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MG Midget and Sprite Technical - how do you test a tacho ?

as it says really. my tacho isn't working and i wonder how you test such a unit. Nothing has changed to stop it, the set up has been the same for years, so i don't think its anything other than age !
The wiring loom was changed about 8 years back, but it has worked fine since.
If its a send it away job, anyone have any experience of who, where and how much ??
72-1275-Mk3
P Bentley

as has been mentioned lots of times on here previously,
RICHARD JENKINS, UNIT 49H, PIPERS ROAD, PARK FARM INDUSTRIAL ESTATE, REDDITCH, B98 OHU 07747 357044
does a very good and reasonably priced job.
David Smith

yep...Richard's workshop is just round the corner from me. Top notch service....
David Cox

P Bentley
If you have a simple battery charger (non electronic) it will simulate ignition pulses. By connecting one lead from the battery charger to one side of the current loop on the tach and the other lead VIA A HIGH WATTAGE 12V bulb OR 3 to 4 OHM RESISTOR to other side of current loop, whilst also having a battery supply to the tach, then it should register 1500 or 3000 rpm depending on charger type.
Bill Bretherton

cheers guys
P Bentley

The most likely cause of failure is a dead capacitor, I’ve also seen the variable resistors fail and broken wires. If you get stuck I can fix these and calibrate them.
R.A Davis

R.A.Davis, many thanks for your offer of help. I will be in touch. . .
P Bentley

R.A. Davies
How do you trigger the tach? I've replaced several components in mine and it works via a battery charger (to simulate ignition pulses) connected to the external loop. I've tried using a signal generator but I presume the current is insufficient (I reckon the ignition current via points is 3 or 4 amps). Do you connect a signal directly to the circuit i.e. to trigger the transistor?
Bill Bretherton

I’ve got a test jig that I initially built to test distributors, I use it to measure advance curves and dwell. A by-product of this is that I can also check tacho’s under realistic and controlled conditions.

It allows me to use conventional points or an electronic version to cover all options.

I power the distributor from a battery and use a coil as the load, I’ve got a breakout point that I connect RVI tacho’s to which I bypass and connect directly to the coil for RVC tacho’s. It also allows me to check positive and negative earth units.

I can spin it up from zero to 6000rpm and I use a conventional tacho for an indication and an electronic device for more accurate measurements.

Bob



R.A Davis

Thanks Bob, that's quite impressive! Of course, the electronic frequency counter is needed as an accurate reference.
Bill Bretherton

I have been trying unsuccessfully for the last three weeks to reach Richard Jenkins by email, and phone. Is he still in business?
Dominic Clancy

I know he has been away, what email address are you using?
Nigel Atkins

woodykeys67@msn.com and the phone number 07747 357044
Dominic Clancy

I got a reply on that email address last week, I don't know about the mobile number as I avoid using mobiles where I can as for over 30 years I've found them and the system a nuisance and unreliable especially when you least want them to be.
Nigel Atkins

Nigel, could you please drop him a note that I am trying to reach him and would be really happy if he could email me back

Thanks
Dominic
Dominic Clancy

Dominic, done.
Nigel Atkins

Thanks Bill/Bob
I built a pulse generator for testing electronic distributors a few years ago but had never thought of using it to test drive a tacho----now all I have to do is find it--lol
willy
William Revit

Thanks Nigel, that unblocked things.
Dominic Clancy

No problem, glad it helped and thank you for saying so.

Not everyone can be blessed with my charm and good looks. :)


. . . oh, and, er, modesty.
Nigel Atkins

The battery charger 'calibration' came as a surprise when I was reassembling the Minor after a total respray 3 years ago.

As a precaution with a newly rewoven loom, I was using the battery charger, instead of the battery, to check all the circuits out, and was very surprised when turning on the ignition to see the rev counter reading 3000rpm!

Picture attached.

Richard Jenkins - he rebuilt the dual oil/water gauge for me a few weeks ago, so very much still in business.

Richard


Richard Wale

Richard,

Battery chargers are in my experience, older ones at least, made down to a price and while they put out about 13.8V to charge the battery OK they have no smoothing capacitors or inductors, only rectification, so the tachometer is seeing the mains frequency and 3000 RPM makes sense. The synchronous speed of a 2 pole induction motor is 3000 RPM at 50 Hz and a 4 pot 4 stroke engine has 2 firing events per revolution.
David Billington

David, actually the tach is seeing 100Hz or double mains frequency because the charger will have a full wave rectifier which rectifies every half cycle and there are 100 half cycles in 50Hz. 100 Hz becomes 6000 pulses per minute which, as you say, is equivalent to 3000 rpm in a 4 cylinder engine (which the tach is designed for of course). If the charger only had a half wave rectifier (unusual) then 1500 rpm would be displayed by the tach.
Bill Bretherton

Hi Guys,
new to this BBS (midget MX5 5speed conversion)
but see your discussion. FYI I use this little optical tacho fro my work and it works great even though it is so cheap. <10 if you can wait for delivery

https://www.ebay.co.uk/itm/Rpm-Speed-Kit-Small-Engine-Non-Contact-Hand-Held-Digital-Laser-Photo-Tachometer/123179395574?hash=item1cae1069f6:g:A7IAAOSwLs9bGhw1:rk:1:pf:0

or just search non contact tachometer
You could readily use it to check your tacho reading against your real engine speed.

hopefully of interest
j c macleod

John, thanks for the link, sounds a good idea.
Bill Bretherton

I know that many people have praised Richard's work, but I will never use him again.

I tried to make contact in October and could get no reaction to phone messages or emails. Nigel kindly woke him up.

I sent Richard three gauges for repair and calibration in early November with the message what needed doing and that I would supply the calibration data as soon as Speedograph sent me a part to repair my 90 degree gearbox on the 5 speed gearbox. Confirmation of their safe arrival was received on 12 November. Calibration data was sent in the third week of November.

I spoke to him (again with great difficulty in getting an answer) in mid December and he confirmed that the gauges were ready to send. I asked him to send the gauges just before New Year. Since then I have been unable to contact him, again no response to emails and not answering his phone at any time of day - even when calling it immediately after it had been used to call me (literally calling back as soon as mine stopped ringing). This level of non-service is a joke. He seems to be a nice guy when I get him on the phone, but the continual apologies have worn VERY thin.

I am fortunate in that I have a second set of the gauges that are in Limbo, but this poor communication seems to be endemic in the gauge business - I had Speedy lose a few gauges, Speedograph is also not terribly reliable although they do answer the phone and didn't send an invoice as they had kept me waiting for so long for the ordered part (six weeks) due to their internal mix-ups !

Now I just have to dig out the other gauges and try and find a reliable restorer again.

@ Nigel - could you please try and wake him up again in the hope that he's not turned into Rip van Winkel.
Dominic Clancy

Domonic I've only previously communicated by email (I loathe mobiles having used them for decades) and have just sent one by your request.
Nigel Atkins

Dominic,
I feel your pain. I had issues with Speedograph Richfield with my Elan speedometer. Had to go back 3 times before they got it working properly. To their credit they were responsive and covered all my postage costs but it made me wonder about their general quality control. I'm unlikely to use them again.
Simon Wood

Hi Nigel

Thanks - You obviously have influence beyond your own expectations as he responded this morning. It was only another long list of excuses, but it seems that my gauges will be in the post on Monday, I'll just have to wait and see.

There is a local guy here but he (literally) took six weeks and destroyed one gauge I took to him, so it seems that the species "gauge restorerus" has international consistent characteristics,

I have had one or two recommendations but also a number of emails relating similar unhappy tales relating to a range of other gauge restorers, which reduces the range of candidates considerably.

Dominic
Dominic Clancy

Hi Dominic,
I've always had lots of charm, good looks and modesty
- but to be honest I just sent an email.

I probably got the same reply as you, my speedo is with Richard too, possibly I'm more accepting to it as I've experience of why people work for themselves, landlords and builders.

I do understand your side as I used to work in what was then (decades ago) referred to as a "public service" so I do understand about the need and value of keeping people informed even if it's not the best news or even bad news but that can involve a different set of skills, learning perhaps even attitude to the set of skills for mechanical repairs.

I've only communicated with Richard by email but he seems a very nice and very friendly and humorous guy.

With classic car ownership I've dealt with three efficient, or very efficient, sets of con-men. Sometimes the best or better quality people aren't always the most efficient but at least they're genuine (perhaps even saints in comparison).

I've not had the fastest service from Richard but I have had good and friendly service at a good price and extra work taken on willing at no extra cost to me so I would still recommend him.

I hope you're a bit happier when you get your gauges back.

If I was having the Ford T9 box installed now I'd change the gearbox speedo cog to get it nearer to the gearing required for the speedo as mine was 25%/33% slow on the needle (but 18% out on odo) with whatever cog is in there and that makes a stretch on altering the speedo needle.
Nigel Atkins

Richard Jenkins repaired/overhauled our Smiths mechanical oil/water temperature gauge last year and, as others have found, was very difficult to contact with lots of excuses/reasons for the repair being rather later than promised.

Having said that he does do a very good job, and if you do use him, just make sure that you don't need to have the repair back by a critical date!

Richard
Richard Wale

Perhps he is getting too busy? I used him about three years ago, before he was getting publicity here, and the service was excellent. I had previousy used a repairer in Kent (not the ones at the Bakery in T/Wells!!) and would not touch again with the proverbial bargepole.
Graeme Williams

This thread was discussed between 17/08/2018 and 13/01/2019

MG Midget and Sprite Technical index

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