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MGB 78 MGB issues

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J

Johnson 78

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Hi! Newbie from norhern MN! Having electrical issues w/my 78 MGB. Tach, blinkers, wipers, temp gauge and fuel guage all quit working! Working then dead!! Any thoughts?
 
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Welcome to the forum! Your question would be more appropriate in the MG forum, some very knowledgeable people there but they don't always read the "New Member Introductions" (which is what this is).

I know very little about MGB, but the schematic at https://www.advanceautowire.com/mgb.pdf indicates that all of those things run from a single fuse, so that is where I would look first. If the fuse appears to be good, use a voltmeter or test light to confirm that power is getting to the fuse (on the white wire), and getting through the fuse to the green wire. Sometimes they can fail and not look like it, or not make good contact in the holder. The holder itself could even be bad.
 
Welcome to the BCF. TR3 Driver gave you some good starting info, but as he said, the marque forum will probably get you more answers, and much faster. Please tell us a bit more about yourself here so that we get to know you a bit better, and maybe add a photo of your rider here as well.

Enjoy the Ride.
 
Yes, welcome indeed, and check in over on the MG side.

Randall's advice is a great place to start and a likely fix. And always remember the cardinal rule for electrics, particularly of the British persuasion: Check grounds; these cars are notorious for gremlins to pop up where black wires attach to the body.

:cheers:
Mickey
 
Had the same issue with a friends 78. One of the fuses is not connecting. It had corrosion on the tabs. Clean them up . I can't remember which fuse though. But my bet is all of them need cleaning.
 
"If the fuse appears to be good, use a voltmeter or test light to confirm that power is getting to the fuse (on the white wire), and getting through the fuse to the green wire. Sometimes they can fail and not look like it, or not make good contact in the holder."

TR3's advice is spot on - the Mark I eyeball is the worst test instrument in the world when it comes to checking fuses. Fuses can often open up at one end or the other where you can't see the break. Get yourself a decent multimeter (doesn't have to be an expensive one) and learn how to use it, then check each fuse using the resistance scale on the meter set to the lowest resistance range - a good fuse will read 0, anything more than that indicates that the fuse needs to be replaced.

The next thing to do is (again, as suggested by TR3, is to go to the Advance Auto Wire site at: https://www.advanceautowire.com/, click on Stock Schematics, then on MGB. Scroll down to the year of your car and make multiple copies of it. Use that to chase the wiring of the section that doesn't work, making sure that all connections are good and solid. Next start looking at ground connections and make sure that they are making good connection (see the article Electrical Connection Preparation in the Other Tech Articles section of my Homepage at: https://homepages.donobi.net/sufuelpumps/ and follow the instructions in the article).
Cheers,
 
Like Dave said, make sure all connections are clean and good contact. The bullet connectors are prone to getting dirty inside and also will sometimes crack inside making the connection poor. Bullet connectors are available through the MG parts distributors if needed. Moss, LBC etc. PJ
 
Thanks for the assistance!! I've checked the fuses and looked for grounding issues, found non (doesn't mean they aren't there). But in checking the fuse holder, I found if I jump a wire on the hot side of the box, all the instruments work, but don't fuction with the ignition! Maybe a bad fuse holder?
 
Not likely to be a ground in this case, IMO, since all those things don't share a common ground. That's not to say you don't have ground issues, just that it is more likely that something else is making all those things not work. (Occam's Razor : the simplest explanation is usually the correct one)

If jumpering on the hot (ie input) side of the holder makes things work, then it probably isn't the holder. Since the AAW schematic shows an ignition relay for 78 onwards, I would be looking at it next (assuming you have power to the fuel pump and ignition warning lamp, indicating that the ignition switch itself is working).

BTW, Harbor Freight has digital multimeters for well under $10. Cheap enough that I carry one in every car. I don't necessarily trust them to be perfectly accurate, but they are good for checking voltage present/not present. Or, they have unpowered test lamps even cheaper.
https://www.harborfreight.com/extra-long-circuit-tester-43700.html
 
Actually, in my experience, the accuracy of those Harbor Fright meters isn't half bad. I have a bunch of them here. Some time ago, I checked the voltage ranges against my laboratory voltmeters and they agreed within a few millivolts at about 10 volts, which is way good enough for automotive work. I didn't check the resistance, current, or AC ranges, but I'd expect them to be comparable.

The only problem for automotive work is that noise in the car's electrical system can really upset them, but that's true of any digital meter. They should be fine for measurements with the engine not running.
 
Sarastro said:
Some time ago, I checked the voltage ranges against my laboratory voltmeters and they agreed within a few millivolts at about 10 volts, which is way good enough for automotive work. I didn't check the resistance, current, or AC ranges, but I'd expect them to be comparable.
Click to expand...
Doesn't surprise me, although mine are much farther off than that. (They even disagree with each other by several percent.) As usual, the problem is a complete lack of quality control @ HF. You got good ones, I didn't, they sell both kinds.
 
Car starts and runs great! Installed new ignition switch when issue first showed it's ugly head! Wipers will jump a little when I turn the ignition to start!??
 
Johnson 78 said:
Car starts and runs great! Installed new ignition switch when issue first showed it's ugly head! Wipers will jump a little when I turn the ignition to start!??
Click to expand...


good news! Now might be a good time to do those grounds as I suspect the reason your wipers are jumping is because some electrickery is finding its way through.
 
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