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78B Elect. System gremlin!


Freshman Member
Ok! Here we go. 78B with something draining the battery. I pulled the Alt.took in to have it bench tested. Alt. came back GOOD! Pulled dead bat.rolled into local chain part store, They ran it through their test, came back good. Had them charge it for me. Came home installed car cranked on 1st. try. As a test I pulled the fuse on the Brown circut before heading off to bed. This Am. went out to the garage and now power!!! I was reading about coils on the prev. post here. I have a coil installed with a tag No ext. resitor required. I do have a resitor but sure if it works!
Could this be the problem I'm looking to solve? Or am I grasping at straws. I have cleaned all the fuse box conections, Starter works like a champ (when I have some juice). What could cause such a rapid discharge?
Any pointers would be helpful at this point!

Ps. We pulled the dash/instuments last week for a good cleaning and reinstalled. Could there be something we crossed in the elect. that would cause this problem?
Thanks again for this site and all of the MG owners out there willing to help the NEW GUY!!!


Jedi Knight
Country flag
The coil is most likely not your problem. If the coil was draining the battery when the ignition was off, then that means the coil is always getting power and you wouldn't be able to stop the engine. Get yourself a wiring diagram, and look at which fuses power which circuits. You might pull individual fuses and try to narrow down which accessories might be drawing power. If you have the owner's manual it should also say which fuses power which devices. I'm going by what I can think of on my Midget, but the MGB of similar vintage should have similar electricals. Off the top of my head, things that are not switched by the ignition are the courtesy lamp, trunk lamp, horn, hazard lights, headlights, parking lights, and cigarette lighter. Are all of your electrical parts working properly? If they don't work, that doesn't mean they can't have a fault that is both preventing them from working and draining the battery at the same time. Since you can't tell (easily) if the trunk light goes out when you close the trunk, try pulling the trunk light bulb out. If you're still draining the battery, try unplugging the alternator, but DON'T start the car unless it's plugged in. If one of the diodes in the alternator is dead, it can still generate power, although not a full capacity, and it can still drain your battery. I don't know how thorough the alternator testers at the part stores, but one hopes they can detect if all the diodes are working. Hope this helps!


Freshman Member
Thanks for your reply! I had pulled the fuse on the brown circut after having the batt, charged wich should have isolated all the constant power acces. I would think. Itook the Alt. into a shop that rebuilds Alt. so I would hope they know to test them. Sounds like I need to take the alt. in to get a second check/opion.
Thanks for the input!!!


I found the schematics at https://www.advanceautowire.com/ to be really good. From your description, it sounds like you probably pulled the fuse for you unswitched accessories. Which would include (but isn't limited to) your horns, trunk light, map light, clock, cigar lighter, and a couple other things.

If you haven't already, grab the MGB, MGC, MGB V8 pdf from the above site and print out the page for your year car. It's in color so it's a bit easier to read than most B&W wiring diagrams. It's possible you have a short in any one of the accessories that's causing a constant draw on the battery. Or maybe the battery (even though tested good) could be on its way out and the power draw from the clock is draining it over night? *shrug* I'd check with the wiring diagram and see if any of the other accessories are not shutting off properly.


Freshman Member
It looks like another fun weekend in the garage! With a cooler of beer!!! /ubbthreads/images/graemlins/cheers.gif
Thanks for your post!


Jedi Knight
Put a multimeter on the battery. Unplug each fuse one at a time and watch the meter, When the meter goes up you found the bad circuit. If you don't get a hit from the fuses do the sme thing with the brown wires going into the fuse box. This works for me and maybe you can use it,too. By the way Sears has a great digital voltmeter for around $10. The same as my Mac one but a lot less expensive.


Country flag
If you have access to a digital multi-meter, there is a simple method to determine if you have a draw on your battery when the circuits should be off.
This assumes the battery has a decent charge.
1. With the engine stopped and ignition off disconnect the earth (negative for this example) battery cable.
2. Set the multi-meter to read amps, for the initial test.
3. Connect the negative meter lead to the battery negative post.
4. Connect the positive meter lead to the negative battery cable.
5. If the reading is less than the milliamp scale on the meter (often 300 ma), reconnect it to read milliamps.
6. I would expect an MG would probably read less than 50 ma, with a radio, clock etc.
7. If the reading is over 160 ma, I would consider this an indication of a problem.
8. With the meter still in place, remove fuses, and disconnect circuits to isolate the general problem area. You will know you found the circuit when the current falls off.
9. At this point you can use the schematics referenced earlier to isolate the individual component.
10 Don't forget to return the meter leads to the normal voltage reading positions when done.

Hope this can help.


Jedi Knight
Perfect Jeff. I think your way is the simplist at this time. Every one should own a Multimeter now that they are so cheap. My first one was outofthisworld in money and used a strange battery. Mine now uses a standard 9 volt, boy times have changed.


Freshman Member
Found the problem! The brown/yellow wire to the Alt. was conceted to the wrong post on the Alt. I learned that you can invert the the wire block conector!! Most other cars I have worked on the conector only works one way. Any way charg. system works as advertized now!!
Thank you to the guys who posted a response!!!!
-Mike /ubbthreads/images/graemlins/thirsty.gif


Freshman Member
79 MGB. No power to battery. New battery, new starter, new ignition switch, fuses good. New here so if this is a double post apologies. All other elect systems work. What would cause this? Bad altenator, bad relays, bad something else. Thanks


Jedi Trainee
Country flag
I am slightly confused on the question - you're saying no power to the battery but other electricals work - which makes me think you do have power in the battery. I guess I'm asking for clarification on what isn't working correctly...


Great Pumpkin
Country flag
Alternator. If it's the original Lucas or the reman, regulator is built in. A VOM in place with the engine running (at a bit over idle speed) should read +13V at the battery or a convenient brown wire in the engine bay.
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