Temp gauge testing...

[Oshkosh1]

Hi all...

After getting my engine running and my Hi-Temp silicone exhaust hooked up, I bought an IR temp gun to spot check for hotspots.

My temp gauge remains at a constant 175(on muffs), but when I hit the actual brass sending unit with the light...it showed only 145. I allowed the engine to cool overnight and all spots checked right at 91, but the temp gauge is showing about 120 or so.

Is the gauge or the sending unit most likely at fault?

Thanks in advance...

 

[achris]

Re: Temp gauge testing...

The centre sensing element of the sender is electrically isolated from the brass outer casing... It will also show a difference in temperature.

The gauges and senders are not exact science, they are indicators.... If you what something that will show you exactly what the engine coolant temperature is, you're going to have to spend a few more dollars. (quite a few!)

That's the other thing... Your IR is measuring the temperature of metal on the outside of the engine. The gauge is indicating the rough temperature of the coolant in one particular place.....

Chris......

 

[Oshkosh1]

Re: Temp gauge testing...

Can I measure the resistive value of the sender to determine it's status? I'm guessing that as it heats, the resistance increases at a linear rate?

Any idea what I should see on the meter?

 

[Silly Seville]

Re: Temp gauge testing...

Oshkosh...I just experienced this same situation after installing my new engine, and was paranoid that I was running hot when I should not be. My temp gauge was showing 165-168 at idle, and I had a brand new 140 degree thermostat installed. I drove 20 miles to get a temp gun from a family member (I now own one and love it) to take readings like you did. I shot the temp sending unit and the top of the thermostat housing and was getting readings of 141-144 degrees. So I knew the engine was at normal operating temperature. I also discovered from reading here, that idle rpm on muff's are NOT an engines best friend when you're trying to keep cool. That engine mounted water circulating pump needs to be a spinnin' to keep water volume up. I did notice that when I throttled up to 1,200 rpms or more, the temp gauge settled down to @ 157 degrees. So, I think you are experiencing exactly what achris just said...The gauges and senders are not exact science, they are indicators

My thought on this is, if your gauge "normally" shows 170, and shows that consistently under all running conditions, then you are OK. If it suddenly does the dirty dance up to 200 or something, now you have a problem, and it's time to take immediate corrective action. Hope this is of some consolation.

 

[Oshkosh1]

Re: Temp gauge testing...

Thanks for the reply...

Yeah, I'm reading around the same(140) at the water block. Not TOO concerned, just making sure things are working correctly.
Everything else looks, sounds, smells good.

 

[achris]

Re: Temp gauge testing...

Gauge sender is (should be) 33-240Ω... starting at 240Ω and getting LOWER as the temperature rises....

Your gauge APPEARS to be reading higher than reality... That's the nice side to be on...

Have fun...

Chris......

 

[Oshkosh1]

Re: Temp gauge testing...

SO...
I tested my sender with a meter...SEEMS to be OK. Heated it up to around 250 and watched the ohms rise as the temp decreased(started around 29, then ended at 240).

I also grounded my sending wire to the block...gauge pegged out hot.

Ambient temp of the motor "cold iron" right now is 87, when I cleaned and reinstalled the sender, the gauge reads roughly 110.

 

[achris]

Re: Temp gauge testing...

If you wanted to 'adjust' the gauge reading you could add a small resistor in series. That would drop the gauge indication down a bit. You'd have to experiment a bit with resistor values... I'd start small, something like 10Ω...

Chris.....

 

[Oshkosh1]

Re: Temp gauge testing...

Good idea...
I wonder if "The Shack" carries them.

I went in there a year ago looking for a rocker switch and thought I mistakenly stepped into the local "Best Buy". TV's, cell phones, whatever. I do remember when Radio Shack carried every little resistor, diode, capacitor etc...one would need, but sadly those days are long gone.

 

[Failproof]

Re: Temp gauge testing...

The IR guns also work off of reflectivity. They will read different surfaces diferently. Shiny stainless steel vs wood vs flat black painted steel. Every gauge has a margin of error

 

[achris]

Re: Temp gauge testing...

Good idea...
I wonder if "The Shack" carries them.

I went in there a year ago looking for a rocker switch and thought I mistakenly stepped into the local "Best Buy". TV's, cell phones, whatever. I do remember when Radio Shack carried every little resistor, diode, capacitor etc...one would need, but sadly those days are long gone.

Yep, me too.. I still know where to find all that stuff, but the almighty 'consumerism' has taken over most of the nice little specialist electronics shops. I went into one of those stores a few years back and asked for 1488's and 1489's (RS232 transmit and receive chips)... The kid behind the counter look at me like I had two heads and green skin.... :facepalm:

Chris
(Just an old fart at heart... )

 

[Oshkosh1]

Re: Temp gauge testing...

Don't know if anyone else has the same issue with their gauge reading either low or high...but here's how I solved it.

1. Went to the local electronics supplier
2. Bought a 1K ohm potentiometer.
3. Wired it in series with the sending wire.
4. Adjusted the gauge to read correct value afte idling for 10 minutes and using an IR Thermometer. Temp stabilized @138.
5. Permanently "set"(read super glue) the shaft

I found that I needed to add 105 ohms of resistance to get a correct reading. I suppose I could just go get the appropriate resistor, but figured after I tucked it away under the dash...it really doesn't matter.