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Tach filter or no tach filter..that is the question by Kitskaboodle
Started on: 03-14-2013 09:58 PM
Replies: 21 (3196 views)
Last post by: tesmith66 on 04-30-2014 07:20 AM
Kitskaboodle
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Report this Post03-14-2013 09:58 PM Click Here to See the Profile for KitskaboodleClick Here to Email KitskaboodleSend a Private Message to KitskaboodleEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have replaced my tach filter before on my 86GT from Rodney when my tach died. The last time I drove my 85 GT I started getting a jumpy tach needle so I probably need another tach filter I believe. Anyways, before I get another one I wanted to ask if a tach filter is really needed? I saw a thread years ago where some guys were saying you don't really need it and they have been running without it and have not had issues. Please explain the purpose of the tach filter. Your input on all this?
Thanks, Kit
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Slowbuild
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Report this Post03-14-2013 10:19 PM Click Here to See the Profile for SlowbuildSend a Private Message to SlowbuildEdit/Delete MessageReply w/QuoteDirect Link to This Post
Sounds like you icm is about to die. It could be the filter I guess. I'd get them both, change the filter and look for changes. throw the icm in the glove box, and tools to change it.

Chay
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Kitskaboodle
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Report this Post03-15-2013 09:49 AM Click Here to See the Profile for KitskaboodleClick Here to Email KitskaboodleSend a Private Message to KitskaboodleEdit/Delete MessageReply w/QuoteDirect Link to This Post
More input please....
Kit
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Blacktree
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Report this Post03-15-2013 11:23 AM Click Here to See the Profile for BlacktreeClick Here to visit Blacktree's HomePageClick Here to Email BlacktreeSend a Private Message to BlacktreeEdit/Delete MessageReply w/QuoteDirect Link to This Post
The purpose of the tach filter is to smooth out the tach pulses from the ICM. If you delete it, your tachometer will be jumpy. When people say that they've deleted it with no problems, what they're really saying is they don't mind having a jumpy tachometer.
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Kitskaboodle
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Report this Post03-15-2013 02:45 PM Click Here to See the Profile for KitskaboodleClick Here to Email KitskaboodleSend a Private Message to KitskaboodleEdit/Delete MessageReply w/QuoteDirect Link to This Post
Ok, thanks. I knew it was there for a reason. Just didn't know what it was.
I am getting two free used ones from my esteemed colleague Shane. (aka "Brastic")
Kit
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Marvin McInnis
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Report this Post03-16-2013 09:54 AM Click Here to See the Profile for Marvin McInnisClick Here to visit Marvin McInnis's HomePageSend a Private Message to Marvin McInnisEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by Blacktree:

The purpose of the tach filter is to smooth out the tach pulses from the ICM.



This is correct. The signal picked off the ignition system is electrically very "noisy," and the purpose of the tach filter is to reduce that noise so the tach only sees the actual ignition pulses. FWIW, here is an interesting and informative short article on the tach filter by Oliver Scholz (fieroluke here on PFF): Tach Filter Diagnosis.

[This message has been edited by Marvin McInnis (edited 03-16-2013).]

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Kitskaboodle
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Report this Post03-16-2013 10:35 PM Click Here to See the Profile for KitskaboodleClick Here to Email KitskaboodleSend a Private Message to KitskaboodleEdit/Delete MessageReply w/QuoteDirect Link to This Post
Thanks for the link.
Ok, on to the next problem. Turns out I had already replaced the tach filter (with Rodney's replacement) several years ago but yet my tach is still jumpy. I had forgotten that I replaced it about 5k miles ago. So, at this point i made sure the tach filter connections and ground were good. (they are fine) So then, why is my tach still jumpy? Anyone have a failure with Rodney's replacement? Could it be something else?
By the way, I should probably point out that by "jumpy" I mean that it will go from actual rpm readings, back down to zero and then back up to the actual rpm I'm turning. It does not "jump" above this. (not sure if this info will help but technically the tach is not really jumping all over the place) Kit
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IFLYR22
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Report this Post03-17-2013 12:37 AM Click Here to See the Profile for IFLYR22Click Here to Email IFLYR22Send a Private Message to IFLYR22Edit/Delete MessageReply w/QuoteDirect Link to This Post
That sounds more like and intermittent signal issue, not (specifically) a tach filter issue.

If the Tach sees a 0 signal (as in a Pulse Per Revolution of 0), it will display it as a 0 RPM. The tach works off a PPR of cylinder count / 2 (4 cylinder / 2 = PPR of 2, 6 cylinder / 2 = PPR of 3).

If the Aircore movement is good, It will zero upon key ignition on (before start).
If your needle is going from known good RPM, to 0, and back to known good RPM, the signal (white wire on C3, pin 18) is interrupted (bad connection).

-Dave

[This message has been edited by IFLYR22 (edited 03-17-2013).]

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Kitskaboodle
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Report this Post03-17-2013 10:12 AM Click Here to See the Profile for KitskaboodleClick Here to Email KitskaboodleSend a Private Message to KitskaboodleEdit/Delete MessageReply w/QuoteDirect Link to This Post
I checked the tach needle with key on. Yes, it moves right to zero. C3, pin 18, white wire? Are you referring to one of the two connectors that plug into the back of the instrument cluster? Or, are you referring to the tach connections to the speedo's flexible pc board?
Kit

[This message has been edited by Kitskaboodle (edited 03-17-2013).]

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IFLYR22
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Report this Post03-17-2013 02:12 PM Click Here to See the Profile for IFLYR22Click Here to Email IFLYR22Send a Private Message to IFLYR22Edit/Delete MessageReply w/QuoteDirect Link to This Post
C3 is one of the two connectors on the back (facing toward the front of the car) of the instrument cluster. It is the clear (translucent white), 18 socket (most wires in it), connector on the right side.
I was giving that as an end point for the White wire (Tach signal wire).

You were describing an intermittent connection, which are the hardest to pin down.
With the needle moving to 0, the power is not interrupted. With interrupted power, the needle would go to it's powered off rest point.

The only thing I can suggest, is to trace down and check every connection from the Tach to the coil.

-Dave
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jaredmurray88
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Report this Post03-17-2013 02:15 PM Click Here to See the Profile for jaredmurray88Click Here to Email jaredmurray88Send a Private Message to jaredmurray88Edit/Delete MessageReply w/QuoteDirect Link to This Post
Kinda sorta related but should a filter still be used with a swap and aftermarket tach?
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Blacktree
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Report this Post03-17-2013 09:02 PM Click Here to See the Profile for BlacktreeClick Here to visit Blacktree's HomePageClick Here to Email BlacktreeSend a Private Message to BlacktreeEdit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by jaredmurray88:

Kinda sorta related but should a filter still be used with a swap and aftermarket tach?

I think it would depend on what ignition system the engine uses. Also, I think some aftermarket tachometers have built-in filters.
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IFLYR22
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Report this Post03-18-2013 12:00 AM Click Here to See the Profile for IFLYR22Click Here to Email IFLYR22Send a Private Message to IFLYR22Edit/Delete MessageReply w/QuoteDirect Link to This Post
 
quote
Originally posted by jaredmurray88:

Kinda sorta related but should a filter still be used with a swap and aftermarket tach?


Blacktree is right. My 3800 with Stewart Warner guages do not have the filter (it is built into the Tach gauge).

-Dave
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zkhennings
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Report this Post03-18-2013 07:47 AM Click Here to See the Profile for zkhenningsSend a Private Message to zkhenningsEdit/Delete MessageReply w/QuoteDirect Link to This Post
I just rebuilt my engine and I went through my wiring harness and my tach was working fine before, and now it either pegs or stays right at 1. When deleting the tach filter do you ground the two wires that were going to the filter or connect them together? I haven't tried grounding them yet, just connecting them. I made a new tach filter from fieroluke's schematic (3kohm to 10kohm with a .047 micro farad capacitor connected to ground at the connection between the resistors) And it doesn't work. Maybe my tach is broken? I doubt it but it is possible.
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Blacktree
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Report this Post03-18-2013 03:55 PM Click Here to See the Profile for BlacktreeClick Here to visit Blacktree's HomePageClick Here to Email BlacktreeSend a Private Message to BlacktreeEdit/Delete MessageReply w/QuoteDirect Link to This Post
You connect the two wires together. Otherwise, the signal from the ICM won't make it to the tachometer.
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Blacktree
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Report this Post03-18-2013 04:09 PM Click Here to See the Profile for BlacktreeClick Here to visit Blacktree's HomePageClick Here to Email BlacktreeSend a Private Message to BlacktreeEdit/Delete MessageReply w/QuoteDirect Link to This Post
Yay, my not-so-smart phone double posted for me.

[This message has been edited by Blacktree (edited 03-18-2013).]

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zkhennings
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Report this Post03-18-2013 04:13 PM Click Here to See the Profile for zkhenningsSend a Private Message to zkhenningsEdit/Delete MessageReply w/QuoteDirect Link to This Post
When they are connected nothing happens besides it pegs... Any suggestions on what is wrong or where to check?
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fiero2kewl
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Report this Post04-28-2014 12:43 PM Click Here to See the Profile for fiero2kewlClick Here to Email fiero2kewlSend a Private Message to fiero2kewlEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have an 86se that's doing the samething...when I fire it up the tach pegs out past 6K but it doesn't go all the way aroung to tripometer.... sure hope somebody has an answer.
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BrittB
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Report this Post04-29-2014 09:01 PM Click Here to See the Profile for BrittBSend a Private Message to BrittBEdit/Delete MessageReply w/QuoteDirect Link to This Post
I have no tach filter and the tach is not jumpy at all. It was removed way before I owned it too!
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Ponnari
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Report this Post04-30-2014 12:50 AM Click Here to See the Profile for PonnariSend a Private Message to PonnariEdit/Delete MessageReply w/QuoteDirect Link to This Post
I agree with blacktree and others that say leave the tach filter on, if bad replace with new one. Here's my reason. One of my cars is an 86 gt. 107k miles. Always runs very well, 98% out of 100%. Tach started showing more rpm than actual, so replaced the filter, problem solved, nothing else was done. Now I know this shouldn't be....... But everything electrical.works better, and engine just sounds better. And less static on radio, which I never really noticed before. I've always been told the TF doesn't affect anything. After thinking about this for a few days I reinstalled the old filter, everything worked fine but wasn't as good as it was with the new TF. So put new one back on. Now it runs just like my other 88GT that only has 65k miles on it. Who'd. Of thought??????
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Report this Post04-30-2014 01:25 AM Click Here to See the Profile for PonnariSend a Private Message to PonnariEdit/Delete MessageReply w/QuoteDirect Link to This Post

Ponnari

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Member since Dec 2011
If you need to recalibrate the tach this thread might help. You can also use a variable resistor instead of the pot he used.

See
http://www.fiero.nl/forum/Forum1/HTML/089365.html
2/3 of the way down, explains tach fix
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tesmith66
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Report this Post04-30-2014 07:20 AM Click Here to See the Profile for tesmith66Send a Private Message to tesmith66Edit/Delete MessageReply w/QuoteDirect Link to This Post
My experience with the tach filter is you need it until it fails, then you no longer need it. I removed mine years ago when the needle started jumping and it has been fine since. I left it off when I swapped in the V8 and again when I swapped in the 3.4 DOHC. Tach works fine without it.

------------------
1986 SE Aero coupe.

3.4 DOHC swap is complete and running, now just have to finish the rest of the car...

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