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Tips

ignition and electrical problems

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gm9142l

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Hi all, my dad and I are trying to deal with some electrical gremlins on my moms new to her 70 B. Did an ignition tune up due to a lack of spark and old components before she starts driving it. New points, condensor, plug wires, cap, rotor, and plugs. Still no spark. Coil is getting 12 volts and points are gapped properly. Now the part that really has us baffled. When we went to try and start it, none of the gauges worked. Is there some common ground or common positive wire between the gauges and the coil. Up until this it had been running, and all gauges worked. No fuses are blown, and the fuel pump still clicks when the key is turned. Any tips to check would be great.

Greg
74 Porsche 914 2.0
82 Prelude
89 Mustang 5.0
 
If you have 12 volts to your spark coil, your ignition system should work regardless of your gauge system. Those 12 volts should be going to the 'plus' side of your spark coil, and the negative side of the coil should be going to the distributor assembly. Try disconnecting the wire at the distributor and do a voltage check there. You should have 12 volts. If you don't then you have a problem in the coil or the wire or something.
Next, with the wire disconnected at the distributor, hook an ohm meter to the distributor lead and the other probe to ground. Open and close the points with your finger and the needle should deflect full swing, or you should hear the audio buzz of a digital type meter.
If the meter stays on '0' ohms, then you have a short somewhere in the distributor assembly. If the meter reads 'infinite' ohms, then you have an open in the distributor.
One thing that is easy to do, when installing the points, is to wire it up incorrectly. The points assembly has 2 stepped insulating washers. These must be installed correctly. The assembly sequence when installing the points is:
an insulating washer with the small step facing up;
the points spring;
the distributor lead wire;
the capacitor wire;
the insulating washer with the step facing down; and the nut.

If you have any of those 2 wires hooked up between the insulating washer and the nut, the system will simply not work.
There is also a ground wire on the advance plate and if it is broken, there is a slight chance that the system might not work. More likely than not it would still work, but it is a good ground for the system.
If everything STILL checks out ok, I would suspect the spark coil is at fault.
As I remember on the later cars, all of the Smiths electric gauges require a voltage regulator except for the volt meter gauge. When you turn the ignition switch on you should get 12 volts to the voltage regulator, and from the voltage regulator it goes to the gauges, and from there to the sending units to ground. The reason that they have a voltage regulator is so that the gauges would read correctly even though the car system varies from 12 to 15 volts. If I confused you, I am sorry - I am going by memory from a Triumph I used to own and my MGA does not have gauges like the newer cars. Let me know if this helps.
 
I got to thinking last night that your MGB probably has an electric tachometer as well. If the wiring to that tachometer got a short, it could cause your ignition system not to work. You could try disconnecting the tachometer wire at the spark coil and that would help isolate the ignition system.
 
Thanks for the tips Peter. I do have +12 to the coil, and it is all hooked up correctly. When a scopemeter is hooked to the primary side of the coil, and the engine turned over, it fluctutes as points open and close. So all of that is working, yet still no spark. Now the primary side of the coil measures the resistance listed in my MGB manual. Should there be conductivity between the primary and secondary side? Im getting around 5-6k ohms of resistance there. And 30 ohms between the secondary side and ground. This is all with the stock coil. Thanks
 
Greg: It seems to me that if you had no spark before you did the "tune-up" and have no spark now,that you are looking in the wrong place. The tune-up didn't fix the lack of spark because that wasn't caused by the parts you replaced. Check the pig tail coming from the distributor plate. This wire has marginal insulation to begin with and if it shorts you are out of spark (ask me how I know)! I think the gauges are another issue altogether. The small voltage regulator behind the dash may have crashed. We have a '71 and a '72 in the family, one of which is reliable. Bob
 
Thanks Bob, Ill have to take a look at that pig tail. Let me clear up a few things. The motor was running (poorly) with the old ignition parts, then the gauge issues poped up around the same time I lost the spark.
 
Bob, I was thinkng about that pig tail. From my other post i mentioned with a meter hooked up to the primary side of the coil, I see the voltage go up and down as I turn the motor over. If that pig tail from the distributor were indeed bad, shouldnt I not see any of that like I am?
 
Probably so, but that wire is so flimsey that just the act of turning the motor could shake it enough to make momentary contact. Goofy idea: If the blade on the new rotor is too short it could limit the current going to the cap???? Likewise the tension on the center contact with the inside of the cap???? I have,(not recently), purchased a new condenser that was no good right out of the box???? How about putting all the old crap back into the motor as a test to see if it works poorly or not at all???? My car is still in winter storage so I can't do any experimenting, sorry. Bob
 
Hi Greg -
That was a good question you asked about the spark coil, and yes there should be conductivity between the secondary output and the primary leads. The spark coils that I have lying around measure approximately 6500 ohms between the secondary and the primary leads. However, there should be no conductivity between the secondary and ground. I am assuming that for ground you mean the case of the spark coil. That 30 ohms that you read should read infinate.
The way I understand it, you probably have a short within the secondary windings of the spark coil and the spark coil case. This, of course, is not good!
Seeing how you changed everything else, and all of the previous components check out ok, you pretty much have that spark coil backed up against the wall with no escape. Let us know how it goes.
Pete
 
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