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MG MGB Technical - Help Please - Lucas Keeps Car From Starting

Well, the MGB (1973 roadster) is reborn, thanks to Moss' supercharging kit. For a few days it drove like a dream (more on the beautiful results later). However, today before I could make it out of the driveway, the engine quits. I try to restart it but it becomes clear that gas and air are not the problem - it was struck by the Prince of Darkness. Simply refuses to even try to start. I'm retracing the electrical system as best I can, but are there any typical failures of this sort? I should say that before (but not immediately before) the engine quit, I had plugged my cell phone charger into the lighter. Could this have fried something on down the line? Is it just a wire or is it the coil? I'm more than happy to investigate this, but I'd appreciate a head start. I'm getting near 12V across the coil terminals according to my multimeter, so I assumed that everything upstream was good - is this assumption correct? Thanks all.
snake

If you have a points distributor, try cranking while watching the voltmeter across the coil. If the points are working, the meter will show it. At least, an analog meter will -digitals may not, but will show that something is different by rapidly changing readings. If it stays steady, the points are not working or not connected.Pull the coil wire from the dist cap and hold it near metal while cranking. If you have spark good, otherwise look into the distributor. Before you blame the much maligned Joe Lucas, it is very likely that the tune up bits in the dist are not Lucas at all, and there are some very crummy ones about. Improper assembly of same can cause this as well. Despite the fact that people replace coils all the time, I've never had a bad Lucas one that wasn't physically broken, but I've had bushels of bad replacement coils.
FRM
http://www.usachoice.net/gofanu
FR Millmore

Hi.

A standard cellphone charger should only take a relatively small current and is unlikely to have fried a wire unless it was faulty.

The lighter circuit is not electrically 'close' to the ignition circuit anyway.

I personally use genuine Lucas spares whenever possible.

Don
Don

Thanks everyone for the help. Here's an update:

I pushed the car in fourth with a voltmeter across the coil, and it went from 11.4 V to zero as it should. So the points are good. Then I grounded the coil wire and got a healthy spark. So the coil (and everything before it) is good. And all the spark wires are good. The only thing I can think of is the dizzy cap or the rotor arm - everything else is in tip-top shape. Is this a likely deduction? Are caps and rotors prone to just going out? I don't have any spare caps or rotors lying around, but I think I need to get some - they seem to be the only weak link.

(And just to clarify: I apologize for my maltreatment of Mr. Lucas. In fact, I do buy all-Lucas parts, the most recent being the points and condenser set, the alternator, and the Sport coil. There's just too many good Lucas jokes out there, and when you're pretty fed up with the electrical system . . . well, it's a weakness.)

Thanks again everyone.
snake

Snake - aren't you a character on the Simpons? Hmmm...

Yes, in this car as in most others, the cap and rotor are subject to failure. It seems that any vehicle with points is more prone than electronic distributors to produce carbon tracking and general fouling - at least that's my experience. A good test of your ignition is to bend the tip completely straight on a good plug and see if the ignition will still jump the gap. If not, you have a bad cap, rotor, or wire. Why not just replace them all?

Good luck!
Jeff Schlemmer

That's an odd way to check the coil, have to think about the meaning. I meant it only as a check of open/close of points. Check coil hot to ground Volts should be 12.5 or so, whatever it is across the battery. If it's lower you have high resistance somewhere and possibly not enough spark under load. Then with points closed and switch on, check coil neg to engine ground volts. Should be zero. If higher, there is a bad white/black connection or something amiss in the points. Isolate with the voltmeter. Did you clean the points before install? Are you saying that you have healthy spark from coil high tension wire when cranking? If so, then it is going to ground either through a bad rotor or cap. Rotor is more likely, the cap usually only takes out 1 or 2 cylinders. Bad rotors happen, esp on new ones. The cause is a bubble or hole in the plastic, sometimes not visible even under magnification, generally between the center contact and the shaft. This WILL drive you nutso! If a new rotor fixes, break the bad one and see if there is a bubble. When you say "plug wires are good" did you check resistance? High wire resistance is the number one cause of bad caps and rotors, other than defects.
Jeff's observation re better service on electronic equipped cars is because the plastic is better due to higher voltage requirements, but running them with bad or disconnected wires will really fry them, sometimes instantly. That 50K voltage will find a way out.
FRM
http://www.usachoice.net
FR Millmore

I cleaned the rotor and the inside of the cap as best I could, including light sanding, but it does not appear to be fouled. No easy fixes tonight...

I had heard of the "across the coil" method before and questioned it, but upon inspection of a good wiring diagram, I found that when the points are closed, the negative side of the coil goes to ground, so I effectively have a hot/ground setup. When they are open, then I get no circuit and therefore zero voltage.

I did start with clean points; I'm pretty convinced that I set up the distributor as best I could. I do have a healthy spark from the high tension coil wire, so I agree that it's losing it between the HT coil wire and the plug wires. (When I installed the new wire set, I checked resistance on all of them, and it was good - lower than the ones replaced) If it is the rotor, you say that it can go out suddenly (which it did) and you cannot see with the naked eye that the rotor has a bubble. For $3, I don't mind trying a new rotor. But you say that these bubbles occur on new ones too - is there a good rotor to buy? (Lucas vs. replacement) Or is this what will drive you nutso? Maybe I'll buy two...or three...keep a few more in the glove box...

Thanks again.
snake

Yes you have a hot/ground setup, but 11.4 v is too low. If you check as detailed, you will find out why it is less than bat volts, which could help.
Did you sand the plastic in the cap? While this may sometimes help on a mildly tracked cap, it is not a good thing. The roughness holds moisture and dirt, leading eventually to serious tracking. The very smooth surface is an important part of the long term performance of cap and rotor.
Bubbles in rotors are nearly always in new ones, because if they have bubbles they fail early. There are bad rotors, and old rotors, but very few bad old rotors! It is a rare condition, and it makes you really batsoid.
I had a customer with a 3.8 Jag. I put a 4.2 in it, rebuilt by me, with all new Lucas bits, drove the car about a thousand miles myself before giving it back. A week later he called from his home, 100 miles away, to say that it died randomly but started up again after a few minutes, running like a train. After a week's worth of LD diagnostics and parts switching, I made the road call. After about 20 high speed runs of 3 miles, it quit. Checking found all in order. Several hours of this with failures occurring at shorter intervals resulted finally in discovering that there was no spark coming out of the dist. Purely out of frustration and lack of anything better, I changed the rotor, presto! Put the old new one back-worked fine for a couple of runs. New new one worked good for many runs. Examination (magnified) inside the shaft hole showed a very tiny dot, when broken there was a bubble as big as the thickness, with about 3 thou of plastic either side.It was nicely balanced so that only a full-load run -5000 in third - would make the spark jump through. The hotter it got, the lower the load required. The more times it did it, the easier the failure.
It is more common that the rotor has a crack caused by improper installation. Any rotor could have these conditions. I have heard that Lucas may in fact not exist any longer as a mfr of some Lucas branded parts, who knows? Bosch replacements seenm reliable. It is good to have spares - even the best of us can put a cap on crooked in the dark and rainy night. One turn of the engine and the rotor is history!
FRM
http://www.usachoice.net/gofanu
FR Millmore

Well I picked up a new cap and two new rotors, popped one of the rotors in (carefully), and she started right up. Thank you all for helping me diagnose this one - I never thought these rotors could go bad like that. I'm keeping the other one in the glove box, just in case.

(And BTW: I checked the voltage from the hot coil to a "real" ground (the grounding strap) and I got around 12.8 V, so I think voltage is OK.)
snake

As a matter of interest I have just guided another amateur through the diagnosis process successfully. A moron formerly fooling with it put string wires on the old style Lucas screw down dist cap/coil terminals. These made such bad connections that it ate 2 coils, one a Bosch Super Blue, normally indestructible. It ran miserably for a couple of years, finally sucumbing to dead rotor disease, all a result of the bad "new" plug leads. Good thing I just had this troubleshooting review!!
FRM
http://www.usachoice.net/gofanu
FR Millmore

If you get 12.8v from the coil +ve to ground (as you should) *with the points closed* but only 11.4v across the coil itself that indicates slightly high resistance through the coil to points wire, points and ground wire in the distributor. Taking voltage measurements though the wrt ground should indicate where it is occuring. Worth doing as the ground wires can fail.
Paul Hunt

This thread was discussed between 21/05/2004 and 24/05/2004

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