FC tach jumping around on decel
 

Hello. I am having a very weird issue, that I'm not sure is really an issue or what. I have recently noticed (not sure how long it has been doing this) that my tach will bounce up and down randomly, on decel only, above ~3000 rpm. I changed my trailing coils and it still existed. There is no difference of feeling, everything is smooth, I don't feel some weird spark on the decel, but it does not does it any other time.

Any ideas?

Run some datalogs and see what the ECU is doing. Log RPM and leading and trail advance. If the log looks clean I wouldn't worry about it.

That was the first thing I did. Not a single instance of any trigger jumping around. We also swapped the trailing coil/ignitor pack. No change. Wondering if it's something goofed on the tachometer. The dwell time is set to 4.2ms.

B

My friend's 2nd gen did the same thing. He got a "tach adapter" from MSD and a host of other stuff. I hooked up a cheap autometer tach to test and it worked fine. Turned out to be a bad tach. Just hook up a cheap autometer tach to test, since it's a haltech all the wiring is easy to get to.

Anthony

a copy of something i wrote elsewhere --

====================

https://img504.imageshack.us/img504/...0620390nn2.jpg

something you may find handy

- its variations of the "HEI corvette tach filter"

they are simple , low pass filters, and nothing complex
( stereo and electronics people will recognise the circuit and its brutal simplicity )
there are more complex types of filter designs
but this one GM found suitable for quelling the bell-ring and RFI that HEI coils would feed to the old points spec tacho

with the bosch HEI DLIDFIS and solid or spiral HT wires, the stock tacho can pick up the occasional noise, and flicker

the monster tach in particular seems even more vulnerable
( with so many variations over the years, its a little wrong to lump them all as one )
- actually a sign the tacho actually has fast response sensitivity
( but unfortunately more susceptible to bell ring and RFI false pulse )

only a few cents worth of components, repro tach filter is $25 US and $15 postage !!!!
( the original discontinued and in huge demand in US and dismantlers know it )
https://img508.imageshack.us/img508/...hfilterap9.gif
if using polarized one way type caps get them around right !!
( i use unpolarised polyester film type )


ima using similar to this ( 3.3 + 10 + .047mf )
( that's going to filter out signals beyond what a V8 single coil needs cut off , V8 single coils work hard )
[ highest frequency cut, IE softest filter ]

but also i have some fall backs if that not good enough
same at .082mf ( a much lower cut off )
[ lowest frequency cut, IE hardest filter ]

and another at 0.047 and combined 20K ohm
( ranked low, medium and high cuts )
two of them if installed reversed, will slightly alter the cut

- all cuts beyond frequency a rotary leading coil would fire at 10000 rpm with divider factor 2
( wastespark )
( actually up around 22000 don't ask for math, its crude and modified around the practical values of components avail at DSE )

knocking out this obviously bell ring or RFI signals will limit the bounce
( the tacho displays an average voltage, and responds to the high frequency flickers )

going TOO nuts with the resisters, may slow down response
( they are required to stop some surges and things on shut down also )
[ but also have other effect in the calculation, but be fee to alter within reason to trim your filter ]

going less than .047 mf is going to raise the cut off and allow lots of noise through
( factory actually uses similar values to quell alt and old dizzy noises )

some recommend cut offs using .1mf for 4 cyl applications
i would consider them as hard a filter as you would like to try

have fun !


PS
look for a factory noise condenser fitted to firewall of FC ( small square block marked 0.047 )
make sure its earthing body to the firewall OK !

Adding another hint to this mystery. I finally have got fuel cut decel to work nicely. Before any time over ~4,000 rpm and on decel the tach would bounce randomly and not register anything weird on the haltech logs. Now when I am over the ~4000 rpm mark and decel, it will jump all over the place for the 1 second until fuel cut decel hits and then it doesn't jump, it drops like it should.

What do you suppose this could mean? My first random guess would mean that my injectors or injector wiring are messing up my trailing spark? Doesn't make any sense..

Any new theories are welcome.

Thanks
~Tweak

PM IAN

He had the same tach jumping issue while racing at Mosport IIRC. Can't remember what he did though.

I find it extremely suspicious that nothing gets logged when the tach's jumping. Its driven from the trailing coils which are driven by the Haltech.....

Good luck :)

I have the same problem, tach will bounce if I downshift real hard. I have no ignition log problems so I just put it off as tach signal issues.

Bumpstart that post is interesting but I am slightly confused, I can get a filter and just put it inline the signal wire to the tach or for the whole ignition system?

Tweakgames,

I have no idea why my car did this. I swapped coils and tried different settings. I think it must be when the car gets driven hard on the track something in the coils heats up causing this weird tach problems. Eventually it didn't happen that often.

I am putting in a sequential setup / Autronic 500R CDI box which outputs to the tach and I fully anticipate this problem to disappear.

Hopefully you will find out what the problem is.

(I was never able to capture this on datalogging because of course this happened each time when I was not datalogging.)

Ian

Hook the tach to PWM 1 and see what it does.

I had tach bouncing (not just tach, but rpm signal on Haltech as well) with overly rich mixtures, causing plugs to not fire. Leaning it out helped and problem was gone :)

im having the same issue.. but itll do it whenever and then itll beep.. is there a battery in the dash? just askin but im on it!