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MG MGB Technical - So what have I done wrong?

Hi, I have a 1974 MGBGT. It has been backfiring slightly for a while. I deided to have a look at it this afternoon. I took the dizzy cap off and gave it a good clean.

I got the engine to run better.

It has had a battery drain for a while - so it being bank holiday, I decided to have a look to see where the drain was.

I placed a multi meter in series from the postive terminal of the battery and saw it had a drain of 12.36v. When I placed the terminals across the battery it had a drop of 12.37v.

I looked at the fuse box to start with, and saw that the wires were all covered in Ziebart - so I took them off and gave them all a good clean.

The brown wire had a small non descript wire pushed onto the spade and held there by trapment of the male clip.(placed there by a previous owner) This wire goes under the dashboard - it is a grey coloured wire.

Being the kind of chap who does not like to see this , I spliced this wire correctly into the brown wire.

When I placed the brown wire (male part) back onto the fuse box (female) - the ignition light came on and stayed on.

I disconected the grey wire and the still the ignition stays on.

What have I done please?

Regards

Edd.
E McGee

THE BROWN WIRE IS ALWAYS LIVE. It comes off the starter motor battery live. The fuse on that line should connect only to purple. The fuse next to it is white one side and green the other. The white wire is an ignition switched live. Sounds like you have juggled them. Personally I abore wires tucked into spades, especially when I don't know what they doing, and they are not fuse protected, as this one now isn't!!! Take it off and see what doesn't work!! If it is needed for something which needs to always be on, put it on the purple side, preferably with a "piggy-back" or put in a line fuse. If it can be ignition switched live, put it to green.
Allan Reeling

First learn how your meter works, and how electricity works.

A voltmeter reads potential between two points - this is NOT a "drain".When you put it across the battery posts, you are reading available voltage of the battery. When you connect from battery hot to "something", and get a reading near the across battery number, you are connected to an effective earth, which is the same as the earth cable of the battery; you are measuring the same thing, and any difference is due to a bad connection.

To measure a "drain", you set the meter to amperes, and connect in series from the power source to the (supposed) "load". You can just as well connect in series with the earth cable, since electricity always must return to whence it came. If there is a "drain" you will see an appropriate amps reading, like 1.2 Amps, or 0.5A etc. >>Note that you must set your meter at or above the highest possible "load" range, or you will kill the meter - usually by blowing the meter fuse.<<

Your nondescript wire is powering something, but nobody can tell what.
One possibility is that something did not work, and the wire is a parallel feed to something that is otherwise still hooked up probably in the ignition circuit (W-G). It could also be something faulty. If you put back all wires correctly as you say, then it may be that you have activated or fried the unknown device at the end of the strange wire.

FRM
FR Millmore

Did you do anything else, such as removing the fusebox?
Dave O'Neill2

Do you have an isolator switch? I had to connect a permanent live in order to keep the clock running as it was a pain to keep resetting it. (not to mention the cost of replacing it in the 1st place!)
This is possibly what the wire was for?
Pat Gregory

First a safety message. The earth terminal should always be disconnected first from the battery, regardless of positive or negative polarity, and reconnected last. The reason for this is that if the spanner touches the car body whilst it is on the earth terminal of the battery (all too easy with the MGB) nothing will happen, as the earth terminal is at body potential anyway. However if you remove the 12v terminal first, and the spanner touches the body, you will get a flash and a bang. It could weld itself in place, and it could ignite battery gasses. But once the earth terminal is disconnected, if you then touch a spanner between the 12v terminal and the body, nothing will happen as with the earth already disconnected there is no path for current to flow.

With the earth disconnected, but the 12v still connected, you can connect a meter in place of the earth strap to look for a drain.

Secondly you *should* do as Edd did and use a multi-meter on its 12v scale to check for a drain initially, again for safety reasons. In voltage mode any meter presents a very high resistance between its terminals, so if you accidentally connect them to both battery terminals it will simply read battery voltage. By contrast in current mode a meter presents a very low resistance between its terminals, virtually a full short. Accidentally connect this across a battery and you are likely to cause a spark, which if there *is* a drain may well have caused battery gasses to be in the vicinity, and and may destroy the meter if it doesn't have overload protection.

If there is a drain somewhere, then connecting an ammeter in place of the earth strap, will cause the drain current to flow, which could be considerable, with the spark and gasses as mentioned before. But connect a voltmeter in place of the earth strap when there is a drain, and all that will happen is that the meter will display battery voltage, as Edd describes.

Normally on a dynamo equipped car there should be no drain with everything switched off, doors and boot closed, etc., hence nothing displayed on the voltmeter. By contrast an alternator always exhibits a very small drain from the reverse leakage current of its diodes, but this should be microscopic and not cause any problems. Typically this drain will display as something quite a bit less than battery voltage on an analogue meter, perhaps only half voltage, but due to the higher sensitivity of most digital instruments these will probably display battery voltage or very nearly so. If there is a significant drain then both instruments will display battery voltage. Note that alarms, radios, clocks etc. are usually pulling something from the battery all the time, so these should be disconnected first if you think you have a drain.

The first thing to do, then, is unplug the alternator! Both analogue and digital instruments should now display zero volts if there is no drain. If they still show battery voltage, then you have a drain, so you have to start disconnecting things. First would be to remove the brown to purple fuse, then brown wires from the ignition switch, lighting switch, fusebox, starter relay and ignition relay (these last two where provided), and the control box in the case of a dynamo. If still there, then you will have to remove the browns from the starter solenoid, and maybe even the battery cable. What you do next depends on which brown you removed caused the drain to go - or maybe none of the browns did!

Note that using a voltmeter in place of the earth strap and not an ammeter is again for safety, as any brown wire that happens to short to earth with a voltmeter in series will simply display 12v on the meter, but with an ammeter connecting a brown to earth will effectively be shorting out the battery.

The ignition light coming on could be because the fusebox is upside down, if you removed it to clean it. This is because when correctly installed the front of the top two fuses (on a four-fuse box) are linked together for the parking lights. Get the fusebox upside down and you have linked the purple and green together, which effectively is the same as turning the ignition on normally. If you *didn't* remove the fusebox to clean it, then something else is wrong, which could be a blown diode in the alternator, unfortunately. With the ignition switched off, see if you have 12v on the white and green wires. If you do, then it is that which is causing the ignition light to glow, so it is a matter of taking wires off to find out which one the 12v is coming from.

Note that normally only the brown wire should have 12v all the time, that goes through the bottom fuse from front to back to the purple wires.

White wires have 12v with the ignition, and that goes through the 2nd fuse up from front to back to the green wires.

The red/green wire has 12v with the main lighting switch in the first position, and that goes through the top two fuses from front to back (it can be connected to either as they are linked) to red wires and the parking lights at each corner.
PaulH Solihull

Thanks for this guys - I did remove the fuse box to clean it - Doh!!!!

I will change it around and see if the igniton light goes out,and see what happens.

As for the drain - excellent advice and I will follow it.

Thank you for your advice - I'll report back and let you know how I get on.

Regards

Edd
E McGee

This thread was discussed between 05/06/2012 and 06/06/2012

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