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MG Midget and Sprite Technical - Battery Drain - Found it.

I've long suspected that the Sprite's battery was being drained somehow when it was idle and today was no different when the not very old battery didn't have enough in it to start the engine. This prompted me to get the multimeter out. With everything switched off it was showing 0.03 Amps.

My main suspect was the horn circuit but, with the horns and connection at the column disconnected the meter still showed 0.03 Amps. It did the same with both of the fuses removed.

Attention turned to the solenoid. No change with the white and red ignition wire removed but a zero read-out with the brown lead removed.

Back to the wiring diagram and the next suspect was the circuit for the radio and cigarette lighter - it doesn't have either! With that lead removed from the fuse box I finally got a zero read-out. That lead will now stay removed and taped up to insulate it.

Some burrowing into the rats' nest created behind the dash by POs is called for to find the ultimate culprit but I'm confident that that can wait until the winter and the battery should now have enough in it to start the Sprite the next time it spends a hole week in the garage.
C Mee

Great result Colin. Well persevered!
Electrics are a nightmare.
Jeremy MkIII

I've always considered electrics to be black magic so I'm always impressed when someone can do what Colin did.
Martin

Colin,
after disconnecting the wire from the fusebox did you check everything else electrical works on the car? (mainbeam?)

Nigel Atkins

Martin, I'm no expert. Having a wiring diagram helped but it was rather like a 5 year old learning to read by pointing at every word as I traced each line on the diagram. I also had to refer to the multimeter's handbook to be sure of how to connect it as I've never tried finding the current in a circuit before.

Nigel, I held my breath and went through switching everything on and off and then breathed a big sigh of relief when everything worked - including the main beam!

C Mee

Colin,

You may be no expert but you're a braver man than I.

That said, I did replace my Sprite's wiring harness last year and everything works but that was a matter of removing the old wires and plugging in the new ones as both harnesses lay side by side.
Martin

OMG!

Serves me right for being smug and self-satisfied!

With the dodgy brown wire disconnected from the fuse box, I've left the car for well over a week to see if the battery held its charge.

I forgot to check the voltage before I tried starting it today, but no problem, the battery was still fine as it spun the engine with no problem and fired on the second attempt. Bingo - then B****r - Tacho not working.

I must have disturbed something when I was looking for brown wires behind the dash. I finally got to a position where I could see the back of the tacho and checked all the connections. They were all firm, but I was a bit surprised to see a brown wire connected to it.

I fitted the brown lead back on the fuse box and fired up the engine. Lo and behold, a working tacho.

I've checked my wiring diagram in the factory workshop manual carefully and, while there is a correctly located brown and yellow wire, there shouldn't be any plain brown wires anywhere near the tacho.

It looks as though I've got a lot of investigating to do. As the Sprite's running really well at the moment with a trip to Harewood this weekend I'm going to leave things as they are for now and just disconnect the battery when it's not in use. I'll put a thorough wiring investigation on the winter To Do list.

POs, don't you just love 'em!
C Mee

The power feed for the tacho should be off the green (=via ign sw and fuse) circuit. Brown circuit is unswitched, and since the tacho is drawing current whenever that terminal is live, feeding it off the brown circuit will indeed flatten the battery.
A simple interim fix would be to relocate the dodgy brown to the green circuit at the fuse box, that way it will be switched off when the key is off. Simpler than disconnecting the battery.
Put a masking tape flag with GREEN written in it to head off confusion.

But in the longer term explore behind the dash to find the original green wire or another (fully) green wire you can loop from for the tacho power.
Candidates are fuel gauge, wiper switch, indicator flasher.
Paul Walbran

Colin,
In the WSM (factory version) are you using the same as I'm looking at - 'Diagram 1' as I guess your car is Mk111 positive earth up to 1967 ?

The Tacho feed is green - the only brown/yellow on the tacho is to the ignition warning lamp.
I think that is what you are saying ?

There seems to be an issue with the diagram as far as I can see, the brown from the fuse board is shown 'dotted' to the optional radio and cigar lighter but also shown dotted from the same connector block where is runs down to the indicator switch ??

Are you sure everything worked previously when you disconnected it ?

Did you try giving the tacho a gentle tap when it was not working ?

My eyes are giving up now !

For my car I enlarged it up years ago on a big works copier so I could follow the lines better.

R.


richard b

I used to employ a simple battery isolator knurled knob sort of thing which I used to isolate everthing after every run. Saved any battery drain and also acted as a simple anti theft device. About a fiver....
Oggers

When OPs have been involved with wiring all bets are off and wiring diagrams are only a broad guide. The brown wire/cable might be diverted from the original loom/harness by OP or it might be some random brown coloured wire/cable put in by OP.

If other than the small drain everything is working well and the wiring has been like that for years then for now it should be OK to remain as is.

Given the history of the car with the literally odd carbs and body repairs a thorough investigation of the wiring will be repaired. The checking, cleaning, securing and protecting and where required repairing and replacing of the wiring and connections will generally improve lots on the car and/or help to prevent future issues. Possibly more productive than the commonly much favoured carb-fiddling.

I'd start with the battery posts and main cables and go forward from there, including all earths of course then the lives and a full circuit one at a time but Op knit one, pearl one can alter the course sometimes.

Good luck, imagine it as carb-fiddling and it won't seem as bad or after seeing the results of just a bit of cleaning (sometimes lubricating switches) it might inspire greater enthusiasm for the work.
Nigel Atkins

Thanks for all the advice.

Richard, Yes, that's the diagram to which I'm referring with the dotted brown line for the radio/cigar lighter. There is a green wire in the vicinity - coiled up and not connected to anything! I didn't try tapping the tacho, but as everything works when that brown line's connected, I'm going to leave it like that until I've got time to investigate properly when I embark on the winter projects: replacing the gearbox and carburettors.

Oggers, I have one of those that I acquired some time ago but it wouldn't fit my battery post when I tried it. One that fits is on my shopping list. I used to have plastic capped quick-release terminals but was advised against them on reliability grounds so currently back with bog standard bolt-on clamps.

Thanks for the encouragement Nigel. At least you can do carb-fiddling in a standing position and I think I'm going to have plenty of practice at that by the time I've finished. I think I'll need to invest in some cushioning before I tackle the tangle behind the dash board unless I take a very deep breath and take the dash off. Which reminds me, my collection of receipts tells me that it had a new dash panel back in 1996, bought from MGOC along with demister tubes for £101.12.
C Mee

Colin

Depending on how much corroded wiring and wiring terminals and any PO dodgy repairs you might find you may decide that a new loom is a good idea (get direct from Autosparks for the correct model and year and they can properly add in extras if you want https://www.autosparks.co.uk/finder/car) - surprisingly good value.

I found it depressing locating endless bodges in my old loom, plus when I started making good repairs I had to cut back a long way to get over corroded wire hidden within its insulation.

Cheers
Mike

PS how is your hillclimb season thus far?
M Wood

Thanks Mike, the loom's on the long-term wanted list already!

I think I know where I'm going now. With the brown wire feeding tachometer, that must make the tacho permanently live so perhaps that's the mystery 0.03 Amp load.

I think I've identified the correct green lead behind the dash so when I've done some continuity tests I should be able to switch over to that one pending a bit of work on terminals due to a mix of bullets and spades. That's a job for next week because I need to focus on prepping the car and fit a window to Mrs. M's new potting shed cum workshop before I go off on Friday for the weekend.

As for the hill climbing, the car has run really well on the half-dozen outings so far this year. I ended my Loton year with a new PB by 1.3 seconds with a time of 71.99. There's a chance that, with a correct gearbox and new carburettors I just might manage a time beginning with a 6 next year. This weekend if my final outing for the year at Harewood, marshalling on Saturday and driving on Sunday; I just hope the dry Saturday / wet Sunday weather forecast's wrong.
Will we be seeing you on the hills in 22?
C Mee

Colin, why not swap that brown feed at the fuses to the switched live terminal instead. (with the white wires)
GuyW

Thanks Guy and Paul. That seems to have sorted it, at least for the time being. All electrical systems working as they should and zero Amps current when everything's switched off.

Interestingly, the brown lead on the tacho has a piece of green tape on it, as do a few other wires behind the dash but no writing on them so goodness knows what's been going on there.

Thanks again for all the comments and advice.

Colin
C Mee

This thread was discussed between 04/09/2021 and 15/09/2021

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