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MG MGB Technical - Rev counters

1972 GT. Electronic ignition module. Wayward rev counter.Very disconcerting to see it swing wildly when indicating at M-way speeds!!
Now I know the later RVC rev counters are wired differently from the RVI units, and operate on a different principle. Question, can the RVI units be modified to work accurately with electronic ignition?
Allan Reeling

You can try removing half a turn from the pickup (i.e. straight in and out rather than in, round and out), but it is internal to the tach on that year so requires dismantling as well as removal. Might be easier just to fit the later RVC tach, they rarely have problems with after-market electronic ignition.

I do have a tach adapter somewhere when I built a Sparkright ignition system in the late 60s, that was probably to correct tach problems (never used it as Mini vans don't have a tach!). I'll have to investigate the circuit of that one day.
PaulH Solihull

Hi Alan,

Try taking the power supply for the ignition from a place other than the +ve terminal of the coil.

What is occurring is that the ign module will have an input capacitor, which alos smooths out the current pulses that the tach should see to trigger the circuit.
See picture, below.

Herb

Herb Adler

Thanks to you both, will try Herb's easier option this weekend.
Al
Allan Reeling

Thanks Herb, it seems to have worked.
Al
Allan Reeling

Dug out the tach adapter, and worked out what it does. It basically switches earth via a 7 ohm resistor on and off. Presumably this is connected to the white coming from the tach, the supply for the electronic ignition coming from elsewhere. I say 'presumably' as I can't find the connection instructions for it or the ignition (Sparkrite 1000), but Googled and found this original Sparkrite SX2000 information sheet http://www.olivegroveapt.ms11.net/sparkrite.html which shows two wires connecting to the 12v terminal on the coil, but specifically states if that causes problems with RVI tachs to move a specified wire from the coil to an alternative 12v supply, as Herb says.

Interesting to see just how many references to Sparkrite there are given it is really a 70s thing. Most of them not very complimentary it has to be said. I never had any problems with mine, but as I found it made no difference, although I took it off the car when I sold it I never bothered fitting it to anything else. Might be fun ...
PaulH Solihull

Thanks for that Paul. Did the Sparkrite come with coil clips. I think mine is still in the roof of the garage!!
Allan Reeling

Yes it did.
PaulH Solihull

You guys seem to know about MGB rev counters, I hope somebody can help. A few years ago I bought a Mk1 1964 roadster that had been converted to negative earth. The rev counter has never worked properly, never reading above 3000 rpm even when driving hard. The exception being pulsing when the indicators are on. Any suggestions?
Donald
DD Carmichael

Sounds like bad connections somewhere, possibly in the earth connection to the tach and indicator tell-tales. Does it do funny things when the dash lights are on or off? Reading low can be down to one of the capacitors inside having dried out and changed value.
PaulH Solihull

Thanks Paul. Car's off the road at the moment but will add checking connnectors to my pre-season list of thiongs to do.
Donald.
DD Carmichael

This thread was discussed between 06/04/2012 and 17/04/2012

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