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Car Forum / Driving, Maintenance, Tuning / General Car Topics / May 2008HELP: Porsche 944 no spark
Hi Group, The other day my car would not start. The battery is fully charged and the starter cranks well. My friend and I did some preliminary diagnosis with which we discovered: 1. There is 12V at the ignition coil (both wires to ground). 2. The resistance/impedance numbers of the coil are within spec. 3. There is no water in the coil, we displaced it with lubricant and the protective boot looked ok anyway. 4. We hooked up a 2 different spark plugs to the output wire of the coil (the wire that goes from the coil to the cap) and then grounded it and turned the key into the ON position (but not the start position) and did not get any spark. 5. There is fuel in the rail. My question is, if there is 12V at the ignition coil, and the resistance numbers are ok, could it still be a bad coil, or does this point to something else like a bad DME/ECU, a bad DME Relay, a bad crank position sensor or speed sensor? I've read in other posts that when these sensors go bad, there is no spark... but did the authors of these posts mean to write that when these sensors go bad, there will NOT be 12V at the coil? Please help. Thanks. > Hi Group, > [quoted text clipped - 21 lines] > these sensors go bad, there will NOT be 12V at the coil? Please > help. Thanks. Is there a pulse at the injectors? > Is there a pulse at the injectors? There is plenty of fuel in the rail. We haven't' checked the injectors yet because we were unable to get spark. So we wanted to solve that problem first. They are unrelated, right? -Jon >> Is there a pulse at the injectors? > >There is plenty of fuel in the rail. We haven't' checked the >injectors yet because we were unable to get spark. So we wanted to >solve that problem first. They are unrelated, right? No. Both the spark and the injectors are commanded by the ECU. If the ECU doesn't know that the crankshaft is turning, it won't fire either one. If your coil were gone, you would not have spark, but you would still have plenty of fuel coming out of the injectors. But you'd have noticed this if it were the case because the exhaust would have smelled strongly of unburned fuel. --scott  Signature"C'est un Nagra. C'est suisse, et tres, tres precis." On May 12, 11:29 pm, jonb55...@yahoo.com wrote: > Hi Group, > [quoted text clipped - 21 lines] > these sensors go bad, there will NOT be 12V at the coil? Please > help. Thanks. Could be any of these, but I would try the DME relay first, this is the classic failure of these vehicles. I had a similar failure on my '88 and it ended up being a combination of bad coil and poor solder joints in the DME (likely caused by the bad coil overheating the coil driver) but if you are confident in your diagnosis of the coil as being good try the DME relay. You can bypass it temporarily, look for a procedure on Clark's Garage. nate > Could be any of these, but I would try the DME relay first, this is > the classic failure of these vehicles. I had a similar failure on my [quoted text clipped - 3 lines] > being good try the DME relay. You can bypass it temporarily, look for > a procedure on Clark's Garage. Yes, but if there is 12V at the coil, should it be sparking? there is 12V at the terminals on the coil, the impedances are correct, but we aren't getting any spark. <jonb55198@yahoo.com> wrote in message news:559d5410-1976-4120-a908- > Yes, but if there is 12V at the coil, should it be sparking? there is > 12V at the terminals on the coil, the impedances are correct, but we > aren't getting any spark. Simple presence of 12 V at the coil does not guarantee a spark, if that helps any. There are a number of different types of ignition circuits around, and I dont know what that Porsche uses... > Simple presence of 12 V at the coil does not guarantee a spark, if that > helps any. So, if there is 12V there, can we rule out a bad DME or bad speed and reference sensors? asked another way: would a fault in the DME, or these 2 sensors cause no voltage at the coil? The spark is actually produced by taking away the 12v. The DMA will ground the coil for every time it needs to spark. You can try testing the coil by using Clark's website suggestion. I tried this and I thought I would see more of a spark. I actually had a hard time seeing the spark so I had to move the coil and battery into a dark room like a walk in closet to see it. My guess is if your coil measures good it probably is good. Do you know about Clark's web site. Here is a link to the troubleshooting section on "No Spark": http://www.clarks-garage.com/ go to "Garage Shop Manual" click on "T" for Troubleshooting and then on "TS-01" BTW my problem was a bad ignition switch. >> Could be any of these, but I would try the DME relay first, this is >> the classic failure of these vehicles. I had a similar failure on my [quoted text clipped - 7 lines] > 12V at the terminals on the coil, the impedances are correct, but we > aren't getting any spark. >Yes, but if there is 12V at the coil, should it be sparking? there is >12V at the terminals on the coil, the impedances are correct, but we >aren't getting any spark. Voltage does not cause the coil to spark. The CHANGE in voltage causes the coil to spark. If there is 12V on BOTH sides of the coil, there is no voltage across the coil anyway. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." The ignition coil works by rapid collapse of the voltage in the primary coil. If the primary and secondary show propter resistance then the coil is probably OK. The sensors can only be tested with an oscilloscope. They may have slipped in position. You need to check this. Go to http://www.clarks-garage.com for more specific data. Have two 944's and have been through this drill a few times. If you have an older model that has been stored outside your problem is probably a bad connection on the blade connectors on the relay panel. Try wiggling the wire mass when starting to see if it fires. >The other day my car would not start. The battery is fully charged >and the starter cranks well. My friend and I did some preliminary >diagnosis with which we discovered: > >1. There is 12V at the ignition coil (both wires to ground). When you try and start the engine, does it pulse? If not, it's not getting proper signal. >2. The resistance/impedance numbers of the coil are within spec. >3. There is no water in the coil, we displaced it with lubricant and >the protective boot looked ok anyway. Makes sense. Coils fail occasionally, but not very often. And usually when coils fail, it's because they're not being driven correctly. >4. We hooked up a 2 different spark plugs to the output wire of the >coil (the wire that goes from the coil to the cap) and then grounded >it and turned the key into the ON position (but not the start >position) and did not get any spark. >5. There is fuel in the rail. Okay, sounds good. >My question is, if there is 12V at the ignition coil, and the >resistance numbers are ok, could it still be a bad coil, or does this >point to something else like a bad DME/ECU, a bad DME Relay, a bad >crank position sensor or speed sensor? The coil is fine. One of these sensors is bad, or the ECU is bad. Pull the codes off the ECU and see what the ECU has to say. >I've read in other posts that when these sensors go bad, there is no >spark... but did the authors of these posts mean to write that when >these sensors go bad, there will NOT be 12V at the coil? Please >help. Thanks. There will almost always be 12V at the coil. --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." > I've read in other posts that when these sensors go bad, there is no > spark... but did the authors of these posts mean to write that when > these sensors go bad, there will NOT be 12V at the coil? Please > help. Thanks. others have explained about coil and sensors. I would just about guarantee the coil is good. I would check that the two sensors at the back of the engine that provide reference and mark information by sensing flywheel position are both properly connected and measure them for continuity - that is the most likely cause outside of a DME relay. You can lift the cover off the DME relay easily and check for bad solder connections where the upper relay in the package is soldered to the little circuit board - this is a VERY common failure and takes all of 5 minutes to fix for no $$ at all ** Posted from http://www.teranews.com ** > others have explained about coil and sensors. I would just about guarantee > the coil is good. I would check that the two sensors at the back of the > engine that provide reference and mark information by sensing flywheel > position are both properly connected and measure them for continuity - that > is the most likely cause outside of a DME relay. It was a bad crank position reference sensor. Thanks everyone for the help. My friend has several E30 BMW's and the sensors are the same. So we tested the resistances on my sensors, found that the crank reference one was bad, swapped in his... and it started right up! Thanks everyone. >> others have explained about coil and sensors. I would just about >> guarantee [quoted text clipped - 9 lines] > reference one was bad, swapped in his... and it started right up! > Thanks everyone. good work - do not discard the old sensor - believe it or not, I've repaired them - just find the break in the coil and fix it ** Posted from http://www.teranews.com ** > Hi Group, > [quoted text clipped - 10 lines] > it and turned the key into the ON position (but not the start > position) and did not get any spark. And you should not. If the engine is not cranking, there will be no spark. If there is no spark when the engine cranks, suspect the ignition module, distributor pickup, or associated wiring. SignaturePaul Hovnanian paul@hovnanian.com ----------------------------------------------------------------------- Have gnu, will travel. DME relay Try using a three-way jumper plug to replace it. This has 3 spade connectors, like the wide ones on the bottome of the relay, connected by two wires. Picture the old McDonalds "M". Don't leave it in when not running the car. It'll explode! Or something. > Hi Group, > [quoted text clipped - 21 lines] > these sensors go bad, there will NOT be 12V at the coil? Please > help. Thanks. |
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