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