Coolant Temp Gauge Reading

[James B.]

Temp gauge in my '67 L79 reads low; the needle moves but when the needle is at the second mark (155) temp at the thermostat out and 'top tank' measure 200 degrees (I-R gun). I know this is a common problem and have done a search of old threads. I am not very good on electrical issues but want to see if I am on the right track.
With the switch off the needle sits at the 155 mark. Ignition on and cold engine it drops to below the 100 mark. If I disconnect the green wire at the sender and ground it the needle jumps to/past the 230 mark. Given the way the needle moves I assume that means the gauge, ground, and green wire continuity are all okay.
I checked resistance from the sender to ground today (I think someone said I should see 700 ohms at room temp. but I didn't remember that until I had started the car); using I-R gun at thermo. out and 'top tank':
130 degrees- 500 ohms
180 degrees- 162 ohms
190 degrees- 141 ohms
200 degrees- 124 ohms
can anyone relate to these numbers? Assume next step should be to take out the sender and check it in boiling water (& make sure no thread sealant was used putting it in). Should resistance be close to zero at 212?
Will appreciate any help/advice.
Regards, Jim Baker

[Timothy B.]

Re: Coolant Temp Gauge Reading

James,

Going from memory, I recall the resistance at 190* to be approx. 100 ohms. I have the same problem that you have with the guage reading cool and engine at 180-190*.

If you read the archives, the first thing to do is verify the guage with it's fixed resistor and wiring is good. This is done by using 60 ohms resistance at the sender end of the green wire and go to good ground on the engine.

This resistance profile should make the guage needle point to the 230* mark. If your dash guage reads OK here the wiring is in tact and guage is properly calibrated. The next thing to consider is the resistance reading of the sender at this temperature as the sender is designed to provide this 60 ohms+- at overheat conditions.

You can install resistors to the sender wire to correct a hot guage but can't make a cold operating guage read hotter without changing the sender.

[John H.]

Re: Coolant Temp Gauge Reading

Jim -

Although "temp gauge" and "overheat" are among the highest-volume posts on most Corvette forums this time of year, yours is unique; most complaints are that the gauge reads 20*-40* high to an I.R. gun shot, not low.

For what it's worth, the graph and charts below are the results of several studies over the last few years of the resistance profiles of most of the currently-available temp senders. The graph is the profile of an original AC #1513321, as used in C1's and midyears. There are several articles in older issues of the "Restorer" also.

[Michael H.]

Re: Coolant Temp Gauge Reading

Temp gauge in my '67 L79 reads low; the needle moves but when the needle is at the second mark (155) temp at the thermostat out and 'top tank' measure 200 degrees (I-R gun). I know this is a common problem and have done a search of old threads. I am not very good on electrical issues but want to see if I am on the right track.
With the switch off the needle sits at the 155 mark. Ignition on and cold engine it drops to below the 100 mark. If I disconnect the green wire at the sender and ground it the needle jumps to/past the 230 mark. Given the way the needle moves I assume that means the gauge, ground, and green wire continuity are all okay.
I checked resistance from the sender to ground today (I think someone said I should see 700 ohms at room temp. but I didn't remember that until I had started the car); using I-R gun at thermo. out and 'top tank':
130 degrees- 500 ohms
180 degrees- 162 ohms
190 degrees- 141 ohms
200 degrees- 124 ohms
can anyone relate to these numbers? Assume next step should be to take out the sender and check it in boiling water (& make sure no thread sealant was used putting it in). Should resistance be close to zero at 212?
Will appreciate any help/advice.
Regards, Jim Baker

Jim,

I had the same problem I end up buying a old correct used A/C Delco sender for $65 and that solve my problem. Your right about the 700 ohms at room temp but you should read around 125 ohms at 180 degrees.

Mike

[Roger O.]

Re: Coolant Temp Gauge Reading

I'm going to be working on a 70 gauge here soon. I'm going to start out with a Lectric Limited sender and see how it does. I talked to a tech at LL and he said their sender will be 110-115 ohms at 180 degrees and 50-60 ohms at 200 degrees.
On the 70 I'm working on I fed in different resistances to test.
55 ohms produced a gauge reading of 235 degrees
112 ohms produced a reading of 140 degrees
These 2 readings were in the middle of the LL sender from what the Tech said.

86 ohms to the 70 gauge produced a dash gauge reading of 183 degrees.

If the LL sender does put out the predicted ohms the 70 gauge is not going to be right. I guess we will see.
I have a feeling I will have to adjust the resistor on the back of the gauge to set an accurate reading at 180 degrees.

[Timothy B.]

Re: Coolant Temp Gauge Reading

Roger,

If the guage reads 235* with 55 ohms the guage sounds like it's calibrated OK.

Changing the resistance profile of the fixed resistor on the back of the guage will make guage match engine temperature at 180 but not correct at higher temps. The guage will read hotter than the engine really is.

On my car after verifying the in car wiring and guage accuracy (230*) at 60 ohms, I decided it's good enough.

[Roger O.]

Re: Coolant Temp Gauge Reading

Roger,

If the guage reads 235* with 55 ohms the guage sounds like it's calibrated OK.

Changing the resistance profile of the fixed resistor on the back of the guage will make guage match engine temperature at 180 but not correct at higher temps. The guage will read hotter than the engine really is.

On my car after verifying the in car wiring and guage accuracy (230*) at 60 ohms, I decided it's good enough.

Good point , maybe if it's necessary to make a resistor change I should pick a hotter point to make accurate. This 70s cooling system is very capable and almost any rise would be reason for concern but a high point to shut it down might make the most sense.I'm hoping the readings with the new LL sender will be decent across the scale.
Too many poorly made senders and too many aftermarket gauges has made gauge accuracy a real pain.
James your not alone. Zero ohms will not produce 212 , when you grounded the green sender wire you gave the gauge a Zero ohms reading and the gauge pegged to hot.
I would clean up your sender put it in water on the stove with the terminal out of the water and monitor the ohms readings and temp readings as the water temp increases and see if it falls anywhere in line with the posted scales. If it does the easiest thing to do is then take your stove setup to your car and again increase the water temp and see how your gauge reads. A pan sitting on the fender with a thermometer and adding hot water in small amounts will work fine.
(I use a step in the middle where I use a series of resistors to check the gauge response.)

[James B.]

Re: Coolant Temp Gauge Reading

Timothy, good point on the 60 ohm resistor; however, I was confused reading earlier posts. Can you go to Radio Shack and buy a 60 ohm resistor or do you also have to specify wattage-there was talk on old posts about buying 1/2 & 1/4 watt resistors.

[Gary C.]

Re: Coolant Temp Gauge Reading

Roger, FYI - might want to check with Jack Humphrey who's a TDB member to see if he has any original temp sends available. Gary....

[Roger O.]

Re: Coolant Temp Gauge Reading

Roger, FYI - might want to check with Jack Humphrey who's a TDB member to see if he has any original temp sends available. Gary....

Thanks Gary, the owner of the 70 already purchased the LL sender so Ill try it and see how it does. If it doesn't I will PM Jack and see if he can help. Thanks again.

[Ronald L.]

Re: Coolant Temp Gauge Reading

James,

My 66 had the same low indication.

I had a correct calibrated sender, the original and an original out of a 66 427 that I had back in '78. They all had resistance readings that did not quite fit the charts.

What I started with was putting it in a beaker of water and heating it to boiling. Checking resistance.

Then I took a wire from 12v to the gauge and then to the sender and then to ground - and boiled the sender. I then started to see the newer and original #2 sender were faster and read a little higher, but 210 was OK for water boiling in Michigan.

I then took the connector off the back of the gauge and wired up the gauge "out of the dash" but still in the circuit> out to the sender and sender in boilding water.

Once confirmed there, I then put the sender of choice into the engine and verified it there.

If you want to shorten the circuit, get a thermometer or heat gun and verify what the real coolant temperature is. On an open system you can get close but not as good as pressurized.

[Jack H.]

Re: Coolant Temp Gauge Reading

You're not going to have the 'test' resistor connected for a period of time long enough to stress it's power handling/heating rating. Any, 1/4-watt resistor (or larger) will do the job for you...

[James B.]

Re: Coolant Temp Gauge Reading

Thank-you all, good advice and this gives me a work plan.