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MG Midget and Sprite Technical - tacho problems with lumenition

I've just fitted a Luminition Optronic system to my 1275 Sprite. The tacho is now very erratic and wanders around all over the place. It looks like its missing pulses as it could be stable for a few seconds then drop down then recover. Has anyone else had this problem. With normal ignition I had no problems.

Steve
Steve Church

Fitted H&H contactless distributor to Anita's midget last year, that does funny things to the tach. Turn the ign on and it reads about 3k rpm, start the negine ad it's fine... Never give a problem with the running and I have to say starting and performance has been better.

Mark.
Mark Boldry

I think the earlier RVI tachos can give probs with elec ignition and the later RVC coded ones are the ones to have.
Mark
Mark Whitmore

Where abouts have you connected your power supply for the unit? if its the positive side of the coil or the same wire that leads to it, then thats your reason. I had the same problem when I tried to use this point for my anti-runon valve.

Nick

Nick - I've had no problems with the r/c with Ignitor.

Are you suggesting that it would be better to have an inductor 'slug' in the 12v supply to the r/c for the Luminition?

Maybe try a slug similar to that found in the supply for car steros?

(BTW - my friend's TR-6 with luminition packed in last summer... turned out to be the 'light bulb' had ceased to function... therefore nothing for the optical sensor to 'sense'; happened quite suddenly - solid towed him home, since he had points & condensor to fit the old system back, but didn't have the orig back-plate. At least with Ignitor, you can just re-install the points/condenser...)


A
Anthony

So does anyone have any idea what is different with the signal sent to the tacho with a Lumenition. Not strong enough? Too strong? Interference on it?
I may put a 'scope on it and have a look.

Steve
Steve Church

Steve

If signal not strong enough, then the coil would also not be fully energised. If too strong, I'd expect this would be fine by the r/c.

Most likley there's another signal imposed - maybe ringing, maybe some drop in signal due to supply dip to the module (ie. do you have a good +12v and earth on the Lum module itself?).

Scopes are always interesting on car electrics... there's always a lot more going on then 12V DC!!

A
Anthony

Some nice 350V spikes on the ringing at the front of the pulse to the coil. Of course just as I was looking for any clues on the scope the tachy decided to work perfectly. The curse of the engineer!
Lumenition indicate that current sense tachys are prone to problems with electronic ignition systems and hint at the fact that some users have sorted this by rewiring, but failed to give any details. Guess I'll have to look for an RVC unit at Spridget50.

Steve
Steve Church

hmm - the orig tacho in my racer does this too - for that reason I've always shied away from fitting the nice expensive Elliot tacho for fear of bu99ering it ! Anyone know if the Elliot works like an RVC or RVI type?
David Smith(davidDOTsmithATstonesDOTcom

Almost certainly they;ll be the voltage type:

- easier wiring loom
- easier to connect/disconnect
- fewer volts drop across the loom
- less variation in connection specification (e.g. no need to worry how many loops).

A
Anthony

Yes the Smiths current loop tacho can be a strange beastie at times. The sensitivity shifts as it ages which tends to give erratic readings and can eventually stop reading altogether. If you have a radio suppressor condensor fitted externally to the live side of the ignition coil then sometimes removing this can give improve the reading as the condensor tends to soften the pulse that the tacho looks uses. This is easy enough to try and who listens to AM radio anyway.

Another less easy trick is to put an extra turn in the sensing coil at the back of the tacho but take care to ensure the wire cannot short to ground anywhere.
Dave

This is a common problem with Luminition and they are aware of it
If you send the Rev counter to Speedy cables they will recalibrate it.
i chipchase

Dave. We have the H&H system on Anita's Atlantis and I fitted an Elliot last year, No problems.....

Mark.
Mark Boldry

Steve

As previously mentioned, if you have the red wire from the Lumenition module taking its power form the +ve side of the coil, that may be causing the problem.

Try connecting it to a spare white feed on the fuse box.
Dave O'Neill 2

Just been out on the St.Georges drive it day. Noticed the tacho behaves perfectly for the first 10 minutes then gets progressively erratic. So probably temperature related. Inductance of the coil changing with temperature perhaps?

Steve
Steve Church

This thread was discussed between 10/04/2008 and 20/04/2008

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