Lets talk tachs |
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Lets talk tachs |
Ductech |
Apr 19 2011, 12:58 PM
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#1
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Member Group: Members Posts: 356 Joined: 16-July 10 From: AridZona Member No.: 11,949 Region Association: Southwest Region |
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.
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SirAndy |
Apr 19 2011, 01:54 PM
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#2
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Resident German Group: Admin Posts: 41,815 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
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. (IMG:style_emoticons/default/popcorn[1].gif) |
Ductech |
Apr 19 2011, 01:59 PM
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#3
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Member Group: Members Posts: 356 Joined: 16-July 10 From: AridZona Member No.: 11,949 Region Association: Southwest Region |
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. |
whatabout1 |
Apr 19 2011, 07:46 PM
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#4
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Toys in Red Group: Members Posts: 403 Joined: 6-March 06 From: Charlotte, NC Member No.: 5,676 Region Association: None |
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. |
Spoke |
Apr 19 2011, 07:52 PM
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#5
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Jerry Group: Members Posts: 7,052 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None |
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) |
jaxdream |
Apr 20 2011, 07:27 AM
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#6
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Senior Member Group: Members Posts: 974 Joined: 8-July 08 From: North Central Tennessee Member No.: 9,270 Region Association: South East States |
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. (IMG:style_emoticons/default/piratenanner.gif)
Jack / Jaxdream |
Spoke |
Apr 20 2011, 09:54 AM
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#7
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Jerry Group: Members Posts: 7,052 Joined: 29-October 04 From: Allentown, PA Member No.: 3,031 Region Association: None |
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. (IMG:style_emoticons/default/piratenanner.gif) 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) |
DBCooper |
Apr 20 2011, 11:12 AM
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#8
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14's in the 13's with ATTITUDE Group: Members Posts: 3,079 Joined: 25-August 04 From: Dazed and Confused Member No.: 2,618 Region Association: Northern California |
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.
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. . |
chuckc |
Apr 20 2011, 03:11 PM
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#9
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Member Group: Members Posts: 196 Joined: 30-August 10 From: Astoria NY Member No.: 12,123 Region Association: North East States |
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. How did the ej22 do compared to the Rx7? (IMG:style_emoticons/default/biggrin.gif) |
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