Faulty Ignition Coil and Aftermarket Replacement Question

[LifterEyes]

I've been trying to isolate a problem where my 944 has been running rough. I have just recently checked my coil and ran the following tests:

I tested the coil first by removing the cable from the distributor and connecting a grounded spark plug directly to the output then cranked the engine. I was able to get a spark but it seemed weak.

The second test I did was the procedure outlined in clarks garage Ignition Troubleshooting article. This test called for disconnecting the negative side of the coil, turning on the ignition and manually grounding the negative side of the coil to test for a spark. With this procedure I wasn't able to produce a spark at the sparkplug. In fact, I was getting a spark from the ground cable when I would tap it from the body.

The Clarks Garage manual said that the coil primary resistance should read between .4 and .6 ohms. With the coil still wired in the primary ready about .9 ohms but when it was completely removed it read 0 ohms. I am unsure if the article meant that this should be the resistance with it wired in or with it removed.

If this sounds like a faulty coil, could I use some aftermarket coil such as an MSD Blaster 2 coil or something similiar? I am asking because these can be picked up cheaper then a factory replacement coil. Also, as far as my understanding, the bosch coil that comes stock with the 944 has a built in ballast resistor. Will an aftermarket coil require this ballast resistor and, if so, what would be the appropriate value such that it will not cause damage to the DME?

[924Sman]

Do a search here on the Blaster2, there are iirc many posts, may get an answer to your question.

Dal

[HondaDustR]

I've been running the MSD Blaster 2 coil for about a year without the ballast resistor and without any problems. I think I saw someone in an old post say they called up performance products asking the same question and they said it doesn't need one, so I just went ahead and tried it. No problems and it works really well...and it's only $40 at autozone! Only issue I had was that it is a little narrower than the stock coil, which meant I had to fold up some cardboard to shim the mounting bracket so it would hold it securely.

[LifterEyes]

Thanks for your reply. I was looking at the specs for the blaster coil and It looks like the primary resistance is .7 Ohms, which is about what the stock one should read anyway (.4 to .6 Ohms according to Clarks Garage). Also, from looking at the 944 wiring diagram, It doesn't seem like there's any ballast resistor at all in the circuit. From what I can see anyway.

I'm going to go check the coil one more time before I go out and buy a new one. It's weird to me that it will run but when I test the coil by trying to manually trigger it I don't get any spark.

Thanks again.

[924Sman]

If it runs why bother?

Dal

[HondaDustR]

Coils probably either work or not, but for the money, you can be sure it's up to spec for not too much, plus it would be a mild performance upgrade. Some people complain iridium plugs don't work well with the lower power stock ignition, but mine with the blaster coil has no trouble firing them. If it's testing out of spec on the meter, I would definitely look into it, since it may not be working quite right with the DME output if the resistance is off, and that could certainly cause rough running from a weak spark (worn plugs, cap/rotor, and deteriorated plug wires will amplify the effect), especially if the more likely causes of rough running are ruled out.

[P944GUY]

the coil either works or it doesn't..don't fool yourself thinking aftermarket stuff will increase your performance..it doesn't....it doesn't......it doesn't..stick with the Bosch...for God sakes they use the same coil in the 911 that puts out 250+ hp...did you think they were wrong...

[P944GUY]

computer may have gone out...it's like the coil..it will either work great or not at all..try swapping a computer..check with your mechanic maybe they can loan you one..

[88Silver924S]

How did your engine come to running rough, suddenly or gradually? What kind of spark plugs (normal, platinum, etc?) and how old (miles) are they? Have you cleaned and checked the gap; how old or what are the conditions of the ignition wires, cap and rotor? On a rough engine look for "carbon tracks" on plugs, cap and rotor; if possible swap your spares in (I recommend keeping spare plugs, cap, and rotor). The tracks may be hairline small but cause problems. On past cars I have seen tracks inside a rotor (hard to see), and inside a cap (easy to see). Inspect the plug insulators on the outside and the insulator inside surrounding the center electrode for tracks. Run the engine in the dark and see if you can see any spark "leakage" along the wires, coil, distributor cap. Make sure your wires are not too close to each other. Years ago I had a 356B where one spark wire made one partial twist around onother spark wire which induced a spark in the wrong wire at the wrong time=roughness; keep separation between high-volt wires.
Years ago just for fun I put a multimeter to the primary of my original coil and got wierd numbers too but since everything ran strong and smooth I didn't change a thing; still going strong.
Good luck.

[88Silver924S]

Also on a rough engine look for vacuum leaks.