Hey Folks:

I have a 69 vert with a 427, running the transistorized ignition. When I rev the engine in neutral, or accelerate, the tach does not seem to increase or decrease at the same rate as the motor, and seems to be behind the actual engine speed in both directions. I don't trust the readings, and am afraid that the tach won't show the higher readings quick enough as I accelerate through the gears, and then - BLAMMO!

I know the tach is driven by the distributor, but my question for troubleshooting this, without replacing everything is - what is the most common source of the problem? drive gear on the distributor shaft? tach drive cable(shaft?)? Faulty tach?

Very afraid of over-revving the motor with incorrect readings!!

Thanks in advance for the advice and help.

Rob

 

It would be a good idea to pull the distributor and insect the condition of the drive gears in it. Also clean and lube your tach cable.

Mike

 

If you want an accurate tach put an electronic one in. After modifying my 6t8 SBC I installed a MSD shift lite with a 7200 chip, The tach was only about 5000 when the shift lite came on. T

 

thanks for the suggestions folks. Was planning to do this over the long weekend, but am under the weather. Will update once i get better and pull the distributor.

Rob

 

I'm sure that would work very well, but i'm going to keep the car looking 100% stock.....

 

If the problem ends up being the tach itself, the early years of HEI used the same looking tach. Some C3 owners have gone that route.T

 

Terry:

Thanks for the great idea! Opens a whole new can of worms for me, but better to be safe than sorry!

Rob

 

The '69 tach is a cable-drive unit driven from gears in the distributor. You can check the tach's accuracy by removing the drive cable from the distributor, then inserting the end of the cable into a hand-held drill chuck. Run the drill (read manual to determine the drill's normal running rpm) and read the value on the tach. I'm sure that someone on this Forum know what the conversion ratio for "drive cable rpm-to-tach reading" should be.

This is basically a 'gear-drive' system with a cable transfer line. The most likely fault is with the gear set in the distributor. But, a damaged cable-end receiver socket or damaged gears in the tach head could also give you this problem symptom.

 

Oh come on now, everyone just loves the challenge of taking the dash out of their car. T

 

I was having a similar problem with my 81, you said your distributer was transistorized does that mean that your coil is in the distributor cap ? I replaced my whole dist and still had same problem actually worse and I found that the problem had been there so long that it had caused damage to the computer, disconnected computer,reset timing to best it could be, popped the dist cap and the place you plug up the wires (three prong clip) behind the bat and tach plug ins with cap to coil off check the red wire from connector to coil, mine was arcing to actual coil leaving a black residue on the edge of the coil itself from thin insulation on wire, replaced it with 50,000 volt coil from Chevrolet, runs great a little rich until I go all the way with a regular carb and a distributor with a vacume advance built in it, gained a little horse power with doug's headers, side pipes, and getting rid of pollution control system, took it to Cleveland labor day weekend, up and back and then some wich is awesome because I kept burning up the ecm's inside the distributor, good luck hope it helps you looked most of the summer and couldn' t find an answer to save my butt, Corvette forum or anywhere.

 

Years ago I converted my 72 tach over to electronic. After watching the original tach bounce around and giving inaccurate readings I went this way. It looked 100% stock but worked a whole lot better. If you want the engine compartment to look stock you can just remove the inner cable of the tach cable and leave it all there.

 

Tach bouncing issues can be caused by several things. The first thing I’d do is pull the coupler out of the distributor and inspect the condition of the tach drive gear. Replacing the drive gear would only be a band aide though. If the small gear is destroyed stripped or worn the issue is going to require the distributor to be removed and rebuild with a new main shaft and possibly other repairs. Early 69 cars didn't have the Teflon bushing in the housing so it's possible the side gear has ate the side wall of the housing out... Late cars had the bushing which makes the repair easier. There is more info on distributors at this link http://repairs.willcoxcorvette.com/?...&s=distributor

But.. Since you said the tach was bouncing I’d remove the cable from the distributor and hook up a drill to the cable in reverse mode. Then run the drill and see if the issue is still there in the tach. Honestly, I think the issue is probably in the tach itself and not the distributor because when a distributor gear tanks the tach usually stops working completely. Since you said it’s bouncing my guess is that the first worm bushing is out on the tach itself (pictured below) so doing the drill test will confirm this one way or the other.

It’s very common for the first worm and bushing to tank and when this happens the first worm magnet will **** to the side and the clearance between the first worm magnet and the speed cup is very tiny, so the slightest **** in the first worm magnet will cause the needle to bounce because it’s hitting the speed cup inside. Usually you hear a tinning sound when this issue is present and my advice to anyone is to disconnect the tach cable before you do even more damage to the gauge.
On another note: I can’t say how many times I’ve posted this but never ever spray grease on a tach or speedometer cable. The cable is reverse wound and it pulls the grease into the first worm bushing and will cause premature failure. Use this link as guide on how to lube a speedo or tach cable. http://repairs.willcoxcorvette.com/1...-instructions/


Willcox

Or you can read more about this issue at this link: http://repairs.willcoxcorvette.com/1...-stop-working/