Tach cables keep failing on my 68

[Frederick W.]

I've replaced the tach cable 3 times on my 68, and it lasts for a few days, slowly begins bouncing the needle and making noise, then fails.
The last time I put a 90 degree angle adapter on the distributor tach drive to remove the acute curve in the cable. Didn't help.

What could cause this?

I'm wondering if there is a problem in the distributor tach drive gears or in the tachometer itself (binding?)

thanks
Fred

[Joe L.]

Re: Tach cables keep failing on my 68

I've replaced the tach cable 3 times on my 68, and it lasts for a few days, slowly begins bouncing the needle and making noise, then fails.
The last time I put a 90 degree angle adapter on the distributor tach drive to remove the acute curve in the cable. Didn't help.

What could cause this?

I'm wondering if there is a problem in the distributor tach drive gears or in the tachometer itself (binding?)

thanks
Fred

Fred------

First of all, does he cable actually break or is the failure mode something else?

[Frederick W.]

Re: Tach cables keep failing on my 68

Thanks Joe,

The failure mode, I believe, is breakage. I haven't done a lot of investigation of the cable when I take it off, but the ends look fine.

I've just taken them off and put new ones on. The current one is still on there, having not functioned since last summer.

Fred

[Terry M.]

Re: Tach cables keep failing on my 68

There is a significant difference in the installed orientation of the distributer between mid-years and 1968 and newer C3s. If a C3 distributor is installed as a mid-year distributor the tach drive cable will have a severe bend/kink. When this happens the cable sheath (I know that is not what GM calls it, but I can't recall the proper terminology at the moment) can be damaged and may damage the cable in the future. This may be your issue.

A properly installed C3 distributor does not need a 90* adapter fitting. We twice printed an item from a 1968 issue of Chevrolet Service News in The Restorer detailing the differences between the mid-year and C3 distributor install. It has been posted here in the past as well, yet I still judge C3s that have been restored by folks who learned their craft on C2s -- and the distributor gets a deduct for Installation.

[Jack H.]

Re: Tach cables keep failing on my 68

Yes, I agree with Terry. The most typical cause I've seen for a Corvette 'eating' tachometer cables is an incorrect indexing of the distributor relative to the block.

When this is done (improper indexing), people also tend to see the vac advance nipple too close or interfering with the distributor shield's upright brace and the tach cable attaching to the distributor almost directly in back where it needs to make a rather sharp bend to clear the firewall...

When indexed properly, these two issues disappear!

[Terry M.]

Re: Tach cables keep failing on my 68

I shouldn't be doing this technical stuff from work for a lot of reasons -- not the least of which is I have none of my reference materials here.

Take a look at the K66 pages in your 1968 AIM. No matter that your car uses points or TI -- the distributor orientation is the same. One of those pages will show the proper orientation of the distributor. As I remember (and please verify this with the AIM. My memory sucks, and it has been way long since I did this.) the #1 spark plug wire goes in the distributor cap hole the first opening to the passenger side of the door for the points adjustment, and that #1 hole is pointed directly to the front of the engine.

You have to get the spark plug wires in the proper holes and the distributor oriented correctly. Of course the distributor has to be installed with the dot on the drive gear radialy aligned with the tip of the distributor rotor as well. Install the distributor with the #1 piston at Top Dead Center with the #1 wire in the cap pointing to the front of the engine (if your verification shows I am right about that). Then as Jack said the tachometer drive cable and the vacuum advance port all fall into place. If any of those three parameters (wire orientation, drive gear orientation, and distributor install) are off the whole program will be in the dumper. There quite literally is only one way to get it right, and a whole bunch of ways to get it wrong.

I know it is a big job, but you might consider replacing your tachometer sheath and then correct the distributor orientation. My bet is that will cure your problem. Of course that is easy for me to say, I don’t have to do the work.

[Frederick W.]

Re: Tach cables keep failing on my 68

Hi guys and thanks for the pointers.

I became aware of the issue with indexing the distributor some time back after I already replaced a couple of cables and bought the 90 degree fitting.

If I recall correctly the 68 dizzy was installed at the factory the "old way" and after issues in service a TSB was issued to change the indexing to remove the acute bend in the tach cable.

Anyway, since I had already installed a 90 degree adapter, and the cable failed anyway, it doesn't seem like this could be my issue. Granted I will need to remove the 90 deg and properly index the dizzy, but while correcting the this will remove the kludge, it won't relieve any stress on the cable.

That's why I've been thinking the problem could be in the distributor or the tach itself.

[Edward M.]

Re: Tach cables keep failing on my 68

Probably not related to your issue, but exactly how long is your tach drive cable outer cover (measured end to end)?

I ran into an issue many years ago with a 73. I had to replace the entire tach drive cable assembly (inner cable and outer cable cover).

I discovered that the reproduction tach drive cable outer cover (and inner cable) was about an inch longer than ther original that I had removed.

That extra inch made it a real pain to get the replacement cable assembly in place, and it put some extra stress on the whole cable assembly. The extra length increased the angle of the bend in the cable assembly. It went from a "gentle sweeping curve" to "an almost radical 90 degree curve".

[Frederick W.]

Re: Tach cables keep failing on my 68

Hmm, not sure the length since it is still installed on the car.
Good thought, though, since so many repro parts have issues like that.

[Richard D.]

Re: Tach cables keep failing on my 68

I've replaced the tach cable 3 times on my 68, and it lasts for a few days, slowly begins bouncing the needle and making noise, then fails.
The last time I put a 90 degree angle adapter on the distributor tach drive to remove the acute curve in the cable. Didn't help.

What could cause this?

I'm wondering if there is a problem in the distributor tach drive gears or in the tachometer itself (binding?)

thanks
Fred

Fred, the attached article by Lars Grimsrud sent by a fellow member helped me out with correct positioning of a correct replacement distributor that I put back into my 68. Scroll through it til you see the pic for 68 distributor postitioning. Rich

[Terry M.]

Re: Tach cables keep failing on my 68

Looks like I had the orientation wrong based on page 4 in his PDF. The illustration on that page is from The Corvette Restorer. Too bad neither Paul67 nor Lars had the integrity to credit the true source of the illustration. Another example of the theft of intellectual property via the Internet.