Temperature Gauge Question

[candcbrown]

I have a 95 Mercury Black Max 150 hp that the temp gauge is not working on. i see two sensors on the motor, one on the left side (brown wire) that has a small cylinder, and one on the right side (brown and blue wire) with a spring mechanism. This looks similar to a thermostat on a car. I have power to the guage, but cant't seem to make it move. The brown wire has power to it when the ignition is on. How can I test to see what is bad? I think the cylinder shaped sensor goes to the guage, but I am not sure.

Thanks for the help.

 

[j_martin]

Re: Temperature Gauge Question

The one on the right is the temperature gauge sender. The one on the left (brown wire) is the overtemp switch which sounds the horn.

Get into the diagrams and check your wire routing.

hope it helps
john

 

[Silvertip]

Re: Temperature Gauge Question

Disconnect the wire at the temp sensor. Using an ohm meter, measure the resistance of the sensor. It should read 33 - 240 ohms. 33 ohms hot and 240 ohms cold. Room temperature should result in a reading somewhere in between. If the sensor is good, the problem is the wire going to the "S" terminal on the gauge, the gauge has no ground, or the sensor shell is not grounded.

 

[j_martin]

Re: Temperature Gauge Question

Disconnect the wire at the temp sensor. Using an ohm meter, measure the resistance of the sensor. It should read 33 - 240 ohms. 33 ohms hot and 240 ohms cold. Room temperature should result in a reading somewhere in between. If the sensor is good, the problem is the wire going to the "S" terminal on the gauge, the gauge has no ground, or the sensor shell is not grounded.

Actually, I believe the sensor ground is by one of the wires from it put under a screw nearby. The gauge would be supplied with key'd power (purple to I terminal) and the current drawn through it (S terminal, tan wire) by the sensor would move the gauge. The ground is for the gauge lamp.

The temp sender wire is routed to near the cannon connector, where it hits a bullet connector. That's often where it gets disconnected.

 

[Silvertip]

Re: Temperature Gauge Question

The gauge requires ground as well and is not just for the lamp. They may in fact, be common and usually are. Without the ground the system cannot work.

 

[candcbrown]

Re: Temperature Gauge Question

Should there be power to the both terminals at the gauge? I only have power to the I, not the S terminal. I am a little confused about the ground wire discussion; if I am looking at the right sensor, there is no ground, and there is not power at the sensor.

 

[Silvertip]

Re: Temperature Gauge Question

The "I" terminal should have power only when the key is ON. The "S" terminal will have voltage on it but NOT 12 volts. The voltage will vary depending on what the sender resistance is based on the how warm the engine is. As I said, measure the REISTANCE of the sender (and that must be done with the sender wire disconnected at the sender). Until you do that you will be chasing your tail.

 

[candcbrown]

Re: Temperature Gauge Question

The "I" terminal should have power only when the key is ON. The "S" terminal will have voltage on it but NOT 12 volts. The voltage will vary depending on what the sender resistance is based on the how warm the engine is. As I said, measure the REISTANCE of the sender (and that must be done with the sender wire disconnected at the sender). Until you do that you will be chasing your tail.

Thanks, so back to the original answer to my question about which of the senders goes to the gauge: is it the one on the left, with a brown wire, or the one on the right, with a brown/blue stripe wire? I looked at some wiring diagrams last night, and it seems like the solid brown (or tan) wire should be going to the gauge... thanks for your patience, and sorry for being so dense.

 

[Silvertip]

Re: Temperature Gauge Question

Look at the color of the wire on the "S" terminal of the gauge. If it is tan (possibly brown depending on your perception) it is no wonder the gauge doesn't work. The tan wire is the for the TEMPERATURE SWITCH. A switch is either ON or OFF. During normal engine operation the temp switch will be open. If the engine overheats it should close thus sounding the overheat alarm. The TEMP SENDER is a variable resistor. Its resistance changes from 240 ohms (cold engine) to 33 ohms (hot engine) thus making the temp gauge swing from cold to normal to hot depending on resistance. Since you are apparently not the original owner, God only knows how the wiring has been mucked with. So once again, you need to make some RESISTANCE measurements. Disconnect the temp switch. Measure the switch to ground. Should show open (infinite resistance). If it measure something between 33 and 240 ohms THAT is the TEMP SENDER - not the TEMP SWITCH. If that is the TEMP SWITCH, then the other unit is the TEMP SENDER. Lastly, go here for a diagram of the temp gauge system.

http://forums.iboats.com/showthread.php?t=461780

 

[candcbrown]

Re: Temperature Gauge Question

Look at the color of the wire on the "S" terminal of the gauge. If it is tan (possibly brown depending on your perception) it is no wonder the gauge doesn't work. The tan wire is the for the TEMPERATURE SWITCH. A switch is either ON or OFF. During normal engine operation the temp switch will be open. If the engine overheats it should close thus sounding the overheat alarm. The TEMP SENDER is a variable resistor. Its resistance changes from 240 ohms (cold engine) to 33 ohms (hot engine) thus making the temp gauge swing from cold to normal to hot depending on resistance. Since you are apparently not the original owner, God only knows how the wiring has been mucked with. So once again, you need to make some RESISTANCE measurements. Disconnect the temp switch. Measure the switch to ground. Should show open (infinite resistance). If it measure something between 33 and 240 ohms THAT is the TEMP SENDER - not the TEMP SWITCH. If that is the TEMP SWITCH, then the other unit is the TEMP SENDER. Lastly, go here for a diagram of the temp gauge system.

http://forums.iboats.com/showthread.php?t=461780

Thanks, so I tested the sender wire at the gauge, and got the 240 ohms, so I grounded the gauge, got nothing. Looks like it is the gauge that is the problem. I can't tell you how much I appreciate your help.

I think the previous owner also removed the temp alarm system. Does the horn go back on the motor? I found a purple wire back there that looks like it could be the wire for this.