51 Champion High Temperature Reading

[cbonner757]

Good morning. I'm looking for troubleshooting advice to pin down why my temp gage says hot but a meat thermometer at the radiator reads a normal 170 degrees. What I've done so far:

Replaced the sending unit (twice) and the single wire running to the gage.
Replaced the gage with NOS. Tested new gage by grounding the sending unit wire, and the
gage pegged, so appeared normal.
New water pump, thermostat, drive belt.
Repaired and cleaned heater core. Replaced defroster core.
Replaced all hoses, cleaned all metal Tees.
Cleaned the elbow at the back of the engine block that leads to the heater hose.
Pulled the freeze plugs and cleaned the block. Got about 2/3 of a coffee mug of sand, grit and some casting wire out, but there was almost no minimal rust scale at all in there.
Tuned up the car and have the timing is dead on.

The car shows no symptoms of running hot: no steam or burping of water, no gurgling.

I was very careful to put the gage wires to the same terminals I took them from,
but if they were reversed to begin with, could it cause this?
The gage reading rises at a quick but somewhat normal rate (does not immediately peg) but
once it reaches 2/3 hot, it pegs very quickly.

I am open to ideas.

Regards,
Clarence in Virginia Beach

[nvonada]

I would be checking grounds. Any voltage difference sender ground and the gauge ground would screw you up. Are you using Teflon tape on the sender?

[Chucks Stude]

Wirebrush the contact area where the ground attaches to the body. Also, make sure there is a good clean ground between the engine and the frame. Cleanliness is next to getting a 6 volt system working!

[jackb]

when I last pulled the freeze plugs out of a 63' V8, I got about 2+ qts. of sludge....You may still have obstruction and sludge out in the back of the block where the sender is located....It may be a hot spot. The front of the engine, at the radiator can be a lot cooler than the back of the block...

[cbonner757]

I didn't use Teflon tape. On the first sending unit replacement I used plumber's pipe "dope," and this time I used a little blue RTV gasket sealer.
The wire I replaced (upper terminal on the gage) runs to the sending unit, apparently the ground wire, since the other one goes to a terminal
marked "IGN" on the back of the gage and I followed that wire to a spot on the ignition switch before I removed the old gage. But
the gage was inop when I got the car, so I can't be sure the wires weren't swapped way back when. Can anyone verify this is the correct
wire routing?
I recently added two additional ground straps - one from a starter bolt to a heater hose firewall bracket, and another from a higher-up timing cover bolt to a radiator mounting bolt. The old one from a different (lower) timing cover bolt to frame was so oily I figured it was not doing so well.

With no actual symptoms of overheating, I'm trying to be optimistic that I won't have to pull the head, or the engine, to solve this.

Regards,
Clarence

[jclary]

If you happen to have an old mechanical temp gauge...why not install it just to confirm that your engine is heating normally. You won't even need to mount the gauge in the dash. Just hang it somewhere under the hood and run the engine long enough to reach operating temperature. This way, you will not need to worry about correct electrical connections, voltage or grounding. If you do have a hot spot in the area, the mechanical gauge should confirm that as well.

[Lothar]

If your meat thermometer shows the water in the radiator isn't excessively hot, I would suggest the "Car Talk" gauge repair method: Cut one or more pieces of black electrical tape to fit over the gauge dial face and forget about it.

[Jimmie]

Clarence-
This is unreal, Three days ago when I finally got my 1950 Champion ready to drive (after 3 years of hard labor) to get the new exhaust system put on, the temp gage went to the peg rather soon after starting it. I removed the new thermostat and still it did the same thing. But it fluctuates from a little less than peg to peg and just keeps changing. A friend suggested that there might be an area such as the heatrer core where there might be air instead of water. Anyway, yesterday after reading another thread, I decided to use my wife's cooking thermometer in the radiator and it showed a little under 150 while idling, when I turned the engine off and when I looked at it a little later it was a little over 150. The heater puts out hot air. A friend has a heat sensing device that he uses for checking the temperature of his tires. I am not sure how high it goes but hope he can check various parts of the engine to see if there are hot spots. It sounds like you have tried everything there is to try, including the grounding. I have been told that it might help to get any air out of the system if I put a 7 pound radiator cap on it rather than the original which is zero pounds. Sure hope you get it figured out soon-things like this since I finally got it going have really frustrated me. If you figure something out, please let me know.

[StudeRich]

An incorrect Temp. Sender would by my guess in BOTH cases, new is not always good!

[cbonner757]

Good morning, all. A mechanical gage did indeed bring GREAT news. It rose to 180 and stayed there, perfect. 170 or so at the radiator, so I'm elated that I don't have to pull the head, or maybe even the engine, to clean out more gunk. I re-checked my wiring, and am certain the wires appear to be in good condition, and I had connected them correctly-- the bottom (IGN) gage terminal got the double wires as pictured in the shop manual, and the other got a new wire from the top terminal to the sending unit. A few inches from the gage, the double wires disappear into a small harness, so I don't really suspect they went bad for no reason (chafing, etc.) and have no means to easily trace them. The original sending unit is dead (makes the gage read zero). I got two replacement sending units, finding out the first of those was a slightly different style, and was for later years by part number (but still in the 6Volt era). Second replacement looked more like the original in the style of electrical connector, but there were small variations in the overall length between all three. I'm guessing the gage grounds through its mount to the dash, and that is same as when I removed it. So now I just don't know-- two new sending units gave consistently the same performance, so now I'm leaning toward the NOS gage being bad.
I welcome opinions.

Regards,
Clarence

[nvonada]

That is good news! I would still try putting the sender in with CLEAN threads and make sure it has a solid electrical connection to the block. The odds of two senders being bad the same way are slim and the gauge rarely fails. Senders are easy to check with a multimeter and some hot water.

[cbonner757]

Thanks, Nvonada, I'll try that. The manual gage was hooked up with nothing on the threads, so maybe it needs to go in without anything that might insulate it from the block?

[52 Ragtop]

My 52 had isues with the temp gauge, I replaced the sending unit and gauge, it would go slowly up to operating temp. then peg all the way up. I drove it like that for about a year, as I knew it was not over heating. I bought a new correct sending unit from SI, and that fixed the problem.
I agree with Rich that it is most likely the sending unit.

Jim

[cbonner757]

Thanks, Jim, but I replaced it twice already- one unit from S/I, then another from a different vendor. Soon as I get the fuel fittings back on the new fuel pump today, I'll know whether installing the new sending unit -without- any goop on it makes a difference. With the old/bad original sending unit, it just lays at zero, so I know that sending unit is bad.

[skyway]

Excessive resistance in what is by now a very OLD wire?