No power to ignition coil.

[flpporsche]

Hi there. i have a 1981 911 sc, with CDI ignition.

problem is, i have the humming noise coming from the CDI box, all leads and wires connected properly, but no power to the ignition coil .

Would be grateful if anyone can help,

Cheers

[Groesbeck Hurricane]

Welcome aboard!

We will be glad to help! Many people will chime in, most will have more experience than I do!

Just a starting suggestion, update your location. You never know who could help out if they knew where you were!

My first assumption is you know where the CDI box is located. Can you tell us what kind you have? Is it Bosch, MSD, Permatune, other? This will help in diagnosis.

If it is a permatune you can go to their site for simple bench testing instructions. IF it is a permatune you might want to consider replacing it with something else. My permatune bench tests as normal and fully functioning. It does not work properly and would leave me stranded if I had not replaced it.

Sometimes corrosion builds up. Have you unplugged and inspected the connectors to ensure they are not corroded?

There is a process for bench testing the Coil. I would suggest doing a search.

Distributor service (Clean and lube) real easy without removing the pinion gear!

I am blatently plagerizing GUNTER

The wire from the distributor to the 6-pin module has an inside (Signal wire to # 7) and an outside (Shield to # 31d)

31/1 = Ground
TD = To Tachometer
31d = Distributor pulse wire shield
A = To positive side of ignition coil
15 = 12V power with ignition ON (from fuse 22?) - 2-pin plug?
7 = Distributor pulse input (Signal wire)

It could also be a failing coil? Bosch black is what you want and maybe someone has an easy way to test the coil? Do NOT test for spark by removing the center cable from the coil and grounding it; VERY BAD!
Bosch warns against this. Use a lead from any plug with a good plug connected to ground when cranking; there should be a visible spark.

Since it is intermittent, can you swap the coil for another one?

To be sure that it isn't fuel-related, you could swap out the red relay temporarily with a black one, replace the 25A fuse #16 making sure it's held tight and the next time it stops running, leave the ignition ON, remove the airfilter and lift the Air Flow Sensor Plate Lever.
Are the injectors squealing?
Meaning: The Air Flow Sensor Contact is working correctly controlling the pump through the pump relay with ignition ON. Bentley page 201-8.

Do you still have the 6-pin CDI module?



The following is a good read and could be related:

The Green wire...

[ClickClickBoom]

I replaced the primary wire with a good ole fashioned one from the local Napa store wire collection. Works fine!
Pull your ign coil and shake, should have liquid(oil) and sound like a coke can, full, not empty. Ask me how I know. Another Napa replacement piece.
eric

[HarryD]

Hi,

Welcome to the best BBS for air cooled porsches. Help us out a bit by adding your location to your profile.

You will not see any voltage at the coil as it only gets a pulse from the CDI.

But moving on...

This thread may be helpful: Ignition problem troubleshooting (electrical)......

Also, from another thread (How do i check a coil with a timing light??

Originally Posted by HarryD

To get a spark to the spark plug in the engine. Several things need to happen. First, you need power to the CDI. Next you need to have a way to trigger the CDI (the points do this by opening and closing). Then the CDI will then send a pulse to the coil which will increase the voltage and send it through the high tension lead to the rotor and to the selected spark plug. You need to verify that each step in this chain is working.

1) Verify that the CDI is getting power. Do you hear it wining (if it is a Bosch unit)? If so, you are good to go.

2) Verify that the CDI is producing power to make a spark. You can do this with a "telegraph" test. Take the high tension lead off of the center of the distributor and put a spark plug on it. Ground the threaded part. With your plug attached to the center lead, open the distributor, remove the rotor, place your key in the run position, and manually open and close the points. Each time you cycle, you should see a spark from the plug. If you do not, you may have a bad CDI.

3) Verify that the coil gets a signal when you crank. Replace the rotor and distributor cap. Leave the plug on the center high tension lead. Crank the engine. Do you see a spark? If so, you are getting spark to the distributor. If not, your point gap may be off or there is some other problem with the connections.

4) Verify that the spark goes to the spark plug wires. Reattach the center lead to the distributor. Attach you spark plug to one of the leads going to the cylinder spark plugs and ground. Crank engine. If you see a spark you may have a timing problem. If no spark, you have a problem with your rotor or distributor cap or your wires are improperly seated.

Do a search here as there are many threads on this topic. Look especially for the ones by early_s_man. Warren (RIP) left a great deal of knowledge on troubleshooting this problem.

[flpporsche]

Thanks chaps. I'll give everything a go and get back to you. Updated my profile as requested, living in sunny London (for a change), normally miserable London!

Yep we're running with the bosch CDi unit, vehicle electrics is not my strong point, can i use power probe to power up the coil, or will i damage other components??

I'll get some photos of her up soon, she's just been sprayed gt3 orange, interior being done as i speak!

[Groesbeck Hurricane]

Verify that the coil has oil inside by removing and shaking. Verify the wyre coming into the top of the coil is not crimped or broken inside the rubber shielding. Below I am copying from autorepair.about.com

determine whether your coil is on the way out, the only proper coil test is with a multimeter. Why? Here's the rundown:
Inside an ignition coil are two coils of wire on top of each other. These coils are called windings. One winding is called the primary winding, the other is the secondary. The primary winding gets the juice together to make a spark and the secondary sends it out the door to the distributor. Either one of these windings can go bad and cause your ignition coil to fail.

Sometimes an ignition coil is bad, clearly bad, as in it makes no spark at all. But if a coil is on the way out, but not dead yet, it can make a weak spark that can cause the car to run rough or wrong. By testing an ignition coil with a multimeter while it's disconnected, you'll be using data and numbers to determine the health of the coil rather than your eyeballs and dead reckoning.

We'll show you how to test both the primary and secondary ignition coil windings using a multimeter.

*You'll need the resistance specifications for your specific coil in order to perform this test. Consult your service manual for this information.

The primary winding of your ignition coil is the first to receive voltage from the battery. Find the resistance specifications for your car's primary coil winding in your repair manual. Then using a multimeter, place the leads on the smaller, outside poles if you have a traditional round coil, or on the indicated poles if you have a newer enclosed unit. If the reading is within the range indicated in your manual, your primary winding is ok and you can go on to the secondary test. if it is even a little out of spec, the coil should be replaced.

The secondary winding of your ignition coil delivers the spark to the distributor to be sent to the spark plugs. If it's bad, you'll get a weak spark or no spark at all.

To test the coil's secondary winding, attach the test probes to the outer 12V pole and the center pole (where the main wire goes to the distributor). Determine the resistance and check to see if it's within the range indicated in your repair manual. If it is, your coil is up to the task. If it's even slightly out of range, your coil should be replaced.



Running the Bosch CDI is accepted as a "best option". As was stated above, if the coil is bad you should be able to easily source a replacement. Best,

[flpporsche]

Hmmm, i've tried two different coils on the bloody thing now, still no spark. contacts in the dizzy cap are clean etc. must have a broken wire somewhere????

i'll continue into the evening (again)

trying to fit a personal job around customers vehicles is a pain, this is going to take a while!

[wwest]

Originally Posted by flpporsche

Hmmm, i've tried two different coils on the bloody thing now, still no spark. contacts in the dizzy cap are clean etc. must have a broken wire somewhere????

i'll continue into the evening (again)

trying to fit a personal job around customers vehicles is a pain, this is going to take a while!

The CDI sound should be a high frequency squeal, not a hum.

At this point I would pull the dizzy and turn it by hand with the ignition on.

[Flat6pac]

see if your wires that are shielded and common to the sheilding. The wire to the dist and coil could be broken down and bleeding over to ground showing as a wire common to the ground shield..

Bruce