MG MGB and MGB GT Tech Talk
So, the 70 was running just fine until I hooked the wiring up to the dashboard assembly. The alt light didn't go out though.
Now, the alt light doesn't come on at all and the car doesn't start. Is it possible for a tach to go bad such that a car won't run? I ask because I did have the two white wires plugged to each other, then unplugged them and attached them to the tach.
The wire through the tach is just a double wire loop creating inductance for the tach pulse pick up. If the wire is open, or if the connections you made to the bullet connectors are poor, then current won't flow from the switch to the coil.
Try reversing the process, and check continuity through the tach.
warmly,
dave
The alt light has two wires to it: white and brown/yellow stripe. White comes from the ignition switch. Brown/yellow from the alternator. BOTH should carry 12v with the engine running at 1500 rpm (say) and alternator working.
The white should have 12v and the b/y have 0v with the key on and engine off. The white feed comes from a sleeve with 4 white wires that is up under the dash just left of the steering column. One of those 4 comes from the switch and the other 3 take the voltage somewhere (fuse box, ignition, and alt light I think). You should be able to check the voltage in the white feed at the fuse box and if there's no disconnection or special resistance, the value should be the same. The b/y comes from the alternator and you should be able to find it there and check voltage there.
If the light flickers at road speed it says that either the battery is stronger than the alternator (alternator not working right) or the alternator is stronger than the battery (battery in bad condition)
(the light comes on when one wire carries more voltage than the other, this allows a current thru the filament and the bulb glows. the greater the voltage difference, the more current, the brighter the glow.)
The tach (on my car, a 73...maybe not yours) has a green power lead (which comes from the fuse box opposite the white) and a white/black lead which comes from the coil. No white (only). Hmm. Yes, your 70 has a white lead from the ignition switch (adding to those mentioned above, I suppose) then white to the coil and the w/b from coil to distributor. (when the coil field breaks down to fire, this must send a pulse back from the distributor to the tach.
So the distributor is getting it's low tension feed from the tach. If you plugged the two whites together you'd have feed for the ignition but the tach wouldn't work. Will it start then?
Are you sure you have the white lead connected to the correct tach terminal?
One is male and one is female. It only goes one way. I will take the tach out of the equation and see what's what.
It's almost game time so this will have to wait. I did disconnect the whites from the tach and plugged them into each other. Still no alt light and and no starting. I'm not hearing the fuel ump now either. I did bump the white/brown on the starter relay and the engine turned over. I'm wondering if the relays always do that, or if this one is defective.
There must be the wire of all wires somewhere under the dash. Thinking this over since the game and checking today, there are no gauges lighting up, no fuel pump, no start, although the starter does crank over by the key. There must be some wire that I have knocked loose.
And now it is running again. But, there is no fuel gauge, oil pressure gauge, and the alt light won't come on due to a bad connection at the socket. There is power at the oil pressure sender on the engine block. Hmmm.
Edit: So if I bump the alt light socket against metal, the light lights which means that the white wire must be hot.
When I wired the alt, I just used the fat brown and the brown/yellow. Maybe I should hook up the little brown anyway? I know the diagram shows all of the browns connecting together at the starter.