Bouncing Tacho

[911MAN]

Hi
I have read all the tacho bouncing stories but am still at a loss.
It goes wild for no reason at all, car is running as always, like a dream (1988 930).Out of all the postings there is still no answer.
This must be the tacho itself? What else can it be. The battery is fine, the car is charging etc etc. Could it just need a simple service? Any info would be brilliant. This will also save me a 4 hour drive to my specialist. The speeding fines would'nt be worth it!

[dd74]

When I accelerate hard and/or shift, my tach bounces around, but settles down when the engine evens out. My mechanic never mentioned it when I had a 2.7 or now with a 3.0, so I'm not sure it's a problem. I'm not sure if this is a problem or a character trait. I've seen many bouncing tachs before. Maybe others have some insight into this. Firstly, is a bouncing tachometer a problem? Should one that bounces be replaced?

Thanks.

[Glasgow 911SC]

This started recently on my car. I've not investigated yet but this thread has a few ideas. Maybe distributer related?

[1980SC]

If the tach swings back and forth with abrupt throttle inputs like what dd74 is describing, the likely cause is the internal dampner in the tach. The speedo repair places can fix this. Mine behaves this way, and becomes useless while autocrossing.

When the tach seems to go crazy for no reason, it's probably an electrical problem. Perhaps a poor ground for the tach, or something (?) in the distributor. Often points in a pre-'78 car.

-Rob
1980SC

[Early_S_Man]

Have tou checked your system Voltage lately with a digital multimeter during normal driving? If your Voltage regulator is causing higher than normal Voltage levels, or surges, say ... above 15-16 Volts, many times the tach goes crazy, too.

[silverc4s]

I had this problem with an early car when I changed to a Permatune CD... Had to put a resistor network somewhere, as I recall. Changed bach to a Bosch Cd and had no further problems. A lot of things that LOOK like a bad Tach head are actually electrical problems.

[911MAN]

Hi Guys
Thanks for that info.
My tacho doesn't bouce at gear change. It does it when ever it feels the urge! No real reason. It does settle after a few seconds. The engine shows no signs of missing etc.
I think I will pull it out, hopefully without buggering something up and send to a specialist. I like the idea of the dampner inside.
Thanks for all the help.

[1980SC]

What I was trying to say in my previous post is that I do NOT think your problem is the dampner in the tach. Your situation sound more like an electrical problem. Check the connections on the back of the tach. Check the battery voltage with the engine running.

Rob
1980SC

[silverc4s]

What Rob said....

[ischmitz]

Here is some insight on the bouncing tach and how I fixed it on my 74 2.7 CIS:

First make sure it is not a grounding/alternator problem. You would see this on other instruments as well, when the engine revs up. Also, a voltmeter would show variations of your 12 volt supply.

Second, you should check the connectors. I had a bad connector on my oil pressure sender. The instrument bounced erratically. After crimping a new one on it was perfect.

Last, it is most likely the dampening in the tachometer. It is dampened by oil on the needle shaft. This oil dries up with time. I was to lazy to take mine apart and mess with it. I put another unit in for testing and voila the bouncing was fixed.

When you have the tach out you can check the dampening or absence of it easily. Just spin the unit so that the needle moves. It should settle back to 0 smoothly. When I compared the bouncy tach with the replacement tach the difference became very obvious:

The needle on the bouncy tach would almost slam back to the stop at zero while the needle on the good unit settled back much slower.

Hope that helps.