1983 911SC CDI Module Output

[jimdandy]

The ignition has failed - no spark. I've done all the usual tests and believe the problem to be the CDI module or the ignition coil. My question relates to the CDI module output.

1. With ignition on, not cranking, should I see a small DC voltage (around 4 volts) out of the module (or across the primary of the ignition coil)? One source of documentation asserts this. I see nothing. Does that represent a definite failure of the module?

2. I have looked at the output when cranking the engine and see nothing at all, expecting a 450 volt pulse train if it were working. I don't have a lab scope or any way to look at the waveform, but should see some activity with my digital multimeter.

I must determine whether the module is bad before arranging it's repair - too expensive if not the problem. Where might I have the work done?

[Jonny H]

Does it whine with the ignition on? If not, the HV inverter circuit has failed.

I wouldn't advise putting a meter on the primary unless the meter is good for 500v.

Be safe.

[wwest]

Originally Posted by jimdandy

The ignition has failed - no spark. I've done all the usual tests and believe the problem to be the CDI module or the ignition coil. My question relates to the CDI module output.

1. With ignition on, not cranking, should I see a small DC voltage (around 4 volts) out of the module (or across the primary of the ignition coil)? One source of documentation asserts this. I see nothing. Does that represent a definite failure of the module?

4 volts might be about correct for an old Kettering ignition system. I know of no reason for having ANY residual voltage across teh coil/transformer of a CDI system.

2. I have looked at the output when cranking the engine and see nothing at all, expecting a 450 volt pulse train if it were working. I don't have a lab scope or any way to look at the waveform, but should see some activity with my digital multimeter.

The 300-400 volt CDI pulse is to short for a digital meter to read and the inertia of an analog meter's D'arsonval movement would be to great.

I must determine whether the module is bad before arranging it's repair - too expensive if not the problem. Where might I have the work done?

As the above poster stated, if the inverter is alive you will hear a ~1Khz whine

[wwest]

Watch for the tach needle to jump, jitter, every so slightly as you crank the engine. That would be an indication that the CDI is powered and the distributor timing output is functional.

[jimdandy]

Battery voltage is seen at the module and the device is drawing 1.4 amps when ignition on (engine not cranking). One service manual specifies between 1 and 1.9 amps here. So, I know it's powered. The distributor pulse generator shows the proper resistance, both wires are isolated from ground. I see a modulated voltage signal out when the engine is cranked.

How can I detect the CDI module output, short of an oscilloscope?

If I was sure the ignition coil is good, I'd send the CDI module off to be repaired/exchanged. The primary and secondary resistances are as expected, but I worry that the device could nonetheless be failing under load. Does experience show that this is the case or should I conclude the coil is definitely OK?

[Bob Kontak]

A fellow the other day had the white coil wire that goes into the 6 pin connector barely hanging on where it connected to the female bayonet connector. It caused intermittent failure until it finally gave up the ghost and broke. Everything else was fine. Worth a look.

You may have already......

[wwest]

Originally Posted by jimdandy

Battery voltage is seen at the module and the device is drawing 1.4 amps when ignition on (engine not cranking). One service manual specifies between 1 and 1.9 amps here. So, I know it's powered. The distributor pulse generator shows the proper resistance, both wires are isolated from ground. I see a modulated voltage signal out when the engine is cranked.

How can I detect the CDI module output, short of an oscilloscope?

If I was sure the ignition coil is good, I'd send the CDI module off to be repaired/exchanged. The primary and secondary resistances are as expected, but I worry that the device could nonetheless be failing under load. Does experience show that this is the case or should I conclude the coil is definitely OK?

A LED would probably respond fast enough to the 300-400 volt pulse that you could see it flash(ing), and a good size resister in series with the LED bulb would protect the LED.

Personally I would be tempted to purchase an inexpensive 12 volt LED bulb at any auto parts store to try that.


Long ago we used a neon bulb for that.

[Jonny H]

The pulse is only a few microseconds at most so you're unlikely to see that on an LED.

TBH, the audible whine is the best check unless you have a scope.

[wwest]

Originally Posted by Jonny H

The pulse is only a few microseconds at most so you're unlikely to see that on an LED.

TBH, the audible whine is the best check unless you have a scope.

I've been messing around, experimenting, with power duty cycle vs light output on LEDs, CREE mostly. LEDs are solid state so there is no reason they would not produce light output in the microsecond power application range.

Visible to the human eye....??

[James Brown]

pelican parts offers CDI rebuild service, check it out