Temperature Guage

[bscheper]

I have a 1992 Merccruiser 5.0 LX. My last outing of the year before the winter storage and my temperature guage or temperature sending unit went out. The gauge went up to 165 and then got stuck. I tapped on the glass and the gauge went way below the lower limit and never would move again. I looked at the back of the guage and it looked like a light brown wire was going back to the engine compartment and it appeared to be correct color according to the Merc Shop Manual. How do determine if the problem is the guage or the sending unit? Also, where is the sending unit? I couldn't find it. Any help would be appreciated.

 

[PitchFork]

Re: Temperature Guage

My TStat is on the Right Side of the Intake Manifold next to the TStat Housing where all the hoses hook to. The DIY manuals #17 pg368 shows how to test the Temperature Sender Tan Wire and the Temperature Switch Tan/Blue Wire that hooks up to the buzzer. Your boat may have a different Manual maybe #15. My buzzer has a red tape strip on it to indicate it closes at 190? - 200?. That Temp Switch is located on the Right Side of the housing looking from the stern.
Pull Temp Sender and test in hot H2O
Water Temperature Ohms Reading
140? F (60? C) 121-147
194? F (90? C) 47-55
212? F (100? C) 36-41

 

[Bondo]

Re: Temperature Guage

Ayuh,... You need Power to the gauge,... Grounding the wire at the sending unit should Pegged the gauge...
If Not,.. It's the gauge, or the wiring....
If it Does, it's the Sender, or Bad ground at the T-Stat housing...

You should be able to find the factory manuals for your motor,+ drive up in the Adults Only section....

 

[bscheper]

Re: Temperature Guage

Well it is Spring and I now have the boat home and trying to apply what I have learned through this forum. I grounded the sending unit and it pegged the guage so I assumed that it was the sending unit. I bought a new sending unit and installed and my problem was not fixed. Note: When I put the tan wire on the new sending unit, I tightened the nut with the socket in my hand. When it got tight the screw began to turn inside the sending unit. Is this ok or is the new sending unit broke? I didn't think it was ok so I swapped it out with another sending unit and installed. It did the same thing. (Sierra sending unit) I grounded out this sending unit also and it pegged the guage also. When I turn the key the temperature guage stays pegged and doesn't go back to zero? Help??

I thought about testing the resistant Ohms as stated below but am unsure of how to conduct the test. (Where to test the resistance/touch the wires on the sending unit)

One other question. The boat/engine is a 92 and does not have a warning buzzer system. Can this be purchased? Can it be installed? How hard is it to install?

All your help is greatly appreciated.

 

[bscheper]

Re: Temperature Guage

I did a lot of testing last night. The new sending unit Ohms were consistent with the specs specified above which were:

Water Temperature Ohms Reading
140? F (60? C) 121-147
194? F (90? C) 47-55
212? F (100? C) 36-41

When installed I was still not getting a correct reading so I took a glass of water at about 140 degrees outside to the boat to do some more checking. Without installing I connected the sending unit wire and then grounded the sending unit to the battery. The gauge would jump up to about 100 degrees consistently when grounded. (guage doesn't read that low but thats about where it indicated.) I then grounded the sending unit to the mounting hole and it did the exact same thing. So I now believe that I have a operating sending unit and a good ground at the mounting location.

My guess now is that the sending unit I have is not the correct sending unit for the guage. The engine is a 92 Mercruiser V8 305 LX with a original aprox 1992 Teleflex guage. Is it possible that the ohm readings above are not the correct specs for a 92 Teleflex guage? How do I find out what the specified ohm readings are for my guage?

I purchased a new guage but did not want to install if I do not have to since it doesn't match exactly with the set. I would prefer to find the correct sending unit. The new guage says for a Class D sending unit on the package. Would the above ohm reading specs be a Class D sending unit?

One other question. The boat/engine is a 92 and does not have a warning buzzer system. Can this be purchased? Can it be installed? How hard is it to install?

 

[bscheper]

Re: Temperature Guage

I have done more research and also found the below specified readings from Don. Any thoughts on which are correct?

---------------------------------------------------------------
Water Temperature Sender: To check water temperature senders,
remove sender from engine. Connect sender to a digital ohmmeter.
Immerse sender in a container of oil with a cooking thermometer. Heat
oil over a flameless source. Observe meter and thermometer. Meter
should indicate:
? 448 ohms ? 10% at 100? F (38? C)
? 128 ohms ? 7.5% at 160? F (71? C)
? 46.6 ohms ? 5% at 220? F (105? C)
----------------------------------------------------

 

[PitchFork]

Re: Temperature Guage

One other question. The boat/engine is a 92 and does not have a warning buzzer system. Can this be purchased? Can it be installed? How hard is it to install?

I had about the same conditions as you when I first purchased my boat. My temp gauage worked but the overheat switch did not and no buzzer system. The buzzer system is easy to install if you have the wires already ran as mine were but the actual buzzer was missing. I purchased Part 816492A 6 off eBay and attached the wires that were under the dash and it has worked ever since.

I believe the wires should be brown with blue strip. There should also be a overheat switch mounted on the TStat housing or intake man I cannot remember and a Oil Pressure switch on the engine either above the oil filter or next to the distributor.

Part Diagram: http://www.perfprotech.com/store/assembly/AUDIO-WARNING-SYSTEM,1014-470.aspx

Hope this helps

 

[MexicoMike]

Re: Temperature Guage

I had the same problem with my temp gauge sending unit combo when I rebuilt my motor. Since I live in Mexico parts are hard to come by. I would suspect that you can go to the gauge manufacturer and they will provide the part number for the correct sending unit. I did not have this luxury, so what I did was measure the resistance using the hot oil method stated above and installed a resistor in the lead going to the S on the gauge. I toyed around with it (I had actually removed the gauge and took it inside the shop and rigged the wiring) at different temps and my gauge is right on. If I could have readily obtained the correct sending unit I would have done so but necessity is the father of invention. Good Luck

 

[bscheper]

Re: Temperature Guage

I have solved my problem and learned a few things along the way that I thought I would share since so many people seem to be confused about temp gauges and ohms resistence and trouble shoot temp gauges.

Lesson
1.) I grounded my temp sending unit and it did peg the gauge which would indicate that the temp sending unit was bad. However the problem was still the gauge. This test does not always indicate the problem.

2.)The below Ohm readings are for dual station (2 temp guages) and one temp sending unit.
Water Temperature Ohms Reading
140? F (60? C) 121-147
194? F (90? C) 47-55
212? F (100? C) 36-41

3.) The below Ohm readings are for single station sending unit.
? 448 ohms ? 10% at 100? F (38? C)
? 128 ohms ? 7.5% at 160? F (71? C)
? 46.6 ohms ? 5% at 220? F (105? C)

4.) The large local marine dealer and shop sold me the wrong sending for a dual station because that is what his merc manual said based on my engine serial number etc. I only figured it out when Teleflex told me the Ohms for each and did the water/heat test and checked the ohms on my old sending unit as compared to the new one. Maybe some day they will figure it out also if they subscribe to Iboats and post. By the way, Teleflex had excellect customer service technical support.