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MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - RVI-RVC Tachometer Conversion

I have a 1965 MGB and sadly my refurbished tachometer than had served well from its refurbishment in the 1990s failed so I cast around and looked for a repair. The Spiyda option was recommended by the MG Owners Club - see https://www.spiyda.com/tachometer-electronics.html.

I eventually took the plunge and bought one of these and although I was somewhat challenged with the installation of it (not being terribly smart when comes to auto-electrics) Chris at Spiyda stayed with me all the way. Thank you Chris and I now have a fully functioning tachometer again - a thing of real joy to see a steady read out!

Highly recommended.

Paul
Paul R Barrow

Paul, I did he same as you did.
My tachometer, also in a 1965 MGB, read progressively higher as in car temperatures rose. It had done this since the car was about five or six years years old, but by about 2006 it would read ridiculously high.
I was told the problem was "a faulty thermistor" by an instrument repairer, but after having it rebuilt and serviced by him, it still misbehaved in exactly the same way.

I too am technically challenged, but like you found, Chris of Spiyda gave fantastic help. I'd send him photos of where I was up to and questions, and I'd have a helpful response in, at most, a few hours.
The result is absolutely perfect, with rock steady and consistent readings.

The one problem I did experience was that sometimes when switching out of overdrive, the needle would jump onto the peg at the maximum extreme of the dial. It would stay there till I switched off the ignition, and the tachometer would then work normally again.
I had an auto electrician investigate the problem. He traced it to the relay on the firewall. Mounting a small diode next to the relay solved the issue and it hasn't happened again since.
T Aczel

Good to know T Aczel as I too have the 3 synchro 4 speed overdrive transmission with the early vacuum style control. I will watch out for that but if you can maybe give details of the diode and show where and how that was fitted it will be helpful.

Thanks

Paul
Paul R Barrow

Sorry, the photos have uploaded sideways, but I think you'll be able to orientate.

The auto-electrician chose an old diode he had that looked "period".
He tried fitting the diode out of sight, but it proved that the diode needed to be mounted close to the relay.

Perhaps you may not have the same problem.
I added the overdrive gearbox in 1985, but the vacuum switch + relay only last year. The history of these components is unknown. But the auto electrician checked the functioning of the relay and vacuum switch, and they seemed OK.
The problem with the tachometer only started after I fitted the relay and vacuum switch and related parts. I did the tachometer conversion about six or seven years ago.







T Aczel

Great thank you. That will be a great help. I will watch for trouble but clearly now have an insight should it happen and I know a cure!!

Paul
Paul R Barrow

That looks like a capacitor not a diode. Used for radio supression and in the distributor.
I cannot see what it will do on the relay except to kill a spike that might affect the tacho.

Colin
Colin Parkinson

Agreed, a suppression capacitor not a diode. Also because it needed to be fitted near the relay, which is how they are fitted to things like coil, fuel pump, instrument voltage stabiliser and so on.

It would suppress a spike as said, but why that spike causes the tach to stay pegged until the ignition is turned off is down to the tach electronics. These cars don't really like added electronics as they are so 'noisy' electrically, unlike modern cars which are designed around their electronic systems. I've known of after-market electronic overdrive controls being triggered by things like brake lights and indicators. Funnily enough a couple of weeks ago I was watching a video about Bugattis such as the type 35 and 59 on long distance rallies, and the support team saying usually the only problems they had were with added electrics of the conventional kind, not even electronic.
paulh4

Thanks for the clarification Colin and Paul. So a capacitor, not a diode. (I did try to imply that I am technically, especially electrically challenged).

I do know the purpose of the, ahem, capacitor is to damp the spike that seemed to be kicked out by the relay when disengaging the overdrive.

As you imply, the original RCV1 tachometer circuitry was not bothered in this way. And the Spiyda converted tachometer was OK till I added the inhibitor circuit. I did this simply in the interest of reproducing the factory layout in these earlier cars with the 3 synch box and D-type overdrive.
Between 1985 and 2019 I had used my overdrive gearbox without the vacuum switch and relay for a considerable mileage, with absolutely no issues, including when the tachometer had been fitted with the Spiyda circuitry.

I'll be interested to hear in due course whether Paul Barrow strikes problems such as I experienced. I still have one more (new) relay and one new vacuum switch in reserve to try, gathered after years of foraging, especially for a vacuum switch. The currently fitted parts are used.
T Aczel

As you say T Aczel - hang on to those spare parts as they are hard if not impossible to come by! I have a spare relay and vacuum switch too - these are not going anywhere other than on my car if needed LOL!

T Aczel - I did try to reply to your email by email but your ISP blocked mine. Sorry about that I know less about the internet vaguries than I do about car electrics! What a wonderful world we live in!

Best wishes

Paul
Paul R Barrow

Hi again Paul
I've checked my junk mail folder and your email is not in there.
I don't understand why my server rejected your email. It obviously let me send you an email. I don't have difficulties communicating with others in the US.

Nonetheless, I'd be very interested in an update from you at some time whether you experienced] similar issues to mine with the Spiyda circuit and your overdrive circuit. My presently fitted relay and vacuum switch came from a chap from who I bought many things, from a complete MGA Mk II to more minor parts (he has since passed). Most turned out to be problematic in one way or another!
But I must say, it's been an absorbing and educational pastime sorting through the many resulting issues. After 10 years, I am now quite delighted with my MGA!
T Aczel

This thread was discussed between 10/04/2020 and 16/04/2020

MG MGB Technical index

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