Talking about a 1966 Olds 98 convertible. Learning will applies to 65-66 big cars, but beyond that there might be some troubleshooting process that will help you.

Over the past few weeks my "COLD" idiot light wasn't showing on startup. Old's design is that this light is illuminated by a signal from the intake-mounted temperature sensor until the engine coolant reaches +/-120°F. On the hot side I think the "HOT" light illuminates around +/-240°F. I have a temperature gauge in the glove box so I don't really pay attention to the idiot lights, but it was throwing me off not seeing it when the car is cold at startup. A "COLD" light on startup is tradition!

First I thought it was the sensor, but switching that to me means draining the radiator & all that mess. I got a replacement, but this is not one of my favorite jobs. So I was trying to think up other things that might go wrong before committing to drain & fill the cooling system. (By the way - is there a way to replace the sensor and _not_ drain & fill?).

Second I thought about a broken wire at the sensor. 57 year old wires break. Or can short out. But I checked it & the connection was not only fine, but both strong & clean.

Third I wondered if I had a dead bulb in the gas gauge cluster Olds called the "Tell Tale". Tungsten bulbs eventually wear out.

The bulb wasn't dead, but it had fallen out of the plastic/brass connector in the circuit board. 1st time in 40 years I've seen a peanut light bulb fall out. I removed the Tell Tale cluster to remove the bulb out after trying a bit of fishing, it's just the electrical connector & 2 1/4" hex heads to remove it. 1/4" deep socket helps here.

In pulling the gas gauge cluster, not only that the bulb had fallen into the cluster, but the rear circuit board had _2_ broken pins on the back: I would have neither "COLD" nor "HOT" lights when the sensor worked just fine.

I've got a pretty decent parts hoard under the house and found another circuit board, problem solved. If you don't have another board, you'll need to try resoldering broken pins back on the board, or otherwise electrically connecting the wires to the gauge cluster. This can work, but it's iffy for longevity unless your skills include circuit board surgery. Mine don't.

Anyway learning from the day - check all those aging electrical connections for connectivity to avoid a messy job with coolant & water.

Cheers
Chris

 

Figure out which wire triggers the cold light, ground it with the ignition on. If the light works, replace the sender.

 

Cool test for whether or not the sensor is working!

Modern sensors like tire pressure monitors, fuel injection this&that, 02 sensors, window up/down, etc, etc, seem to go wrong so often I’m winding up not trusting vastly simpler technology from long ago.

Many thanks
Chris

 

Probably so, but a temperature sensor is still going to be the same and work the same, the associated electronics with newer engines, now thats the rub...

 

The idiot light sensor is just a thermally-activated on/off switch. Typically it's a bimetallic spring that moves when heated. They do fatigue and fail as a result.

 

The two contact switches are for sale on ebay most of the time. As suggested, if you short either contact to ground, the hot or cold indication should illuminate respectively.

Search 'Oldsmobile temperature switch'

https://www.ebay.com/itm/27383450753...Bk9SR9S7q4adYg

 

I think my sender is defect in my 98 also . Grounded the wire and cold lamp works and also hot lamp.
But i think i connect a gauge instead , have a SW gauge in my 88 and this works fine.