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> Help needed diagnosing ignition problem
ewok of death
post Jun 19 2008, 11:01 PM
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I have a '75 914 1.7L that I've had for almost a year now. I got it mostly as a 'project' car but I have practically no mechanical experience, and have just been essentially learning as I go. I very rarely drive it, but last week, I was driving it home it suddenly cut out. After stopping, the engine would turn over, but would not start. Upon later inspection with the tow truck driver by holding the HT cable over the engine cover, I determined there was no spark.

So after scratching my head and poking around inside the engine bay a few times, I have come to the following conclusions:

- The ignition coil is a Accel (Super Coil I believe). The resistance across the LT terminals is 3ohms, and the resistance from the battery terminal to the HT terminal is 13k ohms or so (though I may not have gotten an accurate reading and will recheck this in a bit).

- There is a Pertronix Ignitor system in the distributor cap. These I hear are pretty reliable, but the way it was wired was pretty funky. The lead from the battery was connected to the - end of the coil, as was the black wire from the pertronix unit, and a mysterious blue wire that I assume is the tach? The red wire from the pertronix unit was connected to the + terminal on the coil, as well as two black leads (what are these?).

I switched the wiring around so the battery lead, the red ignitor wire, and the supposed tach wire connect to the + terminal, and the black wire from the ignitor and the other two black wires are connected to the - terminal. I read somewhere that the pertronix would work if its polarity was reversed, but would eventually burn out so this seemed like a possibility. Anyway, car still would not start. I tried disconnecting the mysterious wires as well, with the same results.

Then, just now, I went out to measure the voltage at the input of the coil. It was 8.5V or so, which seems about right. I was about to measure the voltage at the - end of the coil, but smelled smoke and noticed that it was coming from beneath the distributor cap, and lo and behold, the pertronix is burnt. Now I am wondering, was the pertronix really burnt out, or did I just destroy it by messing around with it...?

My next step is going to be to test the coil itself by momentarily grounding and ungrounding it directly, but I need to get some leads first.

Just wondering if anyone has any advice, or if they have had similar issues...

Also, if I want to pull out the distributor and install a new pertronix, how can I remove it? Or should I try to replace it with the distributor in place? I assume if I remove it I'll have to redo the timing, or is there a way to avoid that?
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ericread
post Jun 23 2008, 08:59 AM
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I am ssuming you haev read through the Pelican Parts Ignition document:

http://www.pelicanparts.com/techarticles/9...roubleshoot.htm

Please note:

"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...

...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.

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."

Sooooo, if your points aren't moving quickly, the coil will be not create the high voltages you are expecting.

Eric Read


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ewok of death   Help needed diagnosing ignition problem   Jun 19 2008, 11:01 PM
Cap'n Krusty   Who told you the Pertronix was reliable? Not in m...   Jun 20 2008, 12:03 AM
ejm   :agree: also forget the pertronix, get points and...   Jun 20 2008, 05:40 AM
ericread   I have had a Pertronix unit in my teener and I lov...   Jun 20 2008, 10:38 AM
Nolongermusclecars   I have a '75 914 1.7L that I've had for a...   Jun 20 2008, 11:26 AM
ewok of death   HOW WAS IT WIRED BEFORE????? The previous wirin...   Jun 20 2008, 12:53 PM
ericread   HOW WAS IT WIRED BEFORE????? The previous wiri...   Jun 20 2008, 02:12 PM
Nolongermusclecars   HOW WAS IT WIRED BEFORE????? The previous wiri...   Jun 20 2008, 03:26 PM
ewok of death   When do you get voltage changes??? it needs to c...   Jun 20 2008, 10:42 PM
ewok of death   Checked the resistance again. 2 ohms primary resi...   Jun 21 2008, 12:23 AM
Nolongermusclecars   make sure you have a good battery in your multimet...   Jun 20 2008, 03:32 PM
arvcube   even though the smoke is a good telltale that the ...   Jun 21 2008, 04:55 PM
vsg914   Put in a bosch blue coil, new points gapped at .01...   Jun 21 2008, 05:33 PM
ewok of death   I just got a new bosch blue coil, and did the same...   Jun 21 2008, 07:37 PM
Nolongermusclecars   I just got a new bosch blue coil, and did the sam...   Jun 23 2008, 07:35 AM
ewok of death   it maybe a stupid question but did you also grou...   Jun 23 2008, 01:25 PM
Nolongermusclecars   it maybe a stupid question but did you also gro...   Jun 25 2008, 08:48 AM
ewok of death   take the broken pertronix out of the loop.... di...   Jun 25 2008, 12:46 PM
Nolongermusclecars   take the broken pertronix out of the loop.... d...   Jun 26 2008, 04:19 PM
ericread   I am ssuming you haev read through the Pelican Par...   Jun 23 2008, 08:59 AM


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