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TR2/3/3A Tachometer Malfunction

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M

mxp01

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All:

Over the weekend i replaced the oil pan and differential gaskets. Everything went well and i didn't disconnect anything. Oddly, when I started the car up today the tachometer spun all the way over and is now buried at the max range (by a zero).

I have a TR3B. The tach is cable driven. I disconnected it and re-connected it. Everything looks ok, but it still reads way too high.

Can someone explain what is happening?

Thanks,

Mike
 
Does the needle return to zero when the engine stops?

If not (and I expect not) then it is likely the hairspring is broken or detached.

Here is a good site for speedometer repair (note that the tach is essentially a speedometer w/o the two odometers):

https://triumph.daveola.com/NOTES/Speedo_Repair.pdf

That will get you a sense of what is inside - but I have never dealt with a broken spring.

If the needle does return to zero when the engine stops then it sounds like the magnet bar may be fouling on the drag cup.
 
I can't think of any correlation of the oil pan to the tach issue. It has to be a coincidence. As Randall says, check to make sure the cable is not forced too far into the tach. That can push up on the rotor magnet and cause mechanical contact between the rotor and the aluminum follower disc. If you removed the tach cable during your work, it's also possible to install it backwards and cause the same problem.

Aside from that the tach will have to come apart to fix anything else that could be wrong.
 
John:

I didn't touch the tach cable at all during the work, so yes this must be a coincidence. Is it possible to pull the cable end out slightly at the end (by the engine) to counteract any possibility the cable has been pushed too far into the tach?
 
Last edited:
mxp01 said:
...Is it possible to pull the cable end out slightly at the end (by the engine) to counteract any possibility the cable has been pushed too far into the tach?
Click to expand...

I think if you look at the speedo end of the cable you'll see that the solution (if a too-long cable is the problem) has to be there as there is a fitting that locates the cable against the speedo input connection.

I had a cable that would do this and as a stop gap I added a washer over the speedo end of the cable to act as a spacer and relieve the pressure of the too-long tip.

As I recall I had to start with a washer of a suitable ID and grind down the OD so it would fit inside the ferrule.
 
One end of the cable has a locating ring on it that looks something like an attached washer. What commonly happens is a tinkerer replaces the cable with the locating ring at the wrong end.

The quickest check of a too long cable end would be to loosen the cable sheath nut on the back of the tack and pull the sheath out of the tach 1/8" or so. Then try checking the tach operation with the loose cable. If it fixes it, then you know you are on the right track. The fix would be to try reversing the orientation of the cable, grinding just a tad off the tach end tip, or adding washers as Geo mentions.
 
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