Tach Ain't Workin'

Miniac · 03-02-2002, 09:27 PM

This may be a dumb question, but I'm new to Porsche hood, and was wondering if you could give me some tips on common problems with the tach not working.
Thanks.

bigchillcar · bigchillcar's Garage

miniac,
first of all, welcome to the board

had a problem with my speedo or tach back in the 80's and seem to recall that it was related either to a need to replace the entire cable from the case to the instrument or i seem to remember that there is a 'vinyl plastic' gear at the end of the cable going into the instrument that simply wears out and has to be replaced. don't know why something like plastic was used except maybe it could fail without chewing up the end of the cable when it's service life ended. i'd contact a porsche shop and ask about that.

c ya

1974 911

Early_S_Man · 03-03-2002, 10:56 AM

SC's don't have mechanical cable-driven speedo's!

The tach. is driven by a signal from the TD terminal of the CDI-ignition unit. Corrosion or a loose connection at either CDI-unit or tach could be causing a problrem. Take the tach out of the dash -- carefully, and monior the signal at the TD terminal with a odigital multimeter set on Hz (pulses) or AC Voltage ... to see if you are gettingh a signalat the tach.

Voltage regulator problems can cause the tach to malfunction! Check to see if your system Voltave is in the nominal range of 13,5 to 14.5 Volts with a known good digital multimeter with a fresh 9 Volt battery. It would be best to build a test cable to go from your cig.ighter socket to 3/4" double banana connector of the multimeter ... and monitor the Voltage (have a passenger record the Voltage at one minute intervals during a 10-minute test drive) above idle -- not at stop signs or lights!

Miniac · 03-03-2002, 07:46 PM

Thanks for the tips folks. I'll check into the voltage levels. Any other ideas out there?