drwet

I've been working on tuning the new 427 small block. Still having issues with the temperature gauge reading low. When fully warmed up, the gauge reads about 120 deg. The sender is the same one I had in the old 383. It always read a little on the low side, but nothing like this. I have checked with the IR temp gun. The area around the thermostat reads about 120 deg. If I take a reading on the temp sender itself (which is in the stock location in the left head) it reads about 178 deg. I suspect an issue with the sender. I have read most of the old posts about temp gauge issues and watched the Willcox video on the adjustable resistor for the temp gauge. I honestly don't want to pull my dash apart AGAIN! Is anyone making a temp sender that works yet? Anyone have any other suggestions?

rberman999

If you pull the wire from the temperature sensor and ground it the gauge should read full hot(with ignition switched on, or run position). As long as the gauge does that you'll know your gauge is good. Then you can go from there. Lots of luck, Russ.

carriljc

I recommend that Willcox adjustable resistor. I put off doing mine for years, but it's wonderful. I wish it would have been available ages ago. I think you will be happier if you just buy it and install it.

0Willcox Corvette

I wish I would have made this years ago... it works like a jewel. Roger and I worked on this for a long time but I never thought about making them for re-sale until a few years ago... You can dial the gauge to match the sending unit.

Willcox

You can find it here:
https://willcoxcorvette.com/catalogs...table+resistor

Installation video's are here:
http://repairs.willcoxcorvette.com/?yt=&s=adjustable

drwet

Yeah I checked it out and the installation videos. I just got the car back together and I really don't want to pull the console apart again. I'm going to do some diagnosis - check out the gauge and measure the resistance across the sender at various temperatures before I decide exactly what to do. I will probably end up installing the adjustable resistor. Its a cool part and I'm pretty sure I won't be able to resist now that I know it exists.

drwet

So I did some diagnostics. The gauge reads zero with the sender disconnected and it pegs full field when I ground the sender lead. That tells me the gauge is working, more or less.

I took my IR temperature gun and ohmmeter and read the resistance across the sender at various temperatures as the car was warming up. (I read the temperature at the sender.) Here are some of the values I got.

53 deg. F 4500 ohms
120 deg. F 1400 ohms
160 deg. F 800 ohms
180 deg. F 540 ohms
188 deg. F 440 ohms

At 188 deg. F my gauge in the car reads about 120 which is about 440 ohms.

The only information I could find on what my readings should be was for 68-74. Does anyone know what the resistance values should be for a good sender on a '79?

carriljc

If you order it and just have it at home, then that will provide incentive to go ahead and do it

Also, I read a recent post where yo/u want to inquire about shipping costs when ordering.....don't remember where?

I say, just order it, and look forward to it

cardo0

No but to truly verify what the gauge reads at several actual temperatures you can dip the sensor in an water bath on a hot plate with a simple thermometer in it. Most likely only 2 temperature readings will be good enough but do as many as you feel comfortable with. This will be highly accurate as I never trusted reading the temperature inside a casting using an IR gun on the outside.
Once you can proficiently measure like this it's now a matter of testing different sensors to find a usable match.
No, I have never done this myself but I calibrate power plant instruments for a living. My 2 cents, take it or leave it.

BTW you need to ground the sensor to the block when testing it in a water bath.

REELAV8R

So it appears that the sender is working just the wrong resistance at operating temps.

If you wire resisters in parallel you can drop the total resistance. If appears that if your resistance was dropped then the gauge would read the right temp.

Here is a link to figure what the new resistance would end up being.

https://www.allaboutcircuits.com/too...ce-calculator/

what I would try is getting a variable resistor such as one of these or similar:

https://www.ebay.com/itm/1000-Ohm-10...gAAOSwh-1W6aPM

Then wire it up so that one end in on the same wire as the connector to the sending unit and the other end is grounded (at the block at the sender if you want) and dial in resistance until the gauge reads what your IR gun reads when at full temp.

Then measure that resistance level that you dialed in and buy a resistor of that value to wire in parallel with the sender and you got a sender that reads the right temp at operating temps.

Not sure how accurate it will be at other temps, you may need to experiment to find out.

Just an idea.

7T1vette

You can add resistors in the circuit to get it to read correctly at ONE specific temperature. But, the scaling will be screwed over the rest of the temp range. Doable...but not the best solution.

Rescue Rogers

''''''''''''''''i would check the resistance between the sender and the block. make sure its zero, then recheck the grounds from the block to the frame, maybe clean them up and see if that helps before pulling the dash

drwet

Thanks, that's exactly what I was thinking about doing as a temporary measure, until I have a day to tear into the gauge cluster again. I was hoping someone could tell me what the resistance should be so I could calculate what value resistor to use, but trial and error works too.

Any idea how much current goes through that sensor wire? What will that resistor have to stand up to?

drwet

Thanks. Also something I was planning on trying. I did find about a50 ohm difference in reading when I grounded the ohmmeter at the alternator, vs directly at the sensor. When I installed the sender I used Teflon pipe sealant and that may be acting as a partial insulator. On the Todo list for today.

joewill

you will find that the ohm readings you take at various temps are NOT linear. what i have not seen said here is that the problem is in the incompatibility between the gauge and the sending unit. and not saying which one has the problem ( or better said, which one is calibrated differently than factory).

there could also be an incompatibility in the gauge wiring. too much or too low resitance. or even grounding at the gauge or the sending unit ( too much pipe thread sealer messes with the resistance). these may or may not skew the readings of the factory spec.

I have seen this problem pop up here on various threads thruout the years and it is always an issue I described above.

I hope that the wilcox solution does factor in the non linear relationship between the 2 sensors ohm spec vs only factoring in a resistance adjustment at one temp. which is also discussed above.

gr8t73

Resistor from Willcox is $35.00 the problem I have is they want almost $20.00 to ship an item that in actuality could be sent USPS for $3.00 or less.

carriljc

See this thread concerning the ridiculous shipping costs.... Check out the ebay option, it's only 4 bucks shipping.....

https://www.corvetteforum.com/forums...-resistor.html

drwet

So if I order is it through Ebay shipping is $4. If I get it from Willcox, shipping is $20 ??! It comes from Willcox either way. I don't get it.

Again, does anyone know how much current a resistor connected in parallel with the sender would have to withstand?

REELAV8R

I have no idea what wattage would be required for that circuit, but I’d gamble it’s 1/2 or less.
I have gobs of 1/4 watt resisters and they are super cheap or can be taken from old electronics, but you could wire one up and see how hot it gets, just key on, jumper wires resistor in your hand to see if it gets hot.

Sorta redneck but it would give you an answer.

And I agree that the resistance is not linear, but you can still get a relative reference for temperature, particularly over heat situation, and still a normal indication until proper repair could be made.

If you have a meter that can read amps put one lead on the wire and the other on the sensor ( not connected to each other except through the meter leads) and see how many amp/ milliamperes it’s pulling. Then do the math for 12 volts ( engine not running) to get the wattage being pulled. That will also tell you the wattage rating you need for a resistor.

V x A = W

0Willcox Corvette

Since the Usps updates we've been having issues withe USPS calculator on small items on the site.. We have a meeting scheduled this coming week with them to hopefully get this resolved.

So the best option is probably just place the order on Ebay until we get this issue fixed...

On another note, you can't add resistance to the input wire. If you do this you might be able to get the gauge accurate at one individual point but the gauge is non leaner and this will throw all the other readings on the gauge out the window.

Installing the adjustable resistor will make a linear change to the non leaner gauge which is what makes it unique. Yes you have to pull the cluster out to install it, but in the long run it'll make your dash unit match your sender.

Willcox.

drwet

I was hoping you would chime in on this one. I agree your adjustable resistor is by far the best solution. It would appear that GM used this as a means of adjusting the operational range of the gauge. Unfortunately I don't think I can spare the time to install it for this driving season, and I'm considering the parallel resistor as an interim fix. I just need to know how much current the resistor needs to be able to withstand. (It would also be helpful to know what resistance value I am aiming for , but again, trial and error will get me there if necessary.)