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Electrical issue

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Robby

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I recently purchased a 76 MGB that has some electrical issues thanks to the PO I think. One issue is that the red light on the dash (not sure what it is but next to the High Beam light) stays on when the switch is off but goes off once the switch is turn on and stays off after car is running. Obviously there is a short somewhere but not even sure what that light is for so not sure where to begin.

Any advice will help.

Robby
 
I think you're talking about the "IGN" lamp. To check (easy) disconnect the brown/yellow (small) wire off the back of the alternator see if the light goes out. Bad diode in the alternator can do that, and still charge (causing light to go out). May be marked "IND" for "indicator" on back or alternator.
 
Will do - but why would it stay on as long as the switch is OFF and go off when igntion is ON? Sounds like somethign connected wrong ??

Thanks

Robby
 
Probably is something else wrong. This eliminates any issues with the power to the alternator back-feeding via a bad diode.
Alternators are "hot" all the time.
That "IGN" lamp is fed from the same circuit that drives the coil. You may not have enough current to make the coil hot, or fry the points, but if the ignition switch is wired correctly, we can proceed.
If you disconnect the wire, and the light goes out, THEN use a voltmeter and see what voltage is on any of the fuses in the fuseblock. Realizing that power will be split between fuel pump and coil, all other loads like wiper motor, washer pump, fan, turn signals and emergency flashers will be off, you won't get battery voltage, rather a split load, part of it dropped through the lamp, the other through the loads.
Tell me if the lamp goes out, then tell me what the voltage is (key off). If battery voltage, we need to look elsewhere, but if you have battery voltage present, the car would not shut off.
 
Thanks TOC - will go through the procedure tonight and see what I find out. I assume you mean to disconnect the hot lead at the coil?

Electrical issues don't scare me if I have a good starting point so hopefully you ahve put me in the right direction. Will report back with my findings.
 
No, disconnect the IND lead at the alternator. You still have the electric fuel pump (at least) in the circuit, unfused, if I recall.
 
OK - been playing with the car some and here is what I found. FIrst let me say that I have no idea what the DPO did so colors, etc probably mean nothing right now.

Ihave three wires going to the alternator - a large brown, smaller brown and a smaller green. With the swtich off, the light glows when either of the brown leads are conencted - if only the green lead is connected, the light goes out. With the brown leads off the alternator and key in off position, the voltage at the fuse block is 12.5V on the very bottom fuse (if looking from the top of the engine into the bay) only (don't have a manual yet so don't know which one that is) but 0 volts at all others. Voltage at VR is 12.5V and at coil is 0. Without connecting anything else, when car is running, everything stays the same except voltage at coil is 9.5 volts.

Light - on when switch off and both brown wires connected, off when engine running. Off when both brown wires DISconnected, stays off of course when engine running

Also noticed when engine is running and I connect the large brown wire, the alternator produces a small whine and the engine runs a little rough. Disconnect the brown wire and the noise goes away and engine smoothes out.




TOC said:
Probably is something else wrong. This eliminates any issues with the power to the alternator back-feeding via a bad diode.
Alternators are "hot" all the time.
That "IGN" lamp is fed from the same circuit that drives the coil. You may not have enough current to make the coil hot, or fry the points, but if the ignition switch is wired correctly, we can proceed.
If you disconnect the wire, and the light goes out, THEN use a voltmeter and see what voltage is on any of the fuses in the fuseblock. Realizing that power will be split between fuel pump and coil, all other loads like wiper motor, washer pump, fan, turn signals and emergency flashers will be off, you won't get battery voltage, rather a split load, part of it dropped through the lamp, the other through the loads.
Tell me if the lamp goes out, then tell me what the voltage is (key off). If battery voltage, we need to look elsewhere, but if you have battery voltage present, the car would not shut off.
Click to expand...
 
Oh, geez, I said engine OFF! You open up an alternator while running, it goes to 90 volts and 90 amps and FRIES! NEVER DO THAT AGAIN!
 
Now that I've taken a blood pressure pill, and numerous deep breaths, I'll discuss the findings.
The diagrams say brown and yellow, should have been a smaller wire, and if original, the alternator probably has "IND" above the plug for that wire.
Power at the lower fuse, is from the battery, and is correct. If none other, you have no power bleeds inside the cabin, which is good.
Coil should be 9-10V, as there is a ballast resistor in the feed line to it.
One of the wires from the back of the alternator is the lead to the lamp, only. Doesn't go anywhere else, usually smallest of the bunch.
The other side of the light is white, goes to inertia switch, fuel pump, second fuse up, coil, and after the fuse, everything else that comes on when ignition is on.
The two brown wires both come from the starter solenoid, battery power.
So, the green wire should be it, at the outer end of the plug on the alternator, and it ONLY goes to one side of the lamp.
 
Don't get your blood pressure up too much as I am aware of what happens when you run an alternator with out the connections <span style="font-weight: bold">BUT</span> since this car has been sitting for a while prior to my purchase, I have already purchased a new alternator to install AFTER I fix all the electrical issues. The current alternaotr is going to teh trash shortly. I didn't feel comfortable running on an alternator that I wasn't sure of it's age, etc. The previous owner has screwed up 90% of the wiring in the engine bay, has cut out all headlight, taillight and horn relays and by passed them, and when I went to lookat it, he had the battery disconnected and then connected it as if it were positive ground so I am sure he fried some other things. Disconnecting the lead from the alternaotr while running was of little concern to me = I wanted to see if I had a short in wirign back to the dash or what. Plus all the wiring is screwed up so much that following color codes from a wiring diagram is going to be tough.
 
That's odd because the light will only come on if either of the brown wires are connected and does not change if the green wire is disconnected. Something is not wired correctly apparently

TOC said:
Now that I've taken a blood pressure pill, and numerous deep breaths, I'll discuss the findings.
The diagrams say brown and yellow, should have been a smaller wire, and if original, the alternator probably has "IND" above the plug for that wire.
Power at the lower fuse, is from the battery, and is correct. If none other, you have no power bleeds inside the cabin, which is good.
Coil should be 9-10V, as there is a ballast resistor in the feed line to it.
One of the wires from the back of the alternator is the lead to the lamp, only. Doesn't go anywhere else, usually smallest of the bunch.
The other side of the light is white, goes to inertia switch, fuel pump, second fuse up, coil, and after the fuse, everything else that comes on when ignition is on.
The two brown wires both come from the starter solenoid, battery power.
So, the green wire should be it, at the outer end of the plug on the alternator, and it ONLY goes to one side of the lamp.
Click to expand...
 
I went through just about every schematic for a "B", all use the brown/yellow wire from the light to the voltage regulator or alternator except one or two years of generator which use an NB (brown/black) instead of NY for the lamp, but to use NY to the generator proper from the common post on the regulator. Some earlier B's use a 4-post alternator, with the two for "IND" jumpered externally, one at least uses a 2-post alternator, one for battery, one for "IND". Nowhere have I found a green wire to any alternator in any schematic, period. Green is almost always used inside the cabin for switched loads, like t/s, wipers, washers, radio, etc. Green/White sometimes for ballast bypass off the starter solenoid, and if cooling fans factory installed, solid green for that application. Gren/White for front turn signals, Green/Blue for temp sender. So, either someone has changed the wire out, or something wrong is plugged into the "IND" port on the alternator, which may explain why the light stays lit when the IND green wire on your car is unplugged.
Do you have any other issues of things that don't work? I'd be looking to see where that brown/yellow wire goes.
 
Oh the list of things that doesn't work is long:

Headlight (including high bean indicator of course)
Taillights
Side marker lights - checked switch and it shows good via VOM
Tachometer
Dash lights
Interior light

But interesting enough, the brake lights work as do the back up lights - fule gauge and temp gauge work as well. The parking brake light also works !!

As I mentioned the PO did a number on the wiring under the engine bay. The taillight and horn relays are both by passed and the tail lights are wired in with the headlights at the relay. It is just a mess in there.

But on the positive side, due to the electrical issues I was able to get a running/driving (well otehr than electrical issues) great bodied, no rust MGB from the $2300 asking price down to $1000 so I don't mind tackling the electrical.

BTW, it is definitely a green wire at the alternator. Granted it shouldn't be but it is green
 
Well, first problem is the indicator light, and I am hoping this is really the "ign" light and NOT a high-beam indicator light mis-wired.
So, if it is the "ign" lamp, for sure, one side is ignition switched side of the ignition switch, period. The other side, one wire only, goes to the "ind" port on the alternator. You unplug the wire from the "ind" port on the alternator and the lamp does not go out, the wires go to the wrong place, period. A ground on that wire won't do that (in fact, it will be opposite....key on, light on, key off, light off).
Is the wiring loom open in the engine bay? If not, open it. Follow that green wire back, see where it goes. See if there is a bullet connector in the loom, near the firewall, mis-plugged, Look for a brown/yellow wire in the loom. I know it;s hard, but look under the dash, follow the two wires out of the lamp, one should be white, ignition feed, other brown/yellow, see where it goes, see if it is mis-plugged.
 
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