MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Weak Spark from coil

Getting the B out of winter storage...won't start...cranks over nicely...battery new, fuel is being delivered...pulled a plug and help it against the block and got a very weak spark so I'm getting some power to the plugs...pulled the wire from the coil to the dizzy, cranked it over against the block and got another very weak spark...that spark should be very strong right?
Leads me to believe the coil is suspect as I'm getting power into the coil but a very weak spark out...is there a test for the coil? Coil is about 10 years old but what would make it go bad just sitting in a barn all winter?
Bob Dougherty

Check the points condition and the gap/dwell, plus the coil connections. Wipe out the dizzy cap, clean rotor arm, in fact all the usual stuff if a car has been standing!
Allan Reeling

More likely to be the condenser failed than the coil. Compare the spark you get by flicking the points (*if* you have points, see below) open manually, to disconnecting the points from the coil and tapping an earth on the coil. You should get more sparking and spitting at the tapped connection then the points, and a smaller HT spark from the coil lead connected to a plug laying on the block. If the sparks are the same in both tests it's the condenser. These can fail when left over a period from dampness and resultant corrosion.

If the points flicked arcing is noticeably less and the HT spark better then it could be poor voltage to the coil or many other things.

Check you have close to 10v at the coil +ve or SW when cranking.

For other ignition circuit tests we need to know if it's a chrome bumper or a rubber, standard ignition or after-market, points or electronic, as there are too many variations depending on what you have.
PaulH Solihull

Forgot to mention I have a pertronixs installed...no points...Not sure why I would have a weak spark from my coil wire to ground...I would think that would be a very strong spark...
Bob Dougherty

Bob. "Not sure why I would have a weak spark from my coil wire to ground"

Bad primary windings in coil. Bad secondary windings in coil. Electronic points replacement system malfunctioning and insufficient magnetic build up in windings to induce a strong spark. Bad king lead (coil lead). Poor connections to anything resulting in loss of voltage. Just off the top of my head.

As Paul notes, knowing the year of your vehicle would be a good thing.

Les
Les Bengtson

With a Pertronix remove its two wires from the coil, and any other wire on the coil -ve such as the tach trigger wire. With the ignition on connect an ammeter between the coil -ve and a good earth and you should see about 4 amps, chrome bumper or rubber i.e. not ballasted or ballasted. Much less than that and there is insufficient current flowing through the coil primary. With the earth connected directly to the coil -ve measure the voltage on the coil +ve. Only leave the ignition switched on or the meter or earth connected as long as is needed to take the measurements, leaving current running through the coil will overheat it.

On a chrome bumper you should see 12v i.e. the same as on the brown at the fusebox. Less than the brown means you are losing voltage through the brown - ignition switch - white circuits. If you see 12v you need to check the primary resistance of the coil, all wiring removed from the spades, and it should be 3.1 to 3.5 ohms cold, a bit less if hot.

With a rubber bumper/ballasted system you should see about 6v on the coil +ve with an earth on the -ve. If you see less than this again you are losing voltage as above, or the ballast resistance wire contained within the harness is going high-resistance. *More* than 6v could mean you have a 12v coil instead of a 6v, which will give weak sparking, or the coil primary is faulty. Measure the primary resistance as above and a 6v coil should measure about 1.4 to 1.6 ohms. Also measure the ballast resistance between the white/light-green that goes on the coil +ve and the white or white/brown at the fusebox and that should be similar to the coil.

Also with a rubber bumper ballasted system the ballast should be bypassed during cranking so check that the voltage on the coil +ve goes *up* to about 10v with an earth on the -ve. If it doesn't then that is another reason for weak sparking when cranking, although by rights if everything else on the engine is right it should still start OK. Note that if a chrome bumper/unballasted system the voltage will *drop* from 12v to about 10v during cranking. If you see much less than 10v during cranking then either the battery is bad or one or more connections in the battery circuit is high-resistance.

If all those tests give the correct results then the Pertronix unit could be faulty. With it disconnected connect an ignition condenser between the -ve terminal of the coil and earth. Now tap an earth on and off the coil -ve and observe the spark. If it's noticeably better then the Pertronix is faulty. If the same then it's probably the coil HT winding.
PaulH Solihull

Thank you Les & Paul...it is a 69 B...I will try the aformentioned tests with a voltmeter and report back...unfortunatly, the car is in a storage facility that is only open on weekends so I won't be able to get to it until next Saturday...leaving me a whole week to stew about it!!!!
Bob Dougherty

And the verdict is...
A bad coil! Finally was able to get into the car today...swapped out the coil and she cranked right over...looking at the old coil, the screw inside the coil was rusty...with a volt meter I could get 3.0 from the + & - posts, but no reading when going from the + post to the screw in the inside...had to scrape the rust off to get a weak reading...so a very weak spark was leaving the coil...the coil was about 13 years old, hope the new one lasts as long!!!
Thanks to all!
Bob Dougherty

This thread was discussed between 07/04/2012 and 15/04/2012

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.