I see from today’s postings that I’m not the only one with tach problems.

I’ve been driving my car around town this week, and today after starting my car after my third errand, I noticed that the tach completely stopped working. I have a 1969 L46 coupe. I saw this much older thread on here someone posted years ago about their ‘64’s tach.


https://www.corvetteforum.com/forums...-do-first.html

I have the ‘69 assembly manual and also the ‘66-‘82 shop manual. My version of the shop manual, ignition section at page 4-9, discusses tachometer connections at the distributor. I haven’t actually touched anything yet. This is verbatim what the shop manual says, for “Tachometer connections”:

“In the distributor cap connector is a “tach” terminal. Connect the tachometer to this terminal. Follow tachometer manufacturer’s instructions, as some require a ground lead while others require a hot lead. CAUTION: Grounding the “tach” terminal could damage the electronic module.”

There’s also a diagram.

Whenever I see the word “caution” in all caps, especially in a discussion about connectors being grounded or not, I proceed cautiously.

I have no idea whether this is the original tach wire, let alone what the manufacturer’s instructions for the tach are. Everything on the dash inside is original. Certain things in the engine compartment are new. Because of this shop manual cautionary language, I’m not confident that I should just root around looking for a loose wire at the distributor.

I’m hoping it’s either a loose wire at the distributor, or perhaps the wire itself needs replacing (as opposed to worn or broken gears, or anything requiring me to have to take the dash apart to fix). Does the fact that it’s been working fine up until now suggest that one cause is more likely than another?

Any suggestions (including photos) would be appreciated. I do have a mechanic I can take the car to, but I’d like to try some troubleshooting first.

 

Your 69 uses a tach drive distributor with a cable drive to the tach. If not yours has been converted to a later speedo(75-76).

 

Ignore the wide-ranging shop manual in this instance as it applies to your '69. It came with a purely mechanical tach driven by a cable between the distributor and the gauge. At the distributor, the cable is driven by a "cross gear" that meshes with a gear on the shaft of the distributor -- if all is stock to '69's original design.

Look at the bottom, rear, of your distributor from the driver's side. You should see the heavy cable sheath connected to the dizzy and running into the firewall. A first check is to unscrew the cable from the distributor and rotate the inner cable clockwise (you can use a drill for this). If the gauge moves like normal, then the issue is the cross gear and shaft. This is a very common failure point.

If no needle movement, then the cable could be broken or the gauge. Pull on the cable and see if it comes out. If it does, it's broken as the cable is inserted into the sheath from the gauge end is not normally removable from the engine side. This fix is more involved and back-breaking.

Order copies of the Chassis Service Manual and the Overhaul Manual for '69. This has much more applicable repair processes for your car. The Service Manual is the more used of the two unless you are really digging into the mechanicals of the engine, trans, or differential, but they're both valuable for reference.

 

On Corvettes through 73 ( and maybe a few more years) all the tachometers and speedometers were drive by a steel cable that turns inside a sheath. The distributor had one cable for the tach, and the transmission had another for the speedometer. If you want to be adventurous and investigate what is wrong, use Google to find posts that will guide you on the methods to determine the cause of your problem.

 

Mechanical drive distributors were used in Corvettes up through 1974. There is no tach "electrical connection" in the distributor cap on your car, unless it has been converted to an HEI electronic tach.

Assuming you have the stock tach drive system, it's possible that your tach drive cable has broken. Check that first. That would be a cheap and easy fix, and very lucky. The cable breaks when the distributor has not be re-oriented into the "Service Bulletin Clocking Orientation" to straighten the cable.

More likely, your tach drive cross shaft gear and mainshaft gears in your distributor have stripped out. This is due to the crossgear "digging" itself into the distributor housing thrust surface and locking up. You need a complete distributor rebuild with good quality parts and good machining on the housing thrust surface (with new thrust surface parts fabricated and installed). You also need to install your distributor in the GM Service Bulletin "Alternate Distributor Installation" clocking position to prevent the problem from happening again.

Here's a good deal to solve your problem. You still need to install it per the SB instructions to make sure the problem doesn't happen again:

https://www.corvetteforum.com/forums...-restored.html

Lars

 

Many years ago when my tachometer stopped working, I removed the cable from the distributor and turned it with my fingers to see if the tach needle would move. It did, so next I removed the gear from the distributor and could see that it was worn, along with the distributor main shaft. I replaced both and it's still working today.

 

Thanks. This is all very helpful. It had been working until yesterday, so that’s why my first guess was the cable. I will take a look. I also appreciate the clarification. I, too, thought the ‘69 tach was mechanical until I read the shop manual about grounding, leads etc.

I’ll have to get that specific manual. When I got my car, I went to the various Corvette parts websites, and so many of the books were sold out. All I could get my hands on were the shop manual and the reproduction ‘69 owners manual.

 

First thing I’d do is remove the tach cable at the distributor cross gear assembly, lock the cable in a drill and run it in “reverse”. If the tach works you know the problem is at the distributor side. It’s not necessarily the gear or the shaft itself though…
When the one in my ‘68 quit the cable had cracks in it so as they are cheap and it being cracked it was a safe replacement… that didn’t fix the problem entirely.
Turns out it was partly the cause.
The gears in the cross gear assembly and the shaft were in good condition. The cross gear assembly where it connects with the cable has a square connection that had been rounded out allowing the cable to slip. I installed a new stainless unit and the tach now works as it should.
The cross gear has a 1” wide slot to turn it out by. I cut a wide butter knife to use as a tool to unscrew it’d worked well.


Brass unit is the old piece and shows the square connection thats been rounded out

Old gears on right show little to no wear

An altered butter knife and wrench did the job