'66 Knee knocker tach problem... - Chevelle Tech
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post #1 of 14 (permalink) Old May 6th, 20, 5:18 PM Thread Starter
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'66 Knee knocker tach problem...

Hi all,

My tach has been dead for a long time and I was thinking about getting a new one from one of the repop parts suppliers.
Today the tach was showing signs of life (must of read my mind about it's future trip to the garbage can...), so today cruising around I glanced down at it and it was reading 800rpm but really should have said 2400ish rpm but wouldn't go higher with more revs then sitting idling it read 400rpm when engine idles at 800-850rpm.

Hei has the MSD module with the built in rev limiter that they made 10-12 years ago also.

Any ideas?

Thanks,
D
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post #2 of 14 (permalink) Old May 7th, 20, 12:47 AM Thread Starter
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Re: '66 Knee knocker tach problem...

Hmmm I hoped/thought someone would have some idea what to do/look for.
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post #3 of 14 (permalink) Old May 7th, 20, 1:33 AM
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Bill Wente
 
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Re: '66 Knee knocker tach problem...

DO NOT toss that OEM tach! It has value, even if not working.

Having it repaired or upgraded to modern electronics would be preferable to repop.

I have no experience with how HEI/MSD might affect an OEM tach??
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A few '67s and a '66
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post #4 of 14 (permalink) Old May 7th, 20, 8:01 AM
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Jeff
 
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Re: '66 Knee knocker tach problem...

Frankly those tach's were very hard to read while in motion. I picked up a shift light and hid it under the dash, it lets me know the perfect time to shift...
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post #5 of 14 (permalink) Old May 17th, 20, 11:09 AM Thread Starter
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Re: '66 Knee knocker tach problem...

I'm not really a racer but will play once in a while, just having it work would be nice.
there were a couple things that may have been wrong:

1) I switched from point to HEI a long time ago and cut the brown wire to remove the loop terminal for the points coil
to add plug for the HEI in my youthful haste I just twisted the wires together and taped the up. The connection was
loose so I cut cut and re-stripped wires and soldered this time.

2) Also the bolt that holds the tach to the dash (which is the ground path was not very tight so snugged it up.

I haven't started the car since doing the above because a leaking pinion seal grew to something bigger and is taking the spotlight now.
Thx,
D
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post #6 of 14 (permalink) Old May 17th, 20, 11:58 AM
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Bill
 
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Re: '66 Knee knocker tach problem...

I have an HEI, and an aftermarket knee knocker, all is well, and has been for many years.

66 Malibu

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post #7 of 14 (permalink) Old May 17th, 20, 2:58 PM
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Re: '66 Knee knocker tach problem...

Quote:
Originally Posted by busterwivell View Post
I have an HEI, and an aftermarket knee knocker, all is well, and has been for many years.
I used to have an HEI module (Dave's small body HEI conversion) and an OEM knee-knocker, never had a problem.

I was going to suggest checking all the connections...

If those don't help, perhaps it needs a "tach filter"... I think MSD makes one...

Once you go RAT, you never go back...
TC #1366

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post #8 of 14 (permalink) Old May 17th, 20, 5:12 PM
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Re: '66 Knee knocker tach problem...

Don't think the old tach has any voodoo electronics inside. Any good electronics guy could probably test and repair it for reasonable money. But I'd try a good ground and run a new signal wire first.
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post #9 of 14 (permalink) Old May 17th, 20, 8:57 PM Thread Starter
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Re: '66 Knee knocker tach problem...

One other thing I remembered just a moment ago when I was playing, there was a voltage difference between battery post to firewall bolt for engine strap was 12.6V and orange illumination light wire for tach and the tach housing ground was at 11.8V.

Probably a dozen places where the grounding is bad but any typical spots to check??

Thx,
D
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post #10 of 14 (permalink) Old May 18th, 20, 7:38 AM
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Re: '66 Knee knocker tach problem...

Funny, my 71 chevy truck has a factory in dash tach. The 454 (and prior 402) had HEI, and that tach has always been way off, way high. I don't know why, just put up with it for 24 years now.

66 Malibu

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post #11 of 14 (permalink) Old May 19th, 20, 2:12 PM
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Re: '66 Knee knocker tach problem...

Quote:
Originally Posted by busterwivell View Post
Funny, my 71 chevy truck has a factory in dash tach. The 454 (and prior 402) had HEI, and that tach has always been way off, way high. I don't know why, just put up with it for 24 years now.
On my original GM knee-knockers, there is an adjustment screw on the back (inside a hole in the housing). I once bought one that didn't work (so it was cheap). I hooked it up, and started turning the screw and it started working... Maybe your truck's tach has an adjustment screw?
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post #12 of 14 (permalink) Old May 19th, 20, 4:02 PM
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Re: '66 Knee knocker tach problem...

Other than what Tony said, you might send it to someone like Instrument Repair Service.

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post #13 of 14 (permalink) Old May 19th, 20, 6:05 PM Thread Starter
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Re: '66 Knee knocker tach problem...

I'm still working on the diff, rear control arms, control arm bushings etc. so still no start since the diff repair started last week.
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post #14 of 14 (permalink) Old May 20th, 20, 10:06 AM
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Re: '66 Knee knocker tach problem...

Quote:
Originally Posted by 1966_L78 View Post
On my original GM knee-knockers, there is an adjustment screw on the back (inside a hole in the housing). I once bought one that didn't work (so it was cheap). I hooked it up, and started turning the screw and it started working... Maybe your truck's tach has an adjustment screw?
I didn't check for that, years ago, I just looked to see if it might have some kind of 6 cyl/8 cyl switch or something.

66 Malibu

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