Tachometer not working

[Jerome P.]

Not knowing a lot about the electrical system; will the tach work if it is not grounded? The tach on my car all of a sudden stopped working after a short ride. Any thoughts will be appreciated.

If it is tach drive cable; what kind of cost am I looking at if done by an independent mechanic?

[Mark E.]

Re: Tachometer not working

Jerry, If you're talking about your '72, it has a mechanical tach so it's not affected by the electrical system.

Likely causes are broken distributor cross gear, cable, or head unit. You'll find a lot of threads on this subject on this forum and other sites.

A basic diagnostic is to disconnect the cable from the distributor and spin it with a variable speed drill. If the tach works, you have a bad cross gear assembly, which is probably the most trouble prone part of the system.

A good local Speedo shop may be able to service the cable or head unit. But the distributor/ cross gear assembly warrants a specialist familiar with this Corvette only design.

[Gary R.]

Re: Tachometer not working

As Mark outlined, you should have a tach drive distributor but a lot of guys have swapped over to the later electronic ones. If you still have the chrome shielding over the distributor you can remove it and look closer at the distributor. There should be a black plastic cable screwed into it. It has a coupling nut you can unscrew and look at the cable end. Sometimes the distributor has been repositioned in the engine and puts the cable at a tighter angle that causes the cable to wear, sometimes they break and you might be able to slide it out.

If you follow Marks test and the needle moves steady at the dash then chances are the cross gear and shaft are the problem. There have been several mods to these to make them work better, from boring and plugging to tapping a set screw on the end of the housing to support the gear. I have not had to rebuild one in several years but at the time the new gears on the market were bad. The ones I refer to were the new ones with brass button on the end. I found the gear mesh was bad and was told by the supplier I had to file the brass button to fit the gear. I set it up in my lathe and faced off a few thousands at a time and couldn't get it to correctly mesh without binding. I finally removed the button to test it and it was the same. I machined a new bushing and but it didn't work. I finally found a good used gear in my parts bin, put that it and bingo it fit like a glove and is still in use. I hope this problem has been corrected by now, maybe someone else can comment on using a new one.

If it is the cable and you are willing, I would buy a new cable assembly and install it. The end of the cable that fits into the back of the tach is held by a spring clip. You have to reach up under the dash, find it, press on the clip while pulling on the cable to release it. Then fish in the new one and plug it into the tach and screw into the distributor. With my bad back these days I would have my son do this job!

If it's the distributor you have to remove it and check the shaft gear too as many times the shaft gears is damaged too. The cross gear can be removed with the distributor in the engine but I would pull it to check it.

Good luck, it's not too bad to do but if you never did it ask for guidance here first.