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MG Midget and Sprite Technical - Odd rev counter behaviour.

On my other car now. Today when I switch on the ignition the electronic rev counter leaps to 4500rpm. And that's BEFORE I start the engine. Then it stays there, when I fire up.

It's never done that before. Other than 'cos it can, why would it do that?
GuyW

Cos it needs a repair.

Either a capacitor,-- likely, or a diode or transistor. You have to open it up to see. You may well see a split in a cap', if not you have to test them.


It's basically a volt reading instrument, converted to show revs.

Because of a fault, yours is getting and showing a permanent reading rather than the pulses it should read.

The speed of the engine is converted to a voltage. The voltage is fed to the moving coil. The coil creates a magnetic field. That field of the coil tries to align itself with the magnetic field of the magnets. This causes coil to move against the spring deflecting the needle. By combining input voltage, the number of turns in the moving coil, the strength of the magnet and the strength of the spring a ratio is achieved that the needle will deflect a number of degrees per volt. Etc, bla bla.

anamnesis

what sort it it ? RV1 or RVC ?
richard b

Ok, an internal repair job rather than an external fault - poor connection etc.
That gets me going in the right direction at least!

RV1 or RVC ? I don't know, do they look different?
GuyW

Keep up Guy - been lots of recent threads regarding electronic ign problems with Rev counters !!!

RV1 have white wires running from coil to r/c and back - RVC don't
Its also written on the face of the R/C !

R.
richard b

Would need to know what the car is as some moderns use a type of motor for the needle movement rather than a moving coil meter.
David Billington

Yes I saw those discussions but didn't take a lot of notice of the detail as my rev counter was working perfectly. Too much info, not good for one you know!

RPM
4cyl negative earth
RVI 2430/01
SMITHS
GuyW

Not especially modern, David
1971 Austin Sprite
GuyW

Guy,

If its an RV1 I have a spare in fair to good condition (needs Wellering - new rubber rings to glass etc) worked last time I ran the old engine before the rebuild - so a while ago !

Same deal as usual but not suited to E15 petrol running and no warranty implied or expressed !


Regards,

R.
richard b

The one I have is an RV1 2430/01A

This is the slightly later version with the wire 'coil' inside the unit (not wound around the plastic post as the earlier version).

Not sure which yours has mine was a '72.

Just have two plug on connectors instead - same wiring system.

Someone will know which way around they go !

R.
richard b

Without cricking my neck and straining my eyes to focus close up behind the dash, I can say from memory that the wire to it is white as it goes to the coil, and has an odd cylindrical plug in connector that only fits in one place.

I also think I know why it worked yesterday, but not today.

And thank you Richard, a good offer which I would very much like to graciously accept. Can one do graciousness in an electronic messaging system?
GuyW

Male and female bullet connectors on this one, + a spade connector - it also has a paper ticket 242 stuck on just below the connectors.

R.
richard b

I have now strained my neck. There is a white/red ( panel light, probably),1 white to the odd connector, a green to a spade connector, and a white a white/ brown to the ignition lamp.

And I think I meant gratefully accept, not graciously accept. The latter would imply a reciprocal to a complement, methinks.
GuyW

Sounds like you are a connection short - there should be ;-

Ignition light
Panel light
Power spade
Female Bullet - coil
Male bullet - coil

I will source the necessary box and send it on it merry way !

Here it is and yes Nigel I know its downside up and it won't rotate on the BBS upload ?

R.




richard b

I was assuming it was all connected. You may well be lucky and a conbection has come adrift.






anamnesis

Guy

Yours is most probably fixable - internal component fault is likely. Capacitors are always suspect at 50 years old. Cheap fix for someone who can solder electronic components.
Bill Bretherton

If it was a coil wire off it would not start/run.

Found the box so its moving - more stuff cleared !

R.
richard b

Fixed, well sort of. The white supply in was there, hiding in the shadows above the green. I cannot get my head far enough away for my eyes to focus and see up behind the dash. I need a pair of professional snooker players glasses with the close up lens at the top!

Anyway doing a wire wiggle and it is working again, but only as long as I hold one hand up behind the dash which makes gear changing, and other stuff, a bit awkward. Tomorrow I will ease the SMITHS RV1 out of its hole in the dash and secure proper connections.

A relief, as I thought I had fried it, doing some bodywork welding.
GuyW

That's odd. I've been looking at the circuit to see what might have caused the fault and have not reached a conclusion. The wire loop itself wouldn't do it imv. A loose 12v supply wire or earth wouldn't do it (imv) so what is it? Unless there is an intermittent short that the wiggling corrects maybe.
Bill Bretherton

Well it's fixed. I took the tacho out and cleaned all the contacts. One of the white wires - the male bullet one, was making an intermittent connection when waggled. I cut the bullet off and soldered in a new bit of white with a new bullet on the end. Not sure exactly which bit of that resolved it, but it is now refitted and working as it should.
GuyW

Another job well done.
Martin

Well done Guy, now anybody need an RV1A in need of a little tender TLC - rubber seals for glass and to dash - think it still works- did when stored away !

R.
richard b

Well done.

Checking the boring basics has turned up trumps again.
Nigel Atkins

Thanks, but it is just part of the routine business of running these cars. And a little help from friends here goes a long way! Thank you.
GuyW

This thread was discussed between 01/06/2021 and 02/06/2021

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