Mercruiser 555 - ECM no power?

[PatL1]

I am a skilled auto mechanic helping a friend diagnose a potentially bad ECM. We've pulled all of the available schematics and it is not apparent to me which line provides 12V to the ECM as it is not identified as such on the schematics. Makes me feel stupid that I can't determine such a basic thing , even with the schematics but......

We obviously want to do some basic testing before condemning the ECM ($$$). The engine stopped suddenly. Now when the ignition switch is turned on, the nothing responds (i.e. no fuel pump) Fuel pump does operate when the relay is energized manually.

Grounds have been verified at the ECM. Can someone describe the basic power connection to the ECM? Pin #, wire color, which of the 2 ECM connectors (A or B) etc. My assumption is the wake circuit purple wire should be battery voltage when ignition is on but I am not sure if the wake circuit tells the computer to turn on or if the wake circuit is controlled on by the ECM when it is powered. (this is a little different strategy than the automotive circuits I am familiar with).

Appreciate the basic direction.

 

[alldodge]

Could use a bit more info about the ECM. Is it a first Gen 555 ECM or PCM? A part number and would be even better to have the motor serial number and type?

Het GA_Boater suggest we move this thread to the Mercruiser section. Better chance of getting input from others like muc and Fun Times

 

[alldodge]

Did some looking and without knowing more;
Pin B18 is wake up power and should be purple (turn key ON)
Pin A22 is Main power control should be Purple Dk-Green (supplies power to ECM)
Pin A23 is MPR output should be Pink(ECM energizes MPR)

 

[QBhoy]

Likely silly of me to even suggest it...but I love to keep things simple...hate me for even asking but;

have you checked the main fuse and circuit breaker ?
occasionaly a fuse on or behind the dash too.

And here is the one you will really hate me for...but just in response to the fuel pump energising manually but not by key...is the cut out lanyard switch ok ?

 

[Bt Doctur]

See if there is a white square plastic block at the starters main post, (90Amp link) also see if there is 12 v at the alt output post No 12v means the 90a link is blown

 

[Bt Doctur]

 

[Bt Doctur]

 

[Fun Times]

PatL1
Do the gauges at the dash power up when you turn the key on?... Also wiggle the main wire harness at the engine....What year is the boat?

Wake circuit tells the computer to turn on or if the wake circuit is controlled on by the ECM when it is powered.

The purple wire from the key switch goes the ECM for wake up activation power then the below happens,
Mercruiser ECM 555 ignition system operation:
*When the ignition key is turned to the on position, the ECM 555 grounds the Main Power Relay control circuit.
*Power from the 50-Amp circuit breaker mounted on the engine passes through the main power relay to a 20-Amp fuse and on to the ignition coil driver module and the ignition coil primary winding.
*When the key is turned on to the start position the starter motor cranks the engine.
*The crankshaft position sensor and Camshaft position sensor on late 2005 and newer engines picks up signals and sends them to the ECM 555.
*The ignition coil driver module is normally closed and is providing a ground path for the current that was sent to the coil from the 20-Amp fuse to flow through the primary coil windings.
*The current flow creates a magnetic field that surrounds the primary and secondary coil windings. The period of time that the current is allowed to flow through this circuit is referred to as dwell.
*The ECM adjusts the dwell by look-up maps in the ECM logic that are based on energy required for speed/load point at which the engine is running.
*When the ECM 555 determines that it is time to fire an ignition event it signals the ignition coil driver to turn off the primary winding current to the ignition coil by removing (Opening) ground path and interrupting the current flow.
*The magnetic field, created by the current flowing through the primary coil windings, will collapse across the secondary coil windings; which induces a high voltage in the secondary windings that is sent from the coil to the High Voltage Switch (HVS)- (Flat Dissy Cap) and on to the spark plugs.
This cycle is repeated for the firing of each individual spark plug in the engine.

 

[PatL1]

Thanks everyone for the feedback, it is very helpful. I will be seeing my friend again today to continue the troubleshooting and provide the solution back here when we resolve it.

 

[PatL1]

Did some looking and without knowing more;
Pin B18 is wake up power and should be purple (turn key ON)
Pin A22 is Main power control should be Purple Dk-Green (supplies power to ECM)
Pin A23 is MPR output should be Pink(ECM energizes MPR)

When you say that A22 is the main power control (supplies power to the ECM), I am confused because it looks in the schematic like the ECM should be supplying the ground for the MPR relay coil to activate it rather than receiving power for the ECM. Can you help me understand this better?

My current understanding is that B18 tells the computer to wake up and in turn provide ground at A22 to activate the MPR, and that powers the fuses for the rest of the system. So if I have battery voltage on B18 when the ignition switch it turned on, I should see a ground at A22 (and the MPR should be activating). Am I thinking correctly?

 

[alldodge]

Yes your thinking correctly, in typing I switched the tags, its correct in post 6

 

[PatL1]

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.

 

[alldodge]

Don't understand the "king wire" maybe the coil wire. The wire that goes from the coil to the distributor cap

Does the tachometer show 300 rpm or higher when the motor is being cranked?

Should be no floating ground so to speak. ECM gets ground from A16 and A24

 

[achris]

Yes, sensor/transducer negative is independent of engine/chassis ground. Splice 100 is 'transducer' gnd, black/pink wires, and does not connect to engine ground. You need to measure transducer 5v and transducer outputs with respect to the transducer ground, not the engine block. You'll get 'weird' readings if you use engine ground.

Yes, B1 is independent of engine block ground (it's transducer ground).

Chris......

 

[alldodge]

Sure looks like the same ground, note splice 104 on letter m which is everywhere CAN, sensor and ignition

 

[achris]

Sure looks like the same ground, note splice 104 on letter m which is everywhere CAN, sensor and ignition

Splice 104 is engine ground, not sensor ground. Have a look at this diagram, and see if you can find a link between splice 104 (engine ground) and splice 100 (sensor/transducer ground).... (Hint: There isn't one )

 

[alldodge]

Your got me

 

[PatL1]

Don't understand the "king wire" maybe the coil wire. The wire that goes from the coil to the distributor cap

Does the tachometer show 300 rpm or higher when the motor is being cranked?

Should be no floating ground so to speak. ECM gets ground from A16 and A24

Sorry, the king wire is an automotive term for the main ("king") spark plug wire from the coil to distributor on cars. I had noticed the tach bouncing around from around 300 to nearly pegged during cranking. I asked the boat owner and he says it has always done that.
I got conflicting responses re: whether the ground from the ECM to the sensors through B1 is floating or not. I am not currently seeing continuity to chassis ground from B1 so I am hoping it is supposed to be independent (floating). It could be desiged tha way to help protect the ECM if something shorts out sensor wires to 12V where it is not supposed to. Do you feel confident that the B1 ground for the sensors is tied to the chassis ground?

 

[PatL1]

I just saw the responses re: the differing information ... looks like the consensus is floating at B1

 

[alldodge]

300 to nearly pegged during cranking. I asked the boat owner and he says it has always done that.

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

looks like the consensus is floating at B1

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