Help, my tach went crazy

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Mar 18, 2016 | 07:26 PM

rmgny44

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Help, my tach went crazy

Out driving today cruising along at 60 mph and my tach went crazy. It was bouncing around all over the dial. At idle it doesn't move.
Would appreciate any advise.

Mar 18, 2016 | 07:39 PM

BKbroiler

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From: Lebanon Township New Jersey

First we need to know if it is cable driven or electric. If electric, probably a bad tach filter.

Mar 18, 2016 | 07:51 PM

rmgny44

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Quote:

Originally Posted by BKbroiler

First we need to know if it is cable driven or electric. If electric, probably a bad tach filter.

The car is a '72 completely stock so I believe it's mechanical.

Mar 18, 2016 | 07:55 PM

redvetracr

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From: WI

Quote:

Originally Posted by rmgny44

Out driving today cruising along at 60 mph and my tach went crazy. It was bouncing around all over the dial. At idle it doesn't move.
Would appreciate any advise.

yea, pull the side gear out of the distributor if it`s bad replace BOTH gear and mainshaft

Mar 18, 2016 | 09:42 PM

rmgny44

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Very Strange! Cars been sitting in the garage for a few hours. Went out, started it up and now the tach works fine.

Mar 18, 2016 | 10:45 PM

babbah

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Quote:

Originally Posted by rmgny44

Very Strange! Cars been sitting in the garage for a few hours. Went out, started it up and now the tach works fine.

I have been there as well with the tach. I guess I should not complain as it has never been touched in 49 years & still working!

Mar 18, 2016 | 11:47 PM

rmgny44

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Quote:

Originally Posted by babbah

I have been there as well with the tach. I guess I should not complain as it has never been touched in 49 years & still working!

So from what you're telling me mine should work for at least another 5 years!

Mar 19, 2016 | 01:13 AM

babbah

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Quote:

Originally Posted by rmgny44

So from what you're telling me mine should work for at least another 5 years!

It should! LOL! All my gauges are original and still working in my 1967 (working since my high school days! Not to bad for almost 50 years)
In addition, the clock is original as well and is still ticking. Go figure!

(love that tic tic tic and click of the mechanical rewind sound when parked)

Last edited by babbah; Mar 19, 2016 at 03:41 AM.

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Mar 19, 2016 | 02:07 PM

454Luvr

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From: Austin TX

Quote:

Originally Posted by rmgny44

Very Strange! Cars been sitting in the garage for a few hours. Went out, started it up and now the tach works fine.

My '72 did the same thing (40 years ago). I don't know how a mechanical drive can become intermittent, but that's what it was.

Mar 20, 2016 | 03:52 PM

#10

rmgny44

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Okay, something is wrong! Out for a drive today and after running fine for a while the tach stopped working and shortly after that the car lost power. I managed to limp home but it was like it was not running on all cylinders. Shut it down and started it again after a few minutes and the tach works and the engine sounds fine again!

Mar 20, 2016 | 04:18 PM

#11

454Luvr

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From: Austin TX

Quote:

Originally Posted by rmgny44

Is there any possibility someone has replaced the mechanical tach with electronic?

Mar 20, 2016 | 05:50 PM

#12

rmgny44

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Quote:

Originally Posted by 454Luvr

Is there any possibility someone has replaced the mechanical tach with electronic?

It's mechanical!

Mar 20, 2016 | 07:35 PM

#13

454Luvr

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From: Austin TX

Quote:

Originally Posted by rmgny44

It's mechanical!

In that case, it's the distributor. I don't know how a defective tach gear drive can cause misfires, but that's clearly what's happening. The gears must be binding and causing a problem with the timing somehow.

Mar 20, 2016 | 07:55 PM

#14

mikem350

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From: Sunrise FL

Quote:

Originally Posted by 454Luvr

I doubt this very much. Gear binding cannot cause the dizzy shaft to stop turning, it would cause the tach CABLE to stop. I believe the engine is mis-firing causing the tach to read off. The tach needle is driven by a magnetic coupler that can slip and cause the needle to bounce and read wonky.

Find out cause of misfire!

Mar 20, 2016 | 08:49 PM

#15

babbah

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Quote:

Originally Posted by mikem350

Find out cause of misfire!

Mar 20, 2016 | 09:16 PM

#16

454Luvr

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From: Austin TX

Quote:

Originally Posted by mikem350

I doubt this very much. Gear binding cannot cause the dizzy shaft to stop turning, it would cause the tach CABLE to stop.

I wasn't suggesting that the distributor is stopping. If end play in the shaft is excessive, gear binding might cause timing to change.

Quote:

I believe the engine is mis-firing causing the tach to read off. The tach needle is driven by a magnetic coupler that can slip and cause the needle to bounce and read wonky.

Why would a misfire cause a mechanical tach to read off? The coupler isn't electromagnetic, it doesn't require voltage or horsepower to function. I agree it can fail, but I don't see how that could be related to misfires. Maybe the car actually has two different and unrelated problems, but it's a stretch to believe they could happen on and off at exactly the same time, strictly by coincidence.

Mar 21, 2016 | 07:53 AM

#17

rmgny44

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Car is going into the shop tomorrow morning. I'll let you know what we find!

Mar 21, 2016 | 12:47 PM

#18

mikem350

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From: Sunrise FL

Quote:

Originally Posted by 454Luvr

Why would a misfire cause a mechanical tach to read off? The coupler isn't electromagnetic, it doesn't require voltage or horsepower to function.

I stated that the tach NEEDLE is coupled by magnetism. This is a known failure mechanism for intermittent/bouncy tach needles. My car actually suffers from this, but not as bad as the OP.

Go on the Willcox site for more info.

Mar 21, 2016 | 01:40 PM

#19

454Luvr

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From: Austin TX

Quote:

Originally Posted by mikem350

Yes, I got it the first time. Same question though: Why would a misfire cause the tach to read off?

Mar 21, 2016 | 01:49 PM

#20

mikem350

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From: Sunrise FL

Quote:

Originally Posted by 454Luvr

Yes, I got it the first time. Same question though: Why would a misfire cause the tach to read off?

I feel the OP has an intermittent tach...AND possibly engine not running right. The tack came back to life after sitting overnite.

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