[brianr]

While running around last Sunday, 8) smoothly accelerating, the tach bounced backwards about 1000-1500 RPM when I got in to the 4000-4500 range. It happened several times and I just went home..

No other symptom except this bounce. Like a sloppy speedometer does when it is about to give up the farm. Is it mechanical or electronic?
A cable?

[Trak Ratt]

Bad ground, usually at the transmission to chaise but could be others. Could also be pickup at distributor.

[APKhaos]

The tach on your 85 is electronic - no cable involved. It works by integrating the pulses frd from the flywheel sensor. If the engine did not stumble when the tach dropped, I'd assume the flywheel sensor is OK.

If the tach dropped 1000-1500 then almost instantly resumed correct reading, it could be an intermittent failing conection. It is likely that this fault would appear at any point in the rev range, so if it only happens at the 4000-4500 point, I'd consider this cause unlikely.

Its possible that one of the diodes in the integration circuit [tach internals] is breaking down.

Best bet is to swap in a known good tach. If the fault goes away, its time to find a replacement tach or have your repaired [hollywood speedometer, or perhaps a local VDO repair operation.

[Lucky]

On your car the tach (and speedo) are fully electronic. Tach is driven by the DME computer found under the driver's seat.

You can try cleaning and reseating the big connector on the front of the DME and see if that clears the problem -- if it doesn't, and your car runs perfectly fine as the tach goes nutz, then the problem is likely with the tach (or some other tach-related electrical gremlin).

If the car stumbles when the tach bounces around, then you may have a bad crank (aka flywheel) position sensor or a bad DME computer.

Good luck.

[Mackpipes]

Brian,
You can swap out your tach for mine to check if you want. I have an 85 as well.

[brianr]

Ted, thanks for the swap offer. No repeat performance this past weekend. Maybe it rested itself better.
I'll keep an eye on it. One of these weekends I will try to make more time than to just run it around for an hour and maybe get together to troubleshoot.

[BlackTalon]

This was one of the symptoms of a failing voltage regulator in my car. The seat belt warning light would come on a lot, the A/C fan would surge, and on one fun run the tach bounced around about a dozen times. The tach only did that for that one day, but it involved a total of nearly 3 hours of spirited driving.