I understand the tach needs to be modified to go from a 4 to 8 cylinder.
Will it still work with inaccurate readings?
Currently I have the tach wire (white) from the fiero computer connected to a tach signal wire and filter from my Street and Performance ECM (ODBI) for the LT1.
It shows about 1,000rpm while cranking and 0rpm when running.
Is the wiring correct (ie. can a wrap the wiring and modify the tach later?)

Also a link to the tach mod would be a plus for you.

Ryan

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1986 Fiero2m8 (LT1 NX / 4T60 Roadster Build)
https://www.fiero.nl/forum/Forum2/HTML/071642.html

https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20050119-2-056263.html

I don't think you'll need the tach filter if you're going from the ECM to the tach directly.

wait, what? Fiero computer? Explain please...

[This message has been edited by ryan.hess (edited 08-08-2006).]

If you did nothing else, a 4 cylinder tach would still work with a V8, it would just register higher, because it would be seeing twice as many pulses.

Now you obviously have something else going on. You still have the Fiero ECM installed? Next question, why?

I would connect the tach signal wire from the LT1 ECM directly to the tach input signal on the cluster. Then modify the cluster for V8 operation.

https://www.fiero.nl/forum/Archives/Archive-000001/HTML/20031229-2-037230.html

quote Originally posted by Will: I got the 2% caps and installed a set in my car. My tach reads 6.6-6.7 when I'm on my 6.4 rev limiter.

Classic reason why not to do the capacitor method.

2% is pretty tight tolerances for caps. Normally they're +40/-20%

With the variable resistor method, mine is dead on at 6500. It might be 50 or 100rpm off at idle, but frankly I don't care.

quote Originally posted by ryan.hess: With the variable resistor method, mine is dead on at 6500. It might be 50 or 100rpm off at idle, but frankly I don't care.

I agree. I posted that link without comment, but I prefer the variable resistor method, then set it dead on at the redline. But not everybody has a frequency generator to do a redline calibration.

Thanks guys +

I am using a piggy back method shown in Archie's video and still using the fiero computer / harness for a few things where the harnesses are married together. Maybe I haven't found the correct tach guage wire for my 4 cyl / auto car.
Does the control wire for the stock Fiero guage enter the engine compartment or is it in the center console ?
I had labelled a solid white wire "Fiero guage" but it is probably the negative coil wire that goes to the Fiero ECM and may be the wrong one to connect my new tach signal to...

What is the stock fiero ECM doing that the LT1 ECM can't?

Check out http://www.v8archie.com/v8mod.htm
This is what I intended to do.
Without getting into specifics, the piggy backed computers/harnesses was a way to use all the existing Fiero guages.

Ryan

I used all the existing fiero gauges, and I didn't need to keep the fiero ECM...

/still confused

There is nothing in the instrument cluster that needs the Fiero ECM to operate.

The tach wire from your 4 cyl car came from the distributor, (and from the DIS on later years). That wire was a straight run from the cluster to C500 with no connectors or splices.