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MG MGB Technical - Tachometer connections

Posting this here because I have an electric tach on my MGA. (Austin A60 block, no mechanical take off). The speedo has always read zero, and I am now getting round to sorting it out. It is neg ground, there is power to the unit and I'm thinking the pulse sensor may be wound the wrong way.
I I thought I would check with the experts before I cut into my wiring the reverse the leads.
Art Pearse

Art, your tacho will be either an RVI or RVC, it's marked on the face. I'm assuming it's an RVI as you mention the connections possibly being reversed. There are two types of RVI tacho, one has an internal coil the other has an external coil. If it's the internal coil, the connections to the tacho are 2 wires, one has a plug and the other a socket, you could simply use a couple of short wires with crock clips to reverse the connections as a temporary test. If you have an external coil you could simply wind it in the opposite direction around the plastic former.

Are you running points or an electronic distributor? If it's the latter, that could be causing your problem.

If it's still dead, it could simply be a dead tach. The circuitry of these isn't too complicated but it can be a bit of a challenge if you're not familiar with them. I repair these but you're on the wrong side of the pond and I think postage would be prohibitive. There are modules you can buy to replace the electronics if need be.

Bob
R.A Davis

I presume the original ignition feed to the coil SW/+ve was removed and that via the tach pickup substituted? If the engine starts and runs with the tach wires disconencted then there is another feed to the coil that is bypassing the tach.

For info as far as I'm aware only positive earth tachs had the external pickup, and for the MGB at least the only 4" -ve earth RVI tach was the UK RVI/2430/00.
paulh4

OK, it's an RV1/2401/01 with external pickup. I cut into the contact breaker feed and inserted the pickup feed wires. Normal ignition system
I got it when I swapped my mechanical tach for this one which he said was converted to -ve ground.
If the pulse wires are backwards, does that cause a zero reading?
Art Pearse

Yes it can do
R.A Davis

OK gonna reverse the wires!
Art Pearse

Still no reading! Power wire is +12V, ground is good.
What next?
Art
Art Pearse

So just to confirm, the impulse loop to the tach carries the current that the CB sees. I did not rewind the sensor at the tach, just reversed the wires where they cut in to the CB line.
Art Pearse

Art, if I'm understanding your wiring correctly, you've got the loop for the tach wired in between the coil and distributor/CB? If so I'm not sure that would work. The normal way these would be connected is for the loop to be between the supply and and the coil, see picture. Bob


R.A Davis

Bob, surely the current in the loop is the same? The voltage to ground would be higher, but the sensor is only reacting to the current dynamics?
I watched a youtube by Derek Twist on changing the unit from + to - ground, and I think I will un-can it and check which way it actually is. Just 2 wires are switched apparently.
Art
Art Pearse

This is the loop


Art Pearse

Just had it apart, The power terminal connects internally to a green wire. According to John Twist, that is a neg ground unit.
Art Pearse

It might just be the photo but that loop looks a bit tight, might be nothing but just in case see 'Image 1' below -
"CHECKING FOR FAULTS Impulse Tachometer (all Types). Check that the iron core and pulse lead assembly is as shown in 36. The pulse lead should form a symmetrical loop and should not be tight enough to pull the plastic former out of line, as this may well cause a poor electrical connection at the iron core. Poor connection may well result in intermittent operation of the tachometer."

For 'Image 2' -
"35 CHECKING FOR FAULTS Impulse Tachometer (Original Equipment). Check the connections to the tachometer (35). Also check wiring."

HTH.








Nigel Atkins

Again it might just be the photo or the way the wires are crossed but your loop looks to go in a different direction.






Nigel Atkins

RVI works just as well between coil and CB as in the conventional position between ign sw and coil
Paul Walbran

Nigel, could it be my loop is wound too tight? To me it is strange that it is wound on a plastic former, not round an iron core. Does the bigger loop "cover" the area where the core is?
And doesn't the iron can screen the loop e/m field from what is inside the can?
Is the sensor core just the thin strip of iron that surrounds the plastic bit?
Having had it apart I also realise I need to refresh the white paint around the inside of the can and the retainer ring so I can see the tach at night!

Paul - thanks.

Art Pearse

The loop is only shown loose in the drawing for clarity, it's normal to have it tight as in the image.

The metal band and the screw form one half of a transformer 'former', the half being inside the case with the secondary winding.

The unknown here is that the RVI/2401 was originally a positive earth tach and should say so on the front, you only have the word of someone else that it has been converted, and successfuly, even if you can see the internal power connections to the circuit board have been reversed.

With the tach out rotate it with a sharp flick of the wrist and see if you can get any movement of the needle, i.e. it's not stuck. Another problem with tachs is that they can fail to register possibly from bad connections as well as a stuck movement until given a sharp rap with a knuckle with the engine running.

Then, if you are sure you tried the direction of current flow through the pickup both ways, then with the tach out of the dash i.e. insulated from the car body I would try reversing the power connections, with the pickup wired first one way then the other, and see if one if the four possibilities works.

If it HAS been converted, and you HAVE tried the pickup wired both ways, then the tach is obviously faulty so you have little to lose.



paulh4

Art,
sorry I don't know technical details (of anything) I put the drawing up out of the Smith's care handbook as the loop is stated and shown to be symmetrical and is shown by the arrows to have a define routing which to my eyes varies to the loop and direction that's on yours.

I was hoping others might use the information and drawings to give more details.

To me there's nothing to say whether the 'Red Marker' or 'Black Marker' is the top part of the loop, nor do I know if it matters, Smith's seem very economical with the information in the booklet.

I can't see behind my dash to the rev counter very well and sorry there's no way I'm removing the unit to look for you as it's all a tight restricted jumble behind there giving to much swearing and potential to upset something else behind there as well as me.

I expect you've already done the obvious and checked all wires and connections are clean, secure and protected. Only the other week I was cleaning up connections (bullets mainly) and it was the ones I hadn't expected that needed doing or replacing, big improvement just by cleaning and protecting (and re-securing).
Nigel Atkins

Paul, the needle is definitely free.
There is a label on the can from an instrument shop in Sepulveda California, and some scratchings on the rear indicating it was neg ground, plus a few numbers that don't mean anything to me. I have lost the contact details of the guy I did the swap with, but he was a regular on this bbs.
I notice on your photo the metal band surrounding the loop is different to mine.
Art Pearse

Apologies Art, Paul,
I started my previous post a good while before submitting as I was interrupted and drawn away and didn't see Pauls post so much of it is out of context.

Good point about giving a sharp rap with a knuckle with the engine running - I always have to do that now with mine. I can't remember when I first needed to do this but at one period I no longer needed to do it, for the fault to return a while later and remain. I've no idea what changed to have it working as I didn't record the points at which it started and stopped working again to see what work I'd done or parts replaced from my records as I was just glad it was working and for some strange reason never considered about it stopping again.

ETA: could the differences in bands just be a difference in when the dials were made and changes to supply or make of components(?)

Nigel Atkins

Art, you're correct the current in the circuit would be the same wherever you measure it, if it was a resistive circuit. This is an inductive circuit and although the same rule may apply I was just suggesting things that could cause the problem. As Paul W has stated it will work in either position (I'm assuming from experience) then I accept that's the case.

I think the problem is you have too many variables, has the conversion to negative earth been done correctly, is the tach actually working, is the loop wound the correct way, (and although dismissed) is the tach in the correct place in the circuit.

The band around the coil in your photo is what I would normally expect to see. It's the band that completes the magnetic circuit for the external coil and interlocks with a similar band on the internal coil

Bob
R.A Davis

Going back a couple of days disconnect the wires that go to the tach and if the engine still runs it is being bypassed somehow. That's quite easy to get when the tach is in the coil feed as they were originally, but if it is in the points wire then it should be more obvious. Nevertheless I would still disconnect it to prove the point - you know what they say when you assume something.

Ideally disconnect the two wires at the tach pickup, but if it is a continuous length of wire up from the coil -ve, through the pick up and back down to the points that would mean cutting the wire.

The good tach will work either side of the coil as long as coil current pulses are passing through the pickup, and in the correct direction.

Incidentally I can't see from Art's photo which wire is which, so I cannot tell which way the current is flowing.
paulh4

Current direction in the loop can certsinly be an issue, if you havent already tried (sorry, rushed at present and havent checked all the posts) it then reverse the current direction.
Paul Walbran

Paulh4, the loop is definitely carrying the coil current, just by inspection of the wires I ran.
My next move is to replace one of the condensers in the tacho, as per recommendation I found on line. Cheap and easy to do. I looked on ebay for replacements but did not see anything I liked.
Art Pearse

Our dear 'he who can not be named' has put up some posts, if not a thread on this, these are appropriate photos I hope.






Nigel Atkins

Art, I hope replacing the capacitor solves your problem but I generally find that rather than a complete failure the capacitors just go out of spec. This causes the tacho to still give a reading but not accurate.
R.A Davis

Well, we will see if a $2 fix works! (+$8 shipping!!!)






























Art Pearse

Well I replaced the capacitor but it was OK anyway.
But I found the Zener diode faulty, so that's next!
Art
Art Pearse

It'll be like 'Trigger's Broom' at the end :o)

http://foolsandhorses.weebly.com/triggers-broom.html
paulh4

Art, take a look at this site, it's about Smiths tacho's in Volvo P1800's but they all use the same circuit.

https://www.sw-em.com/Smith%27s%20Tachometer.htm

I'm not sure why this link won't work straight from here but if you copy and paste it into your browser it does work

Bob
R.A Davis

Thanks Bob. I had the circuit, but the explanation is good!
Art Pearse

Interestingly that diagram does show the tach between the coil and the points instead of between the ignition switch and the coil as on the MGB, confirming (if that were needed) it is a valid method of connection.

FWIW the link doesn't work when clicked due to the embedded apostrophe being represented in code, that and everything after it gets 'lost'.
paulh4

Does the MGB have a voltage regulator for the instruments? The MGA does not, and this tach came from a B.
Art
Art Pearse

Later models have a voltage regulator but not in the supply for the tach.
R.A Davis

There is a voltage regulator for the alternator and dynamo, plus an instrument voltage stabiliser from 1964 for the fuel gauge, and later electric temp gauge.

The confusion concerning the tach is that various years have the 12v supply for the tach (and other circuits) daisy-chained off the supply to the stabiliser. But that is only a branching point, the stabiliser does not feed voltage to them. Just as well as the original stabiliser 'stabilised' by switching 12v on and off about once per second!
paulh4

Well, after changing capacitor C2 and the Zener diode, which was definitely bad, it now works!
Had to adjust the pot to get a sensible idle reading, now to calibrate better. Maybe just by roadspeed in top gear, except my speedo wobbles a lot.
Art
Art Pearse

You can calibrate the tach to at least one point if not two using an old-style AC battery charger http://www.mgb-stuff.org.uk/tachtest.htm#1
paulh4

Paul, I tried that originally, but the unit wasn't working then and I thought it was the method!
Had it on the road and it seems "reasonable" vs roadspeed in top gear. I downloaded a speedo app to my phone (GPS). I will find a passenger to do this safely.
I think you can only calibrate it to one point, there is just the one potentiometer?
Art
Art Pearse

Art,
just a warning, my mate downloaded two different 'free' apps to his phone to test my car and they were inaccurate, I could immediately tell as I'd got so used to driving off my rev counter.

I then borrowed a TwatNav off another mate who confirmed it's accuracy against two other devices and that worked fine, and confirmed the two 'free' apps were wrong.
Nigel Atkins

Art,
I've just thought, if you want, if you can give me your tyre size and your diff ratio I can put it into a calculator someone gave me and tell you what revs at what road speed quite precisely (for 4th gear being direct, or with other gear ratios can do the other gears too). I also allow 3% for tyre rolling circumference, speed rounded to one decimal place which should be fine for TwatNav comparison and too fine for a wobbly speedo needle. There are nit-picking very minor variations that could be thrown in but they'd be too refined for what's required here.

With tyre and diff figures and the calculator it's simple maths to go to work out whatever speed or revs.

Nigel Atkins

Nigel, tyres are 165R15 (I think that's 80 profile but it doesnt state, and 4.3 axle.
Art
They are Continental.
Art Pearse

Art,
sorry missed your reply until now.

Couldn't find 165R15 on Continental for Canada (how old are your tyres they don't show width).

So if we take 80 as aspect ratio the calculator gives a nice crisp 17 mph/1,000 rpm, which Google says is 27.3588 kph.

If your tyres aren't 80 then we can rework it or if you need conformation of other speeds at rpm or other way just ask.

Modern tyre wall look something like below.



Nigel Atkins

I got them 21 years ago in Mobile AL. They are sized for VW Beetles.
Yes I know they are old, but most of the time they have not seen daylight. Only done about 6000 miles
since the resto completed.
Art Pearse

21 years ago I'd have still thought the size would have shown as 165/80 R15 even for the air-cooled Beetle but I don't know American tires, think we can put a small bet on them being 80s, but you can measure them to be sure.

As I put let me know if you need more or different figures.

When my tacho was checked a number of years back it was 100 (and IIRC 200) out but not consistently across the whole range and not linearly, yours may be more accurate with fresh parts.

For the tyres if they've been stored correctly they may well be useable but I'm not sure if they'd give their best performance now. I'd changed them for new and accept their 6k-mile use as good test bedding for the rest of the car. I think with a half decent set of new tyres after the first 100 (dry) miles of degreasing the new tyres you'd notice the improvement in braking, (steering perhaps), (suspension feel perhaps), handling, ride comfort and noise.

Only last week I was helping a neighbour with a 20 year old low mileage car and the rear tyres were original, confirmed by manufacture date on them, they were crazed and splitting, they might have lasted hundreds of miles more or split at the next decent pothole, and we have plenty of them.

For safety's sake I'll put up my usual photos of barely used 27 years old tyres that MGB owner R G Everitt originally posted as a warning for others.






Nigel Atkins

21 years? Blimey! Elsewhere people panic about 10 years and 'gold-platers' are vying with each other as to who can come up with the shortest change interval.
paulh4

Thing is there are so many variables. I think things might be a bit more certain now and in the future as the 5 year old tyres I have look older and did so after about 2 years, reminded me of the aged look on a couple of different sets of Pirelli tyres I had about 20 years ago.

If it wasn't for difficulties with other modern made parts I'd have changed my present 5 year old tyres by now as despite there being plenty of tread life left in them they're well passed their best and that matters if you're going to drive the vehicle as intended. Lack of use and the car being permanently outside hasn't helped my tyres.

Nigel Atkins

Art,
putting my last post about performance to vehicle type reminded me, if your 21 year old tyres were for a VW air-cooled Beetle then that might suggest the tyres even when new were more suited to a steady going sedan rather than a sports car.

That's the problem I have with the size of tyre I want as they're normally fitted to very small modern family city cars, that still weight a lot more than the midget without adding twice the number of occupants.

I did once have a good set of tyres on the Midget and what a difference, I didn't realise how good they were until I couldn't get them as replacements and had to change. Unfortunately the size I wanted was one of the first to be discontinued. As I use the car so little now I don't need to worry about tread wear mileage but being a light, not very powerful car only used on the roads tread wear would be too bad anyway.
Nigel Atkins

I seem to be missing something here. Even with known tire size and aspect ration, the rolling diameter will change at least some depending on air pressure. If the goal is to calibrate the tach, wouldn't using a timing light with a built in tach to compare against the car's tach be simpler? it'd probably take two people unless the leads on the timing light are long enough to reach to the driver's seat.

Just musing.

Jud
J K Chapin

Jud,
depends what equipment, and other body you can command, you have at your disposal, and also the accuracy of both, DIY stuff and helpers can have wide margins of precision, accuracy and reliability.

The road running, tyres and GPS are about conformation of connecting a battery charger to the tacho and twiddling it.

We've allowed for air pressure, balanced it out against atmospheric pressure and chunks of tread departing the 21yo tyres.

Were these tachometers that spot on precise and accurate brand new, I don't know, I was still in short trousers.

I know mine was 100 or 200 out when tested but at various points of the range and not linear, I have to tap the glass to wake it up and any (relatively, I've got a Ford T9 box they're not the fastest) quick gear changes will have the needle bouncing around.

Glance accuracy is all I need, I can't take my eyes off the road for too long or I'd forget where the car was heading.
Nigel Atkins

Jud - if you have another tach then yes that is obviously the easiest way of checking the cars tach.

Failing that calibrate it against the speedo using the factory gearing information.

And if you are not sure about the speedo, then check that against a sat-nav.

I pinged the speedo needle off when repairing it and put that back at 30 mph with the rear wheels off the ground and the throttle held at the appropriate tach reading. Subsequently checking against roadside speed advisors and sat-nav it's spot on, so probably more accurate than it was originally, and the tach must be pretty accurate as well.
paulh4

I borrowed a dwell meter with a tach reading, got it spot on at 2k.
Art Pearse

Well done.

Does that mean it's OK at 3,4,5k, surely 2k is only something you see when passing, that's saloon territory.
Nigel Atkins

the dwell only went to 2400
but seems proportional to car speed.
Good enough!
And it agreed at 600 too.
Art Pearse

Art,
well done.
Nigel Atkins

For others, bit late now for Art, this has been posted o M&S.

Attached PDF -
A Gentleman's Guide to Smiths Electronic Tachometers by Alex Miller

With lots of useful info including - Checking an electronic tachometer, wiring a lots more.


Nigel Atkins

And for those that would soil their vehicle with an electronic ignition (oh, the shame) as well as the info and link in the previous pdf here's the link to pertronix on troubleshooting Smiths electronic tachs when fitting their Ignitor.
Nigel Atkins

The Pertronix one has been superceeded by powering the electronics from the white at the fusebox instead of the white at the coil. It's the extra current from the electronics passing through the tach pickup that causes the problem - hence reducing the pickup to half a turn. It's a bit hit and miss, powering the module from elsewhere removes that extra current altogether.
paulh4

Interesting Paul, considering PerTronix claim to have sold over 4m (of something) the general information circulating, er, generally, has often been they don't work on RVI. Aldon even said so in 1998, their unit or anyone else's.

I'll pass your info on, thanks, it's not for me I have RVC.
Nigel Atkins

This thread was discussed between 23/04/2021 and 11/05/2021

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