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.
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.
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
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.
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.
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.
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?
Right, I’ve wired up a 4 pin relay and run a fused supply from the solenoid, with a switched supply from the ignition switch. All seems fine, no blown fuses, and the rev counter is steady.
However, there’s an almost constant clicking noise from the relay.
Is this normal?
The rev counter was returned fully refurbished, and instead of the old inductive pickups, there’s now 3 wires exiting the casing. SpeedCables instructions are to wire the black to Earth, the yellow to the points side of the coil, and the red to a positive feed.
If I do this (using dedicated wiring) … it does work, and consistently. But I have only so far achieved this with an ‘always-live’ feed from the solenoid.
If I try to use a relay, then it works for about 20 seconds, after which the relay starts clicking and the rev counter drops to zero. Then the rev counter picks up, the clicking starts again, etc, etc.
Any ideas… Help!
PS: Don’t like the idea of having a live feed on the rev counter!
Are you useing a relay marked as per diagram and wired the same? if so you should have a constant power feed from term 87 with the igition on, it shouldn’t kickout, what happen’s if you check with a voltmeter, does it loose power when it clicks? you have’nt got a relay with a thermal overload have you? where did you get the relay?
Brian