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Bouncing tach needle - 74 911S

My tach is intermittent while it is running. Is it electrical? Or mechanical? Where does it take the input from? The distributor? Transmission? How easy is it to remove the guage and check the connections?

What should I look for?

Old 10-11-2005, 05:12 AM
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Could be sticking points - might want to lube them.
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Old 10-11-2005, 05:53 AM
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Check your dwell. Your points may need replacing or resetting. Also, check the connections for the wires off the back of the tach.
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Old 10-11-2005, 07:00 AM
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If you haven't guessed already, the tach gets it's signal from the points. You might first try running some clean paper through them if they look like they are set right. Sometimes the get an oil or grease film that's enough to weaken the tach signal, but not enough to stop the car from running...
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Old 10-11-2005, 07:54 AM
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yeh - business card or index card - be sure to get both surfaces

if that doesn't do it then in order...
file them or sand them a tad; lub them
check the dwell
replace the points, reset dwell
check gnds. near the engine & near tach
tach is dying - kiss it goodbye and send it in for repair
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Old 10-11-2005, 01:53 PM
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There is a clip on my distributor, on the back side. It could be a ground or something - really oxidized, and loose. It almost came came off when I pulled the cap last time. I cleaned it off with an X-Acto knife and pushed it back on solid. Did not do anything, just feel better cleaning up what I can.

Be careful not to pull anything off - push the connections to make sure they are on firm.
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Old 10-11-2005, 02:04 PM
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Tach problem

Well I think you guys have solve maybe two the problems on the car.

1) I have been having a problem when the ignition feels like it just quits. It happens going up a hill or being reved up high.

2) My tach does not work at idle, but will bounce a round while the car is being reved.

I have cleaned the connections (spade connector) on the distrbutor. No real change.

I have a new distributor cap, points, rev limiting rotor that I will be installing this weekend. Wires and plugs have been replaced.

Does it sound like my points maybe bad with the ignition cutting out? Effect the tach too. What do you think?

I wish to thank all you guys for the great help.

Again, thank you!!!
Old 10-11-2005, 03:06 PM
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Check the voltage coming from your alternator. If you can find a take off point and attach a multimeter so much the better, especially a meter with a memory. When your tach starts acting wonkie (a technical term) immediately check your voltage output.

Chances are it's way above 12~13 Volts. In which case it is time for a new voltage regulator.
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Old 10-11-2005, 03:09 PM
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If you don't have a dwell meter you should get one. The spec for dwell is 38 degrees, +/- 3. I like to set mine about 35 0r 36. Then as the block wears down, the dwell gradually increases to 40 or 41, when it's time for a change. Check your setting about 1000 mi or so after you've done the replacement. I think this will solve your problem. If not, I would check that voltage regulator as was previously mentioned.
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Last edited by Paulporsche; 10-12-2005 at 08:47 AM..
Old 10-12-2005, 08:44 AM
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tach jumps

Change the points, the pigtail is bad.
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Old 10-12-2005, 10:22 AM
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Re: tach jumps

Quote:
Originally posted by Otto
pigtail
I'm going to assume that pigtail , spade connector , and clip on my distributor are all the same thing.

Looks like every one said points , nice to be on the same side for once.
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1989 modified Scat II HP Hovercraft
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Old 10-12-2005, 10:55 AM
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Tach Bounce

I have been fighting this battle for 2 years now on my 72T with 2.7S motor and I have done the following:

Alt - Rebuilt
Volt Reg (Extrernal) Replaced with new one when Alt rebuilt
Replaced Points
Cleaned connection at distributor

Strange thing is that the needle moves even more when it gets warm and only wanders some from a cold start.

BTW Cars runs very well, no other issues at all.
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Old 10-12-2005, 12:36 PM
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Re: Tach Bounce

Quote:
Originally posted by techguy
I have been fighting this battle for 2 years
The non-rev limiting rotor is now avaliable through Pelican.

Have you noticed any difference between rev-limiting and the older style non-limiting rotor?

I know some have reported having to redo their timing, but that's it - nothing more.

Just asking, not saying there is a connection to the tach bouncing.

Sounds like from what others have said it might time to start thinking about a new distributor. They are expensive, I've looked at the prices already.
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1977 911S Targa 2.7L (CIS) Silver/Black
1998 Chevy S-10 4x4 Pick-Up Truck, Razzle Dazzle Camo
1989 modified Scat II HP Hovercraft
George, Architect
Old 10-12-2005, 12:49 PM
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Tach

Interesting, I did install a rev-limiting rotor after all my above mods, can't say I noticed any difference in tach operations. I also replaced the cap and plugs at the same time. I am rather suspect of the wiring, its not getting any younger. Who knows the tach may also need rebuilding, it definitely has lost the needle dampening capacity.
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Old 10-12-2005, 03:29 PM
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i want one of those...
 
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I had the same problem in my '76...cleaned the points and after a while the tach started to bounce again. Said screw it and replaced the points with the Crane Optical Ignition, and no problems since then.
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Old 10-13-2005, 11:30 AM
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The optical solution has been in the back of my mind....sounds like a good excuse to get one
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Old 10-13-2005, 11:59 AM
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It runs great!

Tach problem solved!

Backfire problem solved!


I replaced the points, rotor and cap. Clean connections.


Thanks again for your help.
Old 10-14-2005, 07:42 PM
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Congratulations!

I've been changing points on cars for nearly 40 years, so no biggie, but lots of posters have changed to the optical systems mentioned above, so points will never again be an issue.

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Old 10-14-2005, 08:25 PM
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