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MG MGB Technical - Rev counter problem / Electronic ignition

MGBGT 73. Fitted with accuspark ignition. 45D4 Distributor.
Rev counter has been playing up since buying car some years ago. Smiths type RVC 2415/00. Sometimes accurate, other times half engine speed or nil. Engine ran fine so left it for a rainy day which never arrived. Lately went to half speed all the time so had to do something. Checked the archives of MGCars.net for advice and subsequently
1. checked/replaced all the LT wiring. No change.
2. noted comments that electronic ignitions sometimes causes this problem. Changed to points and problem fixed.
I like the idea of electronic ignition for reliability and ease of maintenance. I don’t need performance. Are there any other low cost ones that will not cause this rev counter problem? Anybody out there with the SimonBBC/Smiths RVC combination that runs OK?
Thanks.
D Baram

I'd suspect the ignition module. There are known problems with the earlier RVI tachs used 1972 and earlier, and also on rubber bumper cars i.e. ballasted ignition where you need to pick up the voltage supply for the module from an ignition switched supply like the white at the fusebox and not from the coil +ve. But I can't recall problems with the 73 and 74 tach and ignition.
Paul Hunt

don't forget you can search the Archives here for information too

also bear in mind performance doesn't just mean higher power, speed, etc., improved performance means exactly what you have and want, easier engine starting, better idle, more mpg and reliability and no maintenance on that part - the rest of the dissy will still be worn and probably inaccurate and variable (that doesn't effect your rev counter)
Nigel Atkins

As Paul said, take the live supply from the switched live on the fuse box.
But my Tach had a similar problem to yours, i.e., erratic!! Turned out to be a lose screw within the unit itself. Earth is picked up off the circuit board where the die cast "chassis" is screwed to it. Easy fix when you find it!!!
Allan Reeling

I use a SimonBBC on my midget. It's the same type of tach/wiring combo as you'd have. No troubles at all. Been fitted years and over 20000 miles.
Roadwarrior

I have accuspark and smiths rev working fine on my 72. on a 25d4. Have you checked the resistance of your coil as per the install instructions? just that they talk about 1.5/3 ohm (can't remember which way around) and my radio suppressor made the difference so i took it out. just thinking that 1.5 x 2 = 3 which is maybe a coincidence, but maybe not.
if you don't have the instructions, I will dig them out and scan/email them to you if you wish to check. Graham
Graham Moore

in fact, manual is here
http://www.accuspark.co.uk/fitting_guide.html
Graham Moore

Chrome bumper coils are 3 ohms and connected directly to the ignition supply, rubber bumper coils are 1.5 ohms and connected to the ignition supply via a 1.5 ohm ballast resistance contained within the harness.

Some rubber bumper cars have been converted by owners to bypass the ballast resistance, and these must have the same 3 ohm (12v) coil as chrome bumper cars or they will seriously overheat and will damage points and can damage ignition modules.

Conversely you must have the 1.5 ohm (6v) coil if the ballast resistance is in circuit, or you will get weak sparking.

You can't go by what is printed on the coil or the box or what the supplier says if buying a new one, you have to measure them.

Likewise on rubber bumper cars you must confirm whether the ballast resistance is in circuit or not. To do this disconnect any electronic module, have the original two white/light-green wires on the coil +ve, and connect an earth to the coil -ve. Switch on the ignition and measure the voltage on the +ve terminal.

If you see 12v then either there is no ballast in circuit, or the coil is open circuit, or your earth is bad.

If you see 6v then you probably have the correct coil and ballast, but you still need to measure the resistance of the coil as the ballast can go high resistance, which with a 12v coil could still result in 6v.

If you see any other voltage you either have an incorrect coil, or ballast, or both.

It's cars that have the ballast resistance in circuit where you may need to pick up the ignition module 12v supply from the fusebox, as the coil +ve - which is the connection point for 12v coils - is continually switching between 12v and 6v on ballasted ignition systems.

Radio interference suppression capacitors will make no difference to the ignition system itself - points or electronic, unless they are faulty i.e. short-circuit. They should be connected to the 12v supply side of the coil, not the trigger side.
Paul Hunt

This thread was discussed between 06/06/2014 and 07/06/2014

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