Mercruiser 555 - ECM no power?

[achris]

Disconnect the tach (gray wire) and see if it starts. A shorted tach will kill spark.



Not on my side, I see B1 tied to main ground inside the ECM. Only reason (as I see it) to have a separate ground, is to keep noise off the line of the sensors. A floating ground will cause all kinds of variations in readings

Chris may have a different thought

He does. Very common in computer control systems to have isolated negatives, for the exact reason you said, keep noise off sensors and other sensitive circuits.

Love to know how you know the B1 pin is tied to main ground. I'm yet to find any internal circuit drawings of an ECU. BTW, I have checked on my own ECM555, and there is significant isolation between B1 and the engine block.

The ground is only 'floating' with respect to the engine block, not the 5v that is used to power the sensors (and the main ECU CPU too).

Tacho being 'all over the place' during cranking is completely normal. Mine does it, and that's with 2x N70ZZ batteries (paralleled of course)!

Chris.........

 

[achris]

Disconnect the tach (gray wire) and see if it starts. A shorted tach will kill spark.

Not on the ECM555. The tacho signal is generated inside the ECU and not as a result of the pulse to the coil driver. You could ground out the tacho signal, and the engine would still fire.

 

[kenny nunez]

Is there current to the primary side of the coil when cranking? I was wondering if the shift interrupter wiring could have a bad connection or the shift switch could be either stuck in the open condition or is just bad internally. Since there is no secondary ignition output.

 

[PatL1]

Is there current to the primary side of the coil when cranking? I was wondering if the shift interrupter wiring could have a bad connection or the shift switch could be either stuck in the open condition or is just bad internally. Since there is no secondary ignition output.

I checked the voltage to the coil and the ign driver module.....both OK. I'll check it while cranking as you suggested.

I am going to spend a few hours with the owner today and am planning to put an automotive DSO on a couple of the signal lines to see what is going on with the CPS input to the ECM and the ECM output to the ignition driver module. While I am hooked up with the scope, I'll grab the injector signals since it is a 4 channel scope. Should be interesting to see if there is a crank signal and trigger signals to the Ign driver. If I see CPS signal going in and no signal coming out of the ECM, I suppose it its a faulty ECM unless there are other inputs the ECM needs to fire the ignition.

Any other suggestions are welcome.

 

[PatL1]

After more troubleshooting today, the problem is the internal ground circuit (B1). I get no voltage when connecting the 5V+ sensor voltage line to the sensor ground line (B1). When I check the 5V+ sensor pin to chassis ground I get 5V. To diagnose it further, I connected the chassis ground to the pin B1 (sensor ground) and the engine starts and runs fine. Looking at it with the oscilloscope, there isn't any apparent noise on the line but I realize that could change depending upon what else might get attached to the chassis ground.

What are the downsides of splicing the chassis ground to the sensor ground line, other than the potential for noise on the line that may cause intermittent performance?

 

[achris]

...What are the downsides of splicing the chassis ground to the sensor ground line, other than the potential for noise on the line that may cause intermittent performance?

The only problem I see with that is the loss of isolation between sensors and the main power rails. If it works fine, then as long as you don't have a short (internal or external) between anything 5v, or its ground, and anything 12v, or its ground, you should be fine.

Chris......

 

[Fun Times]

If adding a ground circuit doesn't seem to work properly then another option would be to have the ECM repaired internally for $600.00..., https://whipplesuperchargers.com/index.php?dispatch=products.view&product_id=73

Any thoughts of why/how the 12v purple wire was broken?
May have missed it but what year is the boat/engine?

 

[PatL1]

The purple wire is not broken, it is an ECM internal isolated ground for the sensors that is not providing the ground for the 5V sensors.

Earlier on in the troubleshooting /discussion when the ECM was not getting any power at all ,it was from a spade connector on the purple wire under the dash from the ign switch that had come loose.

The owner says the set us is an unusual hybrid (he bought it used). The engine i believed to be a 2002 6.2L

 

[Crownieguy]

Thanks. Once I understood (thank you) the ECM power up strategy, we traced a problem down to a broken purple wire that was supposed to feed battery voltage to the ECM (B18) from the ignition switch. Fixed it and the ECM came to life (big surprise what power will do for an computer!) and the fuel pump is now priming properly on initial ignition switch on. Yeah

Now the engine cranks but won't start. I checked spark at the king wire to the distributor from the ignition coil and don't see any spark. If the logic/strategy is like an automobile system, the ECM needs to get a signal from the CPS to signal the ignition module to trigger the coil for spark. Not sure if that is how the mercruiser 555 works. I proceeded to check the 5V and sensor grounds from the computer and the 5V is good but the ground is not showing continuity to chassis ground.

Question...Does the ECM use a floating ground at B1 for the sensor grounds or should I see a continuity to chassis ground with an ohm meter? Logically, if the sensors aren't seeing a path to ground , then the ECM wouldn't see any inputs from the required sensors (like the CPS/tach signal) and likely not trigger the ignition circuit.

Again, appreciate the feedback.

I am so grateful to the OP. Let me tell anyone with this problem to FIRST check B18, the purple wire that goes to the ignition switch. I spent a week trying to find the problem the resulted in strong cranking but no start. Like the OP I had a wiring diagram but it wasn't obvious which of the 48 wires going to the ECM provided power to it. I have a Seloc manual which told me to test several other ECM connections and the MPR, but not B18. After spending a week and staring a cancelled boating outing with my family on Memorial Day in the face, I spent half the night Googling for an answer and came upon this thread. Sure enough, checked B18 and no voltage. I traced it back to a poor splice someone had made near the throttle control that got pinched and failed. Re-spiced (properly) and the beep came on, the ECM and relays powered right up and the engine started immediately! I thank you, my family thanks you, you saved the trip.