Bouncing Tach

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John Brooks
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Bouncing Tach

#1 Post by John Brooks » Wed Jul 13, 2022 2:36 pm

Last weekend 80 miles into the NW Group Saturday drive, my tachometer needle started to bounce at Low RPM about 2-3 needle widths. No Big deal I thought. After the first stop I noticed a growl in the dash, and lager jumps in the needle. Reached under the dash and disconnected the cable and pushed on. I forgot how much I rely on the TACH on down shifts on twisty roads.

The next day I pulled cleaned, flushed and lubed the cable no help. Then I pulled the instrument and found the drive socket extremely hard to turn by hand. Cleaned it with WD-40 and wood Q-Tip, it was better but no joy. Then I looked at it under magnification, and old dried grease had built up between the drive socket and the outer housing. There is hardly any clearance (.025mm) between the shaft and it was packed with a hard grease, I used a small jet reamer to scrape some out then carefully sprayed a little carburetor cleaner on the end. I could not believe how much junk came out of the socket. But cleaning the socket fixed the TACH. I have never had the cable or instrument out so it was probably original and the grease that had migrated up the cable had just gone bad.

I was extremely careful, keeping the socket pointing down. Using small squirts of the Carb Cleaner and wood Q-tips to get it clean the socket. , There is plastic stuff in there that the cleaner might dissolve. Then a little WD-40 for good measure it works fine now.

Bouncy TACH Needle, try cleaning the Instruments socket.
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Wes Bender
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Re: Bouncing Tach

#2 Post by Wes Bender » Wed Jul 13, 2022 6:05 pm

I hope you lubed it again after getting it nice and clean. I use a boater's winch grease for applications like this. It doesn't harden and it stays put. I'm sure there are others that work equally well.
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Jim Liberty
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Re: Bouncing Tach

#3 Post by Jim Liberty » Wed Jul 13, 2022 6:14 pm

Wes, you just keep on educating me on the fine points of maintenance. Thank you, Jim.
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John Brooks
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Re: Bouncing Tach

#4 Post by John Brooks » Wed Jul 13, 2022 6:32 pm

Yes I use a hypodermic to grease things like this. I have several kinds of grease in them. Pain to fill but last a long time.
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Hypodermic with molybdenum
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Steve Turino
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Re: Bouncing Tach

#5 Post by Steve Turino » Wed Jul 13, 2022 6:50 pm

Good post John, I have changed to an electric tach in my car but my Speedo which was rebuilt recently is very sluggish to respond under heavy acceleration. I wonder if it is a similar situation with the cable or shaft in the casing needing cleaning and lubing.
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Martin Benade
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Re: Bouncing Tach

#6 Post by Martin Benade » Wed Jul 13, 2022 9:13 pm

John, you said only that you lubed it with WD40 which isn’t really a lubricant. You greased it a little also?
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Re: Bouncing Tach

#7 Post by John Brooks » Wed Jul 13, 2022 10:41 pm

Martin

Yes with some grease in the Bell at the end of the cable housing that compressed in to the socket. Not a great deal of lubricant, just touch, I will give it another 60 years to harden again.
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10cc blunt tip. Very handy
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James Rowe
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Re: Bouncing Tach

#8 Post by James Rowe » Fri Aug 05, 2022 9:29 pm

John, I am having a bouncy tach on my 64 C Coupe. I was wondering. If you had any pictures of where the grease was accumulated that you can share? I think I know where you are talking about. How big of a job was it to pull out the tach to clean it out? Time wise? Any help would be appreciated.
John Brooks wrote:
Wed Jul 13, 2022 2:36 pm
Last weekend 80 miles into the NW Group Saturday drive, my tachometer needle started to bounce at Low RPM about 2-3 needle widths. No Big deal I thought. After the first stop I noticed a growl in the dash, and lager jumps in the needle. Reached under the dash and disconnected the cable and pushed on. I forgot how much I rely on the TACH on down shifts on twisty roads.

The next day I pulled cleaned, flushed and lubed the cable no help. Then I pulled the instrument and found the drive socket extremely hard to turn by hand. Cleaned it with WD-40 and wood Q-Tip, it was better but no joy. Then I looked at it under magnification, and old dried grease had built up between the drive socket and the outer housing. There is hardly any clearance (.025mm) between the shaft and it was packed with a hard grease, I used a small jet reamer to scrape some out then carefully sprayed a little carburetor cleaner on the end. I could not believe how much junk came out of the socket. But cleaning the socket fixed the TACH. I have never had the cable or instrument out so it was probably original and the grease that had migrated up the cable had just gone bad.

I was extremely careful, keeping the socket pointing down. Using small squirts of the Carb Cleaner and wood Q-tips to get it clean the socket. , There is plastic stuff in there that the cleaner might dissolve. Then a little WD-40 for good measure it works fine now.

Bouncy TACH Needle, try cleaning the Instruments socket.

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Martin Benade
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Re: Bouncing Tach

#9 Post by Martin Benade » Fri Aug 05, 2022 9:37 pm

James, your C definitely has a cable tach? Some are electric.
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Re: Bouncing Tach

#10 Post by David Jones » Fri Aug 05, 2022 9:44 pm

64 built in 63, must be cable.
If I had known I would live this long I would have pushed the envelope a little harder.
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Re: Bouncing Tach

#11 Post by John Brooks » Sat Aug 06, 2022 1:11 am

James

On the Tach where the cable plugs in. Inside the threaded housing there is drive cylinder the cable goes into. The clearance between the drive and the housing is only a couple thousands. The grease was between the rotating drive and the housing. Mine was hard to turn by hand using a small finishing nail in the beginning. Keep the housing pointing down, you do not want any solvent or junk getting into the instrument. After the shaft was loose and I could see an air gap all around the housing, I used a drill to spin it up, then reinstalled the tach, still works..
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