Tacho not working

[itsricht]

Hi 
Sorry to bring this up again, I have read the large number of posts on this subject I think I know what I need to do but want to make sure.

The wiring on my Elan is a complete mess there is very little that corresponds to the wiring diagram. For instance the ignition switch just has three wires to it, Yellow, Blue and Pink. There is an Orange and Brown wires hanging under the dash marked Tacho but they are not connected to anything at this end. 
I have included a diagram below. The yellow wire, from a black box (fuse box?) on the passenger side (RHD) of the bulkhead, has been cut into and a Red wire added which is joined to a brown wire that goes to the + on the coil.The looped wires. into the tacho, which should be white have been replaced by the two red wires that are not attached to anything at their other ends. There is a white wire that goes from the dashboard to near the coil which is not attached to anything at either end.

From reading the previous posts my solution would be to detach the brown wire from the coil and attach it to one of the red tacho wires and then attach the other red tacho wire to + on the coil. Alternatively remove the red tacho wires, add another wire onto the joint of the red and yellow wires at 1 and attach to the the tacho, then use the unattached white wire from the tacho to the + of the coil. 
Just want to make sure I would not damage anything by doing this, any thoughts greatly appreciated.


Rich

[mbell]

I'd remove the tacho and take a careful look or post some pictures. Your diagram looks like how you would wire a tacho for RVI rather than the standard RVC. So your tacho might have been converted amd obvious from teh wiring someone has been in there.

Also is there a supply and ground connection on the tacho?

[itsricht]

Thanks for getting back to me.
Attached are some photo's. The tacho is RVI, according to the markings on the front. The car is a S4 Elan Coupe 1970. There is grounding on the frame via the mounting screw. I presumed the supply was coming from the ignition, Red then Brown wire looped through the coil?

Thanks for your help.

[Craven]

Blue is 12volt supply, the two red’s are the loop to provide the + 12volt supply to the + of coil.

[mbell]

Yep I agree, looks like it is still wired as RVI. (It saying RVI on the front doesn't mean someone hasn't converted it.)

Based on that photo and previous diagram, I think the tacho has been by passed or just messed around with. One of those red wires should go to the ignition switch for 12V+ source the other should go to the coil to power it. The Blue wire should go to a fused switch live, not the coil or the same 12v feed to the coil.

[itsricht]

Thanks for your replies mbell, very helpful. Bit unsure about fused switched live. Looking at the diagram I presume I would get this supply from a hatched block at the top of the diagram (green wire on diagram), what is this block and where is it in the car. When you say it is switched what do you mean, it is switched on by the ignition?
Apologise for my ignorance.
Rich

[mbell]

Sorry not familiar with Elan wiring so can't advise on specifics. A switched live is one that is switched on by the ignition, you also want it to be fused unlike the power to the coil.

There should be quite a few switched lives behind the dash to the other instruments. I'd be checking there isn't one just hanging around behind the tacho from the factory loom. If not bridge of the supply for one of the instruments or the 10V regulator for the fuel gauge.

It seem like you wiring has been messed around with thou, so you need to take care and double check everything.

[itsricht]

Thanks for your reply, I'll take a look when the rain stops.
Rich

[billwill]

The circuit for an RVI tachometer is actually very simple, Its power circuit is independent of its sensor signal.

[Higs]

Adding some more detail to Bill's post:

Your tacho has an internal coil sensor (earlier ones had an external fitting that a wire looped around it) and this is fed by the two red wires.

In your top photo, the right hand red wire (with a female bullet connecter) goes to the ignition and the other red (with a male bullet connector) goes to the coil. The blue is power for the tacho and this also goes to the ignition switch. It has a normal spade connector.

On my '68 plus 2 with the earlier tacho, all of these wires were Green with a red tracer. The two for the external sensor connected to a while wire that looped through the sensor (which you don't have because you have the later tacho with the internal sensor).

Here is a link to a useful article with pictures:

http://www.mgb-stuff.org.uk/tach.htm

Hope this gets you there,

Richard