Need help with tacho issue.

[shifty6]

Hey guys,
It's not technically a pcm issue but I've got an aftermarket tacho in my VC with injected 304. Tacho was working fine when installed, but after a while developed an erratic bouncing once the revs reach 3000 rpm the needle bounces all over the dial.

Tested tacho in a VR V8 and worked fine. Pulled the dizzy out of the vr and put in the vc made no difference. Replaced coil, dizzy cap, rotor, plugs and leads. Swapped out ecu and ignition module for known working items. Pulled tacho out and used short jumper leads straight to battery pos, ground and coil neg. still no good.
It's doing my head in, it must be interference from something but I can't sus it out.
Tried running heavy jumper cables from block to battery neg as well in case of bad earth.

[antus]

so your saying the vr tacho also had the same problem when on the vc? so both tachos fail at about 3k rpm in that car?

[shifty6]

Nah sorry. It's a speco branded tach.
I'm saying it worked when I hooked it up to the VR in the engine bay with some splice connectors.
Then put it back in the VC and tried the dizzy and ignition module from the VR since it worked with them in the VR.
Just a really annoying problem to have, I've replaced pretty much everything ignition related and check the wiring and earths from dizzy to ecu, ignition module etc.

[delcowizzid]

did you bolt the module down while testing it needs earthed.normally when the tacho starts bouncing its one of the 3 ignition componants on the way out

[Charlescrown]

Does the car run OK with the tacho problem? If not my guess is a faulty module. I had one go into self oscillation once. It triggered then with the engine off key on it continued to spark at a rapid rate.

[shifty6]

did you bolt the module down while testing it needs earthed.normally when the tacho starts bouncing its one of the 3 ignition componants on the way out

yeah the module is bolted down and also has a lead from one of the bolts to another earth location with engine loom earths etc. even tried a jumper cable from module body to block.

ive replaced everything ignition related and as i said it worked on another car no problems and i pulled the dizzy and module straight out of that car and into mine and tach problem remains. brand new bosch coil, cap, rotor, leads, plugs.... only happens once rpm reaches 3000, but it still sounds / drives fine, not jerky or anything like a miss.

[blownvp]

Might be interference from the alternator. Try a suppressor capacitor on b+ alternator or put an inline filter on tacho ignition + supply.does it seem to charge the battery ok.are you using resistor spark plugs.

[Jayme]

maybe look for other devices spliced into the coil neg that can be interfering with the tacho? a dodgy broken tacho or shift light type device might cause enough interference to screw it up?

[shifty6]

Might be interference from the alternator. Try a suppressor capacitor on b+ alternator or put an inline filter on tacho ignition + supply.does it seem to charge the battery ok.are you using resistor spark plugs.

Little more info on specs of what capacitor / filter to use? I know pretty much nothing about electronics.

[shifty6]

maybe look for other devices spliced into the coil neg that can be interfering with the tacho? a dodgy broken tacho or shift light type device might cause enough interference to screw it up?

Only things attached to the coil neg are this tacho and the ignition module. I've run a new wire from the coil neg to the module as well to pull it out of the main loom and isolate it from possible interference. Made no difference. I'm going to try taking the belt off the alternator and see if it still does it.