Over-Reading Tacho
 

My tacho over-reads by about double,, how do I get it sorted? By the way I need the real dummies guide here, better still an explanation that helps me understand when I take it in to the garage for them to fix.
The car’s a 1962 P1800 with retrofit electronic ignition- basic not programmable.. How do I check which Gen is fitted? Ive tried to understand this site; http://www.sw-em.com/Smith's%20Tacho...mmercial_sites
Is there a cheap/ easy fix I can try first? There was some mention on the above site of a sender behind the grill- what am I looking for and how does that work? Is that easy to change and likely to be the issue?
I don’t want to find the garage send the Tacho away for “repair” when there is some detail about the P1800 we need to be aware of.
Anyway enough ramblings- so if you could help with some ideas please keep them simple.
Many thanks
Simon

Having re-read the link the Tacho looks to be a Gen 1 as there is no “4 Cyl Negative Earth” on the face.

It's not set to 6 or 8 cylinder is it? A lot of these rev counters are adjustable for number of cylinders

Thanks. Any ideas how I check?

if it is non standard it ,might be picking up stray pulses from the electronic ignition , perhaps try it with the points distributor .

Try taking the back off and see if there is a switch for 4, 6 and 8 cyl

Simon;

If it the original Gen 1 Tach, and it shows signs of life at all (although a generous reading), you can consider yourself quite lucky (most Gen 1s have died a miserable death, and factory recommendation for repair was to replace with Gen 2 unit) ...knowledge about the inner workings of the remote sensing module located in front of the Radiator is scarce, but I expect that the electrolytic capacitors within suffer from the same drying-out issue that the Gen 2 Tachs, with internal circuit also suffer from, with your symptoms as a possible and likely effect.

I'd first try adding and external 10,000uF filter Capacitor on the Tach power line, and reporting any change in the operation.

All else I know and can recommend is on the linked SW-EM page you have visited.

Good Hunting!