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MG MGB Technical - ignition light doesn't come on

I've got an intermittent ignition fault on a 1973 B roadster. Luminition electronic ignition is fitted.

Sometimes, I turn the key, the ignition light comes on & she starts straightaway. Then the ignition light goes out as the alternator does its thing, as per normal.

More often, I turn the key, no ignition light. The starter turns but no spark.

I suspect I should check out the LT circuit, right?
Starting with the white wire at the ignition switch?

Thanks for any help, Laurence
L Jones

may be intermittent ign switch , does the fuel pump run? Could be further back, the fuse holder may be intermittent on the ign side. If you do not have a voltmeter a lamp + wire will be big help.
S Best

Laurence,

I'm having a similar issue on my '64 so we're going thru this together! I've been having a slight glow on my red indicator and by all accounts, the problem lies with the white wire circuit. Last week, I had the exact situation as you describe--the engine turned over but no start and no ignition light. Intermitent yes, and finally the light came on and it started. She's flagged and in the barn till this issue is resolved. I'm starting to pull all the white side Lucar connectors apart to clean and reassemble with dielectric grease but I have a hunch it's the ignition switch going south. My fusebox was also a bit cruddy and got a cleaning on the white side. White side connection at the fuel pump too needs to be checked.

Paul
Paul Hanley

Paul (& S Best)

Thanks for the comments. Paul, I'm glad to know there's someone else with the same trouble (in an obscure way)!
When there's no ignition light, there's also no "click,click" from the fuel pump. I'll check & clean the places you list, Paul, but I'll also try to find the fault with a voltmeter over the weekend. I wonder if my 73 mgb roadster (chrome bumper) has an ignition relay?

Laurence

PS I lived in Maryland for 2 years - near Greenbelt & then Annapolis. We enjoyed Annapolis hugely.
L Jones

As S Best says it will be a problem around the ignition switch. This branches off one way to the warning light and another way to the coil (and fuel pump) at a connector by the steering column, so any fault affecting both light and spark must be at that point or back through the switch. If you can crank but not start then 12v must be reaching the switch OK.

A 73 does not have an ignition relay, they only arrived in about 76 at about the same time as the electric cooling fan.

A slight glow of the warning light when running can be a precursor to the total failure to start described.
Paul Hunt

Paul,

Thanks for the comments. I've found 3 problems - see below.
Also, one thing I didn't mention was the enormous backfire when the engine cut out initially. Sounded like something had dropped off the car. In the light of day I found the backfire had been enough to push one of the 4 bolts which hold the airfilters right out! as well as a rubber tube on the carb. Presumably not too good...! Do I need to test for permanent damage?

1) There was a loose connection at the battery - the screws clamping a wire to a connector had worked loose.

2) One of the white wires at the back of the ignition switch was loose.

3) Fixed the above, & now I get no ignition light, but it does start & run. Working through the relevant section of your (Paul Hunt's) marvellous webpages (Bee & Vee), I have 12V all the way from the ignition light to the brown& yellow wire at the alternator. The ignition light does come on if I ground that brown& yellow wire. So I guess I need a new alternator.

Perhaps when running, the battery connection was lost, causing all sorts of nasties, including backfire & knackered alternator?

Cheers Laurence

Paul Hanley - Glad to see you're making progress.
L Jones

Agree with the alternator diagnosis, just check the plug connectors are making good connection to the alternator spades first.

There are certainly warnings not run the engine with the battery disconnected, and I have heard at least one claim that when a battery became disconnected at night the voltage soared and blew all the bulbs. That doesn't seem logical to me as especially at night there should be more load from the bulbs than from the battery to act as a swamp for any voltage spikes, and the voltage regulator is inside the alternator, but I don't disbelieve it enough to try it on any of my cars :o)

Don't know what would have caused the backfire in the intake, normally if the ignition is internmittent it causes explosions in the exhaust. Have a look at the needles to make sure they aren't bent and the pistons rise smoothly against the dampers and fall smartly with a 'clink' when released. Then just see how it goes.
Paul Hunt

Thanks for the comments. I'll follow your advice.

Laurence (of Studley)
L Jones

Hi Paul , there is a well known effect called "load dump" in automotive electrical sytems , and an ISO standard for it . As I recall it applies an 80v surge of defined shape for several hundred milliseconds and simulates the effect of a battery going open circuit while the alternator is charging it . I know someone who shamefully neglected his battery to the point where it did this ( not an MG enthusiast of course ) the event did produce the sort of damage you might expect .
S Best

Did a Bosch Alternator changeout four years ago and the first rebuilt unit I installed was charging 22 volts and rising. Battery was throwing a hissy fit (and literally hissing). Glad I checked early and didn't cook anything.

Luigi
Luigi

22v from the alternator is a voltage control failure , as others have pointed out a voltmenter is all you need to monitor an alternator equiped charging system.

Re load dump , I typed into Google and it appears that there is an SAE spec,

SAEJ1113-11 Pulses 1, 2a, 2b, 3a, 3b, 4, 5 (Load Dump); SAE J1455 Load Dump/

There is lot of literature about , the following is good read .

http://www.st.com/stonline/books/pdf/docs/10493.pdf

Bottom line is , do not disconect your battery while the engine is runnibg .
S Best

This thread was discussed between 25/11/2004 and 30/11/2004

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