Yesterday while on the way to and from work my tach needle started to wiggle upwards from its usual reading of about 3400 rpm. Until yesterday I had no problems at all.

On the way home I noticed that if I was under moderate acceleration or deceleration the tach read steady. When I was cruising at a constant speed is when the wiggling would start.

Any suggestions on how to track down the problem and fix it?


Jim

The tach has very little dampening of its needle and also takes input directly from the engine coil. Both of these items will cause some bounce in the tach needle.

So if you have issues with your points or plug firing, the tach will bounce like crazy. Your issue could be pointing to something happening in your engine's ignition circuit.

Does your engine have points or electronic ignition like Pertronix or other? Does the engine hiccup when this happens? If you have an issue with your engine's performance, this could be why the tach jumps.

If your engine is properly tuned and running good, in normal operation the tach needle will bounce somewhat when you rev the engine or shift gears. You can add dampening to the tach by placing a 2000uF capacitor directly across the tach winding but this involves taking the tach apart. I used a 3300uF capacitor on mine and the needle is rock solid like modern cars. 3300uF is a bit much as the needle is a little bit lethargic when the engine revs go down.

Hey spoke, do you happen to have a 2000uF capacitor lying around you don't want? I'll gladly take my tach apart to get rid of the bounce.

Oh, switching over to a good distributor (Like a mallory Unilite) and not using the points helps tremendously with fixing tach-bounce issues.

Sorry, don't have any 2000uF caps. Just a few 3300uF ones but I would not use them again. Radio Shack has 2200uF caps. Voltage rating for the tach is low so a cap with greater than 10V is ok. Maybe Fry's or other electronic component shop.

Since caps this big are electrolytic and are polarized, you will have to test the tach with the face off to determine which side of the winding is positive. Maybe someone knows off-hand which side is pos and which is neg.

If you like a solid tach this little addition will knock your socks off and put a smile on your face.

Cool. Will it cure the wild 1-1.5k swings?

Yep, no overshoots. Back in the day everything was analog and the driving signal (the points) were brought right into the unit with minimal if any dampening. The needle is so free moving that when you rev the engine, the needle rapidly goes to where it should be then keeps going because of inertia, then drops below, then back up; basically oscillating and finally ending up at the proper RPM.

Nowadays the spark unit is digital and microcomputers calculate where the needle should be and put it there. No bounce, no overshoot.

The 2200uF cap dampens or averages the signals from the points and really dampens the spikes and removes the overshoots and undershoots.

Thanks for the replies.

I am running a stock 2.0L D-Jet, with stock points ignition. While I do have the usual tack swings when shifting, I am more concerned with the swings that are going from my cruising rpm of about 3400 and swinging up to 5000 and back. This hasn't happened for the past two weeks. ( I have only had the 914 on the road for the last 2 or three weeks, mostly to and from work. About 56 miles round trip. ) And there is no corresponding change in engine sound or in the feel of the power output of the engine. Everything SEEMS normal, except for the swinging tach.


I will try the capacitor to reduce some of the swinging later.


Jim

Clean or replace the points!