I've had this 85 Berlinetta for almost 9 years and 45,000 miles now, and a few months ago the tach developed a strange habit. When the engine gets up to 3500rpm or so it jumps back and forth between there and 4500. Over 4500 (which I don't do often!) seems OK, as is anything under 3500. Just in that one range it flickers up and down.

The engine doesn't seem to be the problem. I'm a geologist, not a mechanic, but even so I think I'd notice if the engine was surging by a thousand rpm... it feels normal. I did rev it up in park and from what I can tell the problem seems to be less easily reproduced, but does still happen a little bit.

My local mechanics don't really know much about this rare car. Their suggestion was that the instrument cluster itself is the problem. They might be right, and I do have one working spare (that worked a few years ago the one time I tested it) however I'm reticent to take apart the dash to install it, considering how each time I dig into there some other fragile, irreplaceable bit crumbles to dust in my hands.

Thanks in advance for any insight any of you can offer.

 

Sounds to me like there is a ground issue, potentially with the wire that goes from the distributor to the tach. It would be easiest to tell what it is if you could reproduce it without moving, as having someone look around the engine bay whilst you drive at 3500-4500 rpm is not the most ideal circumstances. It could be that the signal is getting disrupted, such as grounding out on the body, but since you say it jumps it could be a voltage spike from a spark jumping from say a plug wire or something. However, if you choose to check the actual gauge itself, removing the cluster and the dash really isn't that hard, so long as you hold on to all the screws. I've done it before, just make sure you take lots of pictures!

 

Well the first thing to do is hookup a known good tach and verify that it isn't the cluster.

If you still get this behavior from a second tach then the first thing I would do is check the distributor. If there is shaft play or a pickup problem it could do things like you describe. I've seen other types of vehicles develop strange tach behavior (5k rpm readings at idle) from excessive distributor shaft play.

GD