yellowone

Have a 1972 CS with 350, 350 auto transmission. Hot light sometimes comes on but with no consistancy. One day yes the next no. shut car off, wait like going to a store or something and restart and no glowing hot light rest of day. It's hardly more than a glow. New sender, cleaned all grounds in dash guages, new buls, led. Steel wool the printed circuits also. Car does not run hot. Kinda irritating but don't know how to diagnos the problem, any ideas please?

oldcutlass

Buy a mechanical temp gauge and hook it up, then you'll know exactly what the temp is.

yellowone

I had put in a mechanical guage to make sure it is not hot. I would prefer to instead use the light in the dash, I really dislike the guages under the dash. So any help to figure out why the light comes on barely glowing at times would be much appreciated.

oldcutlass

If you disconnect the sender wire and the light goes out and its not running hot, replace the sender.

Vintage Chief

I know you replaced the sender/sensor unit. I suspect you have lost resistance - somewhere. They do make different types of sender/sensor units. There is a possibility you have a coolant temperature sender/sensor used for a gauge as opposed for a (idiot) warning lamp. Ensure you have the correct sender/sensor unit. Additionally, you cannot operate BOTH a warning lamp & a gauge on the same circuit.

yellowone

It was a new sender from Napa 6 months ago, and yes the light goes out when i disconnect it. It is a sender for a light. It comes on as soon as the car is started now and stays on, but only a dim glow. if the light is just right in the car, you can't even see it. Baffeled.

oldcutlass

Its not the first time an electrical part is bad out of the box.

Vintage Chief

Thus far you appear to have performed what I'd have performed. One additional piece you should examine is the wire. An electrical wire provides conductivity. However, the ability of a wire to pass a current is dependent upon the wire's resistance. Since, by itself, the term resistance implies the ability to resist the flow of current, a wire w/ no resistance would freely allow a current to flow; while, a wire w/ increased resistance will impede the ability of a current to flow. Therefore, if a wire loses its ability to resist current flow, resistance will drop and current will begin to flow through the wire. In your case it's very possible you're losing resistance - where, is the key question. IMO, the type of issue you might be experiencing is often incorrectly referred to as 'stray voltage'. It's a bad term because in effect what really is occurring is a loss of resistance or an increase in ground differential between two grounded objects. Suffice it to say (w/o further elaboration) if you begin to lose resistance, you will begin to introduce current flow. Examine the wire itself for any chafing, openings, laying across another open wire, insulation falling off, splices which are corroded/oxidized. Since the coolant/temperature sensor/probe/sender is of the NTC (Negative Temperature Coefficient) thermistor type, as resistance begins to decrease (similar to when the coolant temperature rises) current can begin to flow. JS

oldcutlass

The light is looking for a ground. If you put a volt meter on the sender wire when its disconnected what will you read? 12v. Norm, you are wrapped up in technical BS, if the light went out when the wire was disconnected then there is no stray anything.

Fun71

If the resistance begins to decrease too soon (at too low of a temperature), or if the resistance is too low to begin with (faulty sending unit as oldcutlass suggested), there will be a trickle current flowing, which would make the light glow dimly.

Vintage Chief

Of course the light is looking for a ground - that's what the DK GREEN wire is - the ground wire. When you put a volt meter on the sender wire when it is disconnected w/ the IGN ON what will you read? 12V. When you put a volt meter on the sender wire when it is connected w/ the IGN ON what will you read? 12V The HOT lamp will not illuminate in each case - even though it reads 12V when the sender wire is connected. If you disconnect the sender wire and you ground the sender wire w/ the IGN ON will the HOT lamp illuminate? Yes

So, at this point we agree the DK GRN sender ground wire is searching for a ground (which will ultimately illuminate the HOT lamp). You put the wire to ground and there is no resistance - the HOT lamp will illuminate. It's not clear to me why you're either asking or suggesting if the wire was disconnected then there is no stray anything. Of course there isn't, but we're talking about a wire that IS connected.

My point of looking for a strayed, chafed, DK GRN wire laying over another wire or even slightly touching another wire or grounded device is to suggest if the DK GRN (ground) wire is open or modestly in contact with another wire somewhere, it may be finding a slight ground source causing the slight HOT lamp glow (and, this is what is often times referred to incorrectly as 'stray voltage'). The sender (thermistor) creates high resistance (I believe ~10K ohms) and as it reads the coolant temperature the resistance falls off to near zero (lowers). The sender (thermistor) will achieve ground and the HOT lamp illuminate when the sender reads a coolant temperature of 253*F. You might be correct, the sender unit which he replaced 6 months ago may be bad. Replace the sender unit w/ another sender unit and see if the same slight glow in the HOT lamp occurs. If the same slight glow continues to occur, all I'm saying is you should be looking for a decrease in resistance (the ability to find a ground source) somewhere along the DK GRN wire.

oldcutlass

As I explained above, once he disconnected the wire from the sender and the light went out your theory is disproved. If your theory was correct the light would still come on dim.

Vintage Chief

WTF I passed the dufus test on this one OMG Thick as a brick! You're correct, it would still be finding a slight ground somewhere & the light would be a faint glow after disconnecting.

If it's not overheating as you said, probably bad sender. Even if I attempt to suggest one additional item, it would still equate to a bad sender; and, that is - the sender uses the intake manifold as the ground source but inside the sender the ground source is isolated by the thermistor - where gradually as the temperature increases, there is less resistance until the sender achieves ground with the manifold to illuminate the bulb. Might be a chance there is a small hairline crack in the sender - which would still equate to a bad sender.

bry593

Guys, most likely he has a gauge sender instead of a warning lamp sender. These hook up the same, but the gauge sender has gradually increasing resistance. So, once the motor is good and warm, it might dimly light. Or if the motor is cold or on a cool day, it may not light enough to see. I had the same problem in my Caprice. Changed to Dart Heads required a change of sender thread size. I of course got the wrong sender for warning lamps and my light was dimly glowing.