tach needle bouncing with tach filter already installed - Team Camaro Tech
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post #1 of 19 (permalink) Old May 4th, 15, 03:55 PM Thread Starter
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tach needle bouncing with tach filter already installed

I already made the DIY tach filter. It worked great for some time (few months). I had to bring my car into the body shop to install new door hinges, I got a set of NOS hinges and they work flawlessly!!! However the guys in the shop must've left the dome light on and consequently killed the battery. I noticed that the battery mustve been drained because when I drove the car away the ammeter was pegged at 40 amps. Upon driving the car I noticed a random misfire underload so I replaced the 25 year old Mallory Promaster Coil and the 1 year old wire because I wanted a nice and tight valve cover mountede loom. The car drives much better now but still has some "bouncing" of the tach needle.

Do you guys think driving the car with the weak coil could've fouled a few plugs? But if it had fouled plugs would this cause the tach needle to bounce? I'm going to check the plugs tomorrow .

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post #2 of 19 (permalink) Old May 4th, 15, 04:33 PM
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Re: tach needle bouncing with tach filter already installed

If you have a tach feature on your timing light I would hook it up and watch both, if they bounce rpms at the same time the tack and filter are more than likely good.

But they may have fried the electrical system while it was there and the filter and or tach may need service or replacement.


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post #3 of 19 (permalink) Old May 4th, 15, 04:52 PM Thread Starter
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Re: tach needle bouncing with tach filter already installed

Great idea! I do have a timing light with this feature!

I'm afraid of this because the constant jumping of the battery is what caused the coil to die. Car runs much nicer with the new coil.

Also needle only seems to be bouncing around 2500rpm and higher. Works just fine under WOT

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post #4 of 19 (permalink) Old May 4th, 15, 05:14 PM
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Re: tach needle bouncing with tach filter already installed

I would also pull the battery and have it load tested. They may have shorted it out and damaged a cell.


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post #5 of 19 (permalink) Old May 4th, 15, 07:41 PM Thread Starter
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Re: tach needle bouncing with tach filter already installed

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I would also pull the battery and have it load tested. They may have shorted it out and damaged a cell.
I don't think that'd cause the tach to bouce though. Do you?

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post #6 of 19 (permalink) Old May 4th, 15, 07:46 PM
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Re: tach needle bouncing with tach filter already installed

The battery is the heart of your electrical system and if it is having a heart attack everything will feel it. If the cell is bad it may be pulling the alternator on and off causing a spike.


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post #7 of 19 (permalink) Old May 4th, 15, 09:31 PM Thread Starter
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Re: tach needle bouncing with tach filter already installed

Makes sense lol but wouldn't the ammeter be reading a draw?

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post #8 of 19 (permalink) Old May 5th, 15, 03:47 AM
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Re: tach needle bouncing with tach filter already installed

Well in my experience Voltmeter and Ammeter Gauges just report activity and unless defective they don't cause activity.


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post #9 of 19 (permalink) Old May 5th, 15, 08:45 AM Thread Starter
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Re: tach needle bouncing with tach filter already installed

Correct but if there was a constant heavy draw from the alternator wouldn't there be a positive reading on the gauge?
I will get a new battery anyway. Can't check the plugs today. Think I broke my foot last night in my softball game

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post #10 of 19 (permalink) Old May 5th, 15, 10:15 AM
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Re: tach needle bouncing with tach filter already installed

No, no indication on ammeter - if connected by OEM design.
Ammeters are generally hooked in series with the charge line showing the usage/charging of the battery and not the alternator output, ie, BATT stud.

Battery is another parallel circuit with the vehicle's electrical system.
Once alt is working, alt supplies power to all circuits and recharges battery.
Once alt has too low of RPM or more demand than alt can supply, and then battery supplies power.

So, ammeter shows current activity of the battery - unless an ammeter has been installed on BATT stud, then it will show alt activity.

Voltmeter measures electrical system pressure - volts - at any one point of the electrical system.

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post #11 of 19 (permalink) Old May 5th, 15, 05:13 PM Thread Starter
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Re: tach needle bouncing with tach filter already installed

Good news for my foot just sprained ligaments.

Could it be a bad ballast resistor? I changed the coil but not the resistor... Before the coil went out there was no needle bounce after installed the diy tach filter. I will replace my battery, ballast resistor, and tach filter.

Any other suggestions?

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post #12 of 19 (permalink) Old May 5th, 15, 06:46 PM Thread Starter
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Re: tach needle bouncing with tach filter already installed

I replaced the ballast so far. Tomorrow will check the ignition module inside the dizzy as well as the plugs... Gotta order a nice repro Delco battery

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post #13 of 19 (permalink) Old May 5th, 15, 07:17 PM
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Re: tach needle bouncing with tach filter already installed

I wouldn't just replace the battery but have it load tested, it may just need a full deep cycle charge.


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post #14 of 19 (permalink) Old May 5th, 15, 10:12 PM Thread Starter
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Re: tach needle bouncing with tach filter already installed

That makes sense but it's an ugly duralast and doesn't look at home in my beautiful engine bay

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post #15 of 19 (permalink) Old May 6th, 15, 07:11 PM Thread Starter
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Re: tach needle bouncing with tach filter already installed

Pulled the plugs today and they're covered in carbon deposits. Hopefully this is the problem. I'll replace them today. I think the bad coil caused the motor to run richer than normal and fould the plugs...

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