MGB & GT Forum
Electrical Problems
Posted by brg1966mgbgt
brg1966mgbgt
Timothy Macsay
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Topic Creator (OP)
Oct 1, 2014 07:17 PM
Joined 11 years ago
16 Posts
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Battery dead again. During a recent clutch replacement on my 1966 MGBGT (positive ground) I have been having electrical issues Everything was put back together, (properly) and the battery went dead on me. Battery was 7 years old. Replaced. Battery went dead again. Generator replaced. 16 + years old. Voltage Regulator replaced. Original with vehicle. Belt at proper tension. All connections taken apart and cleaned. Tested for power drain. None noted. Noticed that at night the ignition barely glows so I'm sure that it is doing the same during the day when it can't be noticed. When first started the Ignition light takes a while to go out and glows bright at 900 RPM. After driving with R's up it does go out. Took a 100 mile trip and tried to start car three days later and battery dead. Looking for ideas. Electrical not my bag.
Oct 1, 2014 08:15 PM
Joined 15 years ago
4,348 Posts
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I will assume you have made yourself available to these modern, great, MGB electrical schematics:
http://www.advanceautowire.com/mgb.pdf
You will have to find your year model. There are some year models with different wiring in different countries.
If you don't already have one, you are going to need a multi-meter.
Disconnect the ground cable from the battery....if you have six volt batteries, put the meter between the battery that has a cable going to ground on the MGB unibody.
Turn all your electrical switches OFF, including the IGNITION SWITCH,
Put the positive Multi-Meter test lead between the ground connection at the end of the ground cable, with the negative Multimeter test lead attached to the MGB body, where the ground cable is attached to the body metal, under the back seat.
With the IGNITION SWITCH OFF, and all other electrical systems OFF, you should read no current (amperes) on the ground cable.
I am going to assume your old Lucas wiring systems has worn insulation on some of the wiring and/or you have electrical components with leakage to ground, which is why your battery is going dead.
ALSO: After you have charged the battery, start the engine and make sure, with RPM at 1500, your battery should be receiving 13.8 to 14.3 volts DC.
More information:
http://www.aa1car.com/library/charging_checks.htm
Roger N. Tanner, Professional Engineer, Retired
Oxnard, CA USA
roger.n.tanner@gmail.com
Roger N. Tanner
Professional Engineer, Retired
http://www.advanceautowire.com/mgb.pdf
You will have to find your year model. There are some year models with different wiring in different countries.
If you don't already have one, you are going to need a multi-meter.
Disconnect the ground cable from the battery....if you have six volt batteries, put the meter between the battery that has a cable going to ground on the MGB unibody.
Turn all your electrical switches OFF, including the IGNITION SWITCH,
Put the positive Multi-Meter test lead between the ground connection at the end of the ground cable, with the negative Multimeter test lead attached to the MGB body, where the ground cable is attached to the body metal, under the back seat.
With the IGNITION SWITCH OFF, and all other electrical systems OFF, you should read no current (amperes) on the ground cable.
I am going to assume your old Lucas wiring systems has worn insulation on some of the wiring and/or you have electrical components with leakage to ground, which is why your battery is going dead.
ALSO: After you have charged the battery, start the engine and make sure, with RPM at 1500, your battery should be receiving 13.8 to 14.3 volts DC.
More information:
http://www.aa1car.com/library/charging_checks.htm
Roger N. Tanner, Professional Engineer, Retired
Oxnard, CA USA
roger.n.tanner@gmail.com
Roger N. Tanner
Professional Engineer, Retired
mayberry97
Jon Z
Mayberry, NC, USA
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Oct 1, 2014 08:26 PM
Joined 9 years ago
115 Posts
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I don't know if it's normal for the Ignition light to be have like that for your year. My 79 does not do that. And i know you checked for a battery drain, but my gut tells me that's what's happening somewhere. When i bought my B, the switch for the trunk light was faulty. . It didn't always turn the light off when the boot was closed causing a drain in less than a week.
Might want to check for a drain again using Roger's steps above and if you find it, look at the trunk light or stereo components (if you have them) first.
79 MGB with Weber 32/36
It's yellow and his name is Brian.
Might want to check for a drain again using Roger's steps above and if you find it, look at the trunk light or stereo components (if you have them) first.
79 MGB with Weber 32/36
It's yellow and his name is Brian.
brg1966mgbgt
Timothy Macsay
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Topic Creator (OP)
Oct 1, 2014 10:07 PM
Joined 11 years ago
16 Posts
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brg1966mgbgt
Timothy Macsay
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Topic Creator (OP)
Oct 1, 2014 10:27 PM
Joined 11 years ago
16 Posts
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I did the test you wrote about. Disconnected the cable from the plus side post (ground) and first ran a 12 volt bulb test light between the disconnected cable and the battery post. The garage was dark so I could see any light at all. Nothing. Next I used the multi-meter set to 0-20 volts. Using the steps you posted the only time I showed any voltage is when I pushed the horn button, turned on the lights in increments ( parking, headlights) turned on the map light turned on the key. Same results with the 12 volt test bulb as I mentioned above. When nothing was energized the bulb did not show any light and the meter remained at "0". I keep having this nagging feeling that the voltage regulator is not allowing the charge to get to the battery like it should. One time I show the voltage at the battery you mentioned while running then later just the voltage of the battery itself. One other person on the forum said that he bought 3 voltage regulators before he finally got one to work properly. Stupid me tossed the old one I had that did work away because I had a new one. The bottom of the old one was badly corroded and was the original.
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Oct 1, 2014 10:51 PM
Joined 22 years ago
773 Posts
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brg1966mgbgt
Timothy Macsay
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Topic Creator (OP)
Oct 2, 2014 11:20 AM
Joined 11 years ago
16 Posts
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dickmoritz
Dick Moritz
Philly 'burbs, PA, USA
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Oct 2, 2014 11:25 AM
Top Contributor
Joined 14 years ago
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brg1966mgbgt
Timothy Macsay
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Topic Creator (OP)
Oct 3, 2014 09:25 AM
Joined 11 years ago
16 Posts
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Battery voltage after full charge is 12.8. volts Voltage at battery at 3000 RPM's 12.6 volts. Which leads me back to the voltage regulator. A week or so ago I was getting 13.4 volts at the battery at around 3000 RPM's. I am seeking a used voltage regulator now. Stupid me, should not have thrown away the old one. At least it worked no matter how old it was.
lil.red.roadster
Bernie Anderson
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Oct 3, 2014 09:53 AM
Joined 11 years ago
1,293 Posts
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13.4 @ 3000 IMO is still low
Bernie
"discere mutari est"
Wiring Diagrams: http://www.advanceautowire.com/mgb.pdf
Paul Hunts very useful site: http://www.mgb-stuff.org.uk
Manual for download: http://www.geomatique-liege.be/MGJP/DocumentsPDF/MGB_Workshop_Manual.pdf
UK MoT guide: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/518634/mot-inspection-manual-for-class-3-4-5-and-7-vehicles.pdf
Some basic guides: http://www.howacarworks.com/
1970 Roadster (First Reg July 10 1970). Std exhaust (Bell SS) AFAIK rest is bog standard
Bernie
"discere mutari est"
Wiring Diagrams: http://www.advanceautowire.com/mgb.pdf
Paul Hunts very useful site: http://www.mgb-stuff.org.uk
Manual for download: http://www.geomatique-liege.be/MGJP/DocumentsPDF/MGB_Workshop_Manual.pdf
UK MoT guide: https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/518634/mot-inspection-manual-for-class-3-4-5-and-7-vehicles.pdf
Some basic guides: http://www.howacarworks.com/
1970 Roadster (First Reg July 10 1970). Std exhaust (Bell SS) AFAIK rest is bog standard
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