Coil Problems on my 3.2

[Steve N L]

I have a 1987 911 and have just burned out my second coil in 700 miles. In each case, the car was running fine, was shut off and then would not start. Once it will not start, the problem is lack of spark. I have current to the positive terminal of the coil when the ignition switch is turned on.
I checked continuity between the primary terminals and have .5 ohms, which is good. But I have no continuity between the secondary terminals. Both of my coils failed in the same way. The second was a new Bosch that I had purchased from Pelican.

There is a possibility that the old coil simply failed and the new one was defective, but that probably isn't the reason.

I'm just getting acquainted with this Motronic system. Does anyone know what might be causing these coils to fry?

Steve L

[Deadeye]

I just went through a similar problem (87' 911) and traced it to a chafed wire in the DME harness under the seat. Any in eruption in the power supply to the coil , no matter how brief, can ruin your coil. Not easy to track this down. Check all grounds and wire connections for corrosion as well.

[Steve N L]

Thank you for your reply. Since making my initial post, I have taken another look at the resistance for my secondary winding in the coil. I misread the specs and failed to use the proper setting on my multi tester. As it turns out, my coil is fine. But I still have no spark.

So, I should next proceed to look at the DME and the wire that runs from that to the coil?

[Tippy]

I am far from an expert but I suspect there is an issue inside the DME that is causing you fits.

"ischmitz", "Lorenfb", and "Steve Wong" can provide better insight.

Good luck!

[2.70Racer]

Steve,
The ignition system in Carrera's uses two flywheel sensors to tell the DME when to signal the Coil to fire the plugs. One of the sensors is a reference sensor that tells the DME about engine position, the speed sensor tells the DME how fast the engine is cranking.
I suspect you may have a problem with one of these sensors. Faiure of one of these sensors is common with a Porsche 25 years old. The wiring is stiff and brittle.
The Bentley "Porsche 911 Carrera Service Manual" covers the troubleshooting and replacement of these sensors in detail.
I'm confident this subject has been covered on this site as well.
For more info search Reference sensors or speed sensors.
Pelican stocks replacement sensors part number 911-606-215-01, they are a little pricey at $158 each.

[gamin]

Be absolutely sure that the ground connection in the engine compartment is clean and TIGHT. This is the connection at left rear on intake manifold. A bundle of ground wires attach there. If it is not tight you will get no spark. BTDT.

[Tippy]

2.70Racer nailed it. If crank ref/speed sensors test ok, time to move on to DME tests.

[masterdave]

+1 to every thing said here,
I had some spark issues on my 924s which also has the bosch system.
-Grounds are very important on these cars. They corrode easily and are simple to clean and can solve many issue once they are cleaned.
-When you say you have current, how much, is that when the engine is running or when just the power is on to the system. If this bosch system works the way i think it does, when power is on, there should be 12V at the coil then when the car needs to drive a spark you will have 24V at the coil, or it may be 0 and 12 Im not sure. If you have an oscilloscope you can measure it, it should be a square wave.
-I know there was an issue with the transistors that pulsed the coils in the Bosch systems (thats where the issue on my 924s was) i had the ECU rebuilt and it solved everything.
-Check the plug wires and plugs and distributor if there is a short or a no connect you may have a issue of the coil having no where to dump the spark im not sure what this would cause but i feel like it could create issue

best
Dave