Need help diagnosing a tachometer problem

[Mark E.]

I need help diagnosing a tach problem with my '70. Here's the situation:

- The distributor was rebuilt by a third party (see my separate thread about this). The main-shaft, its bushings, cross-gear, coupling were renewed.

- Installed distributor and the tach worked great. (Note: For years before this, car was not started, then operated with tach cable disconnected from the distributor because the stripped cross-gear was removed).

- After 2 hours of run time, I removed then reinstalled the distributor to properly re-clock its position. (I actually re-clocked the position of the cables on the cap, then moved the dist's driven gear one tooth relative to the cam. Before and after this, the distributor housing was oriented per AIM, with the cap's screws at about 11 o'clock and 5 o'clock, and the VAC pointing at 3 o'clock [almost directly to the right]. The tach cable has almost no bend as it comes out of the distributor.)

- The tach needle then developed a "tick"- the needle would dip for a moment every second or so.

- I unscrewed the tach cable from the distributor, checked cable for free rotation by hand; reconnected tach cable. The cable seems to insert into the cross gear with no issues, and the cable housing's end collar fully seats on the cross-gear's brass coupling. The tach then worked fine for about 20 minutes, then developed its "tick" again.

- With the engine running, I moved the position of the cable about a 1/4" where it goes behind the LH cylinder head. The tach then stopped working completely.

- I disconnected then reconnected the tach cable from the distributor again. Within about 5 minutes after starting, the tach went from working fine, to having a "tick" at one second intervals, to not working at all.

What's causing this? Is this a problem with the distributor's tach drive, the cable, or the tach itself?

[Bob R.]

Re: Need help diagnosing a tachometer problem

My tach bounced and just before the inner cable broke it really started to bounce. I would think if it works smoothly then bounces it is the inner cable. They fray before they break and that can cause a bounce.

[Mark E.]

Re: Need help diagnosing a tachometer problem

Fixed it! While poking around I discovered I could wiggle and move the end of the cable housing where it attaches to the distributor.

Why is that? Well, thinking I didn't want to force anything, I only hand tightened the threaded collar to the brass cross-gear housing. And I didn't check if the cable housing was snug.

Fix: Using water pump pliers with the jaws wrapped in duct tape to prevent damaging the collar, I tightened it until the housing was snug.

[Mark E.]

Re: Need help diagnosing a tachometer problem

So my specific tach problem is solved, but in the spirit of learning, here's a more general question:

What's a good procedure (including tips and tricks) for diagnosing the cause of a bouncy or inoperative tach?

I believe the most likely root causes are:

- Bad distributor tach drive
- Bad cable
- Bad tachometer head

Proper diagnosis of the root cause avoids a lot of time, expense, and aggravation. I sure wouldn't want to tear into my dash to repair the tach, only to discover that didn't fix the problem. (BTW, why don't engineers take serviceability more seriously as a design requirement? I mean, how hard would it be to design all gauges so they can be easily removed from the front of the dash? End of rant.)

I'm not an expert, but some thoughts about diagnosing this:

Disconnect the cable from the distributor (easy), then turn the cable using a variable-speed reversible drill. I'm 90% sure (but this should be checked) that the cable rotates counter-clockwise as you're looking at the distributor end of it. Hence the need for a reversible drill.

If the tach does work properly with the drill test, then the problem is the distributor and/or its coupling.

If the tach does not work properly with the drill test, then the problem is the cable or tach itself. At this point, I would probably replace the cable. That's because I've heard (but somebody who knows please confirm this ) the cable can be replaced fairly easily, while removing the tach requires pulling the dash (see rant, above). The procedure I've heard to replace the cable involves unfastening the cable retainer at the back of the tach (by using someone's unnaturally small hand to reach up in the dash), then pulling the cable out of its sleeve from the distributor end.

If the tach does not work properly with the drill test, and replacing the cable didn't help, then after a long cry, I would conclude the problem is probably within the tach itself.

That's my thought process, from a guy with no experience actually going through this.

Suggestions for a better diagnostic procedure?

[Patrick H.]

Re: Need help diagnosing a tachometer problem

I did similar when replacing the speedo cable on an 81 not long ago. The mechanism is essentially the same. Of course, I've also fixed a few tach drives.

In my experience, if the tach turns the cable, then the cable itself is more likely to be broken than the head unit of the instrument. However, I've found the tach drive to be the cause even more then the cable.