Should a Temp control sensor have a ohm reading?

pbgeneral

Seaman
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Jun 14, 2011
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I have a 1996 Merc 200 carb. motor. serial #OG258517

My question is, the book a clymer I believe says in the trouble shooting section, to set an ohm meter to the highest scale and place the lead on the Temp sensor lead, and then a good ground. The book then says I should not have any continuity, if I do the sensor is bad. I understand the sensor is open until it over heats, and then closes sending an alarm. SHOULD the sensor have a resistance reading whenthe motor is cold? It is not closed and I have no continuity between grd. and the lead (only one wire). I got a continuous alarm the other day, but water pressure was in spec, and the water coming out the tell tail was only mildly warm. My thinking is I have a bad sensor, or one that is going bad, any thought or advise from you boat Techs. I did a ohm reading on both sensors and got a 625 on one, and a 1300 on the other while the engine was cool. I do not understand the statement of setting a ohm meter and then looking for continuity. Should the sensor have a spec. resistance reading when cold?
 

schematic

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Re: Should a Temp control sensor have a ohm reading?

There are different types of sensors used in different places. Some sensors are nothing more than a switch. Some vary in resistance with temperature change. Clymer is a generic manual which may not apply to your exact sensor.

Also, when checking with an ohmmeter in its high range, it is possible to misinterpret a read if you are not familiar with mega ohm scales. Even touching the leads will skew your results.
 

Dave1027

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Re: Should a Temp control sensor have a ohm reading?

Continuity essentially means a low resistance value.

I think the fact that you have two sensors with different readings with the same cold temp indicates something is not right. That 625 reading seems kind of low. I would try it with the 625 sensor disconnected to see if that solves it. If no more alarm then you likely found your culprit.
 

tim gore

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Re: Should a Temp control sensor have a ohm reading?

temp sensor failure rare; would inspect t-stat first
 

pbgeneral

Seaman
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Re: Should a Temp control sensor have a ohm reading?

Thanks fella's I will replace the t stats, which is something I have been wanting to do, and take it from there.
 

Silvertip

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Re: Should a Temp control sensor have a ohm reading?

When making resistance measurements you need to isolate the circuit you are measuring from anything else that would impact the reading. In the case of a temp SENSOR ( temperature sensitive switch) it is either open or closed. But to prove that you disconnect the lead at the sender. Now measure from the sender terminal to ground. Continuity (any number) = switch closed. No reading = switch open. Temp SENDERs are resistance elements and are used with temperature gauges. Those are typically 33 ohms hot and 240 ohms cold. Again, disconnect the sender lead when measuring.
 

pbgeneral

Seaman
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Re: Should a Temp control sensor have a ohm reading?

Thanks Silvertip, thats a great explanation. I have sensors I would say. I did disconnect the wire at the termianl strip where the wire from the sensor connects, and on the meter when on the continuity setting, no beep, but do have some resistance reading when set to read ohms. one reads about 1.300 and one reads.625, any thoughts?
 

Silvertip

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Re: Should a Temp control sensor have a ohm reading?

Are your fingers touching the probes when making the measurements? If so, your body is a resistance element itself and is therefore in parallel with whatever it is you are measuring. Both of those readings are very low so that would indicate the sensor is closed. Disconnect the wire at the SENSOR. Then measure resistance at the sensor -- not the wire.
 

pbgeneral

Seaman
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Re: Should a Temp control sensor have a ohm reading?

I went and bought two new Tstats and a new poppet diaphram and gaskets today. When I removed the t stats housings, low and behold there was not much left of the t stats, but the passage was open on both heads. installed new t stats and will install the poppet comp. today. Silvertip, the guy at the boat dealer said that the sensor on the starboard side was a sensor and the one on the port side was a sending unit. but they both go to the same terminal strip on the motor. I also do not have a temp gauge, only a pressure gauge on the boat, which I installed last summer. My question Silvertip, is the one wire coming out of the sensor is molded into the senor, in other words there is no way to disconnect it there is no connection there. Should I pull the sensor out of the head to examine it, since I do not know what it looks like or how the wire is connected? Thanks for the info.
 

Texasmark

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Re: Should a Temp control sensor have a ohm reading?

Just expanding on what Silver said. As I recall the left side is a tan wire and the right side is a tan/lt blue striped wire. The tan wire is a spst switch which closes at 195F....stat starts to open at 143F....stamped in the pellet on the stat. As he said, pull the tan wire from where it is plugged into the harness and measure between the terminal and ground. Since we are dealing with 12v and you need enough current to operate a piezo device (the horn) you don't need to go to the highest ohmic scale. That opens you up to read erroneous things like your fingers touching the contacts.

Seems to me that both are the same thing and you have two as you have two banks of cylinders and a stat in each meaning that if a stat fails in one bank it could overheat while the other bank was doing just fine.

Having both wires tied together at the mating connector supports my assumption....either side could short out and cause a ground for the horn. Another thing supporting what I said is the remote control connector that connects to the engine. There is a tan wire that goes to the horn in that connector but no tan/lt blue in the harness.

If you installed a temp gauge you would need a separate wire and yes the sending unit would have some resistance which would change with temperature to change the current in the meter movement and change your temp reading.

Look around the rear of the block and on the top rear. Look for a 1/8" brass pipe plug. If you find one, this is where your temp sendor plugs in for a temp gauge and most of them stick in the water and have a protrusion for sensing which sticks out beyond where the threads stop. The OT sensor does not touch the water.

My son had a 150 and that is my only association with a V engine. I spent some time in this area but not a lot. Most of what I am saying is common sense, knowledge of electronic things and associated wiring, how my 90 Merc is setup, and looking at the wiring diagram of the Commander 2000 control box which is used with a lot of Mercs.

HTH,
MARK
 

pbgeneral

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Re: Should a Temp control sensor have a ohm reading?

Thanks for the info. Mark, No I have not touched the leads with my finger tips while doing the test. Yes I did use a digital meter, however I will recheck with my Merc meter which is analog. We did check the tan wire where it connects to the harness (unplugged from harness) a bullet connection and that is when we got a .625 ohms. I will pull and replace the poppet this eve. before I move on to the sensors again. But that is all that is left. New impeller, t stats, poppet. no restrictions that I am aware of. I will run the motor after I replace the poppet and see if the alarm sounds again. As of now when I turn the key on, it does the self test of only a few short beeps and then stops, (no alarm for overheating).
 

Texasmark

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Re: Should a Temp control sensor have a ohm reading?

I bought a used 90 Merc several years ago. First outing, before I could get past the no wake buoys, I got a solid beep. I was not familiar with the engine and was essentially in the dark as to what could be giving me an alarm. I got up on plane thinking I would get some relief by the water pressure entering the engine. 10 or so seconds later the horn went off. I motored around a bit and came back to the dock. I had no more got past the no wake buoys on the return trip when the horn sounded again. I turned the engine off and came home.

In short, my engine had a stuck shut stat...fine, smoking gun. The reason I was able to turn the horn off is that the engine has a popoff which opens at around 2500 rpm and allows a lot more water to flow through the powerhead which satisfied the temp problem.

On your .625 reading, on my digital meter, touching the leads together on the lowest ohm scale (200) I get 0.4 ohms of resistance (test lead resistance). If I subtracted that and understand you, you have 0.25 ohms of resistance to ground for your horn, making 48 amperes available to operate the horn (realizing that the horn resistance wouldn't allow that much current to flow). I think you have a shorted sensor!.

Mark
 

pbgeneral

Seaman
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Re: Should a Temp control sensor have a ohm reading?

Thanks Mark, I did not get the opportunity to change the poppet last night, and it will have to wait until the weekend, but I am going to go ahead and replace the sensors anyway. Don't think they are that expensive, (not sure) but if they are not, I will replace them just for good measure. Thanks for the help, and I will report back with my findings. Happy/safe boating.
 

pbgeneral

Seaman
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Re: Should a Temp control sensor have a ohm reading?

Well I changed the poppet diaphram and cleaned all components of the poppet, Took the boat out for a test run, and after about 15 mins of running, no issues. Stopped to fish for about 30 min. cranked it up and was turning about 2500 rpms, and a "No oil alarm" sounds, being at my next fishing spot, killed the motor and began fishing again. After about an hour or so, pulled the hood and cked for oil in the reserve tank on motor, it was full. Decided we better get it in to the harbor, cranked it up and right from the start, a constant steady alarm. Drove the boat at low and mid speed to see if the alarm would clear, it did not. Getting it back home pulled both leads from the sensor and the sender at the terminal strip, and the alarm still sounded. when I disconnect the wire from the warning module Tan/blue wire, alarm stops. Book says module is bad, is there a way to bench test the warning module?

Have Hurricane knocking on the door, so will have to chk back when things clear up and the power comes back on.
 

Dave1027

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Re: Should a Temp control sensor have a ohm reading?

Sounds to me like a bad module. I replaced mine twice and when the last one went out I said screw it and disconnected it. Watching my oil supply and temp gauge works for me. I may rewire the circuit to be like a 3 cylinder which does not use a crappy module. Another option for me may be to melt off the potting and repair/upgrade the circuitry.
 
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