QuestionQUESTION: Hi Barry, when I got my '76 MGB about a year ago, it had a 45D4 with points. After replacing the points a couple of times, I lost my temper with that nonsense several months ago and installed a Pertronix conversion. With it, the shop sold me a 12V coil. Per their advice, I ran that without ballast. It immediately got way hot, so I ballasted it. Then the car, once warmed up, would die after running for 20 seconds. So I went back to unballasted and ran great for about 500 miles. Now I'm dying again, after like 5 seconds running when warm. If I put the ballast back in, it's just slightly better. I get like 20-30 seconds before dying. The coil reads 3.8 ohms primary, 8K ohms secondary. I'm baffled - help?
Thanks!
Billy
ANSWER: Hi Billy. The points system does deteriorate over time, but a properly set-up system works just fine. If you had trouble with a points system, then replacing with a Pertronix will not usually solve the problem, although it may mask it for a while.
The wiring was changed during 1976, so you need to check if the wires to fuse #3 are plain white, or white with a stripe. If they have a stripe, I would try replacing the ignition relay near the fusebox. These often cause problems. If the wires are plain white, you do not have a relay.
When did you last change the spark plug leads (wires)? These deteriorate over time. I routinely replace them with modern aftermarket silicone covered wires.
Can you test your ohm meter? I think you will find that the test leads have a resistance of maybe 0.5 ohms. So, your coil is actually (3.8 - 0.5) = 3.3 ohms.
A non-ballasted coil should have a resistance between 3 and 3.5 ohms. A ballasted coil is usually around 2.0 to 2.5 ohms. If your coil is outside these limits, get back to me.
Finally, please check that you have wired the Pertronix unit correctly, and that the coil is connected the right way round. The power supply must go to the + terminal, along with the red wire from the Pertronix unit. The wires that are white with a black stripe must go to the - terminal of the coil, along with the black wire from the Pertronix unit. Any White wires with a green stripe should be taped out of the way, because they carry the reduced voltage via the ballast resistor inside the wiring loom.
---------- FOLLOW-UP ----------
QUESTION: Thanks for yet another detailed, thoughtful analysis. My meter reads zero ohms touching leads. However, my earlier reading was when the coil was still warm. It now reads 3.3 primary, 7K secondary. So I guess I haven't burned it up. The spark plug wires are 1 year and 1000 miles old. I don't have an ignition relay, which leads me to a possible answer.
I'm not using the stock ignition switch. I have a $20 Napa switch mounted below the stereo. I haven't replaced the proper switch mainly because it's nice not to have my keys hitting my knees. When I first started getting the car running, I burned up a $10 switch in about a week. Thought it was just cheap/defective. I never realized until now that the car doesn't have an ignition relay. Does it seem likely that the cheapo ignition switch, never meant for a non-relayed application, is overheating and not getting ample current to the coil? If so, is there a means of testing it other than replacement?
Thanks!
ANSWER: Hi Blly
If you have any doubts about your ignition switch, then use it to power a relay with the power feed taken off the BROWN side of the bottom fuse. I use relays with a built-in replaceable fuse. The ignition switch on a 76 B can be taking over 20 amps, which is more than most aftermarket switches are happy with.
Please also check that the coil is wired correctly. If you have got the + and - wrong, this would explain everything.
---------- FOLLOW-UP ----------
QUESTION: The coil is wired correctly. I installed a relay, and that did not affect the problem. I bypassed the relay and ran straight from the brown wire to the coil. Problem still there, eliminating the ignition switch and harness wiring as possible culprits. I then tested the voltage at the coil (with the engine running) for the various setups.
White wire to coil: 13.1V
Brown wire straight to coil: 13.8V
Brown wire through a ballast resistor: 12.5V
Is a particular voltage preferable?
I experienced the problem with all 3 setups. Then the problem stopped happening. For now. Argh.
Auto Zone didn't have a relay with a fuse, and after about 20 minutes of operation, the $6 relay they did have burned out. The power circuit fused closed so the car kept running with the ignition switch turned off. The relay is rated for 12V, 25A, and I figured even for a cheap component, such an immediate burnout was weird. I checked the current through the coil, and it's less than 2A. I figure 14V, 2A shouldn't cause that. But this is probably irrelevant.
Once the problem starts up again, I figure I'll replace the coil. Obviously, however, I don't want to keep burning up coils (assuming that fixes the problem). If a different coil doesn't fix the problem, I'll try some better spark plug wires.
Basically, I guess, are there any conditions I may be creating that would burn up a non-ballast coil?
AnswerHi Billy. The voltages that you have given me look fine.
Did you wire EVERYTHING through the relay? With everything switched on, the maximum load on the ignition switch could total nearly 30 amps. Look at the following table:
Component Amps
Coil (engine running) 2.00
Fuel pump 2.00
Overdrive (engaged) 2.00
Fuel gauge 0.25
Turn indicators 3.75
Rev Counter 0.25
Brake lights 3.50
Reverse lights 3.00
Cooling fan 5.00
Wiper motor 1.25
Heater motor 2.00
Modern Radio 3.33
Screenwash 1.00
Total 29.33 Amps
(Obviously, not all circuits are used simultaneously).
Anyway, I think you have a wiring fault. Is it possible that something else wired into the car is taking a big load and causing your troubles?
In order to diagnose the fault, I suggest that you temporarily disconnect all the wires that are currently attached to the coil, and insulate them separately, so that they cannot touch anything. Then hot-wire the coil (+) and the pertronix unit (red) directly from the Brown side of fuse 4 via a separate dedicated switch, and connect the pertronix (black) to the coil (-). If the coil still overheats, then you have a fault either inside the coil or in the Pertronix Igniter unit.
