Cold temp dash light

[STRUTYOURSTUFF]

Just put a 283 in my 65 c10. This motor came out of a 1966 Chevelle.The temperature sending unit in this motor has one pin and one hole. I believe this was set up for a temp gauge not a light. I currently do not have the temp sending unit wire from the harness connected to anything. My cold temp light stays on with the key off. Goes out when the truck is running. My question. Is the light staying on because if I had the correct sending unit connected the unit would act as a ground therefore making the light go out when the key is off? Any help would be greatly appreciated. Thank you

[ray_mcavoy]

With the wiring harness unplugged from the temperature switch on the engine, the "cold" light should not be lighting up at all (key on or off).

The original temperature switch for the "hot" and "cold" lights should have two 1/4" blade terminals ... one terminal for each light.

One side of the "cold" light should be connected to a switched +12V source (pink wire) that is live with the key on. The other side of the light (light green wire) should be connected to the "cold" terminal on the temperature switch. Normal operation is for the switch to be closed (completing the circuit to ground and turning on the light) when the engine is cold. Then as the engine warms up, the switch should open and turn off the light. Turning the key off should disconnect the +12V supply to the "cold" light and make it turn off for any engine temperature.

The "hot" light is wired similarly (with a dark green wire going to the "hot" terminal). Except that side of the switch stays open unless the engine temp gets too hot. At which point, it closes, completing the circuit to ground, and turning on the "hot" light. There is also a "bulb check" feature that uses a grounding contact on the ignition switch to momentarily light the "hot" light while cranking the engine so the driver can see that the bulb isn't burned out.

[STRUTYOURSTUFF]

Ray, thanks so much for the info. Not sure what’s going on. I will check again today. But if I recall the light green wire heading to the bulb was hot. Should the green wire from the harness to the temp sender split to fit the 2 blades on the original sending unit?

[STRUTYOURSTUFF]

Ray, thanks so much for the info. Not sure what’s going on. I will check again today. But if I recall the light green wire heading to the bulb was hot. Should the green wire from the harness to the temp sender split to fit the 2 blades on the original sending unit?

[bloo]

I just traced this all out the other day on my 66 c10 when the speedometer got back from being rebuilt.

ray_mcavoy has it right. Both hot and cold are fed switched 12v from the ignition, and ground through the sending unit to come on. The oil light works the same way. Nothing should be on with the key off. That is just strange, and has nothing to do with the sending unit.

You do need the right sending unit. One for a gauge wont work. The one you need is a giant brass thing with 2 spade terminals in an "L" pattern. There are 2 wires at the sending unit. WIth the ignition on, ground one wire and the cold light comes on, ground the other and the hot light comes on.

[STRUTYOURSTUFF]

Thanks guys for the info. I only have one green wire coming out of the harness. You are telling me 2 wires go to the sending unit. Looking at the schematic it shows 1. What wire would be the second going to the sending unit?

[ray_mcavoy]

Originally Posted by STRUTYOURSTUFF

Thanks guys for the info. I only have one green wire coming out of the harness. You are telling me 2 wires go to the sending unit. Looking at the schematic it shows 1. What wire would be the second going to the sending unit?

That sounds like you might be looking at a schematic for a truck with a factory temperature gauge ... those only had 1 wire (dark green) going to a single terminal sending unit.

The "hot" & "cold" light setup should have 2 wires going to the sending unit. A dark green wire for the "hot" light, and a light green wire for the "cold" light.

[STRUTYOURSTUFF]

Ray thanks again. I will check my harness and see where my other wire may be going. You and the of the guys are awesome

[ray_mcavoy]

You're welcome!

I was thinking about what you said earlier about your harness only having one green wire ... and I wonder if you might have an engine harness that was originally from a truck with gauges.

That would explain the missing light green wire for the "cold" light. And it would also explain why that light is staying on with the key off.

Instead of the light green wire for the "cold" light, the engine harness for trucks with gauges has a black wire that forms one leg of the battery gauge circuit. Both harnesses use the same type of plug so they physically plug in the same at the firewall. But mixing them up would result in the black battery gauge wire of the engine harness getting connected to the light green "cold" light wire of the cab harness. And that black battery gauge wire runs down to the batt stud on the starter solenoid so it's live all the time. That would feed power to the light green wire all the time. And with the key off, the bulb could find enough of a ground through the ignition system to allow it to light up. But when the key is turned on, that puts power on both sides of the bulb, causing it to go out.

So check to see if your engine harness has that black wire going down to the starter instead of the light green wire going to the hot/cold light temp switch.

[bloo]

[STRUTYOURSTUFF]

Awesome! Thanks Bloo. Is there any certain type of thread compound I should use on the threads of the new sending unit?

[bloo]

On threads into water I usually use either teflon paste or something shellac based (like Indian Head or Permatex #2). Don't overdo it. The brass needs to make electrical contact with the manifold for the temp lights to work.

[STRUTYOURSTUFF]

Ok, thanks Bloo