Tach not working again! Need Help!

[Jakester1168]

I had the VDO tach in my 911 re-calibrated a year ago due to the needle "bouncing"...paid $100 at a local shop in Tampa to have it re-calibrated and all seemed fine...but at our last DE, I noticed it was off at the top end by 750RPM...I had the car dynoed last weekend and confirmed the tach was off...it was reading 1500RPM on the tach when the engine was actually at 2000RPM...

My question is: should I take the VDO tach back to the shop in Tampa that did the work a year ago and pay another $100 or more, or purchase an Autometer tach...if I do the Autometer tach, which would be better, the Sport Comp or the Pro Comp?...is there any difference other than aesthetics? I'm leaning towards the Sport Comp due to the white numbering against a black face as its closest in looks to the VDO gauge...I will do something with the silver ring as well..Thanks

[304065]

What is your voltage measured at the battery with the engine running?

[Jakester1168]

Not sure John...will have to check and let you know...what should I be looking for as far as a voltage reading?

[304065]

12.5 or better at the battery engine off
13.8 or better at the battery engine on

My guess is you have too high a charging voltage, but need your test to confirm it. Without going into electrical engineering, if the system voltage is too high the tach reads too high.