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Posted by: Ductech Apr 19 2011, 12:58 PM

I just finished up my ej22 subaru swap. after drag racing a rx7 in the boonies and self destructing my shifter, i have stopped to take a look at all the little stuffs. now my tach is a little bouncy compared to a nice new digital being fed by an obd2 car computer. As the wiring i used was the original tach signal wire, im wondering if a little bit of noise is most likely the issue or if the tachs on these were never that amazing in the first place. Thanks for the your help guys.

Posted by: SirAndy Apr 19 2011, 01:54 PM

Do you still have the stock relay board?

A bouncy tach can be caused by dirty or lose connections or broken solder joints on the relay board.
I had the same issue when i bought my 914-4 12 years ago. Back then you could still buy brand new relay boards, which is what i did and the bouncy tach issue went away. I'm guessing my old board had tired solder joints where they go through the relay board.

There's a few nice write-ups here on how to refurbish your relay board and redo the soldering.

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Posted by: Ductech Apr 19 2011, 01:59 PM

Thanks for the reply, alas i did eliminate the relay board on my conversion to try and get as much old vw/porsche electronics out from between the motor and the stock harness.

I did not ohm the wire in the oem harness so i guess that your just assuring me that a little bit of crappy wiring and the tach will be a little bouncy.

Thanks that makes me feel a little better.

Posted by: whatabout1 Apr 19 2011, 07:46 PM

The tachs were always great but are now 30 + yrs old.

Has the tach been rebuild ? Palo Alto does great work.

What is sending the signal to the tach ?

Hook a O-scope to the tach input and see if it is a nice clean or almost
clean 0 - 12v pulse. Allot of the newer ignitions will send a spike that
may be to quick for these old tachs that are designed to work with points.
Something like the MSD tach driver box may be needed.

Posted by: Spoke Apr 19 2011, 07:52 PM

Can you define bouncy? At steady RPM, is the tach steady or bouncing?

The 914 tach meter has almost no damping such that when revving the engine or shifting gears, the tach bounces or oscillates when going from one RPM level to the other.

Does yours bounce with constant RPM?

If it only bounces or oscillates when shifting or RPM changes, a 2200uF cap placed across the winding of the needle will steady the tach like a modern one.

Below are different responses to a step function (ie., RPM change). The stock 914 tach responds like the MU=6 curve.

MU=1 or lower is what you want for a response from the tach.

Attached image(s)

Posted by: jaxdream Apr 20 2011, 07:27 AM

Spoke , you say place the 2200uF cap across the needle winding , can this be done on the conections on the back , or does one need to dismantle the tach and solder the cap inside ??? Sounds like a great remedy.

Jack / Jaxdream

Posted by: Spoke Apr 20 2011, 09:54 AM

QUOTE(jaxdream @ Apr 20 2011, 09:27 AM)

Spoke , you say place the 2200uF cap across the needle winding , can this be done on the conections on the back , or does one need to dismantle the tach and solder the cap inside ??? Sounds like a great remedy.

Jack / Jaxdream

You have to open the tach to get to the winding of the needle. Here's a pic of how I mounted the cap.

Attached image(s)

Posted by: DBCooper Apr 20 2011, 11:12 AM

If you're making things more modern anyway why not try some more modern VDO stuff? Fits in the hole, same font and bezel style, data records, and even has a nice little programmable LED shift light. And of course a rock-solid needle.

Attached Image

How odd. In the photo the needles look to be different shades of orange and I made a note to myself that I'd need to paint them all the same color. But then going out to the car in person they all look the same. Can't explain that, but at least I can take that one back off the to-do list.

.

Posted by: chuckc Apr 20 2011, 03:11 PM

QUOTE(Ductech @ Apr 19 2011, 02:58 PM)