Mercruiser 496 cranks but will not spark and no rpm on Diacom...

[PettyOfficerBorden]

I have a 8.1L Mercruiser with a PCM555. The engine cranks, but will not start. I have it on the Diacom and there are no codes and there is no rpm reading on the laptop. I thought it was the Crank position sensor, but the sensor passed resistance and contuinity test as did the camshaft sensor. I have checked contuinity on all sensors and wiring. The battery is 12.6v, I have also tried other batteries as I know low battery voltage or a bad battery can prevent the computer from operating properly. There is just no spark from any coil. Thanks for the help in advance!

 

[Don S]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

When you turn the key on, can you hear the main power relay click? Have you checked all fuses? Safety lanyard in? Power getting from the ignition switch (purple wire) to the engine?

Also need the serial number of your engine.

 

[PettyOfficerBorden]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

M025428

I did ALL the mentioned checks until I felt like I was going crazy.

 

[PettyOfficerBorden]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

And yes, I can hear the MPR click. With the troubleshooting manual I keep ending up at "replace ECM/PCM" I just find it hard to believe.

 

[Don S]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

If you followed the manual and did all the tests properly, then I guess the ECM failed. They do at times.

 

[boatguya1]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

I have a 8.1L Mercruiser with a PCM555. The engine cranks, but will not start. I have it on the Diacom and there are no codes and there is no rpm reading on the laptop. I thought it was the Crank position sensor, but the sensor passed resistance and contuinity test as did the camshaft sensor. I have checked contuinity on all sensors and wiring. The battery is 12.6v, I have also tried other batteries as I know low battery voltage or a bad battery can prevent the computer from operating properly. There is just no spark from any coil. Thanks for the help in advance!

There is a test you can do by jumping 2 terminals of the connector for the CPS that will make the ignition fire if everything else works. I am home and don't remember what terminals to jump (jump and remove quickly like the thunderbolt test, tap if you will). If I remember tomorrow I'll look it up at work, probably available via google though.

I'd do alot of testing prior to replacing the ECM.

I'm reminded of a merc school instructor saying Keep your eyedometer on the tachometer while cranking, if the tach doesn't move check the CPS circuit.

If its not the CPS itself it could also be a wiring harness problem.

 

[boatguya1]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

I googled around a bit and didn't come up with it. The test I'm thinking of is in a book from Merc school, I'll try to remember to look it up tomorrow morning prior to starting wrenching.

 

[Fun Times]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

Keep in mind that it is possible to have a bad CPS even though the ohms tested good due to excessive or abnormal wear or dirt. Though this link is for GM trucks with the 8.1L engine, it's the same concept for the actual sensor, Ignore the first few steps of how to get the sensor out,
2002 8.1l V8 Crankshaft Position Sensor - Fullsize Truck & SUV - GM-Trucks.com <<< Edit: On 5/9/2016 I just noticed the images are removed from the first link...Here is a similar 8.1 crankshaft removal, inspection, installation guided covering the crankshaft sensors ware down inspection with working images....https://www.2carpros.com/questions/chevrolet-silverado-2002-chevy-silverado-crank-postion-sensor

Also the CPS for your 496 engine is really GM part, it's not necessarily a mercruiser part for this particular case. This way you can find a CPS at a better price using say a 2002 8.1l chevy suburban if needed. It's the cheaper way of throwing parts at it till you find it. oops, did i just say that out loud!:brushteeth:

I thought it was the Crank position sensor, but the sensor passed resistance and contuinity test as did the camshaft sensor.

I have checked contuinity on all sensors and wiring.

Have you ensured that all the wire pins inside all the connectors have not been pulled or backed out just enough to lose connection with the sensors or PCM?

Also have you disconnected the gray wire off the back of the tachometer?

Typically, no RPM signal is CPS related, but could be Camshaft Sensor or PCM related as well.

Please keep us updated on how this turns out, Good luck.

 

[boatguya1]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

There is a test you can do by jumping 2 terminals of the connector for the CPS that will make the ignition fire if everything else works. I am home and don't remember what terminals to jump (jump and remove quickly like the thunderbolt test, tap if you will). If I remember tomorrow I'll look it up at work, probably available via google though.

I'd do alot of testing prior to replacing the ECM.

I'm reminded of a merc school instructor saying Keep your eyedometer on the tachometer while cranking, if the tach doesn't move check the CPS circuit.

If its not the CPS itself it could also be a wiring harness problem.

I couldn't find it in the book, but in my notes I've got.... 5v tapped on sense wire of cps connector will fire the ignition.

I assume you have already checked the 5v reference voltage and ground at the cps so... tapping a connection between the grey 5v and the tan/blk wires at the cps connector should fire the ignition.

Hope this helps.

 

[Don S]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

Just to confirm, you have done all the troubleshooting steps shown in the file below, correct?

View attachment 332A.pdf

 

[PettyOfficerBorden]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

Don, thanks for your help. I stepped away from the engine over the weekend, took one more look at the CKP, probed the sending (grey wire) with a needle and tested voltage. The engine started with the pin going into the wire and down into the sensor connector. I take this as the sensor sending wire was broken and the pin completed the circuit. I know we aren't really "supposed" to probe wires in that fashion, but it solved the problem.

Now, do I need to replace that entire section of the wiring harness or can I just splice into the three CKP wires with a new connector? I apologize for any improper terminology I use. Thanks.

After re-reading your previous post, I think you answered my question before I asked.

 

[Don S]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

I would just replace the connector.

Oops, forgot the picture.

 

[PettyOfficerBorden]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

Don, thank you! That is exactly what I need. Do you know if there is a sierra p/n, or if Orielly's carries something equivelant? Thanks again.

 

[Don S]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

Have no idea. Get the right marine one, it may seal different than the automotive version.

 

[Fun Times]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

That is good news to hear, glad you kept at it, instead of just buying parts, much less costly on your part.

Have you inspected the connector to see why it's not making a connection? maybe the wire pin has been simply released from the locked position inside the connector.

 

[PettyOfficerBorden]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

The grey wire was actually broken. We had to order the sensor repair kit. Thanks!

Here is another question I have: I have a 1995 Four Winns Candia 214 with a 5.8L OMC EFI and Cobra drive. I need to replace the shift cable, but am overwhelmed with the variety of cables and length options. What should I be looking for? Thank you.

 

[Don S]

Re: Mercruiser 496 cranks but will not spark and no rpm on Diacom...

The grey wire was actually broken. We had to order the sensor repair kit. Thanks!

Here is another question I have: I have a 1995 Four Winns Candia 214 with a 5.8L OMC EFI and Cobra drive. I need to replace the shift cable, but am overwhelmed with the variety of cables and length options. What should I be looking for? Thank you.

You need to start a new thread over in the OMC forum for that.