87 v10 350 tbi tach going crazy

[buzzman351]

Hey guys, new here. I installed a aftermarket tachometer on my 87 350 tbi and when I turn the key the tach buzzes and jumps around 0 for a bit and same at initial startup then goes to the rpm it’s running. Any ideas

[bd]

Some possibilities:  1) bad tach head, 2) where it's connected, 3) how it's connected, 4) poor ground.

[buzzman351]

I’ve checked everything numerous times. It’s some sort of interference. I’m going to install a resistor on the signal wire

[bd]

Tach Filter

[buzzman351]

Yesir

[buzzman351]

my buddy had a sylvania led load equalizer he said would possibly work. i wired it inline with the signal wire. still have the problem. do i need to branch off to ground coming out of the equalizer or it just wont work?

[bd]

If the problem is a dirty signal, a resistor by itself won't work.  You need a resistor in combination with a capacitor to filter the signal.  See my subsequent post or try the tach filter linked previously.

[buzzman351]

my truck is an 87 with the 350 tbi, i have the signal wire running from the ignition coil lead that didnt have anything plugged into it. that is correct right? i dont believe it goes to the distributor cap on this year model

[bd]

The tach connects to the white coil lead.  Copy the filter configuration below and wire it in as shown...

[buzzman351]

So I got the tach filter and same problem. Unhooked my signal wire and problem still exists. Could it be noise on the 12v keyed side?


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[bd]

The only way you are really going to know the cause is to use an oscilloscope.  Outside of that, it's guesswork.  Try unplugging the alternator regulator to see if the symptoms disappear.

[buzzman351]

Will do. I switched the 12v keyed from the ignition source to the accessory source and it calmed down some.


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