1968 MGC Smiths Tachometer Flicker Diagnosis and Fix

[@ellesmerewildwood4858]

I've only just found this video so anything I say is way too late but I hope you don't mind if I add my two cents. I'm glad you realized by the end of the video that the small stabilizer only regulates the fuel gauge and that all those green wires are just a positive hub. Yes, your fuel pump was the problem but not the issue. The vehicle's main voltage regulator is either over stressed by the power draw from the fuel pump or the regulator itself is not working properly. I doubt very much that the tacho was reacting to transients in the pulse waveform from the ignition coil caused by the fuel pump. Rather that the flickering was caused by a savage drop in voltage each time the pump turned on because the main power voltage regulator couldn't keep up. When you turned on the lights this temporarily made the main regulator work a little harder to stabilize the voltage but really didn't help much. When you wired the pump directly to the battery you actually by-passed the main power regulator so the tacho stopped flickering because you effectively took the main voltage regulator out of circuit and the vehicle's voltage was once again stable. It's understandable that you wouldn't have realized what was happening but good on you for going through everything you did to try to find the problem. A relatively easy fix for any future viewers, if anyone is interested, or if you have any further issues, is to remove the tacho from the vehicle's voltage line and run it through a 9 volt regulator IC (LM7809). Although the tacho runs on 12 volts it will still happily run on 9 volts from the LM7809. (google LM 7809 circuit diagram) there are plenty of diagrams out there, they are very easy and the chip itself is dirt cheap. The LM 7809 will provide a steady stabilized 9 volts to the tacho regardless of any power fluctuations in the vehicle's electrics. Just remember, the ignition coil connection remains where it is, the LM7809 only supplies a clean stabilized steady positive 9 volts to the tacho. Hope that helps anyone. But like I said, kudos to you for all the effort you put in.

[@JRsGarage]

Oh my, thank you so much for this. As you may have guessed, I never really fixed the problem as I just kind of cleaned it up a bit by installing new wires and connections. And yes that did have a noticeable improvement on the Tach, but, as you stated, was not my problem. I still have a flicker that is tolerable but not desired. If I understand correctly the LM7809 isn't actually addressing the problem either but masking it. Would I be correct in saying that I should probably replace my alternator. I do not have an external regulator (that I have noticed) and must surmise that my alternator is internally regulated. I have wanted to change it out for quite a while now due to low output at idle and the appearance of the ignition light at times. Thank you again for your detailed explanation. It really helps. -JR

[@ellesmerewildwood4858]

@@JRsGarage No worries, my comment was based on the assump[tion that your car was using an external regulator so an the internal alternator regulator does change things a bit. However, yes your internal regulator can also be having problems, it simply may not be rated high enough to cope with the power draw from the pump. Perhaps the pump itself is faulty and drawing far more power than it should. And yes, the LM7809 is simply providing a clean power supply to the tacho, masking the underlying problem. And since you've sent the tacho to be re-furbished I also have to assume that they did a proper job on it. If you know someone who has an older car with NEGATIVE EARTH with an old school ignition system you can also connect your tacho to any other car that uses a standard ignition coil (not modern computer controlled cars) If it works okay in another car then this will remove the tacho itself from the equation. I would get the fuel pump tested, make sure it's drawing the right amount of current, if so, check the voltage regulator in the alternator or the alternator itself. These are expensive items so be sure they are either okay or faulty before taking any action. However, if you choose to leave things as they are, the LM7809 will supply a clean power to the tacho. There is also an LM7810 (10 volt regulator) out there but all of these regulator chips need a higher input voltage (minimum input voltage) than the output voltage in order to work. For example, the 9 volt will have an input threshold voltage threshold of about 11 volts in order to maintain 9 volts output. The 10 volt regulator needs around 12 volts minimum input voltage to hold 10 volts output. So, if you want you can use the LM7810 for an output of 10 volts. Just keep in mind that if your vehicle voltage drops below 12ish volts the chip will shut down until it's input minimum voltage is back on again. With engine running the alternation should be putting out over 13.8 volts anyway, so as long as your vehicle running voltage is about 2 volts above the desired chip regulator output the LM7810 will work perfectly. I didn't suggest LM7810 because it's a little harder to find, as far as I can see it's only available on line, they're still cheap , but only available on line. This circuit is so cheap it's worth trying it. If it fixes your problem, happy days, if it does, nothing's been lost. Anyway, good luck with it. :o) and keep in touch. Oh, and if you're having trouble finding the chip or if you're a bit unsure about building the circuit yourself, I have a truck load of 10 volt chips at home. I can build you the circuit and I'd be happy to post it to you. But trust me, it's easy to build. Good luck.

[@paulmuff9883]

A very interesting video with great information, many thanks 🙏🏻

[@JRsGarage]

My pleasure.

[@jasonsutton5107]

Great video that has helped me to understand an issue im having with my Tacho and how to diagnose it.

[@JRsGarage]

Happy to help.

[@dr9974]

Thanks for the reply... my symptoms are pretty much the same as what described.

[@chriswhellams8187]

The well known ”C2” issue with Smiths RVI gauges. The C2 capacitor dries out over the years and becomes ineffective. Numerous other components also commonly fail such as the thermistor , other capacitors or the transistors. It’s possible to replace/repair everything but the circuit boards themselves also tend to de-laminate when worked on. In the end it’s often easier to simply replace the guts with modern electronics e.g.the board from Spiyda

[@JRsGarage]

Thanks for the information.

[@hansjurgenschwartz3578]

You tackled a classic EMI issue: Elektromagnetic Interference. Keep in mind that the speedometer just counts the pulses in the current of the ignition coil caused by the distributor points. The fuel pump sucks a pulsing feeding current instead of a smooth DC current, too. Caused by the points in the SU pump. As result your speedometer counted the ticks of the SU pump in parallel. There are two possible causes for Interference : inductive or ohmic. I cannot finally judge which one had the biggest impact in your case. My gutt feeling says "ohmic".

[@JRsGarage]

+Hans Jürgen Schwartz Thank you for the comment. I assume you meant tachometer when you said speedometer. I'm sure the same reason can apply to my non-SU fuel pump which must draw more and less current in a rhythmic fashion as it operates.

[@Martin_in_Cheltenham]

Thank you for this. I am about to start looking at why my tacho doesn’t work at all and I think it is due to some electrical issue like you say. I was recommended to send the rev counter to JDO so interested to hear you send yours to them. Did they do any report on yours to say what was wrong- I sense nothing was wrong with your tacho. Many thanks again.

[@JRsGarage]

No, nothing wrong specifically that they reported. I still occasionally have the issue, much less and usually only when the car is fully warmed up.

[@Martin_in_Cheltenham]

Thanks for the swift response

[@oldbab]

Same thing (erratic behaviour) with my Kubota Tractor MX5100 !...But looks much more complex, here.

[@StuartCarter-g2r]

I had exactly the same symptoms and inconsistent needle. Tried new taco, isolated fuel pump as with this video etc, still didn’t solve. The issue was the wiring onto the voltage regulator. Swapped the all green wires onto the ‘B’ terminal and bingo! So much time wasted without looking at the basics first.

[@JRsGarage]

Happy you got it sorted out.

[@pugwonk]

Really interesting, thank you. But: 20:24 - my gauges light up dimly yellow. What sweet magic have you done to yours?

[@JRsGarage]

I replaced all of the bulbs with LED's. I have a video on that too: kzbin.info/www/bejne/l6rXc4eapMufiqc

[@wyattfigueiredo8153]

Great video

[@JRsGarage]

Thanks Wyatt.

[@davidyoung5359]

Hi, what was the name of the firm who refurbished the rev counter in the UK please?

[@JRsGarage]

JDO Instruments. Link is in the description.

[@DRM4393]

So are you saying to run a new white wire directly from the fuel pump to the fuze box? What happens when you turn the key off, does the fuel pump continue to run because you have bypassed the ignition, have you not? Thanks

[@JRsGarage]

If you run it to the white/green side of the fuse box it will turn on and off with the ignition switch. Half the fuse box is switched and the other half is constant hot. This will only fix your flickering tach if the fuel pump is the source of the interference.

[@wallisparnell4464]

Wonder, how much do there charge you to clean up and go through diagnostic on the tach?

[@JRsGarage]

It was 150$ US equivalent at the time, I can not recall what the GBP amount was.

[@wyattfigueiredo8153]

And I subbed

[@JRsGarage]

Thanks for the sub, can't believe I missed this comment. I try to reply to all of them.