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MG Midget and Sprite Technical - volt meter

hi all. dont know if anyone would know this, but have bought a 12v to 24v voltage gauge that you plug into your cigar lighter socket. when i put it in my new zafira it shows voltage in battery and when running shows voltage its charging. when put in midget shows voltage of battery but when running does not show charging voltage,it does with a volt meter i have. bearing in mind midget has dynamo, would this make a difference.
thanks bob.
bob taylor

Im no electrician.... but yes id think the dynomo is whats holding you up

A couple of thoughts...

Try a higher rpm, I dont think the dyno kicks in till around 1200-1800 rpm... dependant on the size of the dyno pulley

Try a 2nd cig lighter socket and hook directly to battrey... if nothing try between the dyno and the regulator ...just to see what happens

Before any of that... id also hook in an inline fuse as a precaution...use a fast blow fuse

Remember... I can set fire to a spark plug given half a chance... so word of caution to the wise

P
Prop and the Blackhole Midget

"when put in midget shows voltage of battery but when running does not show charging voltage,it does with a volt meter i have. "

Bit confused by that. If they are both volt meters, they will both read the battery voltage.

However, if your 'new' volt meter only has a scale deflection from 12 to 24 volts, it won't be able to read less than 12volts. So if your dynamo is outputting less than 12V that might be why you can't read it. And if it is outputing less than 12V, either you have a bad dynamo, or a bad voltage regulator.

Analogue or digital meter?
Lawrence Slater

if i put digi meter on battery or plug in cigar lighter it reads 12.8 volts and when running goes up to 14 ish so regulator and dynamo working ok. the plug in thingy is also digi which works ok on alternator but not so on dyno. i think it suggests that plug in thingy works only on alternating current where as dyno is direct. is there a way of connecting somewhere else to get a reading.
bob taylor

Alternator o/p is DC(externally). So if it should work on both. Are you sure it's making a good connection on the Midget?
Lawrence Slater

Although the alternator produces alternating current, it is rectified within the alternator itself and outputs DC.
Dave O'Neill2

it must be making a good connection because the digits light up 12.8 volts when you plug it in and when ignition on but no digits light up when you start car. works perfect on my vauxhall.
bob taylor

Not sure I understand why but is your Vauxhall negative earth - you profile shows a '65 Midget that would be positive.

Many digi things are polarity sensitive - but why it works without engine running is strange.

R.
richard boobier

richard, my midget is neg as well. it dont make sense. if i plug my phone in it for my sat nav it still works.


http://www.ebay.co.uk/itm/380534160355?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
bob taylor

Are you sure the fag lighter is actually live when the ignition is on? Keep looking, it can't be magic. ;).
Lawrence Slater

Is the lighter socket on the midget wired correctly (+ & - the right way round). It will be on the zafira, but maybe not on the midget.
Also, where have you connected the working voltmeter on the midget? Have you connected it to the cigar lighter itself, to check that reads charging ok. You could do with a spare female socket just to test directly across the battery.
HALL JOHN

If it doesn't work ok directly across the battery, it's not the socket in the car that's faulty, If it does, the socket is faulty or wired incorrectly.
HALL JOHN

definately live lawrence caus it works with volt meter. will have a look today and check where i wired it to. also check gauge direct on battery. will let you know.
bob.
bob taylor

right.....
just connected digi volt gauge direct to battery and it works, flashing on and off with display but it works and shows charging voltage. havnt looked yet to where i connected cigar lighter to, think it was a red wire in the loom.
regards bob.
is it ok to connect live from cigar lighter straight to battery terminal.
bob taylor

i had the cigar lighter connected to the fuse box, the earth had come disconnected but should have been earthed through the dash. connected cigar lighter up direct to battery and all ok. so where is the best place to wire the lighter into.
regards bob.
bob taylor

Power as you have it, from the fuse box.

Wire the light into the dash lights. Not much point having it on all the time.




Lawrence Slater

I thought the cigar lighter would have been on the purple circuit, so that it was fused...or does it have an in-line fuse?

Where it is connected to shouldn't have any effect on its operation - 12v is 12v.
Dave O'Neill2

May have been purple on the MK2 Midget. The diag' below is for a +ve earth MK3 Midget up to '67, and definitely shows brown.

Yup it would make sense to fit an inline fuse, since that's on the primary side of the f/box. Didn't notice that. Maybe a it daft not have shown an inline fuse, but the lighter element itself is like fuse, and they often go open circuit anyway. Not sure how much current they draw, never measured one.
Lawrence Slater

I've had a look at the BMC w/s manual and it does indeed have it on the brown circuit for positive earth cars.

On the first of the negative earth cars, it also comes off the brown circuit, together with the radio, but with a white/green wire from the double bullet connector. Although the radio has an in-line fuse, the cigar lighter does not.

From chassis no. GAN4 66226 & HAN9 77591, an in-line fuse is added to the cigar lighter.
Dave O'Neill2

first of all thanks for all your comments. on my 2 fuse box i have 3 greens coming out from 1 fuse and purple coming off the other which is the horn. im going to put the live for lighter on the spare terminal next to the purple and run earth. what fuse should i use inline.will only be used for charging phone, satnav or this volt gauge.
thanks bob.
bob taylor

Bob

I wouldn't bother with an in-line fuse, as the purple is fused already.
Dave O'Neill2

right, have now totally give up with volt gauge.???
rewired cigar lighter to fuse box and new earth and gauge still only gives reading when engine off, not running. connect gauge direct to battery and it works, not fully lit but flashes correct charging voltage. now you might say it must be a fault in the cigar lighter but that shows charging voltage ok with proper meter.and gauge works perfect in new car. so nothings wrong.???
totally flumaxed so sod the gauge.
END. bob.
bob taylor

All the fag lighter is, is a plug and socket. The plug has an element in it, which when pushed in, is connected across 12v and earth, and gets hot.

The plug you are using for the likes of a sat-nav connector, probably has some electronics in it, to drop the voltage to 5v dc. Or, if it's for a device that needs 12v dc, the plug is just a straight through connector, and you'll get 12v dc at the end of the wires.

How are you actually connecting the digital volt meter to the fag ligher?
Lawrence Slater

the volt meter has two prongs or spikes for a better word.touching one on the live stud in the end of the socket and the other on the socket casing as the plug would be. shows 12.8 volts and up to 14v when revving engine to 2k.
bob.
bob taylor

V/odd.

So exactly what type of dig vollt meter is it?

You say, "connect gauge direct to battery and it works, not fully lit but flashes correct charging voltage."

Elaborate on "not fully lit". Doesn't it have a digital numerical readout?
Lawrence Slater

lawrence.the volt meter is a good one from a well known telecommunications company. yes the gauge has the red digi number read out. on battery it reads the voltage and on running the numbers flash on and off at the correct charging voltage.no charging voltage when in fag socket.but in vauxhall charging voltage lights up ok. what i think i need to do is when i go to my car show in may is to try it on another car with dynamo. this is the only thing that i think it has to do with or possibly regulator.
regards bob.
bob taylor

Hi guys,


You may find the unit is getting affected by the magnetic field of the dynamo.


My digital multimeter sure does, I usually use some long jump leads attached to the battery to get the meter away from the car when checking the charge.



Just a thought.


Regards Steve
SR Smith 1

will try that tomorrow steve.
bob taylor

Should have gotten yourself a kamasucki there youngnman

Hahaha

That really is strange... what happens if you hook up a differant volt gauge in the same manner

Prop

Prop and the Blackhole Midget

dont know aint got one.
bob taylor

If I have this right. You connect to the battery when the car is running, and the digi vm reads correctly.

You connect to the fag lighter, and when running the vm reads incorrectly.

So there must be something wrong with the fag lighter connectiions in some way.

Pull the live lead off the fag lighter, and measure the voltage on that against a good known earth, when the engine is running.
Lawrence Slater

right lawrence. im killing myself laughing here.going round in circles. hahahaha.

we have volt meter-vm
we have volt gauge-vg

this is all with engine running.

vm on battery-reads ok
vm on fag lighter reads ok

vg on battery reads but led flashing on and off.
vg on fag lighter nothing.

vm and vg on my new car perfect.(with alternator)

hahahaha fink this is right, im loseing it now.
bob.
bob taylor

Glad your ammused. I'm perplexed, -- unless of course it's magic, and then it all makes perfect sense ;).

vm = vg. They both measure/gauge voltage. They can either be a dc meter/gauge, or ac, or capable of both, depending on internal circuitry.

What make and model of 'vg', have you bought?
Lawrence Slater

lawrence, its only a cheap one from fleabay.


http://www.ebay.co.uk/itm/380534160355?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
bob taylor

Ah that makes sense now then. I thought you had two decent digital multi range meters, one with a plug adaptor that fits a car fag-lighter socket. Now I see. A cheap plug in voltage indicator. :). Sorry, I didn't look at the earlier link you gave. I skipped over the text in that post as I thought you were linking to a sat-nav device. That'll teach me to read properly. :).

You're BT (assume that's who you mean) meter is possibly Fluke or Avo? At any rate it will be a decent device with properly calibrated electronics. The plugin meter you bought for that price, is going to have less well calibrated electronics in it, and may well be sensitve (insensitive) to the more variable output from the dynamo. An Alternator will give a far steadier o/p at the same revs, and that's probably why the cheapo works properly on the newer car.

Guessing a bit here, but it could be due to a lower 'sampling rate' of the cheap plugin digital voltage gauge. Also, did you note Prop's comments about the revs? You're dynamo likely doesn't kick in until 1200-1400 rpm, and the voltage may fluctuate too much for the cheap gauge to read properly. Again the decent meter will be more tolerant of that. On the alternator you will get a steady voltage o/p at lower revs.

But that's not the only factor. The voltage regulator (RB106 or RB340) will be switching to keep the o/p steady. Again the more expensive digital meter will be more tolerant of this, as would an analogue meter, but I'm guessing the cheap plug in, just can't cope with it.

-- Irrespective of the advert saying --

①When your car is powered (but not started), the switch of the storage battery is now on, and the normal voltage reading should range between 12.2-12.8 volts. ② When your car has been started, the dynamo is on, and the normal voltage reading should range between 13.4-14.8 volts.

-- I'm guessing they really mean it for use with an alternator. If not, then it's just not working properly.

I'd send it back, or as it was only cheap and free postage, keep it for the novelty value. :) And if you really want a volt meter in the car, fit a proper gauge.
Lawrence Slater

thanks lawrence.
didnt really want to add another gauge to the dash or the gauge pods that screw underneath, just thought this looked like a good idea to keep a check now and again since having problems with over charging. but now regulator adjusted it should be ok. still it will be ok to use to check other cars in the future if batterys go flat to check if they are charging ok.
thanks again bob.
bob taylor

Hey bob

Just a thought...go ahead and run some extra long wires with connectionsand stuff under the dash ... then if you need a volt gauge its easy to hook up, and toss in the passanger seat till problem solved then remove gauge once agian

Prop
Prop and the Blackhole Midget

that looks like the way to go prop
bob taylor

What would be really cool ....if there was an app for that

Just plug in the smart phone, and done

Prop
Prop and the Blackhole Midget

you might have something there mate. ill look into that.
bob taylor

This thread was discussed between 17/04/2014 and 22/04/2014

MG Midget and Sprite Technical index

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