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MG MGB Technical - RVI tachometer

Having just finished a midget Hertiage reassemble, I am stuck on getting the tachometer working. It does absolutely nothing although I am fairly sure it is correctly wired up. I have no idea if it was working when the car was dismantled 20 years ago.

Is there a simple way to test the unit and debug any wiring issues ?
dominic clancy

Have a look at Paul Hunt's website.

Lots of info on tachos and testing.

http://www.mgb-stuff.org.uk/electricstext1.htm#tach
Dave O'Neill 2

OK, there are two types of RVI tachs, one marked positive earth and the other negative earth. Check which you have and whether the polarity is the same as the car.

If this matches, check that the case is well earthed.

Post which type of tach you have.

Herb
Herb Adler

Positive earth, it is well earthed
dominic clancy

I've said it before, the testing/calibrating bit on Paul Hunt's web site is a work of genius. Do look Dominic.
Allan Reeling

I found using a bulb in place of the coil worked better.
Michael Beswick

I have looked at Paul's site and he has given me an idea to test the fuel gauge using a bulb as resistance as well. For the RVI tachometer I didn't realize that the direction of the connection of the impulse wire is important so I will try reversing that as a first measure. Then if that doesn't work I will take it out of the car again and do the bench tests indicated by Paul.

dominic clancy

A quick test of the electronics and the voltage supply to them, i.e. ignoring faulty or incorrect trigger wiring, is to turn the ignition on and off. On my later RVC tachs this always gives a little jump of the needle, regardless of whether the trigger wire is connected or not. The situation on a sample of one RVI tach is a little less clear - sometimes it did and sometimes it didn't, regardless of whether the coil wire i.e. the trigger circuit was connected or disconnected. I'd be interested to know from others with a working RVI tach whether they get the jump or not, both with the white connected to the coil terminal and with it disconnected.
Paul Hunt

Reversing the direction of the loop did the trick, so Paul's site was definitely the right resource

Thanks Paul

dominic clancy

This thread was discussed on 14/07/2014

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