69 Tach Problem Help

[Brad H.]

Guys,
I have just developed a tach problem that I hoped someone might have some insight on. Literally from one day to the next my tack needle has begun to smoothly fluctuate about 3-600 rpm especially as the rpm increases and is most prevalant above 2000 rpm. This is with the engine running absolutely smooth.

Here is the background and what I have done to isolate the issue. First the distributor was rebuilt by Dave Feidler of TI Specialties and has operated flawlessly since it was installed last winter (approx 200 miles ago). The tach cable was new at the same time. The tach was rebuilt (supposedly) by a company called "Tachman" and looks brand new again, but who knows. It was done about 2 years ago and maybe has 400-500 miles on it.

I removed the distributor to inspect the tach drive and it looks fine. I do have the 90 deg drive adapter on the distributor and it was new when the dist was rebuilt. The tach cable moves by hand easily and smoothly and has no kinks. I attached a hand held drill motor to the tach cable and saw the same fluctuation with the drill running at full speed (about 3000 rpm). This seems to all point to the tach itself. I have never had the tach apart so I don't know how its built internally. Does this scenario point to the tach or does anyone have other ideas. Can the internal workings of the tach cause a fluctuation like I have described? It is such a pain to get the left dash panel out of the car, I hoped someone might have an idea before I dive into that.
Thanks in advance as always,
Brad

[Terry M.]

Re: 69 Tach Problem Help

Why do you have a 90 degree tach drive adapter? None were installed from the factory, and if the distributor is properly installed it is not needed. That is a part looking for a problem IMO.

I am not suggesting that adapter is defective, although that is a possibility -- just that your perceived need for it indicates a potential distributor installation issue which could lead to a misrouting of the tach drive cable. If the drive cable is or was misrouted the cable housing might have been kinked, or might still be kinked. This could be your problem.

[Brad H.]

Re: 69 Tach Problem Help

Thanks Terry,
The distributor is definitely installed correctly. There really is no slop in its location due to interference with the shielding or intake if it is installed incorrectly. My tach cable for sure has no kinks in it. I have followed it all the way to the tach and the cable itself turns easily and smoothly by hand. I could install the cable without the 90 deg adapter but it really eliminates some of the bend as it come out of the drive. I have looked at many examples and most have some bend of the cable coming out of the dist drive. I agree its a potential issue so I will probably take it out. I tried the drill motor to eliminate the dist and 90 degree drive from the equation and the fluctuation followed which seems to eliminate them from the problem. I really appreciate your input as I have had a kinked tach cable in the past so I am very cognizant of that possibility. Assuming the tach cable and dist are ok, do you think the tach itself can cause this acute fluctuation I am experiencing because of slipping gears etc. I apologize for not knowing the internal workings of the tach.
Thanks again,
Brad

[Gerry P.]

Re: 69 Tach Problem Help

Brad - I wouldn't expect this of a recently rebuilt tach, but is it possible that the colder fall weather has caused the tach bearing oil/grease to become sticky which will then lead to the tach shaft binding up? I just had the speedometer rebuilt in my '57 and the rebuilder told me the old oil/grease in the sleeve type bearing becomes dried out which then leads to the shaft binding up and a bouncy needle. You might try adding a couple drops of 3M oil to the back of the tach shaft to see if that helps.
Gerry

[Brad H.]

Re: 69 Tach Problem Help

Thanks Gerry,
I'll try that. Now that I have been on the forum for several years, I probably should have used a different rebuilder for the tach. Who knows if he really rebuilt it or just refinished the face? I can spin the tach cable by hand easily and smoothly. I don't know if that means the tach itself is not binding up or not...
Brad

[Joe L.]

Re: 69 Tach Problem Help

Guys,
I have just developed a tach problem that I hoped someone might have some insight on. Literally from one day to the next my tack needle has begun to smoothly fluctuate about 3-600 rpm especially as the rpm increases and is most prevalant above 2000 rpm. This is with the engine running absolutely smooth.

Here is the background and what I have done to isolate the issue. First the distributor was rebuilt by Dave Feidler of TI Specialties and has operated flawlessly since it was installed last winter (approx 200 miles ago). The tach cable was new at the same time. The tach was rebuilt (supposedly) by a company called "Tachman" and looks brand new again, but who knows. It was done about 2 years ago and maybe has 400-500 miles on it.

I removed the distributor to inspect the tach drive and it looks fine. I do have the 90 deg drive adapter on the distributor and it was new when the dist was rebuilt. The tach cable moves by hand easily and smoothly and has no kinks. I attached a hand held drill motor to the tach cable and saw the same fluctuation with the drill running at full speed (about 3000 rpm). This seems to all point to the tach itself. I have never had the tach apart so I don't know how its built internally. Does this scenario point to the tach or does anyone have other ideas. Can the internal workings of the tach cause a fluctuation like I have described? It is such a pain to get the left dash panel out of the car, I hoped someone might have an idea before I dive into that.
Thanks in advance as always,
Brad

Brad------


Corvette mechanical tachometers are, unfortunately, not of the direct drive type. Consequently, if the damping mechanism in the tachometer head is not properly rebuilt, you will end up with just the problem you seem to be experiencing.

[Terry M.]

Re: 69 Tach Problem Help

Thanks Terry,
The distributor is definitely installed correctly. There really is no slop in its location due to interference with the shielding or intake if it is installed incorrectly. My tach cable for sure has no kinks in it. I have followed it all the way to the tach and the cable itself turns easily and smoothly by hand. I could install the cable without the 90 deg adapter but it really eliminates some of the bend as it come out of the drive. I have looked at many examples and most have some bend of the cable coming out of the dist drive. I agree its a potential issue so I will probably take it out. I tried the drill motor to eliminate the dist and 90 degree drive from the equation and the fluctuation followed which seems to eliminate them from the problem. I really appreciate your input as I have had a kinked tach cable in the past so I am very cognizant of that possibility. Assuming the tach cable and dist are ok, do you think the tach itself can cause this acute fluctuation I am experiencing because of slipping gears etc. I apologize for not knowing the internal workings of the tach.
Thanks again,
Brad

Brad,

I seized on the one item that stood out to me. If you have had a kinked tach, cable then you know the ropes. I have never had the chance to take either a tachometer or speedometer apart, so I am not familiar with the internals of either.

[Brad H.]

Re: 69 Tach Problem Help

Joe,
Thanks as always for the reply. I was afraid of that. I have serious doubts as to the quality of the so called tach rebuild. If the car was yours would you buy a new replacement tach from one of the vendors or send it out for proper overhaul? I assume the new units are Chinese made and probably not much better than my 40 year old one. If your answer is the latter do you have a recommendation. I know there were a bunch of threads about that not long ago. I'll look for those, but if you have a favorite, I would really appreciate it.
Thanks again!
Brad

[Brad H.]

Re: 69 Tach Problem Help

Thanks Terry,
I almost wish it was a kinked tach cable compared to removing the left inst panel, but looks like I'll have to.
I appreciate your help!
Brad

[Joe L.]

Re: 69 Tach Problem Help

Joe,
Thanks as always for the reply. I was afraid of that. I have serious doubts as to the quality of the so called tach rebuild. If the car was yours would you buy a new replacement tach from one of the vendors or send it out for proper overhaul? I assume the new units are Chinese made and probably not much better than my 40 year old one. If your answer is the latter do you have a recommendation. I know there were a bunch of threads about that not long ago. I'll look for those, but if you have a favorite, I would really appreciate it.
Thanks again!
Brad

Brad-------


I think I might try once more for a rebuild. I don't have any personal experience with any of the Corvette instrument rebuilders but perhaps others here can offer some of their experiences.