1998 C90 new powerhead tuning

[flounder daddy]

Doing more investigating. The 1998 C40 uses 2 of basically the same style of "pot" shape resistance switches. One they care calling a thermo sensor, the other a thermo-switch...
This is a ebay auction with the seller selling both temp sensors/switches off a 1998 40hp, they look identical to the C90 switch. So maybe it is right.

Yamaha Outboard 40 HP Elrw 1998 Thermo Switches 689 82560 00 00 6H4 | eBay


This is the stuff that keeps me from sleeping at night I swear.

[flounder daddy]

This is really got my head hurting trying to figure it out. What would advance the 10* of timing when on high idle/cold start and sense temp?? I just ran a jumper wire from the pink wire on the harness side to the ground wire and it did buzz the alarm. Im almost wondering if something in the CDI box is not liking cold. Im really lost at this point. Im gonna go ahead and replace the temp switch, but now im kinda doubtfull that its the cause.

[flounder daddy]

Yes, that is the switch. It just used 212* (the boiling point of water) as a ohm test at a high range. That has to be tied to the CDI unit because it will limit RPM and the only thing limiting RPM on the engine is the CDI. Gonna let the engine get cold overnight and try again in the morning. It states that the thermo-sensor provides data to the CDI unit for YMIS operation. Thermo-sensors are not the same as thermo-switches. One has a resistance range, one is on/off only. The sensor has the range, the switch is the on/off.

Now that I have re-read the manual, its showing its main use is for overheat/buzzer. So im kinda wondering now what the heck is sensing heat for the idle warm up.

[boscoe99]

The seloc manual shows that it does control the warm up timing. Says to disconnect the pink wire and fire up the engine and it should bypass the sensor. I just did that on the muffs and sure enough it fired up and idled back down to 900rpm or so and had its normal snap. Its showing that it works on resistance and has a temperature chart showing temps with coresponding resistance values all the way up to 212*F which would be a very hot engine. Manual states that the colder the sensor, it will add timing untill it reaches a specefied resistance and goes to normal idle down mode. I think it has certain settings/resistance for certain tasks. One for cold start, one for normal, and one for buzzer alarm. It goes into the black box with circuits inside, so who knows what is inside the box.

The Seloc manual does not appear to agree with the Yamaha manual.

The Yamaha manual calls for the thermoswitch to close at or about 183 ~ 194 degrees F. Only one thermoswitch is called out for the 1998 C90, whereas the C40 uses two, which are different from each other with respect to opening and closing temperatures. And they have different functions.

Are you sure you have a C90TLRW?

Is this the thermoswitch that you are talking about?



Is this fun or what?

[flounder daddy]

The seloc manual shows that it does control the warm up timing. Says to disconnect the pink wire and fire up the engine and it should bypass the sensor. I just did that on the muffs and sure enough it fired up and idled back down to 900rpm or so and had its normal snap. Its showing that it works on resistance and has a temperature chart showing temps with coresponding resistance values all the way up to 212*F which would be a very hot engine. Manual states that the colder the sensor, it will add timing untill it reaches a specefied resistance and goes to normal idle down mode. I think it has certain settings/resistance for certain tasks. One for cold start, one for normal, and one for buzzer alarm. It goes into the black box with circuits inside, so who knows what is inside the box.

[boscoe99]

Thank you I thought I was going blind or just crazy.

The one with pink wire at the top I think is over heat yes
the one with orange wire is the warmed up switch yes

Apparently your C90 is not the same, as I cannot find another switch on your motor, so I have no idea how it is suppose to know when the motor has warmed up

I don't think it does.

[boscoe99]

Thank you I thought I was going blind or just crazy.

The one with pink wire at the top I think is over heat
the one with orange wire is the warmed up switch

Apparently your C90 is not the same, as I cannot find another switch on your motor, so I have no idea how it is suppose to know when the motor has warmed up

For your blindness there are actions you can take to stop it.

For craziness there is no cure. Comes with being a Yamaha owner.

[boscoe99]

Appears the C90 uses only one thermoswitch for over temperature warning.

Appears the C40 use two thermoswitches. Each of a different kind. One for over temperature and one for telling the ECU if the motor is cold or warm.

Yamaha is certainly willing to try every different type of motor known to man. Maybe more. No two models can ever be counted upon to be similar.

One thing about this website it is a hellava learning tool. Wish I had come here about ten years ago.

[99yam40]

Thank you I thought I was going blind or just crazy.

The one with pink wire at the top I think is over heat
the one with orange wire is the warmed up switch

Apparently your C90 is not the same, as I cannot find another switch on your motor, so I have no idea how it is suppose to know when the motor has warmed up

[flounder daddy]

See if you can find a diagram with the temp switches on the head, I cannot find it on here.
1999 C40TLRX Yamaha Outboard Parts

I know mine has 2 switches one for overheat and the other of motor warmed up, closes at lower heat

1999 C40TLRX Yamaha Outboard ELECTRICAL 1 Diagram and Parts

items 11 and 12.

[99yam40]

See if you can find a diagram with the temp switches on the head, I cannot find it on here.
1999 C40TLRX Yamaha Outboard Parts

I know mine has 2 switches one for overheat and the other of motor warmed up, closes at lower heat

[boscoe99]

My question is to yam about his C40.

[flounder daddy]

688-82560-10-00 YAMAHA THERMO SWITCH ASSY

[boscoe99]

I am not sure how thatC90 is set up to read when the motor is warm, but my C40 has a temp switch on the head that when warmed up will send a signal to CDI to tell it it is warm. Then the CDI changes the Timing to normal.
At start up the idle timing is advanced until the warmed up signal is received ,then it drops back to the idle specs in manual.
Have you tested the stat and temp switches to make sure they operate in spec?

Not sure why you had to replace the power head, but if you did not find and correct what caused it to fail, you will be hurting the new one.

If the carbs were not gone through at the rebuild, they probably need to be done along with the fuel pump

What is the part number please?

[flounder daddy]

I am not sure how thatC90 is set up to read when the motor is warm, but my C40 has a temp switch on the head that when warmed up will send a signal to CDI to tell it it is warm. Then the CDI changes the Timing to normal.
At start up the idle timing is advanced until the warmed up signal is received ,then it drops back to the idle specs in manual.
Have you tested the stat and temp switches to make sure they operate in spec?

Not sure why you had to replace the power head, but if you did not find and correct what caused it to fail, you will be hurting the new one.

If the carbs were not gone through at the rebuild, they probably need to be done along with the fuel pump

Pump is brand new. The old powerhead top cylinder had a tiny crack in it (my fault, got it hot once and blew a head gasket after a bad water pump rebuild) and something got in the top cylinder or it broke a ring. There is peppered damage in the top of the piston and chamber on the old head. Im putting my money on that thermo switch being bad.