Tohatsu MFS25 Engine Overheat Alarm Wiring

[emarshbuck]

I'm still trying to get the overheat alarm to work. It's a new one, part # 3B7-76171-0. Dealer says I can test it by grounding it to the engine block. That doesn't work. I've been told by my boating neighbors that it should "beep" when I turn the ignition switch on. Doesn't happen.

I did have to rewire the switch to the wiring harness due to some water damage. Engine starts and runs fine.

Here's the information that I can provide:

Switch has the # 302 on front, under key slot. No name on the switch.

6 wires on back of switch:

blue connected to blue in wiring harness
Green to green in wiring harness
brown to brown in wiring harness
red (2) one to + terminal, one into wiring harness
black connected to - terminal and black in wiring harness

lt blue, lt green, & white from wiring harness not connected

Overheat alarm wired like this:

yellow/br connected to yellow in harness
red/black connected to red at switch that goes to + terminal

Any advice you can give me greatly appreciated.

 

[pvanv]

First, we should have your actual model, and the exact kind of RC box you have. Could be MFS25A (carbureted) or B (original EFI) or C (current production). It would also help to know whether you have PTT, which would be noted in the full model number.

The 302 refers to the kind of key. Your key number is 302. Replacement keys will also have 302 on them.

Assuming a B or C, and a side-mount box. The CORRECT part number for the buzzer is 3C8761710M.


Your owners manual (available online at https://tohatsu.com/tech_info/manuals.html) explains when and how the alarm is supposed to sound. The beeper does more than tell of overheat. It's also for over-RPM, or low oil pressure. It will also sound if the MAP or water sensor(s) are out of range.

Yes, it should sound for 5 seconds when the key is first turned ON, and before starting. If not, either it isn't getting power, or it is malfunctioning. If the yellow is grounded, and the red has 12v, it should sound. Light blue is for PTT. White is for tachometer.

Here is a diagram:

 

[emarshbuck]

Paul,

Thanks for the wiring diagram. I'm going to print that and put it in my boat file for future reference.

My engine is an MFS25A, 4 stroke w/3 carbs w/auto-by-starter, ignition is a push to choke. Control box is side mounted next to steering wheel. It has an aftermarket PTT with it's own control/wiring system. That explains (to me) why the lt blue is not connected to anything. It has no tach....

So, I need to check that the yellow is grounded, yes? I have a volt meter and can check for continuity. How would I do that?

thx again.

 

[pvanv]

OK, so your motor is from around 2002. At the motor end, you have two sensors which will cause alarms: the overheat (41 in the diagram) and the oil pressure (32). If either is grounded, you will go into alarm. Also, the ECU will trigger an alarm if you rev too high. FWIW, on Tiller models, the buzzer is item 36. See the diagram:

 

[pvanv]

On the non-PTT Remote models such as yours, the RC box wiring looks like this:

 

[pvanv]

With auto-by-starter, there isn't any regular choke; rather there is a heater in the ABS solenoid that causes the needle to extend as it warms, leaning the mixture for that carb. When cold, the mixture is richer. Once warmed up, the carb runs at its lean setting. You MUST make sure that these carbs do not have fuel sitting in them for too long, or passages will clog with fuel residue, and cause hard cold starting. If not running for a few days, either drain the carbs (preferred) or at least run the motor until it stalls.

If you need to know how to use your meter, you should see local assistance; However, using a good ANALOG (not digital) meter, you can check for ohms from the yellow wire to ground. That should be Zero when the buzzer needs to sound. Likewise, the other wire should be 12v when the buzzer is supposed to sound. You can actually disconnect the buzzer from the wiring and supply 12v and ground to the buzzer to see if it works. If it does not, it is bad. If it does, then your wiring or a component at the motor (or harness) is shot.

 

[emarshbuck]

Well, it's been a long time since Nov 13! Ok, here is what I have. I've posted a picture of my ignition wiring. Long story, but my neighbor helped me rewire the switch after several storms, corrosion, etc. I note in the wiring diagram that the overheat buzzer needs to have a red "hot" wire going to it. I have two red wires coming out of my switch. One sits at the 10 o'clock position to my key, the other about 3 o;clock. The one at 10 o'clock goes to the red wire in the wiring harness back to the motor. The other one goes to the battery +.

The buzzer does work. I touched it to the battery terminals to confirm this. When I connect the yellow wire to the buzzer, and touch the red buzzer wire to the red wire at 3 o'clock, turn the key to ON, nada. I'm starting to think that I need to split the red wire going to the harness and also feed the buzzer....

Oh, my control box is a PTT box, it's just that the original PTT stopped working, and I had a Panther PTT installed. It works fine.

What do you think?

thx

 

[pvanv]

The buzzer should get power only when the key is ON. Did you confirm whether the yellow wire triggers ground when switching ON for about 5 seconds?

 

[emarshbuck]

Oh, I missed the part about having the key on when I test for ground on the yellow wire. I'll do that today. The other part is, should I "split out" the wire going from the ignition to the motor and connect that second wire to the overheat buzzer?

thx

 

[pvanv]

Just follow the diagram I posted in #5. That red will be hot when the switch is ON.

 

[emarshbuck]

OK, I'm getting frustrated with this, since I no longer have the bullet connectors coming out of my switch. My neighbor cut those off when he was rewiring the boat after the wires broke leading to the switch.

I did note that when my analog Micronta 22-2220A ohm meter was set to 0, I touched the red lead to the yellow wire and black lead to ground, no movement. Same if I touched black lead to yellow, red to ground, no movement.

With the key off, touch the red lead to the yellow wire of the buzzer, black to + on battery, buzzer sounds. Key on, red to yellow wire of buzzer, black lead to + on battery, buzzer sounds.

Do I need to connect the red wire from the switch going into the harness to the red wire of the buzzer also? I can do that easy enough.



What is the part # for a new switch, or even for the entire wiring harness going back to the motor? I'm to that point....

thx

 

[pvanv]

353760203m

main key switch ***'y (tohatsu)

$55.35

/* for keyswitch w/square connector use 3y9760200m*/

3a3761151m

cord assy b (20 ft.)

$166.02

 

[emarshbuck]

Paul,

OK, set the ohm meter to zero....have it on 1X....key off. Touch red lead to yellow wire, black to ground post, meter reads 20ohms....reverse the leads, meter doesn't move....

 

[pvanv]

Check for 12v when key is ON. I sent you a PM.

 

[emarshbuck]

353760203M Paul, just to clarify, my boat if the MFS25A. It had PTT when new, but I had to replace it with a Panther PTT. I used the wiring that came with the Panther instead of the original wiring. And this key is a "push to choke" switch, yes?

thx

 

[pvanv]

ABS does not have any choke.The switch may have it, but the motor does not.