ULTM8Z

I'm struggling with a bouncy tach needle on my Autometer tach. Large cap HEI distributor. Heavy throttle above 3000 rpm and I get a jittery tach needle.

Autometer recommended a 10kOhm resistor which didn't solve it...

Increasing physical separation between the spark plug wires and the tach wire didn't solve it.

I know I have a good distributor ground.

Tried running the Corvette tach filter and that didn't solve it either (basically re-created it from the schematic off Corvette Forum using parts from Radio Shack).

Wondering if the 7730 has a tach-output that I can take advantage of. Maybe if the signal is coming out of the ECM, it'll filter out the noise.

Six_Shooter

iTrader: (2)

There's no direct tach output from the '7730.

You shouldn't be using a filter with an Auto Meter on a GM ignition. Connecting the green tach wire directly to the coil negative is all that is required.

Have you tried a second tach to verify it's a vehicle problem and not a tach problem? I've used several Auto Meter tachs in many vehicles and never had the issue you describe, which leads me to believe it's likely the tach itself.

ULTM8Z

Good suggestion. I'll see if I can dig up the factory tach and hook it up temporarily...

Saar

to do this properly..... I think you would need to dedicate a TOC specifically for it. the P4 processor doesn't really have any on its own outside of the real-time clock(it is why the VSS output is run through the divider function rather than through a TOC, which has its downsides).

there are some PWM outputs, but they're at fixed frequencies(8, 16, 32, 64, 128hz), which aren't likely to work for running a tach.

the p4 timer chip(U2, 16045148) does have some TOC-like functions though, injector and ignition control, but you can't really remove those...

if U15 were installed(and some of its pins broken out to unused pins on the main connectors).... you might be able to treat the secondary ignition output as a tach signal, but that would require some decent code to be added to deal with those interrupts.

ULTM8Z

Autometer is sending me out a tach filter. They're saying that the aftermarket pickup coils get quite noisy and they're having an increasing number of complaints from folks on bouncy tach needles...


From an e-mail exchange with their tech support...

Also, would you know, or be able to confirm whether your HEI is an actual original GM manufactured HEI, or is it possible that it is an after market produced HEI? I do have a suspicion with all the attempts made to remedy this issue that you may have either an after market HEI, or your HEI may have a questionable magnetic pick up in it. We are starting to see a growing number of similar issues with some of the “less expensive” new after market produced HEI’s.