1960s 1969 DeVille Water Temp Light Dimly Lit

[R Simone]

Rebuilt the original 472 over the Winter/Spring and have had it out on the road less than 1000 miles doing some fine tuning etc.  She is running smoooooth and strong and have mostly had it out on nice cooler late Summer days.  The last couple times I got home just after dark and noticed my "Water Temp" light every so slightly lit. I couldn't see it during the day it was so dim, but it's not on at start up in the garage so I'm not sure how long it's taking to come on.

Along with the complete rebuild I've:
*Double checked timing and its EXACTLY 5 degrees before TDC
*All cooling components were rebuilt or new (Water Pump, Fan Clutch, Radiator, Hoses, Heater Core...)
*New 180 Degree "Fail Safe" T-Stat
*New Cooling Temp Sender

It doesn't "Seem" hot and I wasn't idling around town in hot weather when it happened either. That light lit up brightly at break in when timing was off and she got a little hot.  The plugs look a little bit lean, but she's running so good.  I don't have a temp gauge currently, but man everything is new.  Why would it light up just dimly - Anybody seen this before/have ideas? 

Disclaimer: The one thing I know is wrong is that I haven't installed the small braided ground from the P/S Head to firewall as I couldn't locate/think the spade broke off.  I know a bad ground can cause weird things with lights, but would think it has plenty of other grounds without this.

Thanks,

[Lexi]

Might want to test your rad's pressure cap, or if no acess to a tester-just replace it. With a tester you could also pressure test the entire system and look for coolant leaks. Clay/Lexi

["Cadillac Kid" Greg Surfas 15364]

Sounds like you MIGHT have the wrong sensor. I assume we are talking about the sensor that sits in tec drone center under the AC compressor
Greg Surfas

[TJ Hopland]

Most likely its a bad or wrong sender but you want to be sure so pick yourself up one of those IR temp guns, they are cheap and easy to find these days and even the cheap ones seem to be reasonably accurate.  With the gun you can verify that you are not actually hot enough that the light should be coming on.   

If you have an AC compressor in place you won't likely be able to read the sender but its in the same passage that goes over by the base of the distributor so you should be able to get a good reading there. 

[R Simone]

Radiator Cap is new and not a drop of coolant leaking anywhere.

Yep it's the coolant temp sender under the AC Compressor. It's a "Standard Motor Products TS6" that Rock Auto listed as the replacement part.

My buddy has a temp gun so I'll try testing in that area next time I have it out.

[35-709]

Try putting another at least temporary ground on it and see what that does.

["Cadillac Kid" Greg Surfas 15364]

A SMP TS-6 is the sensor for the temperature gage. The sensor you need for the  warning light  is different. I will look tomorrow for the part number.
Greg Surfas

[James Landi]

You did a ton of motor work and cooling system work--- use that laser thermometer (if it's a good one... most of the cheap ones are less than worthless junk).   Install a temp gauge in your engine compartment.  NEVER trust the idiot lights...(cautionary tale) on a hot sunny day on route 95 in Florida following a complete engine and cooling system rebuild, I couldn't see the idiot light "glowing"--- radiator blew out and engine head gasket failed. While taking out the blown radiator,we discovered a bunch of gunk blocking the radiator passages-- coolant  couldn't pass through the radiator--- so consider a serious back flush and an installation of a cooling system filter.All good insurance for an old engine that could have a pile of accumulated muck in the engine's water jacket.

[R Simone]

My understanding is the water temp sender is under the AC compressor is for the water temp light and the sensor at the left rear (D/S) of the block is the thermonuclear meltdown engine temp light. The engine was taken down to the bare block and flushed Etc so I'm confident everything is clean.  Hopefully either insufficient ground or a bad sender. I did see a negative review on that sender about erroneous readings, I'll have to see if my service manual shows what the "Water Temp" on-temp should be. The extra ground is an easy first check so I'll try that next time.

[Big Fins]

Greg seems to have a list of the correct sending unit parts. He's already told you the one you replaced is for a gauge unit. PM/Email/Call if you know him well enough. He said he would post the correct part number tomorrow, yesterday and we still have 12 hours left, give or take.

It's too damned hot in Texas to be in a hurry right now.

You are aware I'm semi-joking here, right?

["Cadillac Kid" Greg Surfas 15364]

Finally found one of the new sensors I knew I had but were hiding from me. Here is the part number and a couple of pictures

["Cadillac Kid" Greg Surfas 15364]

By the way, the true function of this sensor/light system is "low coolant" indication. When the coolant drops below the level of the sensor, at say 220 degrees F which is within the normal range, steam at radiator cap pressure begins to form and becomes super-heated above the temperature of the water below it. When the temperature of the steam reaches 260 degrees F, the sensor trips and grounds the circuit lighting the light.
Greg Surfas

[R Simone]

A SMP TS-6 is the sensor for the temperature gage. The sensor you need for the  warning light  is different. I will look tomorrow for the part number.
Greg Surfas

Ok, NOW I understand what you're saying.  I didn't realize there was a difference and when I ordered and thought "Heck the one that's .55 more must be the better one, I'll buy that" - I completely overlooked "With Gauge"

Serves me right for being a spendthrift. I'll get this changed out this week and report back.  Thanks!!!

[cadillacmike68]

A SMP TS-6 is the sensor for the temperature gage. The sensor you need for the  warning light  is different. I will look tomorrow for the part number.
Greg Surfas 

1969s do not have a temp gauge. The last year for the gauge was 1968. They had to make room in the dash for the left and right arrows, so away wen the temp gauge.

The 1969s and later had the coolant overflow recovery tank, making coolant loss a lot less of a problem.