[boboli]

I don't dd my 88 tbird any more, but I try to take it out several times a week. I noticed recently, that when the rpms were up in the 3k range,the needle on my tach would drop several hundred rpm and then bounce back up. There was no corresponding miss from the engine. The car just went through a emissions tailpipe test, and passed beautifully!
So I'm just wondering if this is just a problem in the tach itself? I thought I remember that a failing tfi or pip sometimes does this. Mine were replaced about 10k ago with geniune Ford replacements. If it's most likely the tach, then I'll ignore it for now. I just don't want to be out driving and find it was something else failing!

[NigelM]

I thought I remember that a failing tfi or pip sometimes does this. Mine were replaced about 10k ago with geniune Ford replacements. If it's most likely the tach, then I'll ignore it for now. I just don't want to be out driving and find it was something else failing!

 Or a bad diode in the alternator.

Since you have an easily reproducible test case, unplug the regulator connector on the back of the alternator and test it again.

[Ductape]

Or simply a bad electrical connection.

[Mike C]

I don't dd my 88 tbird any more, but I try to take it out several times a week. I noticed recently, that when the rpms were up in the 3k range,the needle on my tach would drop several hundred rpm and then bounce back up. There was no corresponding miss from the engine. The car just went through a emissions tailpipe test, and passed beautifully!
So I'm just wondering if this is just a problem in the tach itself? I thought I remember that a failing tfi or pip sometimes does this. Mine were replaced about 10k ago with geniune Ford replacements. If it's most likely the tach, then I'll ignore it for now. I just don't want to be out driving and find it was something else failing!

Anytime my Tach needle "bounced" it was from low voltage. Happened a few times over the years, always low battery on mine...
HTH
Mike

[boboli]

I do have an aftermarket volt gauge installed in my dash, where the stereo used to be. I will try to glance at it the next time I see the tach acting funny. The tach bouncing  is so random/ intermittent, that when I realize it's doing it, it's already back to working properly.