Tacho reading high?

[Mrozik]

After 30 odd years sitting in various garages I finally had the time to restore my 68 Elan+2. All is well except that at 50 mph my tacho in 4th gear is reading about 5000 rpms.......i know its a long time since I realy drove the car I don't think this can be right and suspect there's either something wrong with the tacho or I reconnected the tacho wiring incorrectly. Grateful for any suggestions.

[billwill]

This sort of thing has been discussed recently.

You can buy a cheap digital tachometer on eBay to check your dashboard one.

See:
viewtopic.php?f=38&t=49104

[Mrozik]

Thanks for the response. I noticed that one of the comments says that 3000 rpm should give 50 mph so I suspect my tacho is malfuntioning......

[Quart Meg Miles]

There is a calibration adjustment (hole in the back of the tacho for a small screwdriver) that might bring it back in line. I expect your idling looks high too.

My tach is defective and I've just fitted a cheap one, as mentioned above, to set up idling and check the speedo which I know over reads.

[Mrozik]

UPDATE

I've checked the rpm indipendently and the tacho is definitely reading about twice what it should....

I also found a few technical discussions and it seems that the inducton loop on the backof the tacho maybe the cause of the problem. As it was working perfectly before I replaced the dash, I now suspect I may be the incorrectly fitted the loop. Does anyone have a photo or diagramme of the back of the tacho showing exactly how the wire should be looped?

Thanks for the help

[HCA]

If you are 100% sure the tach was ok before you removed the dash, and you have not replaced any ignitiom items with non-standard, then I would warn you against trying to adjuct the pot inside the unit.

So it is an external loop type. Then very hard to get it wrong. Only one loop is needed and does not matter which way it is looped.

Of more concern is the polarity. Do you think you might ever have reversed the polarity? If yes, then almost certainly there is a blown cap inside which will cause a x2 reading. If it is, not difficult to replace. Many adverts for this.

[Quart Meg Miles]

30 years is a long time for electronic components to be idle, it is probably just deterioration. That is assuming that you haven't fitted electronic ignition, which screws current pulse tachos, as you sound as though you have kept everything standard. I'm not certain what your standard set up is, even polarity, though I suspect it is still positive earth and dynamo.

My own tacho problem is due to a wrong component which I've been too lazy to replace but It has been powered up frequently, unlike your own.

[Mrozik]

Thank you foor the additional info.

It could well be that the polaity was accidently reversed at some point especially as the car is a positive earth example and I did have a garage replace the starter solenoid so I suspect that the cap has blown.
I'm more of a mehanical guy and I'm not really certain what exactly I need to do (?) and where to find the adverts for a replacement?

Grateful if you could help with a few more detals

Many thanks
John

[HCA]

I think you are doing the right thing in leaving it to those with experience. A long time ago, I attempted to fix a tacho, I renewed the capacitors easily enough, but made a complete mess of putting it back together!

Google 'Smiths rev counter repairs/reconditioning' and several outfits will appear - maybe one near you.

Speedycables though has a good reputation. I have no idea of cost though: https://speedycables.com/instruments/ga ... nversions/