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MG MGB Technical - Electrical Gremlins
|Okay, I am helping a friend work on an MG restoration project (a 73 GT like mine, but his has the 73 engine as well). We have traced voltage from the battery all the way to the ignition coil, but when I attach my timing light to the #1 spark plug wire, I get no light. The timing light works fine (tested it on my Neon). How can I test to see if the plug wires are bad or if it is my ignition coil or distributor? I tried using a multimeter to check for coninuity (checking for a break in the plug wires) but everything seemed to come out fine. My multimeter doesn't go high enough voltage wise to test the coil to see if it is discharging properly (which would seem to point to trouble in the distributor right?). We have replaced the points/condensor in the distributor (and set the appropriate gap). Where to from here? Any suggestions are greatly appreciated, and in the meantime I will continue to search and read....|
|D. T. Barnes|
|Disconnect the coil hi tension wire at the distributor, hold it about 1/4" from a good ground, turn the engine over and watch for a spark. No spark means a coil problem or a points issue, if you get a spark the problem is in the distributor cap or rotor. Rotors shorting thru to the distributor shaft is common|
|D.T. Remove the distributor cap and turn the engine over to open the points. Measure for 12 volts on the movable points contact. If not there remove the wire going into the distributor and check the wire for 12 volts. If good you probably have the plastic insulator incorrectly installed on the points post. The insulator goes on the post with the condenser and input pigtail under the insulator. The nut goes on top of the insulator. If you install the condenser and pigtail leads on top of the insulator the points movable contact will be shorted to ground all the time. If the voltage to the points is good, turn the engine to close the points. Remove the coil to distributor high tension lead from the distributor cap and hold the end of the lead about 1/4" from a good ground. With the ignition switch on use a screwdriver with an insulated handle and separate the points quickly, you should have a good spark. If you do try another rotor and then another distributor cap. Doing these checks should help you locate and correct the problem. |
|D.T. John's method will give quicker results. |
|DT. Test per John's instructions. That is a good "quick check" of functioning of the low tension circuit and the coil. There are tech articles on testing the ignition circuit and spark plug wires on the MG section of my website, www.custompistols.com/ which should answer any other questions you may have.|
|Thanks to everyone for the information,|
John - Will try disconnecting the wire from the coil to the distributor and chceking for spark. Will let you know what I find.
Clifton - Thanks for those steps, they make a lot of sense, I was having trouble figuring out what to check last night as the evening wore on. I will use this in conjunction with John's advice to further narrow down the possibilities.
Les - I have spent a good amount of time on your website acutally, thank you for all of the grweat information. I spent my time checking fuel pump info for my own MG, must have completely overlooked the spark info. Thanks, I will be sure to pass this info on to my friend.
Any other suggestions different than the great information listed above? Thanks again to everyone who checks this thread and especially to those who respond.
|D. T. Barnes|
|They are all right on...have had to use their advice and the web site myself. Paul Hunt's web site is good as well...just google paul hunt mgb stuff and you will find it then look under Spanners and Ignition. Don't expect you will find any more than you see here or on Les's site but sometimes it helps to see different explanations of the same thing...at least for me since I am kinda remedial in mechanical sense and need as much direction as I can get. You're in good hands with these guys.|
|On a 73 you can check the LT by conencting a test-lamp or meter to the coil while cranking. On the +ve you should see 12v (OK, it probably drops to 10v while cranking) all the time, but on the -ve it should switch between 12v and 0v (ground) as the points open and close. That proves continuity of ignition supply, coil, points, distributor ground strap, and correct assembly of condenser, points and coil wire. However there could still be a problem with the condenser causing weak sparking. With your timing light, if inductive, clip it onto the coil lead as well as each plug lead. No flashes on any of them probably means the coil isn't generating HT for some reason. Flashing on the coil lead but not the plug leads means the distributor cap and/or rotor are breaking down, change both. Flashing on some plug leads but not others probably means the cap is breaking down. Also make sure you have the lead that the rotor is pointing to at the top of the compression stroke is going to No.1 plug (at the front), and the others are connected 1,3,4,2 counting *anti*-clockwise.|
|Paul Hunt 2|
This thread was discussed between 10/05/2006 and 11/05/2006
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