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Tach signal for 3.8/3.9 V6

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Paul Huizenga

Well-Known Member
In the process of installing and dyno-testing a Zex wet nitrous system using their PN 82085 traction control window switch on my '04, we had some problems in setup with the tach signal. In the first place, it took quite a bit of fooling around to get the Dynojet (using a spark plug wire induction pickup) to agree with the dash tach. Once we figured out the right settings and had everything installed, I couldn't get the Zex window switch to work properly.

The tach input on the Zex unit is designed to read a pulsed 12v signal, so we wired it into one of the three non-ground wires on the coil pack. Zex's instructions for setup give you the option of coil-on-plug (one pulse per 720 degrees of crank motion), wasted spark (one per 360), or a conventional coil with 2-12 cylinders.

In wasted spark configuration, the tach reading on the window switch jumps at idle, basically doubling every few seconds for just a moment. It also doesn't agree with the dash tach at any RPM. Going through all the possible settings didn't help.

Any suggestions on what I'm doing wrong? Should I just pull the tach signal from the ECU harness, or off an injector lead instead?
 
Windveil Blues

Windveil Blues

OLD FART
I have heard that it's not a great idea to use a coil pack. I don't remember why, though... sorry.

Jim
Paul Huizenga said:
In the process of installing and dyno-testing a Zex wet nitrous system using their PN 82085 traction control window switch on my '04, we had some problems in setup with the tach signal. In the first place, it took quite a bit of fooling around to get the Dynojet (using a spark plug wire induction pickup) to agree with the dash tach. Once we figured out the right settings and had everything installed, I couldn't get the Zex window switch to work properly.

The tach input on the Zex unit is designed to read a pulsed 12v signal, so we wired it into one of the three non-ground wires on the coil pack. Zex's instructions for setup give you the option of coil-on-plug (one pulse per 720 degrees of crank motion), wasted spark (one per 360), or a conventional coil with 2-12 cylinders.

In wasted spark configuration, the tach reading on the window switch jumps at idle, basically doubling every few seconds for just a moment. It also doesn't agree with the dash tach at any RPM. Going through all the possible settings didn't help.

Any suggestions on what I'm doing wrong? Should I just pull the tach signal from the ECU harness, or off an injector lead instead?
Click to expand...
 
wickedstangs

wickedstangs

Chula Vista, CA
Staff member
Administrator
if you use an injector wire your only going to get 1/8 of the signal if I am not mistaken...

Behind your instrument cluster is a connector that you can get the signal from
instrumentclusterwire.gif
 
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Paul Huizenga

Well-Known Member
wickedstangs said:
if you use an injector wire your only going to get 1/8 of the signal if I am not mistaken...
Click to expand...

1/6 - remember I'm short by 2 cylinders! :D

The window switch can be set up to read pretty much any number of evenly-spaced pulses per crank revolution, from 1/2 (Briggs & Stratton, perhaps?) to 6 (for 12 cylinder, single coil setups). I just need a clean signal that pulses a consistent number of degrees of crank rotation apart.
 
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Paul Huizenga

Well-Known Member
Well, I ended up using an MSD 8918 GMR inductive pickup. Less than $50 from Summit, and it clips right on the coil lead without splicing. Works like a charm.
 
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Paul Huizenga

Well-Known Member
Of the GMR? It's not all that exciting, actually, but I guess I'll snap a couple tomorrow. Here's what it looks like:

8918_full.jpg


Just three wires: Power, ground, and signal output. The clip on the end of the sensor goes around the wire you're using as the signal source. It can be an injector lead, or in my case, the +12v supply to the coils.
 
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