Rev counter, coil wiring & voltage regulator

PostPost by: PaulH » Tue Sep 03, 2019 9:30 pm

I have two electrical problems which are:
1. My rev counter isn't working.
2. The red ignition lamp on the rev counter stays on when the engine is running. I've done some searching in the forums and think I may have the answers but wanted to check rather than try a fix and damage something. I'm reassembling my Elan S2. It's a new loom and has positive earth.

1. Rev counter not working: I've got the white wire looped through the fitting on the back of the rev counter. The rev counter is a positive earth type. At the coil I've got the white low tension wire from the loom going to the positive connection and the other low tension wire from the negative on the coil goes to the distributor. Have I got the wiring at the coil the wrong way around?

2. Red ignition lamp stays on when the engine is running. Is that problem likely to be caused by a problem with the voltage regulator?

Thanks in advance.
Paul
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PostPost by: billwill » Tue Sep 03, 2019 11:34 pm

In what manner is your rev counter not working.

If it does nothing at all It sounds like you have not provided any power to it. The white loop is the SIGNAL not the power,

There should be a power supply lead from a circuit that becomes live when the ignition is on to a terminal spade on the back of the tacho, and the case should be earthed.


viewtopic.php?t=18275&p=147217


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PostPost by: m750rider » Wed Sep 04, 2019 1:07 am

Regarding the ignition lamp - it could be the voltage regulator, but in the 40+ years I've owned my S4, the regulator failed and has been replaced once, just a few months ago. Until then it was original. In my experience it's more likely a faulty generator.

The workshop manual describes how to test a regulator.
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PostPost by: PaulH » Wed Sep 04, 2019 6:12 am

billwill wrote:In what manner is your rev counter not working.

If it does nothing at all It sounds like you have not provided any power to it. The white loop is the SIGNAL not the power,

There should be a power supply lead from a circuit that becomes live when the ignition is on to a terminal spade on the back of the tacho, and the case should be earthed.


viewtopic.php?t=18275&p=147217


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Thanks for the reply and diagram. It has the power supply but I will check it to make sure there is power. Do you know if it matters about the positive and negative connections at the coil (my car is wired positive earth) or will it work if it's connected either way?
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PostPost by: PaulH » Wed Sep 04, 2019 6:13 am

m750rider wrote:Regarding the ignition lamp - it could be the voltage regulator, but in the 40+ years I've owned my S4, the regulator failed and has been replaced once, just a few months ago. Until then it was original. In my experience it's more likely a faulty generator.

The workshop manual describes how to test a regulator.


Thanks. I'll start by checking the generator.
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PostPost by: alan.barker » Wed Sep 04, 2019 9:58 am

try the loop cable on the back of Tacho wound the other way. If you have wound it anti clockwise try it clockwise :wink: .
The earth on the Tacho is a brass loop fitted under one of the knurled nuts holding the Tacho in the Dashboard. It's part of the Daisychain earths from one Instrument to another.
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PostPost by: billwill » Wed Sep 04, 2019 11:53 am

Yes, it matters which way the power is applied. You say your car is wired as per original positive earth, So the case of the rev-counter should be earthed (to positive) and the power contact on the back should be connected to a wire with negative 12 volts after the ignition switch.

When a car is converted to negative earth the rev-counter case needs to be opened and the wire from the rear contact to the circuit board and from the case-earth to the circuit board need to be swapped over.

The rev-counter does not need a stabilized power supply as there is a Zener diode inside to do its own stabilization.

I am not sure, but connecting the power the wrong way around may destroy the electronic circuit.
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PostPost by: USA64 » Wed Sep 04, 2019 2:18 pm

This arrived today. It explains why the coil goes one way not the other even though both work. There are a couple of shows in series too if it's not immediately clear.
https://www.youtube.com/watch?v=bl0Z6kb ... mx9jswqvy7

A long URL but I tryed it and it worked. It's from Moss Motors.
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PostPost by: oldelanman » Wed Sep 04, 2019 3:43 pm

Is the generator original to the car ie +ve earth .. if not it may need polarising.
See this thread....
viewtopic.php?f=38&t=28654&start=

Also I've read that the wattage of the warning lamp bulb is significant .. is that correct?
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PostPost by: PaulH » Thu Sep 05, 2019 9:16 pm

Thanks for the replies. Some follow up and answers to questions.

It's a new generator and I did polarise it before starting the car.

I haven't checked the power supply to the rev counter, I'll check it this weekend.

I've found the answer to the coil connections. If the car is +ve earth the +ve on the coil goes to the distributor. If the car is -ve earth the -ve goes to the distributor. Here's the article which gave me this answer: https://mgaguru.com/mgtech/ignition/ig104.htm
The car will run if they are the wrong way around but the article tells you why it's better the other way.

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PostPost by: billwill » Fri Sep 06, 2019 10:38 am

Please note that the wiring of the voltage regulator has nothing really to do with the Tachometer/Rev-Counter.

The mounting studs of the tacho, merely constitute a handy mounting point and earth for the voltage regulator.
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PostPost by: PaulH » Fri Sep 06, 2019 8:08 pm

billwill wrote:Please note that the wiring of the voltage regulator has nothing really to do with the Tachometer/Rev-Counter.

The mounting studs of the tacho, merely constitute a handy mounting point and earth for the voltage regulator.


Update: I've solved the ignition light problem. There were corroded contacts in the regulator which I cleaned with fine abrasive paper. Ignition light now goes out when the engine is running.

I checked the supply to the rev counter which is OK. There is no movement at all (not even a small flicker of life). I've found some good information on the web and it seems there are some electrical components that could fail due to there age. Looks like I'll have to send it for repair, unless someone can explain how I could do this myself.

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PostPost by: mbell » Fri Sep 06, 2019 9:31 pm

PaulH wrote: Looks like I'll have to send it for repair, unless someone can explain how I could do this myself.


I think there are a few posts on here about replacing various capacitors etc. Another alternative would be to fit a Spiyda RVC conversion board to remove the issues with the original RVI design and electronic ignition. The Spiyda board replaces the existing circuit board, so unless there is a mechanical fault with it the tacho it will fix it.

It not a plug and play job as needs calibrating and wiring changes on the car thou (add a sense wire to the coil)

https://www.spiyda.com/smiths-rvi-rvc-c ... board.html
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PostPost by: Chrispy » Fri Sep 06, 2019 10:29 pm

Here's a thread showing how I fixed my tacho. It was working but reading very erratically.

http://www.lotuselan.net/forums/viewtopic.php?f=38&t=41110

After a full re-wire recently my tacho stopped working. The loop on the back was the wrong way around. Took me ages to work that out :lol:
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