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07-27-2017, 07:43 AM | #1 |
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1951 Temp gauge problems
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. |
07-27-2017, 08:26 AM | #2 |
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Re: 1951 Temp gauge problems
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! Last edited by JSeery; 07-27-2017 at 08:42 AM. |
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07-27-2017, 08:31 AM | #3 |
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Re: 1951 Temp gauge problems
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.
Last edited by tubman; 07-27-2017 at 08:39 AM. |
07-27-2017, 08:47 AM | #4 |
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Re: 1951 Temp gauge problems
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!
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07-27-2017, 10:15 AM | #5 |
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Re: 1951 Temp gauge problems
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.
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07-27-2017, 10:28 AM | #6 |
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Re: 1951 Temp gauge problems
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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. Last edited by rotorwrench; 07-27-2017 at 10:34 AM. |
07-27-2017, 09:11 PM | #7 |
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Re: 1951 Temp gauge problems
Thanks guys; now I have some things to try tomorrow at the shop. I was at a lose until I posted this.
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07-27-2017, 09:33 PM | #8 |
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Re: 1951 Temp gauge problems
Remember, the path is from the battery to the gauge to the sender to ground.
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07-27-2017, 09:36 PM | #9 |
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Re: 1951 Temp gauge problems
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! |
07-27-2017, 10:59 PM | #10 |
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Re: 1951 Temp gauge problems
If you have an old "dry cell" 6v lantern battery, that may be more palatable to the little woman.
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07-27-2017, 11:04 PM | #11 |
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Re: 1951 Temp gauge problems
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. |
07-28-2017, 01:23 AM | #12 |
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Re: 1951 Temp gauge problems
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.
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07-28-2017, 08:27 AM | #13 |
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Re: 1951 Temp gauge problems
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? |
07-28-2017, 08:43 AM | #14 |
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Re: 1951 Temp gauge problems
i have seen mech. install temp gauges with teflon tape or sealer .use so much that the senders has no ground.
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07-28-2017, 08:49 AM | #15 |
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Re: 1951 Temp gauge problems
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.
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07-28-2017, 12:36 PM | #16 | |
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Re: 1951 Temp gauge problems
Quote:
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07-28-2017, 12:50 PM | #17 | |
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Re: 1951 Temp gauge problems
Quote:
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08-01-2017, 06:07 PM | #18 |
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Re: 1951 Temp gauge problems
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. |
08-01-2017, 07:29 PM | #19 |
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Re: 1951 Temp gauge problems
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 |
08-01-2017, 09:00 PM | #20 |
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Re: 1951 Temp gauge problems
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.
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