Overheating???

[jeffh1973]

I have read some post for my answer and had no luck, so here it goes. I have a 1988 120 Johnson m# J120TLCCA. I bought it on a boat for just the motor and it run good with good compression when I tried it out. I swapped motors and found the buzzer on the boat I took it off had a wire cut so it would not sound off. Now on my boat the constant overheat alarm sounds off after 20 seconds of run time. For peace of mind I replaced t-stats, impeller and pulled the heads just to take a look(all was great). The same thing , ALARM!!! Infared temps on my heads are 128 to 132 with a 142 between the spark plugs and water discharge is 96 to 108. I unplugged the temp switches restarted the motor and no alarm and the power is not held back.
Am I overheating or do I have bad temp switches?

Thanks
P.S. temp did get to 150 before t stats opened.

 

[Daviet]

Re: Overheating???

The temp switch is just a ground. Disconnect the sender and use an OHM meter and see if the switch is grounded when it is cold, if so you have a bad sender.

 

[jeffh1973]

Re: Overheating???

What about the one with two wires?

 

[Daviet]

Re: Overheating???

What are the colors of the wires on the one with two wires?

 

[jeffh1973]

Re: Overheating???

One Brown and one white with a black stripe.

 

[Daviet]

Re: Overheating???

Found it, the starboard has a single tan and the port has a tan and a black/white. The tan wires are the powered wires that will go to ground in an overheat situation. Disconnect the sensors and check the tan wires to ground.

 

[jeffh1973]

Re: Overheating???

I could not find a ground. If they were grounded wouldn't the alarm sound with the ignition on?

 

[Daviet]

Re: Overheating???

They ground directly to the cylinder head internally if the head temp makes them expand from heat.
If you use an OHM meter and there is continunity from the tan wire to the block the sensor is defective.

 

[jeffh1973]

Re: Overheating???

Is there a chance they could go to ground prematurely? Are my temps ok?

 

[Daviet]

Re: Overheating???

I think your temps are ok, did you check the sensors, are they grounded. When you said that when you disconnected the sensors and the horn shut off, it makes me beleave you have a defective sensor.

 

[jeffh1973]

Re: Overheating???

I checked them on a cold engine and they checked ok. I was thinking that maybe the metals inside the sensors are weak. Could it be something electronic?
I have to work tomorrow, headed to bed. I'll check your reply tomorrow. Thanks for your help so far.

 

[Daviet]

Re: Overheating???

I think this schematic is for your engine, check out the circut and see what you think.
http://www.maxrules.com/graphics/omc/wiring/1988_89_120_140.jpg

 

[boobie]

Re: Overheating???

The sender on the port side with the white/black wire the white/black wire controls Quick Start.

 

[jeffh1973]

Re: Overheating???

Could a bad CD unit cause a false reading?