1951 Temp gauge problems

[tubman]

I've got my '51 back on the road and am very happy the way everything turned out except for the temperature gauge. It worked fine before I removed the old engine. A little background : About 3 years ago, the temp gauge in the car started reading very low (just off the "Cold" mark during normal driving). I followed the manual (it says the test for a bad sending unit is to replace it with a new one and see if it corrects the situation), and found the sender was bad. I had 3 other sensors, so I put the best looking one in and voila', my problem was solved. In the process, I checked all of the sensors with my multi-meter. Maybe it's just a coincidence, but all of the sensors "Ohmed out" at 14 ohms while the bad one was quite different (I don't remember the exact value). I have tried all 3 of the sensors I have left, and get the same results with all of them. With the ignition off, the temp gauge goes to full "Hot". When I turn the ignition on, the gauge goes to full "Cold". I can start and drive the car any distance, and the gauge stays right there. I have checked the "switch" sensor on the drivers head and it has no resistance (closed), just like it should. I have made sure the the passenger side sensor is grounded to the head. Also, I checked the voltage at the wire to the passenger side sensor with it disconnected and the ignition on; it reads 2.7 volts.

I have to believe that the problem was caused by something I did, but I'll be darned if I can figure it out. The wiring is so simple that it's almost impossible to get wrong (I even "Ohmed-out" the wire between the sensors to make sure there was no internal break). The fact that the gauge cycles from full "Hot" to full "Cold" when I turn the ignition on tells me the gauge and dash wiring are OK (I may be wrong here).

Does anyone have any insight to what may be going on here? I went through "JBSeery"'s post on instruments and it was quite helpful, but I still have the problem. ("JBSeery" feel free to jump in here if you feel like it.) One thing that might help me is if someone with one of these cars (shoebox) could take a voltage reading on the disconnected wire to the temperature sensor with a cold engine and the ignition on.

[JSeery]

You checked the sensor wire to the sensor and had 2.7 volts. Did you check the sensor wire to ground and see what that reading was? Might want to check the gauge and see if it is reading ok. You can use a 1.5v D cell. That should give you 1/2 scale. Now, just guessing here, but if 1.5V gives you a 1/2 scale reading then somewhere in the 3.0v range should be full scale (cold in this case). That is in line with your 2.7v reading at the sensor. An open is Hot on the temp gauge, so full scale voltage should be Cold. It's really measuring average current, but not a easy to measure.

Note: the D cell test has been used a lot on Fuel gauges and works well, but I've never tried it on a temp gauge, just guessing it might work the same.

Again a bit of a guess, but the temp senders most likely have the same issue as the fuel senders, points require cleaning after years of use. Problem is, no idea how you could clean the temp sender points!

[tubman]

Thanks "JSeery" (sorry for the wrong name; I must be using too much "JBWeld" lately). This gives me something to do; I'll let you know how the "D" cell test comes out.

[JSeery]

The ohms readings you are getting on the temp senders would be the heater coil resistance plus the points. The points alone should be very near zero and the heater coil resistance fairly consistent from one unit to the next. So I would think corroded points would show increased resistance. But there is still the bi-metal strip, not sure how to check if it is working!

[Ross F-1]

You can test the sender by hooking up a battery, making a circuit that has a small (dash gauge) lightbulb in line with the sender. Put the sender in a pan of water and start heating it. The bulb should cycle on and off with changing frequency as the water heats up.

[rotorwrench]

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There are no repairs for a sealed unit. You can have good Ohms reading through the heat coil but it could also have the points stuck/dirty or something wrong with the bimetallic strip. Keep in mine that the same thing goes for the indicator.

It's great if you have both a sender and an indicator that are known to be in good working order to use as system test tools. Just using a battery to test an indicator only tests part of its function.

[tubman]

Thanks guys; now I have some things to try tomorrow at the shop. I was at a lose until I posted this.

[JSeery]

Remember, the path is from the battery to the gauge to the sender to ground.

[tubman]

Yep. From everything I see here, it looks like I have 3 bad senders! I don't see how that could be, but Ross seems to have a way to test them. Now, all I have to do is to convince the little lady to let me bring a battery or power supply into her kitchen; that's probably gonna be the hardest part of the whole exercise!

I'm gonna try everything I can before I dive under the dash again!

[Ross F-1]

If you have an old "dry cell" 6v lantern battery, that may be more palatable to the little woman.

[JSeery]

6v may be a little much unless there is additional resistance in the circuit to replace the gauge. Think the sensor would be seeing more in the 3v to 4v range. No need to fry sensors attempting to test them!

If you have the gauge in the test setup, then 6v is appropriate and a small battery would make it easier to work with.

[BillM]

I have been testing some temperature senders with a temperature gauge the last couple of days. One of them operated smoothly with heat applied with a hot air gun but a couple of the others went to minimum temperature and stayed there when connected to 6 volt power through the gauge. I found that rapping them on the side with the handle of a screwdriver would apparently un stick the points and get them operating again, maybe not as smoothly as the good one but looked like they may work OK. Worth a try.

[JSeery]

To bad there isn't an easy way to repair the temp senders. It is most likely the points needing cleaned up a little.

Anyone have experience with using a resistance type modern sender with the original gauge?

[richard crow]

i have seen mech. install temp gauges with teflon tape or sealer .use so much that the senders has no ground.

[tubman]

Originally Posted by richard crow

i have seen mech. install temp gauges with teflon tape or sealer .use so much that the senders has no ground.

In regards to this, I have an interesting sidelight. When I installed the sensor, it had a slight leak. I cleaned the threads as best I could, and even took a little cut at the threaded opening in the head with a pipe tap. It still leaked. I knew I didn't want to use a sealer because of just this problem. My neighbor, who is an EE, gave me some thin copper tape designed just for such uses. It is very thin and has a paper backing. Slightly over one wrap of this tape and my leak was history. I checked it with my multi-meter just to be sure, and there is zero resistance across the junction.

[BillM]

Anyone have experience with using a resistance type modern sender with the original gauge?

I don't think that is an option as the temperature gauge operates opposite of the other gauges. With power off the needle is all the way to the right, max temperature. With power on and a cold engine the sensor points are closed for the maximum time and the gauge reads cold until the engine heats up and causes the sensor to shorten the time the points are closed thus decreasing the average current to the gauge; causing the needle to return to the right.

[JSeery]

Originally Posted by BillM

I don't think that is an option as the temperature gauge operates opposite of the other gauges. With power off the needle is all the way to the right, max temperature. With power on and a cold engine the sensor points are closed for the maximum time and the gauge reads cold until the engine heats up and causes the sensor to shorten the time the points are closed thus decreasing the average current to the gauge; causing the needle to return to the right.

Yep, that is true.

[tubman]

OK guys, I finally got back on this. I have 4 sender units; three have 13 to 14 ohms resistance through them, which I believe is normal. One has infinite resistance, so I eliminated that one. I tested two of them today using Ross's method. The first one lighted the bulb when I put the power to it and put it in a container of water. After a half hour, the light stayed the same as far as I could see, even though the water was boiling. The next one I tested was the one out of the engine I removed from the '51 2 years ago. (It was working perfectly when I took the car out of service then.) With this one, the light also came on when I applied power. After a while, the light would get dimmer and then brighter. It would never go completely off. Because this one acted a little different, it had been working, and it was getting towards the end of the day, I decide to put it in the car and hope. Nope, no go; the gauge still reads dead cold. Ross, or anyone else for that matter, when you did the test, did the light just get dimmer and brighter or did it cycle on and off. I've gotten this far into it now, and would like to be able to develop a definite test for these sending units of possible. I was thinking about buying a new sender, but they are a little expensive. I'd like to make sure all of mine are actually bad before I spend that kind of money.

I suppose it could be the gauge or the wiring under the dash, but I never touched anything there. Any suggestions will be appreciated.

[B_man_Al]

You may want to try this test to check your gauges. I am working on an F1 and have been calibrating the gas gauge with external resistors connected to ground with the sender disconnected. If your gauges are similar here is what you should expect.

First check the voltage to the gauge by disconnecting the sender and measuring the voltage to ground on the wire. It should be 6v with the ignition on, and the car not running ( battery voltage). With the car running it will be around 6.5 to 7 depending on the generator output.

My gauges all read 10 ohms across their terminals ( fuel, temp and oil pressure). You can measure this by connecting an ohmmeter between battery and the open, disconnected sender wire , again with the ignition on to provide continuity to the power side of the gauge.

Here are the fuel gauge readings I get with various external resistors connected to the sender wire. (in place of the actual sender)

20 or less ohms = Full
30 ohms = 3/4 full
43 ohms = 1/2 full
55 ohms = 1/4 full
100 ohms or more = Empty

Though I have not measured the other gauges yet, I suspect they will be the same except the indication on the gauge (ie Full= max oil pressure (80) = Hot

Hope this helps with the gauges. I would be interested in hearing your results if you decide to do the tests

Al

[tubman]

Al, Thanks for the information. I think with all the information I have been gathering, I may be able to figure this out and come up with a test procedure.

[JSeery]

I haven't tried the bulb test, but the pulse rate averages around 1 sec. per a Ford manual. So you should be able to see it blink.

Note: just checked, it wasn't a Ford manual, it was in King-Seeley operation description.

[tubman]

Originally Posted by JSeery

I haven't tried the bulb test, but the pulse rate averages around 1 sec. per a Ford manual. So you should be able to see it blink.

Note: just checked, it wasn't a Ford manual, it was in King-Seeley operation description.

Yeah, the Ford manual isn't very helpful; "Change it, and if it fixes it, that was your problem".

"Jseery", since you're still following this, do you think that the test would be better if I ran it at 2.7 volts rather than 6? All I had was my 6 volt battery charger that was putting out 6.3 volts. My neighbor has a variable voltage DC power supply I can borrow that would allow me to run it at 2.7 volts.

Sorry that I seem to be obsessing on this, but I just got the car together after two years, and everything else is perfect, except for the GD temperature gauge.

[JSeery]

If you are running the current from the source through the gauge then to the light bulb and then to the sensor and to ground, then it would be in the 6v range. If you are running just from the source to the bulb to the sensor to ground it would be in the 3v range. You are dropping half the voltage in the gauge and half in the sensor. 6v alone on just the sensor would be overloading it I would think.

[B_man_Al]

As JSeery suggested I think you should test through a 10 ohm resistor, or what ever your gauge resistance is, if you are going to use 6v, as the current will be too high without a series limiting resistor, or use the lower voltage without a resistor
Al

[rotorwrench]

Don't forget that a light bulb is also a load just like a resistor.

The indicator gauge functions just like the later types since all it has is a heater coil around the bimetallic strip that pulls the indicator needle. It can be tested by varying the current to it with a variable resistor. What range is needed would be the big question that I can't answer. Once a person knows the gauge is good then it can be used to test the senders.

The senders wear out. Even if the heating coil is good, the points can be bad or stuck in the closed switch position. One poster mentioned tapping the sender and that might work to free stuck points or it might not. Bimetallic strips generally last a long time unless some part of it cracks or comes loose. The Lincoln Mercury overhaul manual for 1949 thru 1951 has a good test procedure but it only tests for proper function. If it doesn't function then replacing the sender is the only option. I still see NOS ones on the flea-pay now and then but who knows if they work or not. Their may be a reason that the part set on the shelf for all those years.

[BillM]

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My recent measurements of a sender and gauge in series showed a current of about 230 ma at 6 1/2 volts and a duty cycle at room temperature of roughly 10 %; points closed.
Let's make that when the points are open; my meter reads duty cycle as a positive signal and the temperature gauge shows Cold with max current from the sender. I monitored the points with an LED and resistor in parallel with the sender. The LED blinked brightly when the points opened.
If you connected a 6 volt bulb across a sender and gauge it would lite when the points open (and maybe glow dimly when they closed). If you don't have a gauge to use a resistor of 10 or12 ohms would work or put two senders in series; but then you'd have to determine which one is switching.

[langer]

hey fellas... I have a 51 ford delux that i just got... all seemed to be functional and then i noticed my temp guage was pegged on H... and stays there even when the ignition is off... I'll start down the advice in this thread. do you have any other insite on why the guage would stick on H other than a bad guage... the car runs a drives very nicely and doesnt feel " too hot" to the touch...

thanks
jonah

[tubman]

If you have a multi-meter, check the resistance through the two terminal sender (drivers side head). It is only a swifch that opens the circuit (forcing the gauge to go fill hot) if the coolant reaches a certain high temerature If there is no resistance through it at room temperature, you've found the problem.

If that's not the problem, I seem to remember that I posted another thread on the ultimate solution; I'll try to find it.

[tubman]

Check out this thread : https://www.fordbarn.com/forum/showt...es#post1509023.