Temperature Sender/Guage 89 200 hp Merc

[gss036]

I have read everything I can find in search engine, but still have a question. <br />When I turn on my key, I get the normal beep-beep-beep, the temperature guage jumps a little to the down side. I boiled some water & put in an insulated cup, pulled the sender out of the port head and stuck the sender in the hot water, turned the key on, does not move the guage, I know the water is about 140+(checked w/cooking thermometer), but guage still reads about 95 degrees. I put a pin into the brown wire, reattached the black ground to the block and put the commom there and the red clipped to the pin in the brown wire. Checked with digital multimeter set on 20 on the V DC side, When cold I get a reading of 8.2, I then stick the send into the hot water and the reading drops to about 7. According to thge Merc manual, at 140 degrees I should be reading about a resistance of .02/60 degrees about 1.5 (K@). Do I have my meter set correctly? Is the sender bad? Or is my guage bad? When running the engine it never moves off the 95 degree mark. I traced the brown wire from the sender to the guage. Sure could use a little knowledge on this. Thanks

 

[rodbolt]

Re: Temperature Sender/Guage 89 200 hp Merc

hello<br /> follow the brown wire from the sensor to the terminal block or bullet connector. disconnect it there. with the key on momentarily ground the dash panel side of the brown wire. the needle should deflect to full scale. the test you did on the sender must be done on the Ohms function with the gauge disconnected. the resistance should vary inversly from cold,about 2.6Kohms ohms, to about 80 ohms when hot(approx 140*. my numbers may be off due to the age difference of my book and your engine but that is how they all work.

 

[gss036]

Re: Temperature Sender/Guage 89 200 hp Merc

Thanks for the information. I will try to get back to the boat tomorrow and try your directions. I was thinking later that I had the wrong scale, no idea I had to isolate the sender.

 

[gss036]

Re: Temperature Sender/Guage 89 200 hp Merc

Rodbolt: I ran the test this afternoon. Grounded the brown wire on the guage side, the needle deflected to full scale. Set the multimeter to 20K on the OHM side and got 1.7 at 50 degrees and placed the sensor in hot water and got 0.17 +/- 10 degrees. According to the Merc manual,this is acceptable. They show the reading from 0.0 @255 degrees to 3.2 @30 degrees.<br />Now I will have to wait until I get the boat out on a run to see what if anything changes.

 

[gss036]

Re: Temperature Sender/Guage 89 200 hp Merc

Hopefully I will get out by Thursday and run the motor on a fishing trip to see I fixed anything by cleaning all the grounds. Is there any thing else I can check to see which is at fault (guage or sensor)?<br />It appears the sensor is isolated from the head by the plastic insert that it is slipped into. Is it possible it is only geting to about 100 degrees since it is right below the port (new)thermostat?

 

[KCLOST]

Re: Temperature Sender/Guage 89 200 hp Merc

Are you sure you are not messing with the overheat switch instead of a temperature sensor. <br /><br />Is the sensor you are testing brass metal, with the sensor lead connected to it with a nut. Or is the lead a direct connection going into the plastic so it cannot be removed.. <br />If it's like the later, that's an overheat switch...<br />A "temperature" sensor lead usually goes directly to the guage not the terminal block. However the overheat switch lead does....<br /><br />Hope this helps...

 

[gss036]

Re: Temperature Sender/Guage 89 200 hp Merc

Yes, I am sure. The overheat is in the starboard side and is a small snap button thing w/a spring on it and it goes to a terminal block by the starter. The sensor I am messing with goes straight to the guage. I tested the connection and it works. I don't even remember when it worked right, I have always had a good flow of water so never worried about it. I can put my hand on the heads and they do not feel all that hot. The thermostats are 143 degree, so the heads are about the same temp as hot water out of faucet.<br />I just have the time and wanted to mess with it to make sure it worked.

 

[rodbolt]

Re: Temperature Sender/Guage 89 200 hp Merc

hello<br /> you seem to have a handle on it. the temp sensor is a thermistor device. as temp goes up resistance goes down. less resistance means more current through the meter which means more magnetism in the meter movement which means more needle deflection. if you have a bad or corroded connection between the sensor and the gauge that means a fixed resitance and low current. if the gauge is not reciving a good 12v power supply it means lower voltage and therefore less current in the meter movement. I made a tester with a potentiometer from the ol radio shack. use a felt tip and make marks using your DVM for the min and max resistance value you wish to check. remove the send wire from the gauge. hook up the pot between the s terminal and ground. move the pot between the min and max marks you made. if the gauge does not respond it most likly is bad. the pot also works for most trim gauges,oil pressure gauges and such what nots. its also handy to test the overheat and oil alarms on the four strokes

 

[gss036]

Re: Temperature Sender/Guage 89 200 hp Merc

Update, well everything is still the same, temp guage still reads 100 degrees. Back to the drawing board on this one. I think I will need to look at the wiring under the dash.<br />Beautiful day on the water and good day fishing. We got a 13 & 21 Lb salmon, so all was not wasted.