm is correct (I have an old tach/dwell meter that I can use to get a correct rpm reading)

Spartan7

iTrader: (7)

Try it and find out. I do know the LT1 cars are similar (white wire from PCM drives the gauge), and there are quite a few guys that have hooked aftermarket tachs to the white wire with success.

slvr98camaro

iTrader: (1)

I finally got around to this today, the way I wanted to get my factory gauge working worked perfect. If I set the dakota digital box to 6 cylinder and hooked the output to the White wire on the icm I was able to get the tach working.