Gen 1 stock SBC 350, Chinesium HEI, Dolphin Dash Tach, 2 each tach/dwell meters. 12000 miles on the power pack. Runs fantastic throughout the RPM range. Dolphin dash tach remains reliable, stable and accurate. Dash Tach is hooked directly to hei tach signal tab, tight, not corroded, wiring tests fine. However, when tuning, lately both tach/dwell meters (different makes) have started to show random fluctuations of up to 1000 rpm at idle. Reading is stable 750 rpm on all three tachs until the motor warms, then the pair of tach/dwells get goofy with the dash tack remaining sober. Motor idle stays stable 750 rpm, no miss, no vacuum leaks, just happy as can be except for the tach/dwells act drunk. I have also disconnected the dash tach when using either of the tach/dwells (to eliminate possible interference) with the same result.....drunkin tach/dwell meter readings after the motor reaches operating temps. This is a new event, I purchased the second tach/dwell thinking the first was bad, but evidently its not the tach/dwells. Thoughts: Tach filter needed? Chinesium coil starting to let go? Chinesium 4 pin ignition module starting to let go?
Possibly electronic module false triggering extra times, giving the tach more sparks to count... but that should affect all tachos. I'd throw the oscilloscope at it and check what's going on.
True for a lot of people, but a picture of it would allow others to ****yze it for you. Or better than just a picture, the entire recorded file, that allows zooming to small details and so on, when using modern equipment. But sure, it's not a tool many have available, it just happens to be very useful for cases like this - seeing the exact signal the tachos/dwell meters has to work with so you can see the cause of the strange readings. Some may call it overkill when I use the scope to diagnose ignition problems on old Briggs & Stratton lawnmowers with points ignition, I see it as checking all the available data before pulling stuff apart, hoping to find a smoking gun.
Try a different ignition module if you have one, the old GM ones work flawlessly with modern tachometers.
so, the tach in the car works fine, and you're wondering what you need to fix to make the "test" tachs work properly under certain conditions? Huh...sounds like you're making a problem where there is none. But it's a great excuse to get rid of that distributor and put in a proper points type original Delco distributor, or maybe an early Mallory or something.
Wow. You're usually more helpful and less snarky. It's a curiosity question...perhaps the dash tach has a built-in filter, perhaps the average tach/dwell works best with points, perhaps someone else here had an hei take a **** under similar cir***stances and can lend a voice of experience. Rip it out and replace it all cir***vents diagnosing the problem. Couple good suggestions prior to yours that I can start with. Thanks for trying though.
Sorry, just seems to me that you're chasing a non existent problem...I've been chasing real problems on an off topic car lately, I'm perhaps jealous? Any time you're dealing with modern chinese electronics (dist and most likely the dwell/tachs if they're new), and weird things happen, I consider that to be perfectly normal, and not worth pursuing.
Oh...yeah...if you want the real answer, get an oscilloscope and learn how to use it. I have posted here in the past about tach filters, including a couple scope photos to show what's going on. That was for making a modern chinese tach work with points ignition.
My thoughts run pretty well along the line that a lot of older pre electronic ignition test meters don't work well with Hei or other electronic ignition. We had to buy different test equipment in the mid 70's to be able to properly test Hei and other ignition and that included the shop I worked in buying a new Sun scope / tune up machine . If the tester works fine on a rig with points be glad you didn't burn it up and call life good and maybe stick a lable on it that says points only but don't get your ******* in a knot wanting to change the Hei around to work with the meter.
No interest in changing the hei. Just curious if it could be a hei issue or tach/dwell issue. Might try a filter on the tach /dwell meter as a test
GM dist. has Packard connector that snaps into place. How are you jumping the tac/dwell connector into the mix?
Disconnect dash tac at dizzy. Plug in positive lead of tach/ dwell at dizzy tach terminal and take neg lead of tach/dwell to a clean ground
I have found that tach and dwell meters usually don’t play nice with with hei. I have seen even early Sun machines kind of freak out when hooked to hei.
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