Help! Low input Voltage at 960 ignition coils

[AntoineB]

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Dear All,

I own a 1995 Volvo 960 (2.5l L6 manual gearbox) and facing a challenge for which I cannot find the solution. I replaced the wiring harness going to the ignition coils a few months ago, as it was totally cooked. The car has always been missing torque, smoothness and power. A few days ago, I measured the DC Voltage at the connectors feeding the ignition coils. On cylinders 1, 3 & 5 (starting from the front of the car), I do have 13+ Volts DC peak between the two pins of the connectors . But on Cylinders 2, 4 & 6, I only have about 5.4 V peak between the 2 pins of the connectors at idle. But if I measure the Voltage on Cylinders 2, 4 & 6 between the red/green cable of the connectors and the ground (engine block), I do have 13+V.
I understand that those cylinders are controlled by ignition control module N°2. I have replaced this module with a new one, but still the same situation. I have cleaned the ground point below this intake manifold, where normally both ignition control modules are connected : no change. I have checked with a multimeter the continuity between the ground pin of ICM N°2 connector and the ground point : its is fine.
I have cleaned all the other ground points I could find (on the frame near the battery, the two points on the block close to the injectors, the one going from the frame to the Camshaft position sensor). I noticed the the camshaft position sensor had a broken corner, the one with the ground connector attached (see picture) and I also replaced it. Starting the car with our without the camshaft position sensor does not seem to make any difference to the engine behaviour (but I did not drive without it). I also seem to have a faulty lambda probe (warning light), I have the part but could not replace it yet as the old one does not move an inch, but I do not see why the lambda proble would cause this low voltage issue at the ignition coils. Does anyone already experienced this? I have no more ideas about where to look for. Thanks, Antoine

[jimmy57]

The red/green is power supply and the other wires that have a different color on each coil is the ground from the ignition power stages. The power stages are transistor amplifier circuits, 3 in each power stage. The grounding from those power stages is never a direct ground as the power stage is designed to keep a constant wattage to each coil. During starter operation when battery V is reduced the ground connection through power stage will be better and the V measured between pin at a coil will be higher. The V you measure at a coil is an average voltage as this is a brief ground each time the coil is charged and spark will discharge when that ground pulse stops. You shouldn't be measuring 13 between the two pins on the coil two wire plug. The 5.4V is what I would expect measured there. 12.5 V is what you should have if you measure between green/red wire and ground at each plug with key on, higher(13.5-14V) once alternator is charging if you have engine running. If you have key on, engine not running and you use a test light between battery positive and the signal wire at each plug the light should not be lit. There should be no grounding for each coil until engine is cranked or running when spark from coils will commence. If you have a lit test light in that check on 1, 3, and 5 then I would disconnect the front power stage connector and see if the test light goes out. I think you may have a problem with power stage 1 shorting internally. Coils 1,3, and 5 may have failed from being grounded continuously. You can remove coil(s) and insert a spark plug into coil boot and then lay the spark plug on top of engine so it touches for ground. Now if you crank engine over there should be a spark. I suspect you have no spark at 1,3, and 5.

[AntoineB]

Dear Jimmy57, many many thanks for your kind help and detailed explanations about the functioning of the ICM, You are totally right! I was expecting 12v because this is what is needed to feed the ignition coils. But the digital multimeter might not be able to read those short peaks at idle. Today I have swapped the No. 1 ICM with the No. 2 ICM previously replaced by a new one. Now I also have around 5.4V on cylinders 1, 3, 5, same as cylinders 2, 4, 6. I never had cylinders without sparks, but now the engine is running much better, fuller. Still not perfect, but I have the warning light with the Lambda Probe, which I will replace, I believe it must have an impact on engine management and performance. I already have the part, but could not yet unbolt the old probe, almost soldered to the catalytic converter. I changed all coils with 2nd hand original Volvo ones I had, for safety. I still have a doubt about the MAF, I cleaned it with a good contact cleaner, I have not changed the MAF yet "as standard", because it is an expensive part, and also because this winter, when the weather was cold, I could not start the car after disconnecting it for trial. So, somehow, it is still alive. But in summer I can start and drive the car with it is disconnected. Once again, many thanks for the time you have dedicated to me.