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MG MGB Technical - STARTS!...almost

The Prince has reappeared out of nowhere.
Symtoms: Ignition turns on, energizes the fuel pump, turns the engine over, but as soon as the key is released the engine dies, never having started.
Cure: A wire directly from the battery positive pole to the coil allows the engine to run. I drove it home that way.
I need a permanent fix for this conundrum. Has anyone experienced a similar problem?
All replies are welcome.
Cheers,
Randy Olson
1977 MGB
PS This might be unrelated, but the day this no-start problem arose I connected the new radio to the purple wire at the cigar lighter to allow the radio's pre-sets to be saved when the ignition was turned off.
PSS Another, possibly unrelated, problem also showed up when the car didn't start. The driver side rear tail light was out. It's not the bulb...something else.
randy olson

Hi, Randy
sounds like you've got a coil with a ballast resistor failure....
I'm not too familiar with the later B's , but the symptoms you're describing match up with what I've experienced on other cars. The way the ballast resistor (a special wire or a separate actual resistor) works is to drop the coil volts to 6v or so for normal running. When you start the car, a separate wire from the starter solenoid feeds straight 12v to the coil to boost the spark to give better starting. When you release the key, that connection is broken and the 6v connection reconnected. hence if there's no 6V supply, the car won't run....
Bypassing the 6v as you have will let the car run, but it could be overvolting the coil and will burn it out if you do it for too long.....
If you've got access to a wiring diagram, you should be able to find the wire colours for these wires and check them out.
HTH, John
John Marr

Randy - just remembered the link to an excellent site...
Try this:

http://www.mgbexperience.com/electrical/ballastschematic.html

(you may need to cut&paste into your browser...)

John
John Marr

Some coils take 12V all the time, others drop to 6V as John descibes. Can't recall which was earlier and what year the change was, but no doubt someone will.

Iain
I D Cameron

The ballast resistor had terminals that were fairly corroded, but cleaning them didn't solve the problem.
Then I checked Moss, and the ballast resistor is shown but Not Available.
Is there a way to check the ballast resistor?
randy olson

Are all the circuits working when in the on position? I had a similar problem, the ignition switch failed.
Kimberly

I had a similar problem where the engine started but when the key was released it dies again. It turned out the ignition switch was slightly rotating within its housing causing the ON position of the key to not quite make the correct contacts. A new switch and all was well...

Stuart
Stuart Haigh

I think a low wattage 12 v light bulb should glow via the ballast resitor , say 5 watts = 30 ohms , the coil is about 2 so the ballast must be =.
Put it at either end , should shine normally at the 12v end, with the coil disconnected it should not dim very much at the end that would feed the coil , lumens per amp is non linear , but up to 50% reduction should be OK .No glow R is open .
A multimeter would make this a breeze , 12v one end 6 v the other with the coil in circuit .Open resistor 12v one end 0v the other .
S Best

In checking a few things last evening, when I turn on the key the fuel pump no longer starts clicking. Before it did, even though the car wouldn't start because when the key was released it quit.
Things are pointing to the ignition switch as the source of the problem. Can this be tested using a test light or voltage meter?
I played around with the test light and at the starter relay switch the light only lit when I touched the BROWN wire. The other wires didn't light even when the key switch was turned to the ON position.
All the fuses (4) are replaced and most of the connections on the coil and assoiciated wiring have been cleaned and a thin coating of dielectric grease has been applied.
If the ignition switch is the culprit, does the whole assembly including the steering lock assembly need to be replaced? If so, its much more expensive, and there are two bolts that appear to be difficult to remove...drill them out?
Anyway, testing procedures are the first step. Any thoughts?
Cheers,
Randy
1977 MGB
randy olson

Randy,

The harness connector for the ignition switch is about where the steering column meets the dashboard. So if you own a volt/ohmmeter and can read a wiring diagram at all, you can probably test the switch in place.

The electrical part of your switch is separate from the keylock (a.k.a. steering lock). If it's anything like my '74, there's a tiny screw screwed upwards into the very bottom of the steering column right under the ignition switch. That's all that holds the plastic electrical part of the switch in place. I bought one of those a couple of years ago, it was about $45.

Matt Kulka

This may be unrelated, but the problem with the car starting coincidentally began right after I connected the radio pre-set station wire to the car. I connected the radio to a PURPLE wire in the console that was part of a plug with several wires in it but wasn't connected to anything...I believe it was a plug for connecting to the radio. I simply cut the PURPLE wire in this bundle and connected it to the radio.
It seemed harmless enough at the time, and I still don't see why it would be a problem with the car's ignition, but...
Any encouraging words would be appreciated.
Randy
PS The radio works great!
randy olson

Purple wires are for miscellaneous things like courtesy lights and annoying buzzers, so it shouldn't have made any difference in your problem. The plug that wasn't connected to anything could be the key: While installing your new radio, you could have accidently disconnected the plug...it may have not been properly connected in the first place, or it could have deteriorated over the years.
R. L Carleen

The plug that wasn't connected to anything had been like that for a long time, months in fact. So I don't think it's a factor.
Randy
randy olson

IT could be an ignition switch or wire failure, BUT:
This is the characteristic failure of the Ignition relay in front of the fuse box. There is no ballast resistor as such - it is a wire built into the harness. Any resistors you see are part of the original electronic ignition, or artifacts of that. When running, the power goes through the resistance wire to the coil, giving 6Volts. On start, the ign switch feeds power directly to the coil giving 12V, and the car starts on that. When you release the key, the key turns the relay ON, and power from the relay goes to the resistance wire and almost everything else the key turns on. The relay is a standard ISO relay, which many cars use, cheap in any parts store ($10 not $30 as Lucas charges). Battery power (Brown) goes to terminal #30; Control Power from the key (white)goes to #85; #86 is ground (black); Power out to fusebox hot side is #87 White/Brown. Frequently the relay failure also involves the wire terminals which get hot and make bad connections, which cook the new relay -check them and repair as needed.
When purchasing a relay, if you get a 5 terminal one with an 87a terminal, you have an emergency run feature. The wires are connected as above, with 87a left empty. If you have this failure, or a keyswitch/wire failure, you put the W/N wire from 87 on 87a, which hotwires the car.
FRM
http://www.usachoice.net/gofanu
FR Millmore

This thread was discussed between 20/07/2004 and 23/07/2004

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