#1  
Old 09-12-2021, 10:30 AM
markpj23 markpj23 is offline
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Default Temperature Idiot Light Operation

New to me 67 Catalina has an early 70s 400 in it. No gauges just idiot lights. When the ignition is turned to ON I get the oil light but nothing else.

I disconnected the water temp sensor line at the engine and ran it to ground. Turn the ignition ON and now I get the TEMP light also as expected.

My question: Isn't the TEMP light supposed to come on prior to engine start as a check that the idiot light is working? If so, how is that sensor supposed to be wired?

I hate being left to guess if the bulb is still good every time I start the car. Will add temp / oil pressure / voltage gauges in the future but until then...

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  #2  
Old 09-12-2021, 11:45 AM
LPete LPete is offline
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The circuit to the temp sensor is grounded by the ignition switch in the start position to test the bulb.

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Old 09-12-2021, 03:34 PM
markpj23 markpj23 is offline
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Quote:
Originally Posted by LPete View Post
The circuit to the temp sensor is grounded by the ignition switch in the start position to test the bulb.
Thanks. Now off to find the ground lead on the ignition switch....

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Old 09-13-2021, 01:28 AM
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I believe it's internally grounded. Make sure the switch housing is grounded to the dash

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Old 09-14-2021, 03:08 PM
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Originally Posted by markpj23 View Post
Thanks. Now off to find the ground lead on the ignition switch....
If the oil light comes on, the ignition switch ground is not the problem. You need a new sensor

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Old 09-14-2021, 03:31 PM
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If the oil light comes on, the ignition switch ground is not the problem. You need a new sensor
I don’t think so…the oil pressure light comes on with the engine off because there’s no oil pressure and the oil pressure switch is closed. The temp light comes on in the start position because the ignition switch grounds the circuit.

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Old 09-14-2021, 03:51 PM
markpj23 markpj23 is offline
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While troubleshooting this, I saw that the oil light comes on with the key in the ACC position.

So is it mis-wired to the ignition?

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Old 09-14-2021, 07:49 PM
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Both switches are Gradual Grounds. If the lights were gauges, they would be actual volt meters with oil pressure or temp in degrees.
So with key on, or acc on some circuits you get 12 v through the Ign,FUSE box, to the gauge or idiot light. other side to the sending unit that grounds it.
The Fuel gauge works the same exact way, the sender is a gradual ground.
You can have a "dim" temp light .
This isnt rocket science.

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Old 09-14-2021, 07:56 PM
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Quote:
Originally Posted by Formulabruce View Post
Both switches are Gradual Grounds. If the lights were gauges, they would be actual volt meters with oil pressure or temp in degrees.
So with key on, or acc on some circuits you get 12 v through the Ign,FUSE box, to the gauge or idiot light. other side to the sending unit that grounds it.
The Fuel gauge works the same exact way, the sender is a gradual ground.
You can have a "dim" temp light .
This isnt rocket science.
No, sorry, but they’re not. A temp switch is a real switch — on or off. A temp sensor is a thermistor, which varies its resistance depending on temp. Same with an oil pressure switch vs pressure sensor. One is a switch, one is a variable resistor.

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Old 09-14-2021, 08:07 PM
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Originally Posted by LPete View Post
No, sorry, but they’re not. A temp switch is a real switch — on or off. A temp sensor is a thermistor, which varies its resistance depending on temp. Same with an oil pressure switch vs pressure sensor. One is a switch, one is a variable resistor.
Yes,I agree, but simply stated, they are BOTH means to a GROUND
which was the point I was trying to make for troubleshooting.
A "real" temp switch can give you a dim light... thats not really on or off,
and showed up at my Dealership a couple times a summer.
Your point is well taken, was just trying to simplify.

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Old 09-15-2021, 08:41 AM
markpj23 markpj23 is offline
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I get the whole theory behind these.... but the oil light should NOT come on with the ignition in ACC position. Must be miswired, correct?

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  #12  
Old 09-15-2021, 08:56 AM
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That’s correct. I wonder if the “run” and “accessory” wires are switched at the ignition switch. Is the ignition coil also hot in accessory? The alternator field? They shouldn’t be.

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Old 09-15-2021, 12:28 PM
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Quote:
Originally Posted by markpj23 View Post
I get the whole theory behind these.... but the oil light should NOT come on with the ignition in ACC position. Must be miswired, correct?
I have seen folks put in a radio, and hook the hot wire ( memory) and accy wire together. This can possibly cause this same type of situation.
I would be more apt to look at what has been done, vs a ignition switch. Not many would rewire that connector that plugs into the Ignition switch.

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Old 09-15-2021, 03:48 PM
markpj23 markpj23 is offline
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Quote:
Originally Posted by Formulabruce View Post
I have seen folks put in a radio, and hook the hot wire ( memory) and accy wire together. This can possibly cause this same type of situation.
I would be more apt to look at what has been done, vs a ignition switch. Not many would rewire that connector that plugs into the Ignition switch.
Good tip - thanks. Car does have a Kraco stereo installed.... ugh

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