Faulty temp gauge, sending unit, or...?

M

mcgarrett

Well-known member
Here are the stats: '40 Tudor, 6 volt system, stock radiator, no thermostats - running only straight water with VP Cool Down coolant additive/corrosion inhibitor, Speedway water pumps, belt adjusted correctly. No signs of water leaks or blown head gaskets. Temp gauge is original to the car.

I replaced the temp sending unit with a new one from Third Gen Auto 5 years ago. I noticed that the gauge needle never really went to full COLD when first starting up after the new sender was installed - it's just shy of the COLD mark. After driving long enough for the engine to be fully warmed up the gauge tends to read between the NORMAL and HOT markings on the gauge and sometimes on a hot day it edges nearer to the HOT marking.

When up to full temperature, I inserted a mechanical gauge probe into the radiator neck to get an accurate reading and it only showed 180*. Don't see any signs of blown head gaskets, water leaks, or coolant on the ground after shutting the engine off and no boiling over indicating an overheating issue. I'm wondering if the sender is the problem or is the gauge out of calibration? Should I switch it out to another gauge to see if it reads differently? Just makes me nervous seeing the gauge reading so high.

Your thoughts are appreciated.
 
Was the 3rd Generation temp. sending unit a genuine OEM unit or a reproduction?
I have read that the repro. units are not really dependable. NOS genuine units are quite expensive but are available from private sellers who have collected them. I doubt if your gauge is faulty. Check all your wiring connections to make sure it's ok.

There are 1940 OEM King-Sealy temp sending units on Ebay............not cheap.
 
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Put the thermostats in, they are a designed part of the cooling system to warm up the engine and keep it to the proper operating temperature. I see you are in Texas where I know from experience it gets very hot. I have 2 40s with 170 degree stats and the needle on a normal day is just under the mark that is about 3/4 of the way to Hot. On a hot day around here (90+) the temp gauge while the car is moving is maybe a needle width above the mark before Hot. If I am sitting at a stop light it will move a little higher but never get to the Hot mark and immediately go down once the car is in motion. The crank driven fan is actually a bit too low. Do you have all the engine "splash pans" in place? If your car isn't overheating and boiling over maybe you are looking for a problem where there might not be one. Calibration is probably slightly off, the service notes tell how to calibrate the gauge to the sender. I read it a long time ago but you put a specific voltage to the sender (maybe 1.5) and there is a mark on the gauge face up high where you can't see it when installed in the car that the needle should come to. Has to be done with the gauge and sender on the bench everything grounded..
 
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With the ignition off, where is the temp gauge needle? It should go above the hot mark, but not completely out of sight. If out of sight, the gauge "zero" needs to be adjusted. That is not a trivial task. If it were me, I would not mess with it and just be happy knowing what the various gauge indications represent in actual temperature.
With that I will confess my annoyance at the inaccuracies of the reproduction senders, and paid a high price to get an original that indicates correctly.
 
drolston said:
With the ignition off, where is the temp gauge needle? It should go above the hot mark, but not completely out of sight. If out of sight, the gauge "zero" needs to be adjusted. That is not a trivial task. If it were me, I would not mess with it and just be happy knowing what the various gauge indications represent in actual temperature.
With that I will confess my annoyance at the inaccuracies of the reproduction senders, and paid a high price to get an original that indicates correctly.
Click to expand...

I agree with drolston. I left a message for you on the V-8 Club website also. One of my suggestions was to add a ground wire to the sender to see if that corrects the problem when cold. The sender may not be getting a proper ground , especially if you added some thread sealant to it when you replaced the sender. Not reading exactly correct is a minor problem and not worth the extra trouble to try to correct it.
 
Thanks to all for your suggestions.


The Third Gen sender was a repro; not OEM. I will check out NOS or good used KS sending units on ebay.

I may try thermostats to see if that makes any difference.

I've never had engine pans for this car. :(

When ign. sw. is turned to ON with the engine cold, the needle climbs its way
s-l-o-w-l-y to the COLD side of the gauge, but the needle never really points fully to the COLD marking on the gauge...just to the right of it.

I will also add a ground to the sender for insurance. I don't use thread sealant tape where there is a need for a good ground.

I agree with not messing with it too much; could make matters worse. I'm planning to add an additional analog temp gauge just to keep a close eye on it.

:)
 
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