tachometer issue

A month ago, my original tach cable gave up and snapped a few inches up from the tach drive connection. I bought a new cable and it worked fine for a month. Now it reads correctly up to 5000 RPM's, then drops to zero. Occasionally it will bounce back up, then drop to zero again. I've used the search function and didn't find these symptoms duplicated, so I am asking for opinions....

Lord knows I'll have to take the tach apart because of my obsessive curiousity... comments please...

The tach dropping to zero suggests a complete de-coupling of the drive cable to the tachometer 's magnetic drive.

please clarify: does the tach work normally under 5K rpm? Does the tach consistently drop to zero upon exceeding 5K rpm, then return to normality upon reducing rpm below 5K?

Have you examined the square drive recess in the tach input? Does it appear to be wallowed out?

Slick

thanks for the response. the tach works normally under 5000, then will drop to zero at high rpms. It will go back to normal if I slow down and the rpm's drop.

I haven't looked at the insides of tachometer yet. I thought I would ask for advise before I started disassembling it....

o0norton0o said:

A month ago, my original tach cable gave up and snapped a few inches up from the tach drive connection. I bought a new cable and it worked fine for a month. Now it reads correctly up to 5000 RPM's, then drops to zero. Occasionally it will bounce back up, then drop to zero again. I've used the search function and didn't find these symptoms duplicated, so I am asking for opinions....

Lord knows I'll have to take the tach apart because of my obsessive curiousity... comments please...

Click to expand...

Sounds to me like the cable could be too long at instrument end. I can't remember the figure off the top of my head but only a certain length of cable should be sticking out beyond the housing (3/8"??) and many aftermarket cables are too long and exert too much pressure on the instrument internals. If this is the problem, and you keep running this cable, you will damage the instrument to the point that it will have to be rebuilt. Ask me how I know!!

Or too short and it's dropping out. Had that with the speedo'.
Has the nut slacked off?

Dave

tpeever wrote:. Sounds to me like the cable could be too long at instrument end. I can't remember the figure off the top of my head but only a certain length of cable should be sticking out beyond the housing (3/8"??) and many aftermarket cables are too long and exert too much pressure on the instrument internals. If this is the problem, and you keep running this cable, you will damage the instrument to the point that it will have to be rebuilt. Ask me how I know!!


My memory says 7/16" is the proper length ... or was it the maximum length? Check it out ... it is true damage occurs if it is too long.

Slick

thank you. I'll report back when I get this sorted out.

texasSlick said:

My memory says 7/16" is the proper length ... or was it the maximum length? Check it out ... it is true damage occurs if it is too long.

Click to expand...

7/16" would be the correct length for Smiths instruments up to 1970 (Norton green blobs) with the 'capped' end fitting (right, below). For later green globe/NVT/Veglia instruments (left, below) these have a deeper recess-so are 9/16".

I have the capped end fitting tachometer with the blob face. My cable end sticks out 11/16" past the butt end of the cable. (not the knurled nut that fastens the cable) Is it a forgone conclusion that I have already damaged my tach because I ran the cable which is too long??

I am thinking about adding a doughnut spacer between the cable shroud end and the capped end fitting on the back of the tachometer. That would reduce the projection of the cable to the proper length but also shorten the threaded contact length of the cable nut and capped end fitting .... OR, I could just cut it the cable end...

Thanks again...

o0norton0o said:

I have the capped end fitting tachometer with the blob face. My cable end sticks out 11/16" past the butt end of the cable. (not the knurled nut that fastens the cable) Is it a forgone conclusion that I have already damaged my tach because I ran the cable which is too long??

I am thinking about adding a doughnut spacer between the cable shroud end and the capped end fitting on the back of the tachometer. That would reduce the projection of the cable to the proper length but also shorten the threaded contact length of the cable nut and capped end fitting .... OR, I could just cut it the cable end...

Thanks again...

Click to expand...

I would Dremel cut it off providing you still have 7/16" length of square drive left after the cut. You may have to dip solder the end if the cable wants to splay.

Since you report the tach performs normally under 5K rpm, there may not be any significant damage. Why not just run it until a problem rears it's ugly head?

Good luck.

Slick

That's exactly why I only buy cables from these people...Cheaper than repair to an instrument. ...

http://www.vintagebritishcables.com/

I shortened the cable to 7/16ths inch projection and the tach seems to be working fine now. The cable end is solid metal so shortening it didn't cause any kind of fraying of a cable end.

One of the interesting things that I found was that all 3 screws that hold the guts of the tach solidly in it's housing were gone and the tach was just floating around inside it's housing. 6-32 screws threaded about 4 threads in and then bound up so I know that they are the wrong threads, but probably pretty close. I temporarily threaded them in with locktite and used a grommet washer to space them out so the length would work... I am currently looking for the screws for the early smiths tachometer....

Thanks everyone for your help sorting this out. I would have ruined my tach without your advice.

**Also, I wouldn't rule out that the cable I bought came with instructions to shorten for the various tachometers... that I would likely have ignored.