Lumenition Optronic affecting tacho

smudger

Member
Anyone had any success stopping Lumenition Optronic interfering with the RVI type tacho fitted to the early Series 2 V8? Mine wavers a bit, although not too far out it would be good to cure it. Lumenition recommend having internals converted to RVC type.
Did the last of the V8s have an RVC tacho and has anyone ever converted that way?
Had a look at other threads but couldn't see any useful answers.
 
I think 1973 onwards cars have RVC tacho's. They're identified by 1) having RVC in the bottom of the display face, and 2) having only one sniff terminal (a male bullet) at the back (RVI's have 2 sniff terminals (one male and one female bullet))

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There does need to be some wiring change(s), but it depends on how your RVI tacho is wired. (i.e. if the sniff terminals are the wired between the ignition switch and the (possibly still ballasted) supply coil + terminal (like so with the "pig-tail" female bullet from the female spade which supplies the tacho in the following two pictures)

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or...

wired between the coil - terminal and the "points" (more precisely, the trigger wire which is normally connected to the coil - terminal in the case of an electronic ignition module).

dmblbit replaced his RVI with an RVC last year :- viewtopic.php?f=16&t=9738

Just out of interest, if I may ask, how exactly have you got the optronic ignition wired at the moment? By that I mean where does it get it's +12V feed from and where/to what have you wired the purple or brown trigger wire on the electronic power module?
 
Thanks for the info mate, looks like all I need especially as the RVI - RVC conversion has been done before.
The Lumenition came with the car as bought wired up with the 12v power feed coming from a spare Lucar on the fuse box - the 8A fuse, not any of the 3 35A fuses. The purple wire is connected to the coil negative terminal.
I guess the tacho is wired up as standard for a July '71 Chassis Suffix A 3500 - doubt if it has been modified. Although would the fitting of Lumenition have required a change to the wiring connections? Never had Lumenition before so not really familiar with it, no wiring diagram came with it.
 
There are 2 problems with with electronic ignition modules retro-fitted to cars that have RVI tacho's:

In the case where the tacho is wired between the coil - terminal and the points, the problem is that people connect the lead that the instructions/wiring diagrams say goes to to the coil - terminal directly to that terminal (which is natural, seeing that's what it tells you to do!). This means that the current flowing from the coil - terminal to the "points" (i.e. the wire which connects the coil to the electronic switcher in the ignition amplifier - the purple one in your case!) never goes through the tacho, so the tacho can't display the engine speed.

However... you say that your purple wire is connected (directly? is there just that wire connected there) to the coil - terminal, and the tacho does display? That tends to suggest your tacho is wired between the coil supply line, OR it's RVC already.

In the case where the tacho is wired between the coil supply line i.e. wired between the 12v supply and the coil + terminal (maybe via the ballast), the problem here is that many people will connect the power supply wire for the ignition amplifier to the coil + terminal. If there is a ballast wire and the supply for the ignition amplifier is piggybacked onto/from the coil + terminal, then that can potentially be a problem.

Apart from the ballast wire causing a volt-drop, which could impair the action of the amplifier - but many have reported that some ignition amplifiers seem to operate satisfactorily even when powered via the ballast - the main problem is that the current flowing through the tacho's "sniffing" terminals is not only the coil-current, but also the amplifier's running current. It's likely that the amplifier's running current going through the tacho will impair it's ability to sense the number of times the coil-LT current fluctuates... therefore impair the ability of the tacho to display the engine revs.
 
The purple Lumenition wire is wired straight to coil -ve without any other connections on the way. Nothing else attached to coil -ve.
Connected to the coil +ve terminal are the two original white/yellow wires piggybacked together as original (shewn in the Rover wiring diagram) and a 1uF capacitor to a good clean earth. The ballast resistor has been removed so presumably one white/yellow wire goes straight from the coil to the tacho as it should.
Tacho is definitely an RVI one, it says so on the front. Doubt if it has RVC converted internals.
12v supply from the 8A fuse goes straight to Lumenition module with no other connections.
Tacho does read round about the right figure on normal acceleration/idle but it wavers by about 250rpm either way. It becomes less accurate on say kickdown when it will just swing up and away and read high until 'box changes up, then it'll drop back to something more accurate again.
Without removing dash I don't know what wires are feeding to the back of the tacho but again I doubt if it is anything other than the original as shewn in your photo's.
Reading all that you've said it seems a miracle the tacho works at all with the connections I have........ :?
 
Hmm, seems likely your RVI is wired in the coil-feed line then! (You can never be 100% sure unless you see what the wiring is behind the dash! - somebody might have gotten it converted but it still has "RVI" on the face.)

If it's wavering, then perhaps it would be best to change it for an RVC type. Having said that, I also wonder if it's not actually a case where your tacho is "worn" and the Lumenition is not really the cause, though it may well be exasperating the problem; many might well say "oh my tacho reads fine now it's been converted to RVC" or something to that effect... but converting it to RVC means changing the internals... which in turn means it's effectively been reconditioned!

Don't get me wrong, RVC is the best bet for retro-fitting electronic ignition in terms of least-problems-of-wiring and compatibility, it's just that I'm not totally convinced (yet anyhow...) that RVI itself is the problem, more than actually how they've been connected to the the ignition circuit.

All the best bro!
 
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