• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

MGB 1980 mgb no power from ignition switch

Share this page

H

HelpingTRresto

Freshman Member
Country flag
Offline
Hi all, need help with electrics. some one had reversed battery polarity. what got fried. new battery installed but positive terminal gets warm like a short or back feed. i put volt meter between post and cable with cable off of battery and get 12.5 volts with everything off like something is draining battery. there is no power coming from ignition switch when turned on. no dash lights, will not crank over. Thank you for your help.
 
Not certain anything would melt and cause a dead short just from swapping polarity. Something else is going on IMHO. The brown wire at ignition switch is supposed to be +12V, not sure of source on a 1980, the older (pre-'68) ones got battery voltage from the solenoid, IIRC.

Try the diagrams on Advance Auto Wire for good info. Sounds like you've a mess on your hands, good luck!
 
Later cars may have a starter relay and or fuse inline. As doc advises, consult a good diagram to trace what sort of electrical equipment you car has.
 
If the + terminal on your battery is getting warm something is pulling quite a bit of current (like wiring-harness damaging amounts). What I always do when hunting down suspect wiring (or testing wiring I have not examined fully myself) is use a hand-made current limiter. A standard sealed-beam headlight works well - put it in series with the + of the battery so even if you get a dead short, the worst that can happen is the light comes on full strength. If the light is off or very dim, there is only a small amount of current flowing. Using a simple rig like this, its easier to find stray current leaks by disconnecting various things until the light goes off.

My first suspicion is reversed polarity could have damaged the regulator inside the alternator (which is connected directly to the battery and is unfused). I would immediately unplug the connector at the back of the alternator. If you use a lamp as a current limiter, the light would go off when you unplug the alternator if that is the source of the drain.

I've been using that kind of a rig to test the wiring in the car I'm working on now since I wasn't sure what had been damaged by the previous owners. Its a lot less stressful testing unknown circuits when you know the worst that can happen is the headlight bulb comes on, and you'll never draw enough current to melt a wire or start a fire.
 
I've always just used a 12V test light ( Snap-On, yes) for quick diagnosis. If it gets more serious, isolating all harness wires and a VOM to check each for crossover, continuity or inappropriate ground.

Had a look at the wiring diagram, Yakko has a good point. Pull the alternator plug and see if the draw disappears. But the brown wire at the ignition switch should be 'hot' regardless. Sounds like that's not the case unless the switch is faulty.
 
Grantura_MKI said:
The test light I suggested is fitted with an internal volt meter as well as the bulb.
Click to expand...
Very classy! Mine is the Ancient "vintage" variety... CT4D. :giggle:
 
YakkoWarner said:
I just was making do with whatever the heck I had laying around - a couple chunks of wire and an old truck headlight.
Click to expand...
Same principle (and a lot less money)! Back in my salad days I soldered a couple wires to a 12V single filament taillight bulb.
 
Thanks for all your help. I disconnected the altinator, no change, still drain on battery. my friend told me that before the electrics died the red light on dash glowed bright, that probably started happening when the battery was reversed. Am I correct that this car 1980 is neg earth? even though ground cable is red? I checked volts to main relay on inner fender toward front of car by the fuse block. Brown wire 12v with ign switch off. switch on volts 4.9, pos post got warm again. Thanks for input.
 
Some random thoughts: Ignition light on with engine running (meaning the alternator is being spun) usually means the regulator or the alternator brushes are defective. And yes, the 1980 MGB is negative earth. Have you copied the wiring diagram on the Advance auto wire site? With your test light in between positive battery terminal and cable, you will have to disconnect things one-at-a-time until the light goes out. I'd start with the ignition switch feed (brown wire). Leave the alternator plug disconnected, pull the fuse in the fusebox that has brown wires into it. That takes out (purple) horn, seatbelt module (these have been problematic in the past) and cigar lighter. When the headlight goes out you've found the offending circuit, not necessarily the root cause.

All the brown wiring should be +12V regardless of the ignition switch position. Relays are generally polarity insensitive, solid state ones being an exception.

EDIT: Pulling fuses will also indicate circuit, but not which component is the problem.

Page 33:
Advance Auto-Wire
 
Last edited:
Some folks have found it useful to have the appropriate diagram printed large and laminated by a place like Office Depot or a print or sign shop.
 
Hi folks. Good news, all is well. alternator had to be rebuilt and new regulator. found a small ground in positive cable where it makes a bend under the body where it exits the battery box. also positive cable had a aftermarket battery clamp installed that had rusted and corroded where it connects to cable. replaced it and cleaned cable and post. installed new starter relay. Success! Thanks to all for your help. Cheers, Jim.
 
Friend of mine did it in his Jag. Ruined voltage regulator, generator and wiper. Glad you found it. I got burnt on the wiper motor is how I found it.
 
We've found electrical issues that boggle the mind. Pop-rivets thru a harness, tail lights went bizzaro. Ground straps corroded and the throttle cables melted, just to mention a couple.
 
Yep, Doc, find all sorts in owner fixed and repaired cars.
 
Back
Top