Ok, this may apply to other dash swaps out there with people running an engine swap using a HEI ignition system (ie: 4.9, carb SBC, etc).

Sympton: tach bounces, or work fine till the higher rpms.

Why: the modern tachs are driven off either the ECM or a DIS system and supplies the tach with a nice clean signal - the older systems such as the HEI have quite a bit of noise on the signal and the 'modern' tach can not deal with this noise.

This circuit should eliminate any noise and send a cleaner square wave to you tach input. Edit: fixed picture


I kind of took the lazy way out by hacking up an old Fiero tach to get the filtering circuit out of it. I also have no ideal if the component values are the same as the above diagram (never cheacked).

Sorry, I didn't have a working digital camera to take pictures as I cut up the board, but you should be able to get an ideal of where I cut it from the pictures.

Front view, the large round hole in the lower right is the input to the tach, the resistor pack (white chip) is part of the one that get removed when doing the V8 tach mod.


Rear view, shows a better ideal of the cut in relation to the traces. The brown wire is ground and the upper yellow wire will feed your tach. The lower yellow one is the input from the distributor.


Anyways, there you have it.
Tim

[This message has been edited by Mickey_Moose (edited 09-29-2006).]

Tim,

As always you are way out there in the forefront of figuring out these issues. I knew there was a reason I kept that box full of tach boards that I had laying around. If only I had my 4.9 here to test it out. Thanks.

Jonathan

Is there a way to use the 98 Firebird V8 tach with out using the Fiero ECM?

I am not using the computer in my V8 install.

Thanks.

John

quote Originally posted by 1986GTV8: Is there a way to use the 98 Firebird V8 tach with out using the Fiero ECM? I am not using the computer in my V8 install. Thanks. John

I'm not sure I understand the question...the Fiero ECM doesn't do anything for the Fiero gauges, on the contrary the Fiero gauges output a signal to the Fiero ECM. If you are running an HEI or other engine (with no ECM) , use that signal directly from the engine, (and Tim's filter) to drive the tach directly.

If you have cruise, I have no idea how you are going to make that work.

[This message has been edited by jscott1 (edited 10-20-2006).]

The reason for my inquiry was after reading the quote below which I pasted form the opening information.:

" Why: the modern tachs are driven off either the ECM or a DIS system and supplies the tach with a nice clean signal - the older systems such as the HEI have quite a bit of noise on the signal and the 'modern' tach can not deal with this noise."

I took that to mean, I needed a signal from an ECM, weather it was from the Firebird or Fiero.

Thank you for the clearification. You guys are a tremendous help to those of us who are not as sayv with electrical theory, & calcilations.

God Bless you one and all.

John

PS: I hope to one day personally combine all of the Firebird dash info into one compiled thread for all to enjoy since ther are more than a few threads on this subject.

THANKS again to all for puting this info out there!

quote Originally posted by 1986GTV8: " Why: the modern tachs are driven off either the ECM or a DIS system and supplies the tach with a nice clean signal - the older systems such as the HEI have quite a bit of noise on the signal and the 'modern' tach can not deal with this noise." I took that to mean, I needed a signal from an ECM, weather it was from the Firebird or Fiero. Thank you for the clearification. You guys are a tremendous help to those of us who are not as sayv with electrical theory, & calcilations. God Bless you one and all. John PS: I hope to one day personally combine all of the Firebird dash info into one compiled thread for all to enjoy since ther are more than a few threads on this subject. THANKS again to all for puting this info out there!

John,

In the future, please post questions to this thread:

https://www.fiero.nl/forum/Forum1/HTML/023174.html

Most everything is already compiled into that thread.

What Tim meant to say is the "the modern tachs are driven off either the ECM, DIS or in the case of Fieros witih distributors...the distributor". Fieros with distributors send that signal through a tach filter and then it goes right to the tach.

Consider it done....

Thanks.

John

No, as mentioned you feed the tach directly from the distributor - you just need the filter as I posted. I was just pointing out that the Firebird tach is fed from the ECM or coil system (depends if the car was a V8 or a V6, respectively) in the Firebird.

The problem I was having was a result of feeding the tach directly from the distributor. For some reason, the stock Fieros tach signal seems to work with the Firebird tach as is, I just found that my HEI system did not.

Sorry for any confusion.