Got a good one for you all...

About 3 months ago I had posted a question here about why my tachometer needle 'jitters', particularly at idle (never varied more than 200 rpm either way and it did it mostly around idle to 2000 rpm). Someone had replied, telling me they had the same problem on the very same year T/A, and it turned out that the tach had gone bad. Since then I just decided to let it go and let the tach go out first, then yank it and send it in for repairs once it did....

This weekend tho I changed out my spark plug wires (they were about 9 years old). After having done that the tach has been running rock solid ever since....

This makes absolutely no sense to me. On a conventional ignition system this couldn't happen at all, so the only thing I can think of that COULD cause this is that the electronic system, coupled with the possibility that I wasn't getting consistent spark to the plugs (the car shows a definate improvement- some random, occasional mid-RPM miss that I had been experiencing has gone away) was throwing off the tach signal. But even so, this is only a educated guess on my part. Is this correct, or is there another answer to this?


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Original owner - '84 T/A w/stock 305HO 5-speed

 

DT,

I vaguely remember the original post. Thanks for the update. I'll try to remember that. There was a TSB on '87 cars with the divorced ignition coil regarding interference from the coil and wire causing such problems. The recommended solution was to relocate the coil and reroute the coil wire away from other wiring, but it's pretty tough to move the coil on a pre-'87 distributor and there is no wire to reroute.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Now I can see that possibly interfering, but since I have the 5.0 I've got the internal coil, making it more difficult to relocate

I doubt I had a loose/corroded connection somewhere that I might've inadvertantly knocked back into place while changing the wires. Also, about a month ago I had replaced the rotor and cap and this did not affect the tachometer behavior at all.