Lotus Elan

Testing a Tacho

PostPost by: vincereynard » Sun Feb 25, 2018 4:24 pm

Anyone know how / if I can test a Tacho with a simple multimeter.

I have an old one of a similar vintage out of a Triumph -

50-feb.-25-14.10.jpg and


The resistance reading were as shown.

Does this mean anything? What could I expect of a good unit?

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PostPost by: john.p.clegg » Sun Feb 25, 2018 5:21 pm

Vince
I don't think the resistance readings account for much,just wire it up and fire her up...

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PostPost by: billwill » Sun Feb 25, 2018 6:12 pm

Hmmm, the two sensor contacts are just the two ends of the same wire would around a ferrite transformer core inside the instrument as far as I am aware. If so they should be insulated from anything else such as the case contact that you measured.
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PostPost by: billwill » Sun Feb 25, 2018 6:17 pm

This is the circuit of the Smiths Tacho as drawn up by a bloke who didn't use quite the normal conventional layout, but it is the most commonly available diagram.

Note the input loop at top left is just a simple wire with both ends leading outside the case.

Image
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PostPost by: oldelanman » Sun Feb 25, 2018 6:42 pm

billwill wrote:Hmmm, the two sensor contacts are just the two ends of the same wire would around a ferrite transformer core inside the instrument as far as I am aware. If so they should be insulated from anything else such as the case contact that you measured.

That's true for an RVI tachometer...could be wrong but Vince looks to have an RVC type from the pic.
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PostPost by: vincereynard » Sun Feb 25, 2018 10:42 pm

Thanks for the replies.

The image is only there for illustration purposes - its of a Triumph Dolly Sprint.
The tacho. I am having trouble with is a normal +2 one which I believe is an RVI?
It has just stopped working and I was just wondering if there was any test I can do to determine it is bust.

Such as an open circuit between either of the connections?
Electronics are not a strong point so a circuit diagram means not a lot.

I could wire the dolly tacho up with test +12 / earth / signal then if it works replace the genuine one.
If it still works the fault is in the existing tacho, if it does not it is in the wiring?

That sound like a plan?
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PostPost by: mbell » Sun Feb 25, 2018 11:06 pm

If the fault in the current one was wiring then your car likely wouldn't start, as power to the coil goes via the tacho....

So most likely just your tacho at fault.

I'd test the temp one via test leads, if it works then stick it in while you sort yours but this will likely mean running an additional wire.

Your current tacho (RVI) works by sending current pulses to the coil, hence the coil power is rooted t through it. The RVC one works by sensing voltage on the coil Earth connection. So it's connections are +12v, gnd, and coil negative. Your current tacho has 12v, gnd, coil power in, coil power out. So you need a extras connection to the coil gnd and to bridge the coil power lines together.

It's likely a fault capacitor on your current one so you'll need to decide if to repair any faulty components or buy a replacement (RVC) circuit board to fit, e.g. sypida.
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
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PostPost by: elanfan1 » Mon Feb 26, 2018 12:04 am

Vince Call Speedy Cables in Ystradgynlais. See if you can get trough to a technician and ask them for a testing procedure so you can check it before you send to them for a repair. You can get a cost at the same time.
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PostPost by: billwill » Mon Feb 26, 2018 12:41 am

The contacts look like an RVI one on that photo as it shows TWO contacts so the power to the spark coil goes in one and out of the other. An RVC tacho only needs one contact for the signal.

However it might have been converted to RVC, of course.

I haven't been able to find a good diagram of the circuit of an original RVC tacho. Plenty of newer circuits using a LM555 chip of course.
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PostPost by: rgh0 » Mon Feb 26, 2018 8:58 am

Buy a modern digital timing light with a tacho built in. You need it anyway and you can check your tacho calibration against it. If the tacho is not working at all then its time to send it to someone who knows how to rebuild it.

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PostPost by: vincereynard » Mon Feb 26, 2018 2:23 pm

The demo tacho was indeed a RVC, it states it on the dial.

I have a timing light with a tacho so it will be interesting to compare them. A pressie from Mrs R!

http://www.ebay.co.uk/itm/391346184301

Having removed Toad's tacho it would appear that it has already been to Speedy Cables a couple of times.
53-feb.-26-13.45.jpg and


The last in 2010. I imagine it was converted at that point. Certainly the dial is virtually mint.

So it looks like it may have another visit. I'll try the Dolly one with the existing wiring first.
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PostPost by: bob_rich » Tue Feb 27, 2018 1:54 pm

Hi All

Its cold here and we are snowed in ( well 5mm (1/4") deep) but it is very cold--- too cold to work on the car.

I have an old RVI tacho and so I though I would play in the warmth of my shed and dream up a simple test
and calibration check for the RVI type tacho.

I ran a test and the set up which as you will see in the attached note is a a real lash up but it proved the point.

The tacho came up fine at 3000rpm after being in the spares shed for over 5 years.

I have attached a note that may encourage those with minimal electronics experience to have a go and have attached some oscilloscope print outs that may interest to all including those with more electronics experience.

I quite like the RVI tacho it is easy to mend and the usual problem is the 2.2uF electrolytic capacitor fitted across the transistor base emitter ( easy to find its only capacitor in the unit) is easily replace with a modern 2.2uF 15V or higher voltage tantalum bead capacitor.

My only adverse comment is that it sometimes undereads when it is cold but soon warms up and then is fine

Hope this helps regards Bob
Attachments
Simple Test for an Smiths RVI Tacho copy.pdf
(230.55 KiB) Downloaded 1115 times
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PostPost by: Mazzini » Tue Feb 27, 2018 2:28 pm

elanfan1 wrote:Vince Call Speedy Cables in Ystradgynlais. See if you can get trough to a technician and ask them for a testing procedure so you can check it before you send to them for a repair. You can get a cost at the same time.


I've had a load of stuff at Speedy Cables since August last year, every time I call or write they tell me it's all done apart from a couple of things and it'll be ready next week, this has gone on for the last three months...
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PostPost by: vincereynard » Wed Feb 28, 2018 9:04 am

bob_rich wrote:Hi All

I have an old RVI tacho and so I though I would play in the warmth of my shed and dream up a simple test
and calibration check for the RVI type tacho.

I ran a test and the set up which as you will see in the attached note is a a real lash up but it proved the point.

Hope this helps regards Bob


Excellent Bob. Many thanks, even I understood most of it. What if the Toad Tach is not an RVI? It may say so on the dial but it looks totally different on the rear. Converted at some point when on a trip to Speedy Cables perhaps?

Mazzini wrote:I've had a load of stuff at Speedy Cables since August last year, every time I call or write they tell me it's all done apart from a couple of things and it'll be ready next week, this has gone on for the last three months...


Not that Speedy then! They are probably still on Druid time.
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PostPost by: JonB » Wed Feb 28, 2018 10:08 am

Vince

Not sure if you saw this but it is a document about repairing the tachometer. I found it on another thread.. may or may not apply to your tacho.

tach_repair_rev5.pdf
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