After firing the parts cannon at my overheating problem I've come to the conclusion that either the gauge or the sender in the head is faulty. Is there a method to check the gauge? The sender is new but I know that doesn't mean it is good. Stock '54, 239, still 6 volts, pos ground.
Been there, done that, bought the t-shirt. Grab the cheap infrared temp gun from Harbor Freight and point it near the sender. You can also go around and check before and after the thermostat to see if it is opening when it should.
After replacing the t-stat and the radiator (a whole different story) I finally dug out my ir temp gun, which is what I should have done first. What fooled me was the gauge would only show the engine hot during a drive around town, never while just fast idling in the garage. Yesterday, after installing a new radiator, it went hot even faster and didn't go down. That is when I pulled out the temp gun and realized the gauge was the culprit, not the radiator. So, is there a way to isolate the gauge from the sender?
You can test the sender with a multimeter set on ohms. Disconnect the wire and connect the multimeter lead to the sender. Check resistance when it’s cold and then start it up and observe the resistance as it warms up. It should go up or down I honestly don’t remember. You can check the gauge with a portable power supply. Apply one volt to the wire terminal and increase slightly and observe what the gauge is doing. However; I’m not sure what voltage is used for 6v gauges but I know most 12v systems step the voltage down to 5v for the gauges. Hopefully someone will chime in regarding whether or not you have a positive ground system.
I'll try the ohms check. What I have done so far is, when the engine is cold, I turn the key on and the needle falls from H to C. If I disconnect the wire from the sender the needle stays on H. I don't get any ohms reading with the sender, hot or cold it appears the terminals go direct to ground. I sure hope it is the sender, I don't have another gauge. With the key on the terminal is receiving about 3 volts. Not sure if that is good or not.
I think it’s more important to see the ohms through the sender change from near zero continuity to full continuity… if that makes any sense. I don’t know exactly what values to look for, electrical stuff is one of my many weaknesses lol.
It makes perfect sense but I have full continuity both hot and cold. I'm 99% convinced the sender is bad, I'd just like to hear from someone whose temp gage works and can tell me what readings they get.
I would think that since the gauge it at H with no power and goes to C with cold coolant and power applied, that the cold sender would show lower resistance (more ground) at C and decrease as temps increase. I don't see why a 12v neg grnd or 6v pos grnd would make any difference in how it operates. I can check my 12v this afternoon if it would help.
I don't know if voltage matters either, but there is voltage sent to the sender when the key is on. I checked a similar sender on a ***mins engine I have in my shop and it shows resistance between the single pole and the body of the sender. The one in my Ford shows next to none, like .0002 ohm. If it isn't too much trouble I'd love to hear what yours shows.
You should pick one of these up, that way you can tell the temps in different areas. https://www.harborfreight.com/121-infrared-laser-thermometer-63985.html
I have one, but didn't employ it until after needlessly replacing the radiator. That is what told me that either the gauge is wrong or the sender is bad.
Some of those old senders were NOT a variable resistance unit, they had a bi-metal strip that would open and close with current flow. The average current flow would heat an element in the gauge and the heat would cause the needle to move. Very different setup than the later gauges used and cannot be measured with an ohm meter.
I had this issue with a Studebaker years ago, I installed a radiator cap with a temp gauge in the cap as a diagnostic tool. When on a cruise the dash gauge would read high, I would pull over and the gauge in the cap would read 165 so I soon realized the car wasn't really overheating. Not a fix for the dash gauge but I now use it anytime I suspect I have an issue and put the cap on when first driving a new to me vehicle for the first couple weeks. Just a thought.
Nosford sparked my memory, Ford used the King-Sealey gauge system that wasn't resistance based. I did check my sending unit readings. Using a Fluke meter on ohms read open no matter what the temp was. I switched to voltage and the reading jumped around from nothing to 5.8v. These readings were all with the gauge wire connected to the sending unit. Sorry I couldn't be of more help.
Actually that does help a lot 40cpe. Instead of resistance I'll look for voltage. Considering the price of the new radiator a new sender is pocket change so regardless of the outcome I'll probably be firing the parts cannon again and ordering a new sender. I have a lot of parts from a Victoria parts car but not a temp gauge. Figures.
Just fyi I have been dealing with the same issue. My car has been changed to 12 volts but still has 6 volt gauges. I ordered a 55 ford sending unit from Larrys T bird. Installed it using a voltage drop on the gauge. The gauge, with engine cold, didn't go all the way to cold. As the engine warmed to normal the gauge went to hot. Using the flashlight battery check,the gauge checked good. I had a second gauge I had removed from a driving car. I installed it with the same result. I have read several people using voltage reduces and gauges working fine. In my opinion the new sending unit is defective.
My 55 is converted to 12 volts and I am using the gauge voltage regulator from a 66 mustang. The gas gauge works fine but the temp gauge is wonky. I have tried more than one sending unit and they read different but no where near correct. Because of the previously mentioned King Sealy gauge system which is essentially the same thing as the voltage regulator (bi-metallic strip in both that make and break circuit) I am in the process of converting over to a later model gauge and sender. I am planning to do a write up when I am done and will post it here when finished. The only difference you will see is at key off the gauge will go to cold instead of hot.
This shows how the Mustangs were wired up using the gauge regulator. If you converted to an Alternator do not hook up the generator light, convert to a Volt meter.
I checked voltage and it jumps all over the place, even with the engine off, key on, engine warm (160 f). I ordered both a new sender and an NOS gauge I found on e-bay. I'll check back in if one of the two fixes it.
Voltage jumping around is normal for that type of system, as the points open and close the voltage will change. Picture starting with 6 volts (original system 1954 voltage) and you have the power applied 50% of the time and off 50% of the time, the average would be 3 volts. That is how that system worked, as the sender heated up the on/off time changed and the gauge moved in accordance to the average voltage (current flow). A rather wonky way of making it work, but it did work for a long time!
Well, here goes! When I converted my 55 to 12 volts I used a Mustang gauge regulator to drop voltage to the gauges. Ford used this regulator on most 12 volt cars and trucks for 15 years. The fuel gauge works okay but the temp gauge has not read correctly. I tried different stock sending units but it still wasn't close to being accurate. So I went looking for a fix and this is what I did. I had to find a later gauge that would work with a resistance type sending unit but fit into the stock location in my dash. The gauge had to have a needle that pivoted at the bottom and be approximately the same size as the stock gauge. I had a spare 55 gauge cluster I picked up at a swap meet to experiment with and found a gauge cluster from an early Falcon (1962?) that had a gauge that looked like it might fit. The face of the Falcon gauge was larger and the mounting ears were in the wrong location. The face of the gauge is thin aluminum and trimmed very easily with a heavy pair of scissors so I cut down the gauge face to the correct diameter. The mounting bracket had holes for screws at the 9 and 3 position when the gauge was level on the Falcon, the 55 mounting holes were at about 1:30 and 7:30 (diagonal) but it turns out the mounting bracket could be swapped between the Falcon gauge and the 55 gauge without modifying anything. It fits in the original hole perfectly, the only difference being the needle starts on C or cold when the key is off instead of H or hot like the stock 55. I used a coolant temp sender from a 1966 289 Mustang, these are under $20 at any part store. The pictures are not great (especially the one of the gauge mounted in the 55 cluster) but you should be able to get the idea. Works perfect, 180 degrees is just about half way or in the center of the sweep. Mark EDIT, the gauges that have the FORD script printed on them are the Falcon gauges. You can see one trimmed and one untrimmed next to each other in one of the photos. You can also see the straight across mount and the diagonal mount next to each other in another picture. Hope this helps!
Jeff, if you think this gauge modification for the temp gauge would be a help to anyone please feel free to post it in the sticky pages or anywhere else you feel it would be relevant. Thanks, Mark
Thanks Mark. I'll keep that in mind. Maybe we can get together for a beer this summer. My grandkids live in Salem and one of my grandsons will be graduating college.
A new sender fixed it. I feel really stupid for letting the Concours guy convince me I had a radiator for a 6 cylinder. But it's prettier than the stock radiator, not one ding in it.
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