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Old 05-01-2013, 07:28 PM
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temp guage help

Hi there, I'm trying to get the factory temp guage working in my 85 f body.
Car is carb'd, and had a set of 920 heads on it. The temp probe was installed in the drivers side head and worked no problem. When i switched heads to alum the hole for the sender is smaller, its actually a factory size as opposed to the larger sender that was in there and working fine. I ordered a new factory sender, installed and hooked up the wire. Now it wont read. I then replace the wire as far up in the harness as i could go thinking maybe I pinched it when I had the motor apart, still no dice. I thought maybe it was not grounding well through the head, so I ran a wire from the threads on the sender to a ground, and all that did was make the guage go full pin. I'm lost here, and really don't wanna go after market if I can avoid it. Any suggestions?
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Dave

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Old 05-02-2013, 10:29 AM
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test

My first reaction is you have the wrong sender.
The easy way to find out is to measure the resistance of both senders as you are heating them in water.
They can be connected together for the ground. And a small wire on the sensor stud will enable connection to the ohmmeter.
The resistance of both old and the new senders should be the same throughout the temperature changes of the water.
The gauge going to full pin says that you have a switching sensor (used with idiot lights) and not a resistance sensor and the threaded connection into the head is being insulated by the pipe thread sealer.

vicrod

Last edited by vicrod; 05-02-2013 at 10:35 AM.
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Old 05-02-2013, 02:53 PM
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Vicrod mentioned thread sealer - if you used Teflon tape, it will insulate the sender, keeping it from working.
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Old 05-02-2013, 10:17 PM
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Quote:
Originally Posted by vicrod View Post
My first reaction is you have the wrong sender.
The easy way to find out is to measure the resistance of both senders as you are heating them in water.
They can be connected together for the ground. And a small wire on the sensor stud will enable connection to the ohmmeter.
The resistance of both old and the new senders should be the same throughout the temperature changes of the water.
The gauge going to full pin says that you have a switching sensor (used with idiot lights) and not a resistance sensor and the threaded connection into the head is being insulated by the pipe thread sealer.

vicrod
I did order the correct sender for my make and model. But your right, it actually came already thread sealed, should I remove and clean off some, or all the thread sealer? But at that point would the piece of wire I wrapped around the exposed threads that aernt in the head, then ran to ground, should that not serve the same purpose? And what should I use as a sealer if not typical Teflon tape or goo?
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Old 05-02-2013, 11:00 PM
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I missed that you mentioned adding the wire. It seems to me that the wire you added should function just fine to ground the base of the sensor. Generally, if a sensor already has thread sealant on it, it should be of a type that will not interfere with grounding the sensor. Liquid Teflon thread sealant (not the tape) should work. The tape can interfere with the sensor grounding.
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Old 05-03-2013, 12:24 PM
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I missed that you mentioned adding the wire. It seems to me that the wire you added should function just fine to ground the base of the sensor. Generally, if a sensor already has thread sealant on it, it should be of a type that will not interfere with grounding the sensor. Liquid Teflon thread sealant (not the tape) should work. The tape can interfere with the sensor grounding.
So any suggestions where to look next?
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Old 05-03-2013, 01:31 PM
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I am at a loss here too. All the factory temp sensors I looked up are variable resistance types, but have a connector with two leads which makes me think they don't ground to the head/block, but that one of the leads goes to a ground. Do your sensors have one or two leads?
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Old 05-03-2013, 03:35 PM
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I am at a loss here too. All the factory temp sensors I looked up are variable resistance types, but have a connector with two leads which makes me think they don't ground to the head/block, but that one of the leads goes to a ground. Do your sensors have one or two leads?
Sensor has just a one wire lead, I know it worked until I switched heads. The old heads were an old casting, so the sender wasn't original. When i swaped heads, I simply ordered a sender for an 85 firebird, and installed, plugged in and that was it. Then I thought maybe the PO had installed some form of resistor in case the sender read differently then the factory. So I traced the wire back into the harness and could find nothing. I just went out now, and pulled all the thread sealer of and reinstalled, no change.....so frustrating
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Old 05-05-2013, 05:50 AM
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Get a VOM (Walmart has some for about $10) and start checking the resistance and continuity.

- With the sensor installed, check resistance between the sensor body and a good ground on the engine block. It should be zero. If not, then you need to ground the sensor and/or the head to the engine. This verifies that the sensor is properly grounded.

- With the sensor installed, check the resistance between the sensor terminal and engine ground. From what I recall, it should be 0 ohms with a cold engine, and rise to 90 ohms at the max scale. If it changes with temperature as the engine warms up, then the basic sensor operation is good (although sensor/gauge calibration may still not match).

- Disconnect the sensor wire and with the ignition on:
-- Verify that you have 12 volts on the sensor wire when the ignition is on.
-- Put a 10 or 20 ohm resistor from Radio Shack between the wire and engine ground. You should get a low gauge reading.
-- Then try the same thing with a 90 ohm resistor and you should get a high reading.

If it passes all the above tests its basic functions are working; however, if the gauge itself is off calibration or the sensor has the wrong resistance range (e.g. Fords are different) then you may still be getting readings that are off.

Bruce
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Old 05-05-2013, 07:53 AM
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Originally Posted by 75gmck25 View Post
Get a VOM (Walmart has some for about $10) and start checking the resistance and continuity.

- With the sensor installed, check resistance between the sensor body and a good ground on the engine block. It should be zero. If not, then you need to ground the sensor and/or the head to the engine. This verifies that the sensor is properly grounded.

- With the sensor installed, check the resistance between the sensor terminal and engine ground. From what I recall, it should be 0 ohms with a cold engine, and rise to 90 ohms at the max scale. If it changes with temperature as the engine warms up, then the basic sensor operation is good (although sensor/gauge calibration may still not match).

- Disconnect the sensor wire and with the ignition on:
-- Verify that you have 12 volts on the sensor wire when the ignition is on.
-- Put a 10 or 20 ohm resistor from Radio Shack between the wire and engine ground. You should get a low gauge reading.
-- Then try the same thing with a 90 ohm resistor and you should get a high reading.

If it passes all the above tests its basic functions are working; however, if the gauge itself is off calibration or the sensor has the wrong resistance range (e.g. Fords are different) then you may still be getting readings that are off.

Bruce
Thanks for the guidance Bruce, will give that a try today
Dave
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Old 05-05-2013, 01:55 PM
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mixed

According to the AutoZone | Auto Parts & Accessories | Repair Guides & More schematic, that car has (3) separate temperature sensors.The above has been verified with a separate shop manual (Haynes).

1 is for an idiot light. It is a switching type. A dark green wire is connected to it.

next is for the gauge. It is the resistor type. It also has a dark green wire connected to it.

And the third sensor is for the ECM. It has black and yellow wires connected to it.

The sensor for the idiot light can be tested by grounding it to see if the light lights.
The sensors could be mixed up.

vicrod
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Old 05-08-2013, 09:43 PM
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Thanks for all the help guys, really appreciated the efforts. Found the solution.
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Old 05-09-2013, 07:00 AM
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What was the solution? Always good to know what fixed it.
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Old 05-09-2013, 10:43 AM
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What was the solution? Always good to know what fixed it.
Lol....the "solution" is the autometer temp guage in the pic. I had the right wire hooked up, I had a factory sender, and a factory temp guage that was working and now isnt and way too much time trying to fix something that likely wasn't real accurate to begin with. So I went after market instead.
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Old 05-09-2013, 06:06 PM
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That works! Thanks for posting the answer.
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