1967 Water Temperature Guage Test

[Timothy B.]

Re: 1967 Water Temperature Guage Test

I tested the temperature guage today with 60 ohms resistance added to the sender wire and engine running, the guage went to approx 230* mark, This guage has 100* then 210* in the middle and then 250*.

I have a question.. After turing the engine off checking the resistance of the green sender wire to ground there is approx 65 ohms resistance. Am I seeing the resistor that's installed when the guage is calibrated or do I have a corrosion problem at the firewall connection? I checked the resistance at the sender end of the wire to ground.

The AIM shows the sender wire going throught the firewall connector directly to the guage so I think no resistance unless this calibration resistor makes a difference.

What's the best chemical to use to clean the firewall connectors if I have to go there..

[Stephen L.]

Re: 1967 Water Temperature Guage Test

Internal to the gauge is a bridge. You are measuring the leg that is attached to ground.

[Timothy B.]

Re: 1967 Water Temperature Guage Test

Stephen,

Are you saying there should be some resistance and what I am seeing is normal.. The sender wire has to go through this bridge to ground is that correct.

I just looked at my notes again and the resistance in this sender wire is 80 ohms not 65. The firewall connector has never been off and it looks sealed with that black goo..PIA for sure, I don't want to go there for no reason..

[Stephen L.]

Re: 1967 Water Temperature Guage Test

I checked my gauge as you did, and measured 50 ohms. My gauge reads fairly close (as indicated by an I.R. gauge at the sender location). My connections have been recently cleaned up.

Your gauge may very well be within the tolerance of the system.... I don't know.

These systems were not very accurate at best... being a general indication with repeatability so that you'd know when things were amiss if you knew the normal indication. In other words, I would find it highly unlikely that the system accuracy was better than +/- 10 degrees.

[Timothy B.]

Re: 1967 Water Temperature Guage Test

According to my readings this guage is way off in the mid range. With verified engine temperature at 175, the guage reading is to the cool side of a middle mark that's between the 100 and 210*.

Looking at the guage I would say it reads 140*, that's why I questioned the resistance reading in the sender wire. I am really not concerned as long as the hot reading is accurate.

I am going to check the guage again with my 60 ohm radio shack resistors at the sender wire with the engine running/charging voltage but I think it's working OK. All suggestions are appreciated.

[Larry M.]

Re: 1967 Water Temperature Guage Test

According to my readings this guage is way off in the mid range. With verified engine temperature at 175, the guage reading is to the cool side of a middle mark that's between the 100 and 210*.

Looking at the guage I would say it reads 140*, that's why I questioned the resistance reading in the sender wire. I am really not concerned as long as the hot reading is accurate.

I am going to check the guage again with my 60 ohm radio shack resistors at the sender wire with the engine running/charging voltage but I think it's working OK. All suggestions are appreciated.

Tim:

I have a similar problem with my 1967 327/350 HP car. The temperature readings were "spot on" ten years or so ago, but have gradually fallen with time so that I now read 150 F on the gage with the engine at 180 F.

I believe that my problem is in the firewall connector, as others have said above. I also don't want to remove the connectors and clean, but will do it this summer when the weather (and wiring) is warmer and wiring is more pliable.

My amp gage is also DOA, and I believe cleaning the firewall connector will solve this problem as well.

Recommend using a small brush or pipe cleaner, and electric contact cleaner/lubricant that is plastic safe. Some folks have also used a Dremel with a fine brass wire wheel for terminals that show significant corrosion. I think they removed the individual terminals to get best access for the Dremel wire wheel.

Larry

[Timothy B.]

Re: 1967 Water Temperature Guage Test

Larry,

Other than the dim dash lights which I think are the headlight switch, everything works fine. I don't want to fix something that is not broke. If this guage goes to 230+- with the 60 ohms I put on the sender wire why should I even fool with the mid range. I did not think the mid range would be off this much but it's been like that since I owned the car, 1984..

Does the sender wire on your car show resistance to ground?

[Joe C.]

Re: 1967 Water Temperature Guage Test

Larry,

Other than the dim dash lights which I think are the headlight switch, everything works fine. I don't want to fix something that is not broke. If this guage goes to 230+- with the 60 ohms I put on the sender wire why should I even fool with the mid range. I did not think the mid range would be off this much but it's been like that since I owned the car, 1984..

Does the sender wire on your car show resistance to ground?

Much has been written on this subject.

The bottom line is, that most replacement senders are not correctly matched to the gauge. The best that you will be able to do, is to have your gauge read accurately at one point, and one point only. Since the gauge response is not linear, and the resistance varies, then any difference will increase exponentially as you move away from the "sweet spot" where the calibration of the gauge matches the resistance of the sender.

Go to Radio Shack and buy a low wattage 0 - 100 ohm variable resistor. Wire it in series with the sender wire (since your gauge reads too low................if it read too high, then you would have to trim the internal resistance of the gauge), under the dash and near the gauge, where it can be hidden. Get the engine to operating temperature, and verify the temp of the thermostat housing with an IR gun. When that temperature stabilizes at (about) 180 degrees, then vary the resistance of your trim resistor until your temp gauge reads 180 degrees.

[Timothy B.]

Re: 1967 Water Temperature Guage Test

1, I checked the resistance readings again today. With the engine off the unhooked sender wire has 60 ohms resistance to ground so I made a mistake reporting above.

2, I installed radio shack 60 ohm resistance in the sender wire to ground, the guage will read 230 (about the width of the needle to the cool side of the yellow area). The engine is not running at this point. I think the guage is calibrated fine.

3, If I just ground the sender wire the guage will peg off the scale on hot.

4, I need help here, when I install the 60 ohms on the sender wire and ground the wire and check resistance through the installed resistors my meter reads 30 ohms. Like the built in guage resistor is in parallel with the radio shack resistors I installed on the sender wire. Is this so or am I not understanding something.

5, I ran the engine with 14.5 volts (alternator charging) and installed 60 ohms on the sender wire to ground and the guage position is the same as when the car is not running like #2 above, just to the cool side of the yellow area about 230*.

Warmed the engine and checked the coolant temperature at 175* and that's as hot as the engine will go, car has 180 thermostat. With everything hooked up the guage reads to the cool side of the middle mark that's between 100 and 210. Shut the engine off and checked the resistance of the sender at 130 ohms.

Do I even have a problem? I would think the middle range would be closer than this. Sorry for the long winded post.

[Jack H.]

Re: 1967 Water Temperature Guage Test

Look, it's not all that hard... When you disconnect the sender wire and ohm resistance to ground, you're looking back through the wiring to the gauge. The gauge is a small ammeter. You're measuring the winding resistance of magnetic inductor that drives the pointer needle that's inside the gauge.

You already confirmed your gauge is calibrated properly (60 ohm load to ground = 230F pointer needle response). So, it's not a matter of questioning the accuracy of the gauge now.

The temp vs. resistance profile of the factory original AC 1513321 has been given. It's in the archives. I can't post it again. There WAS a factory acceptable tolerance profile for this sender that's wide/sloppy at low temps and tight/narrow at high temps.

For mid-band readings, say 180F, the system was 'reasonably' sloppy with even factory original temp senders being allowed to vary approximately 18-ohms in value. That translates to a temperature band of roughly 170-190F on the gauge.

This was NOT a high accuracy system. The 'competition' for it was the 'idiot light' which essentially came on to tell you SHUT 'ER DOWN NOW...

If you demand mid-range accuracy, then you're stuck with hand sorting old/original AC temp senders to get a better match for your car/your gauge.

Understand there are some who say go 'trim' the temp sender you've got by either adding series or parallel resistance. But, they don't tell you what the effect of that modification could be to a gauge that's properly calibrated under a bona fide engine overheat situation! If you tweak too much, you just might CASTRATE the high end of the dial accuracy of your system...

Now, all of this (accuracy consideration) has been COMPOUNDED over time because the temp vs. resistance curve of the original temp sender underwent running changes to make the sender 'acceptable' for other applications.

PLUS, there are non-AC replacement temp senders out there that have an even larger temp vs. resistance deviation. Often, these are marketed with statements to the effect of meeting SEVERAL Ford, GM, Etc. specs. Guess what, that doesn't work for the OVERALL temp vs. resistance range of a factory original AC 1513324 sender made in 1967...

[Joe C.]

Re: 1967 Water Temperature Guage Test

Look, it's not all that hard... When you disconnect the sender wire and ohm resistance to ground, you're looking back through the wiring to the gauge. The gauge is a small ammeter. You're measuring the winding resistance of magnetic inductor that drives the pointer needle that's inside the gauge.

You already confirmed your gauge is calibrated properly (60 ohm load to ground = 230F pointer needle response). So, it's not a matter of questioning the accuracy of the gauge now.

The temp vs. resistance profile of the factory original AC 1513321 has been given. It's in the archives. I can't post it again. There WAS a factory acceptable tolerance profile for this sender that's wide/sloppy at low temps and tight/narrow at high temps.

For mid-band readings, say 180F, the system was 'reasonably' sloppy with even factory original temp senders being allowed to vary approximately 18-ohms in value. That translates to a temperature band of roughly 170-190F on the gauge.

This was NOT a high accuracy system. The 'competition' for it was the 'idiot light' which essentially came on to tell you SHUT 'ER DOWN NOW...

If you demand mid-range accuracy, then you're stuck with hand sorting old/original AC temp senders to get a better match for your car/your gauge.

Understand there are some who say go 'trim' the temp sender you've got by either adding series or parallel resistance. But, they don't tell you what the effect of that modification could be to a gauge that's properly calibrated under a bona fide engine overheat situation! If you tweak too much, you just might CASTRATE the high end of the dial accuracy of your system...

Now, all of this (accuracy consideration) has been COMPOUNDED over time because the temp vs. resistance curve of the original temp sender underwent running changes to make the sender 'acceptable' for other applications.

PLUS, there are non-AC replacement temp senders out there that have an even larger temp vs. resistance deviation. Often, these are marketed with statements to the effect of meeting SEVERAL Ford, GM, Etc. specs. Guess what, that doesn't work for the OVERALL temp vs. resistance range of a factory original AC 1513324 sender made in 1967...

One can tweak the gauge for accuracy at any point in the range that one chooses. It can be trimmed so that it reads accurately at incipient boilover, as well. I choose to trim it so that it reads accurately at normal operating temperature since the needle should spend most of its time resident in that position. I can note any variation from nominal operation and act accordingly.

The key to remember here, is that these old nonlinear gauges will only respond within tolerable accuracy (say, plus/minus 5 per cent) within a very narrow band. It is up to the owner/driver to determine where he/she wants that accurate range to be.

[Timothy B.]

Re: 1967 Water Temperature Guage Test

Thanks Jack and Joe,

The reason I am questioning this wiring is because I don't want to unplug the firewall connector if it's not necessary. If 60 ohms to ground on the sender wire makes 230* and that verifies the guage and wiring that's good enough for me..

I believe the sender is original to the car. When the engine is at running temperature the mid range seems cooler with the pointer needle approx 140-145*, (just below the middle line that seperates the 100-210 mark), that's 30* below running temperature.

I plan on verifing the overheat condition (ohms) of the sender in a pot of boiling water, if that's near spec I feel comfortable knowing it's just the senders wide tolerance..

Sorry for the long winded post, I have learned to ask first before doing unnecessary work...

[Joe C.]

Re: 1967 Water Temperature Guage Test

Thanks Jack and Joe,

The reason I am questioning this wiring is because I don't want to unplug the firewall connector if it's not necessary. If 60 ohms to ground on the sender wire makes 230* and that verifies the guage and wiring that's good enough for me..

I believe the sender is original to the car. When the engine is at running temperature the mid range seems cooler with the pointer needle approx 140-145*, (just below the middle line that seperates the 100-210 mark), that's 30* below running temperature.

I plan on verifing the overheat condition (ohms) of the sender in a pot of boiling water, if that's near spec I feel comfortable knowing it's just the senders wide tolerance..

Sorry for the long winded post, I have learned to ask first before doing unnecessary work...

"Overheat" or "overtemp" refers to a condition where the coolant temp is at a point somewhat higher than "nominal". It is a rather vague and meaningless term. I prefer to use the term "boilover" or "imminent boilover" which more accurately describes a situation which must be avoided at all costs. This point, of course, is determined by one's radiator cap relief pressure and coolant mix ratio. I believe that "boilover" usually occurs (with a 15 psig cap and 50/50 mix ratio, and fresh coolant) somewhere around 265 degrees F.

[John H.]

Re: 1967 Water Temperature Guage Test

The indicated temp rises as you increase resistance, so, in the hypothetical case that you have a poor connection at the junction block, then that would cause a rise in indicated temp, NOT a drop.

Joe -

The indicated temp goes down as the resistance in the circuit increases, not up.

[Joe C.]

Re: 1967 Water Temperature Guage Test

Joe -

The indicated temp goes down as the resistance in the circuit increases, not up.

Post amended.