The Stovebolt Site Forums The Shop Area The Electrical Bay Zeroing temp gauge to sender

I just installed the temperature sending unit I bought from Chevy Duty. The gauge needle slowly moves over as the engine heats up and goes all the way to the right and stays.
I called Chevy Duty and they said the guy that does their gauges said that you sometimes have to zero the gauge to the sender. Anyone know how to zero the temp gauge?

Disconnect the wire at the sender , that should allow the gauge to go cold , if it doesn't you have a short to ground somewhere in the wire between sender and gauge head .

Well, Old Blue, you and I are seem to be going down the same path. Previously I had posted the same problem. Didn't get a great response about the solution. Called Chevy Duty and they said that I had a bad ground at the sensor due to using thread sealant. Took all the sealant off and no change. I inserted a resistor inline with the sending unit back to the gauge (30 ohm) and it solved the problem of going all the way to the right, but it doesn't tel me if the engine overheats that it will read the increased temperature correctly. Measured the temperature of the water at the thermostat with an exterior mounted hose sensor and the temps seem to be correct, but again, who knows when the engine temp gets too hot? The sensor operates with a temperature resistor (thermistor) but I don't know what the ohm rating is. Am still awaiting an informed response to this problem.

Sounds to me like you guys are getting the run around from the parts vendor.Must be the wrong sender.I might have one of those senders around here in a 235 engine. I'll get back to you if I do.

49, where can I get on of those resistors you mentioned? What does it look like? I would like to try one.

Guys, I took an original AC sender out of a 56 235 car engine I have laying around in the barn. I set my ohm meter on the 2k scale.At room temperature the sender read .710 as measured between the wire terminal and the sender body.I then held the sender tip in boiling water for one minute. The reading was .145 ,still on the 2k ohm scale.If you want to try it in your engine I will mail it out to one of you guys, just as long as I get it back eventually.Its the larger thred style,looks like 1/2 inch pipe thread.
Oh yeah, do Chevys use a voltage stablizer to control the voltage to the guages? Its a small gizmo somewhere on the back of the dash.VWnate probably knows what they are.

[ 05-25-2001: Message edited by: Tony ]

Hey Tony,

I think that is called an NTC thermister or Negative Thermal Coefficient. Sounds like room temp is 710 Ohms and 212 degrees is 145 Ohms.

If putting a 30 Ohm resistor in series with the sender keeps it from pegging on hot, sounds like the sender is internally shorted out.

Old Blue;
You can get any resistor from Radio Shack. However, this is just fooling the actual resistance from the temperature sensor by a set value. Fiddled around with a variety of resistance values and most didn't change the reading significantly until you got a resistor over 100 ohms. Then the meter never rose above empty, Figured that the resistance was too great then. I know that temperature vs resistance varies along a set curve based upon the requirements of the range so I keep thinking that the sensor from the vendor isn't matched to the range of the dash gauge. I do temperature control work for a living and know that there are lots of thermistors (temperature sensitive resistors) in a variety of ranges available. Unfortunately, they don't come in immersion styles for our engines. Think I will experiment with a variety of thermistors which I have in stock and see if any one of them matches my readings from my temperature meter. If I have any luck, I'll let you know. Otherwise, the gauge looks great sitting under 220 degrees and the engine isn't hot so all is well.

Tony, will do the same test on my sensor this weekend to check the values you recorded. Do you remember what the "room temp" was when you tested yours? Would like to match mine if possible. Maybe the sensors that Old Blue and I got a calibrated in celsius not fahrenheit since they were probably made in """Taiwan"""????

49,
Chevy Duty said the guy that answers their gauge questions said that you sometimes have to zero the gauge to the sender. But he wouldn't elaborate on how to do it. Kind of a secret I guess. If you could zero the gauge to the sender you could use any sender you wanted. I thought maybe there was a screw on the back of the gauge you could use to move the needle back some after reaching operating temp. I didn't see one. Could be the inline resistor thing.

I would say the room temperature was around 65 degrees.I dont think 10 degrees one way or the other should have a big effect on the guage reading. With 700 ohms ,there is a little room for error.The measurements were done on an older digital reasonable quality meter.I would bet that a sender from a much newer GM vehicle might work. GM is noted for parts interchange.

Tony - the 6 volt gauges do NOT use the voltage stabilizer , as I recall that came in around the mid 60's . I missed where anyone said what year truck this is all happening on .
I have NEVER needed to zero an electrical gauge other than fuel , this merely involves bending the float arm , since the temp. gauge works off resistance it should work correctly right out of the box if they sell you the right sender .
sorry I took so long to respond but I've been off line for over a week due to DSL being down .

[ 06-05-2001: Message edited by: vwnate1 ]

Well I finally got around to looking at Radio Shack for that resistor. They had 2 types. One is a small glass cylinder, too small I think. The other was long and rectangular, made out of ceramic. There is an uninsulated wire coming out of both ends. Is this the resistor I need? How would I wire it up? Do you just cut the wire between the sender and the gauge and put each wire end to each end of the resistor? I've never connected an insulated wire to an uninsulated wire. I guess just twist together and cover with tape.

Sorry Old Blue, been off line for a while. The resistor to use should look like a "ceramic" fuse with colored lines encircling it. The colors indicate the ohms resistance of the unit. Should be a open wire coming out of each end. Attach one end to your sender connector and the other end to your wire going to the gauge. You can leave them bare to start with just don't let them touch metal as the reisitance will change. Let your truck warm up and see how the gauge reacts. By the way, I just purchased two senders off of Ebay. The senders look just like the one we got from Chevy Duty. Plan on putting the two loose ones out on the radiator overnight and then measure the resistance of each one as well as the unit in the thermostat housing. If the two loose ones measure differently than the one installed in the housing, I'm going to swap it out and check to see if it works more accurately. More on this in about a week.

Greeting All,

Interesting post... I too am having the same problem with my "temp gauge" on my 64K10. Truck warms up, needle moves up and then swings all the way to the right. Have installed all new wiring harnesses (and a new temp sending unit from Chevy Duty, hmmmm...). I have also swapped out the gauge and tried again only to have that one "pegged" also. Then pulled the sending unit and removed the teflon tape, no change! I then installed a mechanical sending unit / gauge and it reads about 190 degrees (with 180 dg thermostat)after the truck is throughly warmed up. I checked with the local NAPA store and they are going to order me a new sending unit, cost is about $12. Hope to have it by this weekend, will let you know if it solves the problem.

64K10

To All,

Well I installed a new temperature sending unit today... The gauge needle now sits just to the left of the "Hot" indicator bar. The sender from Chevy Duty always pegs the needle to the extreme right!

The new sender is NAPA / Echlin part #TS6469.

Check your engine and dash grounds guys... the sender grounds through the block, and if the ground is sucky your gauge will never be right.
That's assuming your sender and gauge are ok. Chevy duty may be selling the wrong part.

I have the 1945 GMC ½ ton with the 302 gmc straight 6. My question is, I think I have the wrong sending unit in my motor. I may have mixed a chevy 235 sending unit with the one that should have been installed. To verify, what should the high and low range ohm’s be on the stock 45 GMC temp gage? Any help is appreciated.

Back in 1945 GMC was using an electrical temperature sending unit???? Wow, that's a new one on me!
Denny Graham
Sandwich, IL

Yup. Technology has not changed that much. Any help on identifying the high and low limit resistance to match my gage?

I'm working on that aspect right now. Been measuring ohms at different temps and testing gauge reading. I have an original AC temp gauge with a 100-212 degree range and there is no spot on it to do any tweaking or zeroing of the gauge itself. The only spot where the needle can be altered is to bend the brass tab a bit. I have some ohm reading on another recent post in this section of the forums.

Hi Geezer,
Thanks for the reply. Where is the other POST. I did a search uner ohm's, but did not come up with any results. This may be a good link for others going throught he same difficulties.

I believe I found the POST that you were referring to, but it does not have a conclusion on results. If I am wrong, please advise. I really could use the high and low limit values for my gage (45 gmc 1/2 ton) to spec out the right sending unit. THanks again!!

Here's a link to the other post. No, don't have resistance values as yet. What I am pursuing is finding the original GMC gauge (either part number or unit) and an original sender(same). Then if I am successful, I can measure the ohm readings at at least three different temperatures. Then I could post them so others could measure the senders they have to see if they are within a reasonable range to work with the gauge. The best scenario would to be to identify a current part number of a sensor and people could buy that. Will keep on working at it.


http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=3;t=002882

Thanks Geezer. Here’s an update on my end. I pulled my gage from the instrument cluster to get access. I first kept the two wire connection “as is” on the back of the gage with no results. I then switch the hot and sender wires on the back of the gage. The gage was hanging sideways from the wires below the instrument cluster, and as a result, the needle was showing in the middle of the range with no power. The needle seems to move freely, depending on how the gage is rotated and has a tendency to be in a 9 o’clock position naturally so as you rotate the actual gage around physically, the needle again will show a horizontal position. I pulled the sender from the motor. Once power was turned on, and a simulated grounding to the sender with the sendor wire connected from the gage to the sendor, the needle moved to the right. When I up righted the gage to it’s normal viewing position, the needle was pegged to the left, almost like the entire gage is mis-aligned, or the resistance is too high?

I purchased a few resistors. #1 measures 10 ohms, #2 measures 119 ohms and a third #3 reistor measures .295 K ohms. Now, I tried to put the three resistors in line one by one to see the effect. I pigtailed the resistor to the lead on the sender. Then connected the sender wire from the gage to the resistor. I tried all three above, and #1 did not change the overall needle movement from the “un-resistored” sender. #2 changed the needle movement to ½ the previous movement (meaning the needle moved 1/8” vs. 1/4"”) and the #3 changed the needle even less. What does that mean??

So, I believe the gage is working, and the sender is working, it’s just the resistance must be so high to start that it does not even register.


One more note, I did heat up some water, dropped the sender in, and the checked it against a cold sender installed. There was a larger needle movement on the heated sender than the cold (meaning, the needle moved another 1/16 -1/8” ). Makes sense, but the needle movement was in my opinion slight. Meaning, the needle only moved about a ¼” at its tip while just the sender was installed (again, while the gage was on its side allowing the needle to start off in the gage range of view.)

Any thoughts on where I am, and what I should do next?

I also tried various resistors in line with the sensor with either no additional movement or a total peg to the hot position. Don't think that is the way to go. I've got a couple of inquiries in to some NOS parts suppliers on a gauge and sender. Waiting for a response from them. By the way, the needle on the NOS Chevy gauge that I have also reacts the same way as you turn it around. Moves freely between 100 and 212 degrees. So your gauge is probably fine.

It's interesting I guess that I do temperature control systems as a living. I decided to call up one of my sensor suppliers who I purchase NTC thermistors from all the time. I posed the question: "If you have a thermistor of a given range and you apply 6 VDC to it or 12 VDC to it, what will be the measured result?" He said, UM........
However, he stated that thermistors are rated in applied voltages and are self-heating at those voltages. The ones we use in our business are 0-5 VDC as applied to the temperature measuring circuit. This would translate closely to 0-6 VDC in the truck. He also stated that if you apply 12 VDC to the same circuit, the thermistor would then tend to overheat, causing its internal resistance to fall which would translate to a higher reading on the gauge itself. I do believe that this may be what is happening in my case. I am applying 12 VDC to the original sender circuit and the gauge is reading 135 degrees when the actual temperature is 180 degrees or so. Therefore, I think that a 12 to 6 VDC voltage reducer is needed in the circuit to the gauge. This might possibly change the final reading and make it more correct. I'll experiment with this next.

Geezer,
I have a in line 12 to 6 Voltage reducer currently. If what your saying is true, this would not help my situation any. I am trying to get my truck ont he road for the first time, but need to have confidence that the motor is ok. I actually have a show on Saturday, so my time is limited. I wish I could come up with a ttemporary fix for now. Keep me posted as I will do the same.

Set up a test station on my workbench today. Used a battery charger that delivers either 6V or 12V. Ran the hot to one side of the 100-212 NOS gauge I have, then left the other side of the gauge to the center terminal of the sensor and clamped the negative of the charger on the sensor threads to complete the circuit. Put the sensor in a shallow pan of water and heated the water from underneath. Tried a TS-6 sensor first (which is what you get from the repops). No reading on the gauge until it got to about 195 degrees at the sensor (I used a thermometer to measure the water temp) and then the gauge spiked to 212 degrees. Let it cool down and the gauge moved a bit downward then dropped totally to the other end of the scale. Kinda shows what others have said their experience is with the senders in their trucks. Tried three other sensors and pretty much the same deal. Then switched the voltage to 6v and repeated the process. On 6v power, none of the sensors registed on the gauge no matter how hot it got. I looked at three of the sensors and they are marked 12v. I'm thinking that I'll purchase a 6v sensor (if I can find one) and try that to see how it reacts. If my guru at the factory is correct, a 12v thermistor is calibrated to be accurate at 12v. Therefore, a 6v thermistor should be done the same way. I scored a NOS GMC 100-220 temp gauge yesterday so once I get it, I'll test it as well.

Hey Geezer and TOny,
Any updates? I was wondering if there a company that can take the gage and sender and calibrate?

I seem to remember that there are some nuts on the back of the gage, that are inked with paint shut. I wonder if these are the adjusting screws referred to on this post??

Well, I just purchased a NOS AC Service parts GMC gauge part number 1511960 from a supplier in Canada. 100-220 degree range. I'm looking at the back of it now in my hand and there are two areas where a small hex bolt head is covered with yellow paint. It looks like once the hex head is loosened, it allows you to slide the interior wire wound resistor coils approximately 1/32" either way from center. I believe that by doing this, you can adjust the range of the gauge to match the range of your sender. However, there are no markings to tell where you are. When I look at the wire wound resistor coil at the 220 degree end of the gauge, the hex screw is all the way to the left or to the outside rear of the gauge face. At the 100 degree end of the gauge, the hex head is about 1/64" away from the far right side of the rear of the gauge face. I would surmise that at the factory this is the way AC Service Parts "zeroed" or calibrated their gauges. In order to make your gauge work properly with the sender in your truck, I would guess that you would have to remove the gauge and sender, set up a test stand like I mentioned above, put the sender in water and heat the water to 100 degrees, zero the gauge, heat the water to 180 degrees, adjust the gauge and heat the water to 220 degrees and readjust the gauge. Lots of work but it looks like it can be done. A winter project if I want to try it for sure. I'm now trying to find an original GMC electrical sender from '49 to '52 or so. One contact told me that the same sender for GMC trucks was for either 6v or 12v gauges. Will let you know if and when I can find one.

Geezer,
I have not heard from you on this? Is it away for the winter? Sure would like to solve my sender/gage issue. Let me know..


Thanks

49 Geezer,
I hope you can remember how you solved your gauge problem because I have the same issue. My gauge moves slowly to about half way and then pegs full hot. Do you remember the solution to this problem?