Unsteady tach 69 S4 dhc

[snatchblock]

Hi, I've just managed to get the lotus out of the barn after almost two years of standing.
The old girl fired up first time, but I have a couple of questions for the experts.

1. I had a slight air leak on the forward carb. Pulled them off to find that the "O" rings were split. It was only the front carb (weber) that was "damaged". There is no mice damage, or any other sort of damage that I can see. Any ideas as to the cause??

2. The tach "bounces" when at idle, between 800 & 1500 rpm, and shows a max of 2200 rpm, engine is attaining far more than that! The ignition light stay on throughout. Any suggestions as to where I should start fault finding?

If these are the only two problems I have after not touching the car for 2 years, then I will be most happy!!

Thanks in advance for any suggestions!

[m750rider]

1. Entropy. Rubber degrades.

2. Not sure the two problems are related. Check the output of your generator. It should be putting out 13+ volts, I would just measure it at the battery terminals with the engine running above 2500 rpm.

[upnorthelan]

I also have an S4 that exhibits the same behavior, only during high revs. If I'm over 5 grand it bounces all over the place.
???
Mike

[simonknee]

RVC or RVI (it says which on the smiths dial)

[snatchblock]

Sorry, been away for a while, Dial says RVI.

[simonknee]

So here's the options:

If you have electronic ignition of any sort then you need to swap to an RVC version or get it retro-fitted with alternate circuitry by Speedy Cables (UK). Even if it once did work OK (mine used to) with electronic ignition once it starts to play up you need the alternate circuitry.

If you are on points then it depends if you are handy with a soldering iron. The RVI version is fairly simple inside and first step I would take is to pop it out the dash (steering column gets in the way, just loosen it enough). Now open up the tach and re-flow all the solder joints. You may be able to see a "dry" joint where a component leg is loose. Whilst you are their you should replace any capacitors with matching values as these are going to be very old now. However transistors work till there are dead and resistors are very unlikely to cause trouble. If this fills you with dread then send it to Speedy Cables (UK) and they will get it up and running for ?40 or so.

If you do send it to Speedy then you could have the RVC conversion done so that you are ready for electronic ignition in the future.

Simon

[snatchblock]

Hi Simon,

Thanks for that, would have wasted many hours not finding that! I do have electronic ignition, and as you said, the indicator used to work, but now skips about, what has happened within the gauge?

Will get in touch with Speedy Cables and see what they can do for me.

Buc

[simonknee]

The rule of thumb is that an RVI unit will not work with an electronic ignition. This type senses current flowing down the white cable from ignition switch to the coil +ve side. Once you have electronic ignition there is too much switching noise and the circuitry in the RVI misreads what is going on. The RVC type works measuring the voltage pulses present on the -ve side of the coil. This technique always seems to work fine.

Why the RVI can work fine with electronic ignition and then stop I could never figure out.

The Speedy conversion adds a small pcb with modern components inside the unit. However there is a change to the wiring loom. The white cable from ignition switch to +ve coil should no longer connect through the tach it just goes straight to the coil. You can do this easily as the two white cables you remove from the back of your RVI tach are ready to just join together - one is a male and the other a female bullet. You then need an extra cable from the -ve side of the coil to the input of the tach. You can run this all the way around the bay with the loom or find a route to poke it through the bulkhead as conveniently as possible.

[snatchblock]

Thanks for that clear and helpfull explanation, even I could understand it, Tach will be sent off this winter when I take the car off the road for the snow. Now do you have any smart suggestions as to how to get the handbrake working well enough to pass my Norwegian MOT test? I have adjusted it so that the cable is as tight as I can get it without being on. The pads are new, but still won't generate enough holding power on the rolling road!

Thanks
Adam

[rgh0]

The rule of thumb is that an RVI unit will not work with an electronic ignition. This type senses current flowing down the white cable from ignition switch to the coil +ve side. Once you have electronic ignition there is too much switching noise and the circuitry in the RVI misreads what is going on. The RVC type works measuring the voltage pulses present on the -ve side of the coil. This technique always seems to work fine.

Why the RVI can work fine with electronic ignition and then stop I could never figure out.

While it may be a rule of thumb it is not universally true. My orginal current sensing tachs works well with electronic ignition on both my Elan and Plus 2. You may fix a problem by changing the tach type but the problems are typically caused by 1 of 2 reasons

1. Taking the power for the electronic ignition from the coil positive terminal as shown in most of the wiring diagrams provided with the electronic ignition kits. This injects extra current pulses into the coil supply that the tach senses

2. Dried out capacitors in the orignal tach electronics leading to increased sensitivity to noise or inaccurate readings

Both can be easily fixed without totally replacing the tach internals

cheers
Rohan

[collins_dan]

Rohan, How do you fix the dried out capacitors? I made change one and am supplying power to the electronic ignition and rev limiter from the fuse box, but do still get some bouncing around. Thanks, Dan

[billwill]

Rohan, How do you fix the dried out capacitors? I made change one and am supplying power to the electronic ignition and rev limiter from the fuse box, but do still get some bouncing around. Thanks, Dan

Assuming you can recognise an electrolytic capacitor, you simply replace ALL the electrolytic capacitor that you can see on the printed circuit board, with equivalent ones. This can be a good fix for any old electronic device that doesn't work anymore. Saves on the long drawn out process of trying to work out what failed.


You can buy a big bag of electrolytic capacitors from Maplins (in the UK) or equivalent suppliers in the US.

[collins_dan]

Hmmm... that could be a problem. Does anyone have a picture of the guts of one of these and could point out what needs to be replaced?

Dan

[billwill]

Not on hand, but here are lots of pictures of electrlytic capacitors.

http://www.google.com/images?hl=en&sour ... =&gs_rfai=


Electrolytics are generally a capacity of 1 microfarad or more and have a voltage rating. They are polarised, one terminal marked PLUS and the other MINUS. they must be inserted the correct way round.

You would need a fine point soldering iron about 15 to 25 watts and a solder sucker pump.


http://www.google.com/images?hl=en&gbv= ... =&gs_rfai=


NB: in replacing you should find a new capacitor with the SAME number of microfarads, and the new voltage rating the same or greater than the original.

[saildrive2001]

How do you tell if it's an RVI or an RVC?