Lotus Elan and Lotus Elan Plus 2 Forums

Tacho finally back from SpeedyCables.
Wired it up ........... works better, but surges and then dips, hardly ever remaining steady for much more than a couple of seconds at a time.

Before I ring up and "rant and rave", have I missed something ? (which I'm sure is much more likely)

In the returned package was a revised wiring diagram. The normal spade connectors were on the back of the casing, but instead of the old bullet connectors, there were now 3 wires coming bare, out of the casing.

Yellow: to be connected to the points side of the coil
Red: to be connected to the 12volt +ve switched feed
Black: to be connected to a 12 volt -ve

I found the old inductive male & female bullets and connected both to the Yellow, found a feed that was only +ve when the ignition was switched on and connected that to the Red, found an earth and connected that to the Black (I'm only connecting the old wiring leads from before the tacho was sent away) .

I've made no other connections, and not even earthed the case.

What have I missed, please?

Regards,

Stuart.

Hmm,
I had mine refurbed by speedy but I have an aldon system. I find that it works okish up to say 4000 then drops and is erratic. I intend to send it back under warranty. However, I will try a really good earth first.

cheers
Mark

Your tacho has been converted from a current sensing device to a voltage sensing device in the rebuild. You need to change the wiring in accordance with the instructions.

By connecting the orginal bullet connectors to the yellow you have effectively connected the yelow to the coil positive side instead of the negative ( points side) and thus it is not getting a propoer voltage signal.

Connect the bullet connectors together and run a new wire from the coil negative side to the tacho yellow wire and it should work OK

Rohan

Rohan,

As soon as I read your reply (and many thanks), I realised my mistake.
Must have been half asleep last night!

Did as you said, but although better, not right.

It will now hold steady for a few seconds, sometimes even 10, then dip widly, perhaps to zero, or even spike to say 500 rpm above what it 'sounds' it should be reading.

Need to check it against a datum, but I think initially that even when steady it's reading low.

Have also tried a dedicated Earth to the chassis, without success.

Used to be able to adjust these for sensitivity, but unless there are other ideas, I think I'll ring Speedy before doing anything untoward.

Regards and thanks,

Stuart.

Stuart,
I'd check the charging system first. The voltage may be fluctuating.

On second thought I'd check the ignition switch too. Try jiggling it.

Stuart
You've tried a dedicated earth,how about a dedicated 12v supply
John

Stuart

Had a similar problem with a friends 7 recently at the last race meeting. The new voltage sensing tacho ran OK at idle but at revs under load it jumped around. Switch back to the old current sensing tacho and the problem went away.

I suspect the problem was RFI ( radio frequency interference) pick up probably from ignition wires to the new wire he installed from the coil negative back to the tacho voltage input. When he had a chance he was going to try a shielded wire for this connection but I have not seen the results yet.

Rohan

Rohan/John,

Thanks both (and all) for the help. Now seems to be rock steady.
Maybe not calibrated properly but that's to be established.

It seems it was the +ve feed causing the problem (as you sussed).

I'm still rectifying those rotten cill/lattice strengthening sections, so this is a good time to bring through a dedicated +ve feed and earth from the battery into the bulkhead area for future use.

If I do this via each cill, I'll have future options on each side of the engine bay.

Regards and thanks,

Stuart.

Sorry,

Just to say that the only revision I made, was to run a dedicated supply from the battery, and then ...bingo !!

Well, for now, ......... you know how it goes!

Regards.

Okay I give up. Cannot find an existing switched +ve supply that will work the tacho acceptably.

Anyone any idea how to neatly add one?

I'm pre-supposing that it needs to be dedicated, but switched via the ignition, rather than being shared, and/or permanently live?

Regards,

Stuart.

Stuart,

How about adding a relay and switching it from the ignition? It can then control a dedicated power supply from the battery.

Hamish.

Hmm ...... a Relay...

I dimly remember these. They happened once a year (generally in Summer), and someone was often ribbed for dropping something called a 'baton'?

Seriously though, the relay is acting as a switch?

If so, then it would need an always-live feed (from the solenoid, say), a switched feed from the ignition switch, and an output to the rev counter (which would only be live when the ignition was on)?

If I'm right, where does this idiot quickly get a relay, and what sort, please?

Equally importantly, what are the pin connections to the relay?

Regards.

http://www.ado13.com/techs/relay.htm

Brian

Many thanks,

Regards.