Trying to bypass Trim Limit Switch on 2006 MCM 350 Mag MPI, with Volvo SX outdrive.

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
I have installed a Mercruiser 350 Mag MPI engine in my boat that has a Volvo SX outdrive. Everything is working correctly, except that the engine is in Guardian Mode and is limited to 2,800 RPM. Apparently I have to install or mimic the Trim Limit Switch so that the computer will not put the engine in Guardian Mode.

My Service Manual 31 shows an accurate engine electrical diagram, but the transom shield/outdrive components are included. I have not been able to find more complete wiring information online. The diagram for the trim switch is below and should be accurate. I believe +12 VDC flows through the wires, but how that interfaces with the engine I do not understand. If someone could shed some light on this I would be most appreciative. I do know that with the drive down, there is no resistance between the wires of the Trim Limit Switch.

 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
What I really need to know is what wires on the engine tell the computer that the drive is down? I am attaching some images of unused wiring near the rear of the engine. I suspect that the "Transom Harness" connection may be involved but I do not want to guess.



 
Last edited:

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,093
got the engine serial number, never heard of the trim being involved with a guardian mode.
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
OM675521.
I was at a Mercruiser shop on Monday, they called MCM. Mercruiser told them that the Trim Limit Switch not being hooked up is causing the motor to be limited to 2,800 RPM. This matches my experience trying to solve the issue.
 
Last edited:

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,093
you say the motor is mercruiser and the trim pic is mercruiser and you have a Volvo drive and Volvo trim?
what makes you think their compatable?
 

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,093
UntitledDS_zpsazztde9s.jpg
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
That is close! That is the connection for the trim sending unit, for the gauge on the dash. What I need is info for the trim limit switch.

The trim limit switch looks the same as the sender, but is bolted to the opposite side of the outdrive. When the drive is down the switch is closed. The trim limit switch plugs into the transom pump and gear shift system (see diagram at top).

The missing information that I do not have is how and where the engine harness connects to the transom system/Trim Limit switch.
 
Last edited:

Bt Doctur

Supreme Mariner
Joined
Aug 29, 2004
Messages
19,093
just how would you connect a volvo digital sender to a merc ECM or do you just want to plug in a merc sender to the ECM
 

cdiamond

Petty Officer 3rd Class
Joined
Aug 20, 2008
Messages
94
i have a feeling bt doctor's initial comment

was correct.

merc trim limit / trim sender wires

rot out very often in salt water

never heard of that causing a guardian fault
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
That diagram is most helpful. I do have a connector with 2 rows of 5 positions, but only A,B, D, and E are being used. This is the connector in post #2 above. There are two other flat style connectors at the rear of the engine, I will double check the tags on them.

Below is an end view of the connector in post #2.


-Steve
 
Last edited:

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
Using the Merc. engine wiring diagram and the "Transom Harness Connector (to Engine) diagram I traced down the wires in the photo above. The wires represent Transducer ground, +5 VDC, Pitot Signal, and Steering signal. This does not appear to match Bt Doctur's diagram.
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
There are two other connectors on the rear, port side. I will double check those in a few minutes. Thanks for all the help, solving this will be a huge help to me.
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
The diagram above is from Service Manul #36, Rev. 1. ECM 555 Diagnostics.
 

Fun Times

Moderator
Staff member
Joined
May 16, 2009
Messages
8,776
Could be wrong here but from my understanding of the transom harness assembly on a mechanical throttle and shift engine model is if the transom harness is not connected then we're supposed to just ignore the faults that you always see while reading a scan tool even though they say active faults do to not being connected. They are not supposed to put the engine into guardian mode unless the transom harness is actually installed and there was a problem with one of the sensors back there. Digital throttle and shift might be a different story and need the transom harness connected as that would typically be part of the completed smart craft system including the smart craft transom assembly.

The other side of a possible problem is maybe if the engine was/had been connected to a transom harness that actually utilized sensors, the PCM might be putting itself into guardian mode because it knew to recognize them at one time and now it's not.

Do you have the engine serial number by chance as it may or may not help figure out my thoughts above. Also by chance would you of happen to know if the original Merc transom assembly was a smart craft assembly or maybe it had some digital sensors on it in the past. < Serial number?

Are all the rear mounted sensor plugs that you are finding capped off or open possibly indicating if they were being used in the past.
 

Fun Times

Moderator
Staff member
Joined
May 16, 2009
Messages
8,776
Using the Merc. engine wiring diagram and the "Transom Harness Connector (to Engine) diagram I traced down the wires in the photo above. The wires represent Transducer ground, +5 VDC, Pitot Signal, and Steering signal. This does not appear to match Bt Doctur's diagram.
Does your main engine wiring harness that's between the engine and dash use a 10 pin black rubber type plugin or a 14 pin connector that would screw into a locked position?

Or did you have to buy the adaptor something like this, http://www.bing.com/images/search?q...324&thid=JN.n+ceyJxKIJq4ImWpKzLc0A&ajaxhist=0

There are a few small changes in the wiring between the 10 and 14 pin connectors.
 

c1steve

Petty Officer 3rd Class
Joined
Aug 3, 2015
Messages
84
Fun Times,

I just emailed the prior owner and asked about DTS and Smartcraft gauges, hopefully I will hear back today. The engine came with the 10 pin connector.
Serial number is OM675521. I do have some fault codes:

94, Pitot tube pressure, low
75, Fuel level circuit, high
104 Lake/Sea Temp circuit, high
119 Steering position circuit, low
130 Trim sensor circuit, high

I have been told that these codes will not trigger the Guardian Mode. I am including a section of the wiring diagram found in Service Manual #31, it appears to match my engine. There are three other capped plugs:
--Paddlewheel sender and temp
--Fuel level 1 and 2
--Gear indicator (taped off).
--also the CAN connection

Regarding the Gear Indicator, that is taped off. It is possible that I should jump the two connections. Also when I have run the motor in neutral, it will rev. to 5,000 rpm. The Guardian Mode only comes on in gear and under load.

 
Top