Tachometer issues after distributor swap, Rebuild or repair? Options

[Ace Le Count]

Hey guys,

I've been having some strange issues with the tachometer in my 914 and was hoping to get some advice from people who have experience repairing these gauges.

The tach has always read about 500 RPM higher than the engine's actual speed, but the engine itself runs perfectly throughout the entire RPM range. The problem is with the gauge, not how the engine runs.

When I bought the car, it had a Bosch 009 distributor with a Pertronix unit (or some other aftermarket electronic ignition module) installed. That setup eventually failed, so I swapped in an original Bosch fuel-injection distributor from my parts stash, along with conventional points and a condenser. I'm assuming it's the correct general type of distributor for a D-Jet 914, although I haven't verified that it's correct for my particular engine.

After setting the timing, the engine ran perfectly, and the tach still worked, although it continued to read high. A few weeks to a month later, however, the tach started acting strangely. It would get stuck around 3,500 RPM and wouldn't register any higher, regardless of how much the engine was actually revving.

Then, on one drive, it seemed to get past that point, but afterward the needle would indicate around 5,000 RPM even when the engine was idling. Eventually, the tachometer stopped working altogether. Now it intermittently comes back to life, but when it does, it still seems to read around 5,000 RPM regardless of the actual engine speed.

Interestingly, the needle now returns to zero when the ignition is off. Previously, it would sometimes remain stuck around 400 RPM with the car shut off.

I'm wondering whether there's something mechanical going on inside the gauge, or whether it's an electrical issue. Is this something that can be disassembled and repaired at home, or is a professional rebuild the best option? I've found a shop in California that rebuilds these, but the quote was around $500. I'd rather not spend that much if it's something I can reasonably diagnose and repair myself, especially since I don't know much about the internals of these gauges.

The other question is whether switching from the 009 with the Pertronix to the Bosch fuel-injection distributor with points and a condenser could have damaged the tachometer somehow. The gauge continued working immediately after the swap, so I'm not sure whether the timing is just a coincidence or whether something about the ignition setup could be contributing to the problem.

One other oddity: one of the green turn-signal indicator arrows in the gauge doesn't illuminate. The corresponding exterior turn signal works normally, and all the turn signals and hazard flashers function as they should. I'm not sure whether that's related or just a separate issue.

Has anyone experienced similar tachometer problems? I'd appreciate any advice on what to check before sending the gauge out for a rebuild. Thanks!