MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

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 - pulsing tacho

When I switch on the ignition in my '68 B the tacho swings to full scale. I've noticed it only does this if the points are closed. Also when the car is running any electrical device will cause the tacho to pulse when it is switched on if it coincides with a points closure. Putting the indicators on for instance will only cause this pulse intermittantly if it coincides with points closure. I have earthed the tacho well. Any suggestions as to what this may be. It's a '68 car but with a modern alternator conversion, i.e. it was fitted with an alternator that originally had an external control box.
Steve Church

Steve, it is possible for the tacho to flick when the ign is turned on as the current flow through the points and the coil passes through the tacho sensing coil. Full scale seems a bit high but who knows?

Have you done any alterations to the wiring that would cause other circuits, such as the indicators, to be fed through the tacho? Does this happen when the lights are switched on, as this circuit is not fed via the ign switch.

Tony
Tony Oliver

I've done no mods since I've had the car. I haven't checked the current through the tacho when things get switched. maybe something to look at.
Yes the tacho pulses when the lights are switched on. In fact it goes to full scale and then stays there even after the lights are turned off. It only goes back to zero when I turn the ignition off. This is true for whatever I turn on. Tacho goes to FS and stays there. Very odd.
Steve Church

Try disconnecting the white from the coil and seeing what happens then. If the effects cease then it would appear to be pulses through the coil causing it, and seeing as how it only happens when the points are closed it would seem a fair bet. Measure the voltage as various points around the car using the battery earth as the reference point, like the car body, distributor body, fusebox and write down all the readings.
PaulH Solihull

Steve. The theory of operation is that it is only a change in the current through the tacho sensor that makes the needle deflect. If turning the lights on sends the needle full scale until the ign is turned off is something I am unable to explain. Once the current becomes constant the needle should drop to zero. I'll have a think about it in the morning as it is near midnight over here.

Tony
Tony Oliver

Hi Steve,

My thoughts are that the 0.25 capacitor in the electronics has failed, and once the tacho is triggered, by any current hiccup, it stays on.

Herb

Herb Adler

I think Herb is on the money here. I couldn't narrow it down to one component but I am sure it is a tacho fault. Unfortunately the easiest way to prove this is to try another tacho but who has a spare one of those in the shed?

If you have any electronic knowledge, or know someone who does, pull it out and have all the components checked.

You will find a few on evilBay.

Tony
Tony Oliver

Tony, Herb, many thanks for the feedback and also the circuit diagram. I've spent 35 years in the electronics trade so should be able to source a 0.22uF cap somewhere. I haven't seen a circuit with germanium transistors in it for years! I have a road run around the churches of Eastern England on Saturday so will check the problem out after that.
Steve Church

Steve, you should be able to find a few "modern" transistors to replace the old ones. If you are going to attack it with a soldering iron you may as well update it a bit to more reliable and stable components.

Good luck.

Tony
Tony Oliver

I can't see it being a component inside the tach because it *should* be the type with inductive pickup on the back. As Tony says it is only a current pulse through the trigger wire that gets into the electronics, steady-state does nothing regardless of whether the points are open or closed, and apparently this problem only manifests itself when the points are closed. It could be an internal component if it was a marginal voltage level issue i.e. with the coil energised and something else on the voltage drops just enough to cause it, but then it would only happen with the engine stopped. Presumably the system voltage rises from about 12v to more like 14v when the engine is running, which should prevent a fault like that from happening.

That would be for an RVI (printed on the dial) tach though used up to 72, if it has been retro-fitted with the later voltage triggered RVC type used from 73 it would be a different matter.

So, Steve, does it say RVI or RVC on the front?

PaulH Solihull

Paul, I see your point about the behaviour when the engine is running but I can't for the life of me think of anything else that would cause the problem. The engine running condition is a mystery. I guess it would still be possible for the current in the sensing loop to drop slightly with the engine on, (lights and other bits draw a heavy current for a few milliseconds on switch on) and thus trigger an extremely sensitive trigger circuit, but then it should go to full scale and stay there just as it does with the engine not running.

For the tacho to go to full scale when the lights are switched on and stay there until the ign is switched of is a sure indication that the tacho internals have "locked" in the meter on position.

I can explain what happens (in theory only) with the engine not running, it is a common fault with the type of circuit used.

Tony
Tony Oliver

Well, I took the tacho to bits this morning and it's not like the circuit diagram that Herb supplied.
Have a look. Number RVI 2340/00 on the front. Seems to be one tranny, two resistors, a pot and what could be a cap or diode (it is marked + at one end). The coil has three take offs on the secondary side.

Steve Church

Another view.

Steve Church

And another. Also forgot to add there's a thermistor too.

Steve Church

Steve, it looks like you have the later version that is voltage triggered not current triggered. The Current type had a loop of wire held in a plastic mount on the rear of the case. That wire ran from the ign switch to the coil. It induced a voltage into a coil mounted on the cct board to trigger the one shot. Yours will have a wire from the points side of the coil to the tacho
I have never seen the insides of the type you have and I can't recall ever seeing a cct diagram for one, but I can't imagine how a one shot works with only one transistor.

It does tend to completely destroy all the theories I had worked out as to why it behaves as it does as all my theories needed the current trigger tacho.

Back to the think tank.

Tony
Tony Oliver

That will teach me to write before researching. Found this

http://www.mgexperience.net/phorum/file.php?1,file=85273,filename=MGB-tach-revA.pdf

I am guessing it is the type you have. Now to try and work out why it goes full scale.

Tony
Tony Oliver

Good lord. The link won't work.

Try

http://www.mgexperience.net/phorum/read.php?1,1402150

and click on the attachment in the first post.
Tony Oliver

Tony,
Many thanks for that, it looks like the one I have. I certainly have a 50R pot and 39R resistor in there somewhere. So that polarised device is an electrolytic cap.
Steve Church

My pleasure Steve. I have learnt something new, a one shot that only uses 1 transistor! The explanation seems convincing enough. Also it is a current sensing cct. The back of the case threw me off as I thought it only had the one terminal, but I see now that it has 2.

Now the only problem is why your tacho behaves as it does. More reading of the theory of operation is required.

Tony
Tony Oliver

Hi Steve,

If you log on the MGB forum at MG experience and insert the following into the search field you will find the thread.

MGB-tach-revA.pdf

The link tells it well.
My only concern with the description is the purpose of the 2.5uF capacitor, for a couple of reasons. Electrolytics, at that time , were notoriously variable in value, something like -10% to +100%, plus as they aged their value decreased, due to drying out of the electrolyte, so it would be a bad choice for a timing element. Further the voltage across it is clamped by the base emitter, to the transistor's turn on voltage,which means that it would not be able to supply any further drive to the transistor when the voltage from the transformer dies.
My guess is that it is meant to quench short spurious pulses from turning on the transistor.

Back to your issue.

As the problem only happens when the points are closed, the circuit is being triggered by a current pulse, as it should. As far as I can see the only thing in the circuit that could then keep the meter at full scale is the transistor. It may well be faulty, and latch once it is turned on.

Herb

Herb Adler

Aaaah, the vagaries of electronics. I agree with all you say Herb but the thing that threw me from the start, and still does, is that when the engine is running the tacho "pulses" when, for instance, the indicators are operating. But when the engine is not running the meter goes full scale if the lights are turned on. I was trying to work out why this would happen and had almost convinced myself that I could explain it, and then the circuit changed! I had the same thought as you, that the transistor (in either cct) that turns the meter on was latching in the on state, but then under certain conditions it doesn't.

Most confusing, but my advice to Steve would be change the transistor and see what happens. It is by far the most likely cause.

Tony
Tony Oliver

Thanks for the comments gents. I had a car run today in the rain so had to use the wipers. You should have seen the tacho needle! It looked like it was having an epileptic fit. I shall have a look at various voltage drops that may affect the current through the meter. Now that I have repaired my 'scope (not a good idea to drop one) I can see a bit more what is going on. Also check any connections. I found a bad bullet connection in the horn circuit yesterday so likely there may be more. In the mean time I'll look up a likely replacement for the tranny. I also repeaired a friends DAB radio recently which failed due to faulty electrolytic smoothing caps so maybe I'll look at replacing the cap also, especially if it is meant to suppress any spurious noise.
Steve Church

Steve's first image *is* of the earlier current-pulse tach, you can see the white wires coming from male and female bullets and going to an internal pickup. There were at least three configurations of RVI pick-up - external where a loop of white from the main harness was routed through, external with a short length of white with two male bullets, and Steve's.
PaulH Solihull

Solved. I changed the 2.5uF capacitor and the pulsing has stopped. I'm not sure why but presumably the cap stopped spurious pulses. Hopefully this will help any others with this problem. Thanks for the input guys and especially the circuit diagrams.
Steve Church

This thread was discussed between 02/08/2011 and 14/08/2011

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now