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MG MGB Technical - Tachometer

I’m pretty sure I have a failed tachometer. Engine starts and runs fine. I have 12 volts on the green wires and a good ground but no movement at all on the tach. Any suggestions? Anyone rebuild these or do I need to buy a new one?
The number on the tach is RVI 1433/00
Thanks.
Tony
ATM Tony Mason

Speedy Cables can rebuild any instrument. So can JDO Instruments.
Mike Howlett

Thanks. Anyone in the US?
ATM Tony Mason

Tony, I had a tacho issue, it was stuck on zero. So I pulled it, cleaned it then Paul Hunt sent me this link to set it up again, it's brilliant!!
http://www.mgb-stuff.org.uk/tachtest.htm#2
Allan Reeling

Sorry Tony. I didn't notice you were in the USA!
Mike Howlett

If you are sure you have 12v between the green spade and the body of the tach, then unless you fancy opening it up in case anything obvious appears then it probably will have to be sent away. The wires have to be connected for voltage checks, taking the wires off and testing them then can hide bad connections.
paulh4

Tony, Forgot to mention the main problem with my Tacho, and that it was very easily found and fixed.
The circuit boards in the unit are grounded by means of the screws and spacers holding it all together. THEY WERE LOSE!! I cleaned everything, re-tightened the screws and as a belt and braces effort I soldered extra earth connections. After 20 mins with Paul's battery charger rig, calibrating it, everything has been fine for the last 5 years or so!
Allan Reeling

Re the link ...

QUOTE "On a 4-cylinder tach a full-wave charger should display 3000rpm with the UKs 50Hz AC supply (3600rpm at 60Hz). A half-wave charger should display 1500rpm (1600rpm).

The last number in brackets should be 1800rpm
R G Everitt

By Jove you're right! The main page is correct, I've edited the detail page. Thanks.
paulh4

Maybe is problem wiring not tacho itself? Run few parallel wires and check if tacho maybe works. Does your tacho have 3 or 4 connections?


Toni Kavcic

The Tach on my '66 wouldn't register either when I start the engine. I tap it on its face and then it works. For some reason the pointer sticks in the 0 rpm position.
bhall1

An RVI 1433 will have 12v, earth, ignition in and ignition out wires. If either of the ignition wires were faulty the engine wouldn't run. And as said earlier you need to check the voltage on the 12v and earth terminals on the tach with the wiring connected, removing the wires and testing them is no good.

Stiction is quite common. Either a dry joint internally or stickiness of the movement.


paulh4

This thread was discussed between 06/06/2018 and 10/06/2018

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.