Temp Sender testing

Kenneth K

Sustaining Member
Blogs Author
My temperature gauge just stopped working. Doesn't stay at 0, but goes to about 100 and just stays there whether the engine is cold or running at temperature.

Is this a valid test of the temperature sender?
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I thought it worked by changing its resistance as temperature changes. I get 2600 ohms between the body (ground) and the Center Post on a cold engine.

Anyone know what the resistance range should be on a cold sender?
 

Frank Langer

1984 Ericson 30+, Nanaimo, BC
I don't know the answer to your question, but I replaced mine many years ago and it was very inexpensive. If they are still cheap, that might be an easier solution.
Frank
 

eknebel

Member III
A ballpark check is pretty simple to point you to gauge or sender. You do need a meter to test the sender, but only a wire to test the gauge. Which you do first is up to you. If I don't have a suspect, I go easiest first. You will only will need an additional wire long enough to go from a good ground or battery negative to the top sender connection to perform the temperature gauge diagnostics. That nice shiny bolt in the bottom left of the picture is likely a good ground (it doesn't look like a good connection where the dvm lead appears to be on the solid paint in the picture). Not to repeat myself, but the ground has to be good for the testing to be valid. A small gauge wire is all that is needed, like alligator clips, or "normal" gauge(18-26) speaker wire lying around. Bigger is OK, just sometimes harder to work with. Whatever wire you have around.
Since the sender is bolted into the engine, it's base is in a good ground. The principle is that a good sender will decrease resistance(ohms) as it heats. Cold, the resistance is high, but will decrease as the sender heats. This decrease in ohms is detected by the temperature gauge. So when the sender wire connected to the gauge has no resistance(and is at battery/engine ground), the temperature gauge should read full scale hot.

Testing temp gauge: engine not running for safety, but gauges powered on. Connect aforementioned well grounded wire to cleaned top sender connection. If the temp gauge reads hot=gauge, wire to gauge, and connections at gauge are good. If temp gauge doesn't read hot, one of the three is bad.
Testing sender: engine off and cold. disconnect existing sender wire from top of sender. Connect one lead of dvm to top of sender, the other lead to a good ground. Since your meter doesn't auto-range, move the dvm selector knob to the smallest ohm setting that doesn't give a over range display(20k like you had is likely ok). record the reading on the dvm. Verify there is no risk of wires or anything getting tangled, start and run the engine till warm. as the engine is running, watch the digital ohm meter. If you do not see anything that is more than aprox 200-400 ohms in difference, the coolant temperature sensor is defective and will need to be replaced. I am remembering the ohm values from a long time ago, but if the temp gauge tests ok, it is likely the sender. I can take measurements on mine tommorrow, which is a 5416, aka m20. I would be surprised if the sender is different.
 

eknebel

Member III
Results of testing on universal 5416 temp sender.
0.784 k ohm cold sender
0.430k at around 100f
0.085k at 180f

Conversion to just ohms
784ohm cold
430 ohm 100f
85 ohm 180f

always happy to give back to this site! Good luck!
Ed knebel
 

Tin Kicker

Member III
The appropriate range can vary by instrument. While I'm sure eknebel is correct for the one he showed, I know for at least one version used, the range is 301-22 ohms.

Regardless, 2,600 ohms is essentially an open circuit so yours is probably shot.
 

Kenneth K

Sustaining Member
Blogs Author
Thanks all for the replies. I'm going to do more troubleshooting today.

And Frank, that's a valid approach to just replace the sender and the gauge, but in my case, each is less than three years old. And, I just did a rewire of the panel last year, so I'm a just as concerned about understanding the WHY as I am about making it right again.
 

Kenneth K

Sustaining Member
Blogs Author
Based on the information I received above, I went out again to troubleshoot the engine temp indication system. Following Ed's writeup about checking the gauge and sender, I was quickly able to see that the gauge (and related connections) are fine--the gauge pegs out at 250 degrees when the signal post is grounded out. So, on to the sender:

Ed, your resistance values were right on for a Faria type temp sender. This link shows the resistances of all Faria senders:

https://fariabeede.com/site_manuals/IS0085d.pdf (for water temp, 100 deg = 450 ohms / 175 = 99 ohms / 250 = 29.6 ohms)

I checked the resistance of my sender at various temps and got: cold = 1245 (last time, it read 2600 cold) with resistance lowering to 240 as the engine warmed up. That makes sense why the gauge would be showing about 120 degrees when the engine is fully warmed.

Interestingly, when I went out for a sail yesterday, the gauge briefly indicated 180 degrees while the engine was warm. A few minutes later, it was back to showing 120 degrees.

I ordered for a new sender a two days ago. Thanks for the help.
 

eknebel

Member III
Good Work! Nice link too. My thought is that the first cold reading of 2600 ohm was false coming thru the paint. As a side note, when testing, i realized my center sender post turns as the nut is tightened. I am not overly concerned, but after 40 years, maybe it is time.;) So now I have something else to replace, likely the next time i need a free shipping minimum.
 
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