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I received this by email from Donna S, who repaired her own tach by replacing a bad capacitor. So now we know that capacitor problems extend beyond speedometers and into older model years! Thanks, Donna: great job.
The car: 1988 VOLVO 740 GLE with 244,500 miles. Symptom: dead tachometer with no needle movement except for one single blip immediately after installing a new coil. Diagnostics: Using the wiring diagram, I traced all the circuits back to the tach itself and they all functioned correctly. I concluded that the tachometer gauge itself must be faulty. I removed the instrument panel and the tachometer. I took my reading glasses, a magnifying glass and flashlight and examined the guts and circuit board inside the tachometer. Voila! The problem was a burnt out capacitor on the circuitboard inside the tachometer gauge. The capacitor had a small break between the capacitor body and one of the little metal capacitor legs -- just enough of a break that given a large enough spark ( i.e. new coil) allowed the current to jump across the break to make the rpm needle move that one time. I went to Radioshack, bought a 473 mFcapacitor (the burnt out capacitor was a 333mF which they didn't have). I took out the soldering iron, melted the solder on the tachometer circuitboard for the offending capacitor and removed it. Then I soldered the new capacitor in its place. Now the tachometer works great. It pays to examine the tiny components of the circuit boards that affect the gauges: a $1 capacitor replacement is much less expensive and more reliable than a replacement tachometer or instrument panel. [Editor] You can buy the exact capacitor you need from Digikey or Mouser electronics. Look for an aluminum radial capacitor with a 50V rating.
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