Help !!! No spark to coil on 930
 

I just returned from a 1500 mile road trip with my 87 930 Cab. The car ran great. When I arrived home, I let the car idle for a couple of minutes in the driveway to cool the turbo down. While idling, the engine shut itself off, and now won't start. I can hear both fuel pumps, have checked to see that the rotor is turning, jumped pins 80,87 & 87a on the relay at the back left side of engine. You can hear the high pitch of the computer when the key is turned on.

There is no spark to the coil. Does anyone have any ideas on what to check next ? Could it be a dead coil ? The Porsche dealer doesn't think so as it is rare that they fail. How do I test the coil ? Does anyone have a wiring schematic ? Any help would be greatly appreciated....

Thanks,
Robert

I had the exact same thing happen to my 914 2.0 when idling in the driveway, except that there was smoke coming from the fuse panel when the car shut off. Also, I had to jump start the car in order to get it running. Ever since it shut off though it won't start even when jumped. I replaced the spark plugs and think the problem may be the coil. There is a way to check the coil spark in the Repair Manual. It says to make sure the cap is clean, then remove the HT cable out of the center of the distributor cap. With it still in the coil hold the free end about 3/8 inch from the fan housing and turn the engine over with the starter. There should be a spark from the cable end. As the voltage is several KV use rubber gloves or a pair of insulated pliers to hold the wire to prevent an electric shock. If there is no spark check the voltage at terminal 15. (ignition on) It should be at least 9 volts. This is what is says to do on page 65 of the Haynes 914-4 cylinder manual.

Just went through the same thing on my 88 turbo. IT was the CDI unit... was making the high pitched whine like it should.... still was bad. Also, people say that the green wire that goes to the distributor goes bad on occasion.

Good Luck,
Chuck

Thanks for the info... My CDI unit is making the high pitched whine like it should also. Is there a way to test it ? It sure is bizzare that I can drive 1500 miles without a problem and then it dies in my own yard. I've checked the primary and secondary resistance on the coil. Primary is good, but I'm not sure about the secondary as I'm not certain I have the right specs for the 930 3.3 litre coil ( I'm using the 3.2 coil specs ) . Also, When the key is on I have no power to the coil. How do you check the green wire to the distributer ( Resistance, continuity or power ) ? It sure would be great if someone has a wiring schematic for the 930...

Thanks,
Robert

I no longer have my car or the manual, but I remember that there is no voltage to either terminal of the coil pack (only the center one). One terminal is the RPM wire, and the other is ground. Do you have a meter to confirm one of the terminals is ground? At this point it appears your problem is either the CDI unit or coil pack. I hope it's the coil pack since the CDI units are really expensive.

For what it's worth, my CDI went bad, but would only act up while hot. :confused:

Because silcone junctions can break down under heat, but might work just fine when cold. I don't believe there are any IC's in the CDI unit, there are diodes and transistors.

My Turbo did the same thing... died while idling in the driveway. I think you can check the CDI box if you know what to measure where on the pins... it was way beyond me. Usually people suggest swapping with a KNOWN good unit. I say "KNOWN" because a friend gave me a unit he swore was good... it wasnt. Kept my car off the road for some time. The boost monitoring relay... that yellow one.. seems to be a culprit on occasion as well. If you have a friend with a CIS car you could probably swap the box and try it. IT's a quick job... once you get that darned airbox off!!

Good Luck,
Chuck

Robert,

If you plan on having your car for a while, it will pay to get the shop manual for your car. This won't be the last problem you have. Not trying to sound negative, yet to be proactive. Sounds like your a DIY kind of guy.

Hey Brent,

Yes, I do plan on keeping my car for a loooooong time ;-) Do you know where I can purchase a shop manual ? I have a couple ( Bentley & Chiltons ) for my 84 911, but it doesn't cover the 3.3 litre or the 930 itself. I tested my coil today and had low resistance on the secondary circuit. I ordered a new one and have my fingers crossed that I didn't waste money... Too bad my 3.2 litre coil wouldn't work to test things.

Robert

Robert

Not off hand, I got mine through Ebay. You will just have to search around.

The secondary coil has verrrrry low resistance.

I've seen 2 CDI boxes go bad that still made the noise, and I'd bet yours is bad.

You need to check the resistance of the green coax wire that goes from the pickup coil in the distributor to the CDI box - if you pull the 6-pin connector off of the CDI box (you should be able to reach in and get it w/o pulling the IC off even though it is pretty tight), you can pull the rubber boot back a bit and see the pins for the green coax wire, they are both on one end of the plug for pins 7 and 31D. Check the resistance across these 2 pins - it should be ~600 ohms. Then, switch your meter to AC volts and check these same 2 pins when someone cranks the motor over - you should see a low A/C voltage that WILL vary a lot (basically the pick up coil and magnetic trigger are sending a signal to the CDI box).

Then, with the ignition on, make sure the box is getting ~12V at pin 15, and that pin 31/1 is a good ground.

If all of this checks out, unplug the tachometer connections (pull out the tach and unplug it), and try to start your car - if it runs, the tach drive circuit in the CDI box is bad. If it still will not run, the CDI is bad. To verify that the CDI box is not producing any spark, you can measure the output voltage on pin A of the CDI harness (white wire at the coil) while everything is connected - you should see over 400v A/C at the white wire (also a changing voltage as each spark signal is sent, not a constant voltage).

Good luck,

Pat K
87 930

So... if you have an 84 SC, why don't you try the CDI box out of that one on your turbo?? Easy way to figure it out. It's 3 bolts and a plug.

Chuck

Oops nevermind ... 84 is motronic huh? And not an SC. Forgive me... been a long day..

Chuck

Hi Pat,

First of all, thanks for the great info. Here are my findings...

Resistance of the green coax wire from plug to distributer - ~594 ohms

Pins 7 ( White) and 31d ( 2 wires together Brown/Wht ) ~.6 ohms
Pins 31/1 ( Blk ) and A ( Grn) are the same as the green wires resistance ~594 ohms
- I think you may have gotten the ends reversed

Pins 31/1 ( Blk ) and A ( Grn) are 240 to 250 mV while cranking the engine over

Pin 15 has 12.14 V, but only while cranking the engine over
Pin 31/1 appears to be a good ground ~0 ohms

Tach has not been pulled yet. I have to figure out how to pull it out...

To verify the CDI is producing spark, I measured the output voltage at the white wire on the coil with everything reconnected and engine cranking. The voltage at the white wire was 10.2 mV A/C. It is no where close to the 400v A/C you stated.

What do you make of these readings so far ?

Thanks,
Robert

The CDI box has an output of ~460V ac. This is why it has all of the warnings on it. If you're not getting that at the coil (white wire), there's no way that you can get a spark at the plugs. IMO it is fried. Most folks swap to an MSD or Crane CDI setup rather than buying a new or rebuilt Bosch. Search the 911 technical forum for threads on this - it is cheaper than buying an Bosch, and not hard to do.

And, if you do swap out your Bosch box for an aftermarket unit, I would like to try and diagnose it - I have 2 other bad ones here that I'm looking to rebuild.....

Pat K
87 930

Just go buy a MSD 6AL and be done with it. It'll only set you back $250. :)

I definitely will look into the MSD 6AL as an option. Are these 100 % compatible ? Do I have to cut any wires off of the old harness, or is there and adapter available ? Is a new coil required also ?

Thanks,
Robert

A new MSD coil is required IMO, although it may 'work' with the stock coil. If I were converting to an MSD unit, I would make a spade connector 'jumper' harness that would plug into the stock plug on the CDI harness and connect to the MSD unit. That way I could easily revert to a stock CDI (or whatever), if needed.

Pat

I did an MSD box on a 914 I used to have. I too thought the MSD coil was a requirement... until it went bad. Went back to the Bosch and never had any more "coil" related problems :)