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Ignition Troubleshooting

Pelican Technical Article:

Ignition Troubleshooting

James K. Thorusen


4-5 hours






DVOM, decent size jumper wires, electrical tape, flathead screwdriver

Applicable Models:

Porsche 914 4-cyl (1970-76)

Parts Required:

Plug wires, distributor cap, rotor, etc., depending upon what proves to be defective

Performance Gain:

A 914 without electrical issues that starts with ease

Complementary Modification:

Replace the wiring harness
What follows is a general theory / practice for point-type ignition systems. While specifically written for the 914 / 4, it should be applicable to any non-electronic ignition.

I have a belief ( I may be shaken out of it someday, but for now I persist ) that if one understands the theory involved, one can usually discover the problem with any system... point type ignitions being no exception.

Attend me...

In order to develop the high voltage necessary to produce a spark capable of jumping the gap in a spark plug inside an engine, a means of storing and concentrating energy is necessary. The device that does the energy storage and concentration is the coil. The basic laws of physics state that a coil of wire (more so when wound on an iron core) is an inductor, i.e. it exhibits the electrical property of inductance. Inductance is the ability of an inductor to oppose any change in the amount of current flowing through it. In other words, when a voltage is first applied across an ignition coil, if it were a pure resistance, the current would virtually instantaneously arrive at a value determined by Ohm's law... I (current) equals E (electromotive force, or voltage) divided by R (the resistance of the coil in ohms).

Since our coil is not a pure resistance but an inductance, then it follows that this is not the case, and in fact the current flow lags the applied voltage in time. What is happening is that the current through the coil of wire inside the coil housing is establishing an accompanying magnetic field in the iron core of the coil. As this field expands through the coil structure, it's lines of force cross the windings at aproximately right angles, and in so doing, induce a voltage that is opposite in polarity to the voltage we are applying to the coil.

This is an unavoidable law of physics, and is the reason for the time lag between applied voltage and applied current. This is really a good thing, however, because as the magnetic field expands, energy is stored in it. We need this energy to make our spark, and this is how we do it.

Because of the slow expansion of this magnetic field, there is comparitively little voltage induced by it into the many turns of wire that compose the coil's secondary winding. The laws of physics again state (check with Josh hadler for the mathematical formulation if you are interested) that the speed at which the lines of force intersect the windings is one of the determining factors in the voltage produced; the faster we can move our field, the more voltage we will produce. The situation is exactly analogous to an old-style bicycle lighting generator... the faster you pedal, the more voltage you produce, and the brighter the lights.

In order to induce a large voltage, we need to collapse our magnetic field quickly. Because we can collapse it more quickly than we can create it, it is the collapse direction that we use to generate our high voltage. The energy is still present in the field, and regardless of whether it is expanding or contracting, the magnetic lines of force still intersect the coil windings... the trick is to make the relative motion between field and windings as rapid as possible.

This is done by OPENING the points in the distributor. The points remain closed for a considerable time ( 55 degrees of rotation for each ignition pulse if memory serves) and during all that time the coil is storing up magnetic field energy. When the points open, the current through the coil primary is ABRUPTLY terminated. This results in an almost instantaneous collapse of the magnetic field, with the consequent generation of very high voltage in the secondary winding of the coil. We put this voltage to work for us to fire our spark plugs, and that's basically all there is to it.

Now to practical matters: First, the condenser. This name is a holdover from the 1920's when all capacitors were called "condensers". The one on the side of the distributor is a capacitor too. A capacitor's job is to store energy in the form of electric charge. I won't go into as much detail as I did for the coil, but suffice it to say that as I mentioned above, an inductor opposes any CHANGE in the current flowing through it. When the points open, the coil tries to keep that primary current flowing. The opening points are seen as an increasing resistance in the circuit, and the voltage across them will rise to several hundred volts before the energy in the coil is dissipated.

This high voltage will cause considerable arcing across the points and soon burn them up, or would if it were not for the condenser. The condenser furnishes a place for this energy to go... it goes into charging the condenser, instead of burning up the points

Now, you have enough information to test your coil, or any other. First, in order to get enough energy to store, we need a good "stiff" source of 12 volts. Remove both the green wire from the distributor and the tachometer signal wire from the coil minus terminal and connect the coil minus terminal to ground with a good connection (decent size wire, no marginal alligator clips) and turn on the ignition. Measure the voltage at the coil plus terminal. If it isn't close to 12 volts, then you have a high resistance in the coil primary circuit, which must be remedied before you proceed any further.

[This step verifies that we have energy to store.]

Now, being mindful of what I said earlier about the voltage in the primary circuit when it is opened, be sure that you are touching only the insulated portion of your temporary wire and disconnect it as quickly as you can. Grab it in the middle and yank firmly!) Have the coil secondary lead taped down to check for spark as Dave D. suggested when you do this. (Dave D. suggests removing the coil secondary wire from the distributor center tower and using tape to secure it to the engine tin or fan shroud so that there is a gap of about '" between the metal end on the wire and ground that can be easily observed.) If the coil secondary lead insulation is in good shape (if you are not sure, buy a new ignition wire set) and you still get no spark, the coil is defunct. Period. End of discussion.

[ This step verifies that the coil itself can store and release energy.]

If you now get a spark, then you need to decide whether the problem is in the coil primary circuit (points, condenser, 12 volt supply) or in the secondary circuit (distributor cap, rotor, wires, noise suppressors).

Note: Ignition ON for the following tests:

If it is the primary circuit that is suspect, rotate the engine to a point where where the points are closed and measure the voltage on the coil minus terminal. It should be less than ' volt. If not, either the points are burned up, the little braided wire that connects the breaker plate to ground inside the distributor is broken, or the ground strap from the back of the gearbox to the chassis is open. These are the only causes that I can think of.

(By the way, in order to catch that last cause, you need to make all your voltage measurements with respect to the battery negative terminal post... notice I said post, not the clamp or the wire that connects to it. You may find some surprising ground faults that way, and those will need to be fixed too.)

[The above steps verify that our switching circuit (distributor) will pass enough current to allow us to store the necessary energy for making sparks.]

Now, rotate the engine again to a point where the points are well open.

Measure across the coil from the plus terminal to the minus terminal. If you see any voltage at all, either the condenser is shorted, or something inside the distributor is.... replace the points / condenser, inspect the inside of the distributor for foreign matter, and try again.

If the above checks are all OK, get a dwell meter and measure the point dwell, and set it correctly. If you can't, get a new set of points and try again.

[ These tests verify that we can abruptly open the circuit to release our stored energy.]

If the primary circuit is OK, then the only thing left is that the high voltage is leaking away through defective insulation. Replace the wire set, the distributor cap, and the rotor.

[These steps verify that the energy can be delivered to where we want it.]

By this time, it has to run. Or at least, it has to spark.

Last, reconnect the tachometer signal wire to the coil minus terminal, and verify that everything still works. If this kills your ignition, then either the tach is bad, or there is a problem in the wiring harness.

Good Luck,

Jim T.

Comments and Suggestions:
Rony Vish Comments: 1975 914model 1.8 won't start. New battery. lights for radio and oil came on and immediately shut off when switched on to start the car. Then everything went dead. Battery was hooked up properly but not registering any current. Disconnected battery and it now shows full charge. Negative ground wire was hot when disconnecting from battery. Hooked up battery again but nothing when key turned to start the car.
September 28, 2015
  Followup from the Pelican Staff: Perform a voltage drop test on all the battery connections from the battery to the engine and fuse panel. - Nick at Pelican Parts  
IGTARD Comments: Purring. Shalom IGTARD
March 21, 2014
  Followup from the Pelican Staff: Thanks, got it. - Nick at Pelican Parts  
IGTARD Comments: Car in question is a 1974 Porsche 2.0. The heat syptem rings a strong possible. Shalom
March 16, 2014
  Followup from the Pelican Staff: Not sure I understand the question - Nick at Pelican Parts  
IGTARD Comments: My starter does nothing. Starts and runs on compression. Shalom, IGTARD
March 4, 2014
  Followup from the Pelican Staff: Not sure what vehicle you have. Check for power and start signal to the starter motor. - Nick at Pelican Parts  
craig Comments: problem with starter engagement when car / temps are HOT. After cool down all OK
April 25, 2013
  Followup from the Pelican Staff: That sounds like a faulty starter or battery cable. The resistance could be changing when the temp rises. - Nick at Pelican Parts  
Spin'n Wheel Comments: While useful for cars, that are still equipped w/points & condenser, These cars are more & more being upgraded with a simple electronic ignition like the "Pertonix".
September 15, 2012
  Followup from the Pelican Staff: Thanks for the feedback and additional info. We appreciate it.
- Nick at Pelican Parts

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Page last updated: Fri 1/19/2018 02:20:35 AM