MG MGB and MGB GT Tech Talk
I have power to all gauges and they work. Problem is when I shut the car off, it stays running until I pull the hot lead off of the coil.
The ignition switch, which is new, seems to be OK. There is no continuity on the white wire from the brown when it is off. Is when it's on.
Could this be the ignition relay?
DD
There's is 12 volts to the white wire, in the "off" position. Are we accustomed to seeing shorts in new ignition switches? Or is there another possible explanation?
:(
Dan, What was the last thing you worked on before this started. That will sometimes give a clue.
John
Could be the hand-brake warning light diode? Mine had the same issue, but as yours is a US-market B it could be different.
Best way to check is to see if dropping the hand-brake stops it. If it does then that's your problem
Hi JD,
I had to re-wire the brown circuit to the starter and installed the ignition relay. All seems to be wired properly. I've removed the relay temporarily, but I still see voltage on the white wire coming from the switch, with the switch in the off position, by placing the meter leads in between the brown terminal and the white wire.
From a continuity standpoint, I see none between the brown and white wires but I suppose that's not the point here.
Car starts fine, with a new coil and new plugs....crummy gas and all.
Just seem to have a short............with constant power to the coil.
Dan, in 76 there appears to be 2 grn/wht wires on the switch side of the coil. One has a ballast resistor (limits voltage to ~9v when the car is running, not starting) in it and comes directly from the switch. The other one comes from the starter solenoid, it is there to provide 12V to the coil during start. Use an ohm meter an check the voltage on both of those wires independently-you may have a short in the solenoid wire, or you might not.
Check for voltage on the white wire when the car is not running. If it is still present then you have a short. If it goes away, then you have got an current path that is sourcing from the alternator. This 'sneak circuit' is normally present through the alternator indicator lamp. Do a search here in the forum for "sneak circuit".
perhaps buy yourself a copy of Rick Ashtley's MGB electrics book. There's a sneak current he describes in there atat causes this problem on later cars.
It does appear to be the sneak current. With the NY wire out of the circuit the car shuts down. Thanks to all on this and to John Davis who just spent the better part of his afternoon on the horn with me!!
DD
The thinner of the three white/brown wires at the fusebox needs to be snipped off and joined to the single white wire on the ignition relay. This will eliminate the sneak current. This was a wiring fault that MG eliminated sometime in 1977/78.
edit: It would be great if you could post a photo of your wiring so that we can confirm the current layout - I don't want to send you off snipping wires if it isn't necessary for your particular application.
All,
I have 20 barrier rectifiers to resolve this sneak current, coming in the mail. If anyone needs one, just let me know! Early Christmas gifts!!
http://www.mgexperience.net/phorum/read.php?1,1061278,1061909#msg-1061909
More diodes is not the answer.
Move that white wire. Or sometimes wn.
Morris, That wire that Ed was talking about! Where does the other end of it connect? I'm not sure which one he is refering to.
John
Not sure which way to go here. The library article seems to be opposed to the post that Morris refers to.
I should point out that my car has no intertia switch (long ago removed), so when I wired the white from the ignition switch, I just bundled all exisiting whites together...that would be the pump, a white to the ignition relay, and whatever was left from the previous inertia switch.
http://www.mgexperience.net/article/mgb/ignition-run-on.pdf
or
The proper 'fix' is NOT to use a diode but instead locate the white wire from the ignition warning lamp at the Inertia Switch, clip it and extend it with some 16 or 18 gauge wire routed back to to the #3 fuse (or optionally, just find a spare GREEN connector and connect it there).
There were originally three white/brown wires at the fusebox. Two of them are 14 gauge, one of them is 16 gauge. It is bundled with one of the 14 gauge wires and attached to the fusebox with a spade connector. Snip the 16 gauge wire off, leaving only two 14 gauge white/brown wires feeding the fusebox. Take the wire you just snipped off and swing it over and join it to the single 16 gauge white wire feeding the ignition relay. This will solve the problem on a stock vehicle, but again I don't know the exact details with your car.
Diodes should not be necessary!
Ed, Dan has already fed the coil from the white and not the Brn/W wire. I was telling Dan, I had a 78B that had what appeared to be a factory installed diode at the alternator. It looked exactly like those on the handbrake circuit. Rectangular in shape with 1/4" terminals on it and it worked. It must be adding a little bit of forward resistance to the circuit.
John