120 MerCruiser 74'-77' trim pump wiring questions

T

tkcraft

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Joined
Jun 5, 2025
Messages
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Hello Everyone,

Engine serial number - 3955648 - 120hp MerCruiser

I have been searching the threads and I see a lot of posts talking about wiring of the trim pump. However my trim pump is the old two solenoid version which does not have a clear wiring diagram from what I have found. I bought this boat to restore and I believe the trim pump wires may have been messed with by a previous owner.

My controls do not have a trailer button, however my hand control and dash controls just do trim up and down. I traced wires and hooked it up from scratch. I am able to control the up and down function. However when running trim sender and limit switch wires I run into the issue.

I also know it cannot be the solenoids, because I ended up totally replacing them after these pictures where taken but it all works setup exactly how it is in these photos.

Trim sender - one on purple white and one on ground for trim gauge on dash
Trim limit - into bullet connectors on wiring harness, and do not do anything.

My trim will go up as far as it will go and I do not want to damage anything. I would like to set the height measurement.

Could you help me figure out where on the wiring harness to connect the trim limit switch please. I would really appreciate it.

The pictures below show how I have it currently wired up.

Blue - UP - has green/white
Green - Down - has green/brown

Trim sender - purple/white + ground
Trim limit?

Thanks again for any help.
 

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C

Chris1956

Supreme Mariner
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Mar 25, 2004
Messages
28,290
That PTT system probably can with the original three push button panel. You would push the center button for trim, and when the trim limit switch opened, the trim would stop. Push top and middle buttons together would tilt the OD up.

If you have a simple toggle switch to control the PTT, then you cannot have the separate trim and tilt function, as you need another switch of some kind. Some controls came with a separate "trailer" button for tilt. That would work for you, provided you trim limit switch is still functioning.

You should do a test of your boat's PTT system. Run her on plane and trim the OD up until she ventilates. Now, without changing the trim, slow down and turn the motor off. Look at how far the trim is up. You should find that the OD is still in the trim limit, or at least the vast majority of OBs and ODs ventilate before they reach max trim.
 
T

tkcraft

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Joined
Jun 5, 2025
Messages
4
Chris1956 said:
That PTT system probably can with the original three push button panel. You would push the center button for trim, and when the trim limit switch opened, the trim would stop. Push top and middle buttons together would tilt the OD up.

If you have a simple toggle switch to control the PTT, then you cannot have the separate trim and tilt function, as you need another switch of some kind. Some controls came with a separate "trailer" button for tilt. That would work for you, provided you trim limit switch is still functioning.

You should do a test of your boat's PTT system. Run her on plane and trim the OD up until she ventilates. Now, without changing the trim, slow down and turn the motor off. Look at how far the trim is up. You should find that the OD is still in the trim limit, or at least the vast majority of OBs and ODs ventilate before they reach max trim.
Click to expand...
Thanks for the reply Chris! My system just is solely trim up and down, no tilt. So my system is setup on dash and hand throttle with a rocker that does up or down, I do not have a trailer button which I do not mind. However I would just like to set the trim limit so that it is the recommended 21-3/4 in. Max. (554mm) between nuts on the trim rams.

It is honestly more peace of mind to me that I have a safety not letting me trim up past that measurement so I do not damage anything. I rebuilt it all and would hate to ruin it being dumb or someone else trimming up too high.

Thanks!
 
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Chris1956

Supreme Mariner
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Mar 25, 2004
Messages
28,290
If your trim limit switch still works, and you do set it to that measurement, you will not be able to tilt the OD up when you pull the boat, or to work on it.

I.E when your prop hub fails in November, 5 miles from the dock, and you want to change the prop, the OD will be far underwater.

The best advice is to not overtrim the OD. Trim it up until the steering is light, and she is not ventilating. If you want to trim for more performance, listen for the motor to rev up suddenly, or watch the tach.
 
T

tkcraft

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Joined
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Messages
4
Chris1956 said:
If your trim limit switch still works, and you do set it to that measurement, you will not be able to tilt the OD up when you pull the boat, or to work on it.

I.E when your prop hub fails in November, 5 miles from the dock, and you want to change the prop, the OD will be far underwater.

The best advice is to not overtrim the OD. Trim it up until the steering is light, and she is not ventilating. If you want to trim for more performance, listen for the motor to rev up suddenly, or watch the tach.
Click to expand...
Roger, so you would recommend maybe not setting the trim limit and keep it as is? Because right now I can trim up as far as I want with no limit.

Sorry I am new to boats as this is my first boat for myself, but I grew up with a family boat.

Thanks!
 
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Chris1956

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Messages
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Gee, run the test in the third paragraph of post #2. It is likely that the prop will ventilate before it trims above the limit Now all you need to do is listen for prop ventilation, which everyone should always do.

Alternatively, add a tilt switch to your boat.
 
T

tkcraft

Recruit
Joined
Jun 5, 2025
Messages
4
tkcraft said:
Hello Everyone,

Engine serial number - 3955648 - 120hp MerCruiser

I have been searching the threads and I see a lot of posts talking about wiring of the trim pump. However my trim pump is the old two solenoid version which does not have a clear wiring diagram from what I have found. I bought this boat to restore and I believe the trim pump wires may have been messed with by a previous owner.

My controls do not have a trailer button, however my hand control and dash controls just do trim up and down. I traced wires and hooked it up from scratch. I am able to control the up and down function. However when running trim sender and limit switch wires I run into the issue.

I also know it cannot be the solenoids, because I ended up totally replacing them after these pictures where taken but it all works setup exactly how it is in these photos.

Trim sender - one on purple white and one on ground for trim gauge on dash
Trim limit - into bullet connectors on wiring harness, and do not do anything.

My trim will go up as far as it will go and I do not want to damage anything. I would like to set the height measurement.

Could you help me figure out where on the wiring harness to connect the trim limit switch please. I would really appreciate it.

The pictures below show how I have it currently wired up.

Blue - UP - has green/white
Green - Down - has green/brown

Trim sender - purple/white + ground
Trim limit?

Thanks again for any help.
Click to expand...
Here are some extra wiring pictures if anyone has any ideas.

Green/orange and green/white look like they are jumped together and then blue/orange.

Current trim limit is connected via bullet connectors to the blue/orange and the two jumped green/orange and green/white.

Hope this makes sense.

Thanks!
 

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Chris1956

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Mar 25, 2004
Messages
28,290
If your trim limit switch has been jumpered out of the circuit, it means either it broke or when the engine control was replaced, and the new control did not have a trailer button, and as such, it needed to be bypassed.

You might put an ohm meter on the trim limit switch. It should show zero resistance when the OD is down, and open when the OD is ready to clear the side supports of the transom support.
 
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