Wacky 84 Targa RPM Gauge

[JCoop]

The RPM guage on my 84 Targa has a mind of its own. The engine runs great as always, but recently, the RPM guage fluctuates wildly and will drop to 0 or flip to 7k; sometimes it will reflect the normal range of rpm's during idle and shifting, although the baseline will be at idle 1200 when it normally would be around 800. Help! Any suggestions?

[wwest]

With a CDI, Capacitive Discharge Ignition, a jerky/jumpy tach is generally an early indication that your battery is being over charged.

[darrin]

Originally Posted by wwest

With a CDI, Capacitive Discharge Ignition, a jerky/jumpy tach is generally an early indication that your battery is being over charged.

Voltage regulators are a frequent failure item on our carreras -- have you smelled any sulpher-like odors, seen your seat belt light randomly illuminate or noticed your dash lights getting unusually bright at times?

I'd check the area around your battery for acid damage too.

This is pretty easy to check using a voltage meter -- it should be jumping around a fair bit if the regulator is going bad and steady at 14 volts or so if OK.

I was clueless on all of this when it happened to me years ago and suffered a failed tach, needed to get my dme rebuilt and have some battery area corrosion fixed after letting my failed voltage regulator situation linger too long.

[scottb]

Originally Posted by JCoop

The RPM guage on my 84 Targa has a mind of its own. The engine runs great as always, but recently, the RPM guage fluctuates wildly and will drop to 0 or flip to 7k; sometimes it will reflect the normal range of rpm's during idle and shifting, although the baseline will be at idle 1200 when it normally would be around 800. Help! Any suggestions?

My 84 Targa did the EXACTLY same thing....right before it died from a bad alternator. As others have suggested, check your alternator output. Be prepared to pull the alternator and have it rebuilt, though.

HTH.

[Joe Bob]

Get a multimeter and test the battery at the terminals.

12.5 at rest
12.5-13 at idle
13.5-14 at 2500 rpms......

Over 14 and you have a regulator issue, under 12.5 your alternator is going....