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MG MGB Technical - Common Shorts

Hello team and thanks in advance for your help. Got the old MG insured yesterday and took it for a spin today. Ahhhh! What a feeling....anyway...I was testing if I had a short and the voltmeter between the negative battery terminal and the negative (ground) lead showed 12.5 V. I took out one fuse at a time to see if I could isolate the problem but to no avail. The 12.5 V current was still present. I feel bad for not having checked out the old wiring diagrams first but I need the problem corrected quick. Any ideas or things that I should look for would be great. Thanks in advance.
Matt
Matthew Bennett

Matthew:

What year B? {Cardinal rule when posting! ;-) }

Whatever your problem it does NOT sound like it is very high amperage or you would likely have found your problem rather easily. If you have a radio with memory for tuning, that would be a likely source for a very low amperage current drain as you describe. If you do have that sort of modern radio and most of us do, then a small "leak" is normal and to test for any further "short" you would have to first disconnect that "memory" line to the radio. If you do this and still have the 12.5 V drain, then you really have something to hunt for and a good place to begin your quest might be in the archives. Best of luck!
Bob Muenchausen

Matthew.
"I was testing if I had a short and the voltmeter between the negative battery terminal and the negative (ground) lead showed 12.5 V"
From your statement it isn't clear to me where you were measuring voltage. Can you be more specific? The way I read it, you checked the voltage between the negative battery terminal and ground and read 12.5 V. on a negative ground system? If so it should read 0 V. Clifton
Clifton Gordon

Switch your meter over to 'Amps' (10 amp range if you've got it). Disconnect the 'HOT' side of the battery and measure current flow between the cable end and the battery post. If you get a reading and don't have a 'memory' radio or a clock in the car, start disconnecting things like the alternator, also check for things like a boot light staying on, etc.
Ken Lessig

Sorry team the MG is a 1971 B.
There is no radio memory as this is the same radio that was originallly installed.
I am measuring the voltage between the negative battery post and the negative battery cable.
My understanding, which isn't very much, is that measuring the voltage ( and amperage which I will measure and get back to you) between the negative battery post and the negative battery cable is just as effective and should give the same reading as measuring between the hot post and the hot cable. Is this correct?
I agree that if I am testing this way and there is no short then the voltage reading should be 0v. I thought, again I could be wrong, that if there is any voltage passed then there is a short somewhere.
I hope that this helps. I will measure the amperage passed and get back to you.
Matt
Matthew Bennett

Matt,
I could be misunderstanding, but if you are measuring for voltage between the negative battery post and the negative cable coming off the battery, and you see around 12 volts, then I don't think that indicates a short. To me, it shows that there is corrosion between the battery post and terminal clamp, causing them to be insulated from each other. Otherwise, you couldn't, under any circumstances, get that high of a voltage between these two points. If there's some way it could happen, I'd like to know how? My only suggestion is to take the cable clamp off and clean the post till it's shiny.

Joe
Joe Ullman

Matt
I agree with Joe's comment about corrosion between the battery post and terminal clamp, but you didn't say whether the clamp was attached to the post or not.

Either way a circuit is being made when you measure the voltage. Do you have any permanently powered equipment such as a clock? This would provide the rest of the circuit. If not you'll need to track down a fault somewhere.

Hope this helps.
Steve
Stephen Lee

Okay team;
sorry for being so vague...With the voltmeter connected between the negative battery post and the negative battery cable with the cable not attached there is a voltage of 12.5 V and 4 mAmps. The car is a 1971 MGB which has no radio memory settings or clock. This voltage and amp reading does not change after removing the four fuses from the engine bay one at a time. I hope that this is more clear...sorry about that. I guess my question is...does the fact that the voltage remains after removing the fuses give an idea of where the problem is located?
Thanks again,
Matt
Matthew Bennett

"COMMON SHORTS"

FRUIT OF THE LOOM
( . )

Wasn't me that said that (I wear Hanes myself).
.

Matt. You don't have a problem. The small current you are reading is caused by current flowing through the multimeter when you attach the common lead to ground. When you measure the voltage from the negative battery post to the disconnected ground cable a simple series DC circuit is created by the battery and the multimeter. The multimeter has a high internal impedence/restance and the battery has a low internal resistance, consequently most of the voltage is dropped across the multimeter and very little across the battery, that is the reason you read 12.5 v.
FWIW, Clifton
Clifton Gordon

Matt, I don't think there's a problem either, if you're only seeing 4 mA (4 1/1000's of an amp). But if you're still concerned, try unplugging your alternator while the meter is in line. If the 4 mA goes away, you may have a diode starting to degrade.
Ken Lessig

Thanks guys...I guess the ultimate test is if the battery eventually goes dead. I guess I shouldn't look for work that isn't needed to be done (I think that that is one of Murphy's Laws).
Thanks again,
Matt
71 MGB
75 Austin Marina
Matthew Bennett

Measuring voltage as you have described above should show a voltage only if there is voltage leakage ahead of the battery.

In theory, if your battery is disconnected from any devices which allow current flow to a ground, then you should not be showing any voltage. A simple test of this notion is to simply disconnect the battery cable from the Positive post of the battery, and then measure once again the current flow (voltage) between the Battery Neg. post and the battery grounding cable. You should read Zero volts.

Most all devices except the "memory" of a modern radio, in a car like ours, will be switched at some point to prevent unnecessary current drain from the battery. These switches all lie ahead of the Battery Positive post and begin their branching out at the post on the Solenoid where the main battery cable connects to it. From that point forward you will find that some branches (Browns) are "hot" or "live" all the time, the ON/OFF switching being accomplished within the device to which the brown wire is connected such as horn button sometimes. Other devices, usually green or white wires, have current applied to them ONLY when the ignition switch is in its ON position. These devices may also have a dash switch or their own internal switch (thermostatic?, lever, etc.) to switch the unit on/off WHEN the ign. switch is ON (heater fan, Wipers, etc.)

If any of these branches has a short which circumvents the intended switch, then you should indeed see a voltage, even if it is measured between the Battery Neg post and the ground strap.

You are not wrong in assuming that you may have a leak. My guess is that your best shot is to eliminate any device connected to any of the brown wires before digging into anything else. If those prove OK, then, as mentioned above, take a look at the wiring for your alternator. It is possible for it to develop low amperage leaks, especially if you have a bad rectifier pack. Or you might have some other internal short in the internal voltage regulator.
Bob Muenchausen

Matthew is using the voltmeter as an ammeter *in series* with the ground cable. This is a standard and safe test to see if there is any leakage. A dynamo-equipped car should show zero volts in this test, an alternator-equipped car will show a few volts which is the reverse leakage current through the alternator diodes of micro-amps and is normal. If the meter shows 12.5v in this test then there is a significant leakage, one that may well be greater than 10 amps and I for one wouldn't risk blowing up my meter by trying to measure the current. If you connect a test-lamp in place of the meter and you only get a dim glow then you know the leakage is small, but still significant. But if it glows brightly then you know you have a major leak.

Back to the cause. Make sure all loads are switched off including doors and boot/trunk shut. If it's still there unplug the alt. If it's still there remove the bottom fuse in the 2- or 4-way block; ditto the hazard in-line fuse. If it's still there you will have to start pulling connectors in the brown circuit, which feeds things like ignition switch, main lighting switch, starter relay (if fitted), ignition relay (if fitted).
Paul Hunt

It looks as if I gave some bad information on my post above stating that Mathhew has no problem. Paul Hunt has given some good advice that should pinpoint your problem. Please ignore my comments, sometimes our good intentions backfire, I should have spent some time in the archives before giving advice. I apologize for not doing better. Best regards, Clifton
Clifton Gordon

Clifton, welcome to the club! I have had to eat my words more than a few times too, as you probably know. I appreciate a lot more all the times you ARE right on the money. :-)

Paul is the final word on this stuff and I always appreciate it when he gives us the benefit of his clear thoughts on how to get to the heart of a problem.
Bob Muenchausen

E&OE!

Which means 'Errors and Omissions (from me) Excepted!'
Paul Hunt

This thread was discussed between 26/03/2002 and 30/03/2002

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