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Bouncey Tach

I have had an issue wuth a bouncing tachometer that has gotten progressively worse. It only works about half the timenow. I have not yet had the time to look into it, but what is most likely my problem?

BTW, I tried searching for a previous thread on this topic but didn't find one.

Thanks,

Ed
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Old 06-27-2006, 07:58 AM
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Tach signal comes off of the DME. Might want to check all of the connections on the back of the tach as well as the plug to the DME. If the connections are good then it's the tach.
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Hugh - So Cal 83 944 Driver Person
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
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Old 06-27-2006, 09:58 AM
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I have the same problem with my speedo. It is quite anoying, cant tell if im going 20 or 30.
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Old 06-27-2006, 11:05 AM
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Change the cable from the front left wheel to the speedo. The cable is dry and binding. Might be able to make it last a bit longer squirting a liguid graphite down it followed by a blast or two of shop air (100 psi).
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Hugh - So Cal 83 944 Driver Person
NOT a 'real' Porsche -- Its Better!!!!
When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
New Users please add your car's year and model to your signature line!
Never break more than you fix!
Old 06-27-2006, 12:05 PM
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Thanks socal you are a huge help to the 944 community. By the way how do you know so much? just picked it up?
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Old 06-27-2006, 01:32 PM
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Quote:
Originally posted by Ford-o-phobic
Thanks socal you are a huge help to the 944 community. By the way how do you know so much? just picked it up?
And it is free too! Just look at what happens if you ask on sites that charge.

Dal
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Old 06-27-2006, 02:47 PM
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Yeah, thanks for the *free* help SoCal Driver!

Ed
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Old 06-27-2006, 04:53 PM
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Ah yes; "free" help. Worth every penny. Just wait. I'm making a list and will be visiting each one in turn in my later years. Kind of a retirement fund-thing.
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Hugh - So Cal 83 944 Driver Person
NOT a 'real' Porsche -- Its Better!!!!
When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
New Users please add your car's year and model to your signature line!
Never break more than you fix!
Old 06-27-2006, 05:52 PM
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Quote:
Originally posted by SoCal Driver
Ah yes; "free" help. Worth every penny. Just wait. I'm making a list and will be visiting each one in turn in my later years. Kind of a retirement fund-thing.
You better hurry then because I am spending my... or your retirement as we speak, so to speak! But you had better charge extra for those turbo wanna be's

Dal
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Old 06-27-2006, 07:12 PM
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You know my speedo just kinda bouces around for the first 25 MPH or so. After that it picks up and is fine. I thought it was just inherent with the old style speedometers (My car is an 87 but it has the old style pre 85.5 dash).....
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Old 06-28-2006, 05:56 PM
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Mine is the same Schumi. To a tee. May be normal on our beasts.

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Old 06-29-2006, 02:50 AM
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My speedo also bounces around at low speed from time to time, not always though. I have yet to check the tach connections, and I am going out of time this weekend, so it may be a while before I get to check it. I'll post the results when I get the chance.

Ed
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Old 06-29-2006, 06:16 AM
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"time" should have been "town"........

Ed
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Old 06-29-2006, 06:17 AM
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My tach needle bounces too. Been doing that for years. It does eventually stop oscillating and shows the proper engine revs. Doesn't bounce enough for me to delve into fixing it. (Then again, my dash lights are out - rather fix those first!)

-Z.
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Old 06-29-2006, 06:29 AM
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I speculate that modulating current from the Speed sensor could cause supply Voltage noise to Tachs on the pre85.5 944's and all 924's using the Speedo Cable. The Voltage from the Sender is common with the Voltage to the Tach Circuit. Perhaps by putting a cap maybe 1000uF (available at and Radio shack) we could reduce/eliminate the Bounces of the Tach. (Polarity DOES matter!) This could be installed as I've pictured.

Sounds like the Speedo cable needs to be replaced on mine as well. I tried putting some WD40 on both ends which helped but didn't have the air hose like SO-Cal has suggested. Or... Perhaps Oscillation on the Coil feed (Green wire #6) could be causing voltage supply noise to the speedo Sender. Rotation of the cable from wheel creates both current and oscillating Square Waves. Thus the Square waves read by meter could be getting interference from Coil waves at RPM frequencies.

Tachs Pins:
1- {Black/Yellow Engine} Speed (ignition) switched +12V
2- {Green or Green/Yellow} Injection Signal (wave) (internal connection to HG)
3- {Gray/Blue} Speedo Hall sensor (wave signal)
4- {Blue/Yellow} Ground , Coolant Temp gauge is feed from this pin
5- {Black Switched} +12V (is internal connection to H+)
6- {Green} Ignition pulse (neg side of coil and typically Green DME Wire #1

Speedometer Connections to Tach:
HG {Green/Red} Hall sensor Pulse (Square wave 1-6 Volts generated by Hall effect on Front left wheel spinning)
H+ {Black Switched} 12V supplied jump on pin 5 and internal VDO connection to H+

Haven't tried any of this my self. Just adding some ideas.

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Old 06-29-2006, 11:40 AM
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Quote:
Originally posted by Schumi
(My car is an 87 but it has the old style pre 85.5 dash).....
Nice swap! I like the pre 85.5 dash much better than the new one. The newer one looks to plain alot like a Tauras' dash. The pre '85 is totally unique, except for the 924.
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Old 06-29-2006, 12:36 PM
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Quote:
Originally posted by Ford-o-phobic
Nice swap! I like the pre 85.5 dash much better than the new one. The newer one looks to plain alot like a Tauras' dash. The pre '85 is totally unique, except for the 924.
His 924S comes from the factory with the same dash as the early 944's and the early 924's. While the 924S has the narrow fenders most of the rest of the car is a later 944 under the sheet metal. Even has aluminum trailing arms and the side cover on the trans for the speedo sender. Still uses the cable from the left front to drive the speedo.
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Hugh - So Cal 83 944 Driver Person
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When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
New Users please add your car's year and model to your signature line!
Never break more than you fix!
Old 06-29-2006, 01:15 PM
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From a previous post, if it helps..

"It may be the cable. I had this same problem (intermittent tick, tick and the needle bounce) a few years ago. It kept getting worse till it became constant. In my case a replacement cable made no difference. I ended up getting an ebay speedo that worked fine till the odo gear broke. I decided to let the VDO shop in Milwaukee fix/refurbish it. When I told the guy that I had two and described the problem(s) with both he said that the one with the tick/bounce would cost "much more to repair" than the one with the broken gear. Just my $0.02.. YMMV"
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Old 06-29-2006, 01:58 PM
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my speedo bounces for the first 30mph or so.. then is normal... i guess socal is right, it binds until the wheel is spinning fast enough for it to not bind anymore...

oh well.. who needs a speedo anyway?
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Old 06-29-2006, 02:43 PM
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I use mine to test the new road side "Your Speed Is" signs that are posted around school intersections. Not when schools are in session.
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Hugh - So Cal 83 944 Driver Person
NOT a 'real' Porsche -- Its Better!!!!
When was the last time you changed your timing and balance belts and/or cam chain and tensioner?
New Users please add your car's year and model to your signature line!
Never break more than you fix!
Old 06-29-2006, 03:24 PM
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