Tachometer is "twitching"

Hey there all !

I have a Pontiac 1994 trans sport that is generally in good shape and getting better by the day wiith all the efforts going in to it. My current issue is that the tachometer seems to be "dancing" whenever I drive at about 2000 rpm's or greater ... The engine is smooth, there are no alternating lights, the speed is constant, and all the other gauges are fine ... The tach just starts to "twitch". Drives me nuts !!!

Any suggestions or diagrams would be appreciated.

Thanks again for the all the help.

Best regards,

Henri

Try checking out the charging system.

Try checking out the charging system.

Thanks for that reply, but it turned out to be something unexpected ...

I am the latest owner of this 1994 Trans Sport since August 2005 and I was lucky to have bought it so cheap, because I spent a fortune getting it up to driveable standards.

The "twitching" became a big issue. As the climate got colder here in Canada, the engine started to hesitate and almost cut out entirely.

After checking the electrical systems, the alternator, and battery. We took the air intake and the injector system out to change the distributor cap, rotor, wires, and plugs. We then checked the wires and plugs and got quite a shock.

The dealer I got the car from had a history of the repairs done for the previous owner because he (the previous owner) had the work done there. There is a bill dated June 7, 2005 that clearly states the 6 sparkplugs were changed for an amount of 259.00$ (amazing someone would accept to pay that much, eh?).
We are now almost exactly 4 months later, and I can clearly see by the accumulation of rust, burn marks and generally severe wear ... That only the front most accessible plugs were ever changed, they are new ... The back three, must have been there for years, some of the wires were broken by someone too lazy to use the proper tool. It was just shoved back into place.

I feel bad for this guy (the previous owner), he got shafted in so many ways ... It dissapoints me to discover more of this stuff. He must have been running on intermittent cylinders for a long time! ...

Anyway, 6 new Platinum NGK G series gapped to .045 and a new set of BOSH stainless plugs later and the the thing is just purring along with every song.

No hesitations, no stumbling on acceleration, no lack of power, and ... You guessed it ... No "twitch" in the "tach".

... only the front most accessible plugs were ever changed, they are new ... The back three, must have been there for years, some of the wires were broken by someone too lazy to use the proper tool. It was just shoved back into place.

I feel bad for this guy (the previous owner), he got shafted in so many ways ... It dissapoints me to discover more of this stuff. He must have been running on intermittent cylinders for a long time! ...

259.00$ - I wouldn't mind paying that much IF the work had been done properly. Getting to the rear plugs can be a real pain.

One tip I usually give folks when work is done by any shop is to ask to see the Old Parts when the job is done. The shop I go to most of the time is now "conditioned" to do that for me.

I don't use this technique as an "integrity check" at the shop I go to, but I like to do "Failure Analysis" on the parts. I will often take pictures of what went wrong and store them for future reference.

You did a Great Job of troubleshooting and Failure Analysis!

Take Care,