Temp Sensor and Temp Gauge

[72Highboy]

Hello Everyone,

Some of you guys may have saw my posts in drivetrain about getting my truck running. Well it all worked out perfect got her started up on the Sunday that just passed with my custom exhaust that I did by myself. I bet my neighbors were like

OH OOH his truck is running

because its been down for about 3 months. Anyways I have been doing little odds and ends around my truck to get my dad to stop naggin me about whats all wrong. Yesterday I was working on my Temp Guage. I had one wire hooked up to the sensor on the manifold, but not one on the thermostat neck. My gauage gave a little bit of movement but not much untill I hooked a wire up the the sensor on the thermostat neck sensor. That got my Temp guage working perfect. Just trying to help out not sure if you guys new that already but I am guessing the guage needs both hooked up to work properly. So if you have only one hooked up try hooking up both.

[Thunderfoot]

The Temp gauge only has 1 wire to the 1 sensor, it is a Red/white striped wire... There is also 1 wire for the oil pressure gauge/light it is a White/red striped wire.

[72Highboy]

Umm cant remember if that was the color because the wires were pertty toasted so I replaced them but I have two sensors and when only one is hooked up it doesnt work but when both are hooked up it works. Ill go take pics really quick. Like I said its worth a shot

EDIT
Thermostat

Manifold

[fordman]

lets see the rest of that wiring that was replaced. where does it splice into the old wiring.

[ForingaMex]

Thunderfoot wrote:
The Temp gauge only has 1 wire to the 1 sensor

fordman wrote:
lets see the rest of that wiring that was replaced. where does it splice into the old wiring.

That sensor on the thermostat housing must me an aftermarket adition by someone for some reason. At least I've never seen that before. If you use two sensors for one gauge it would give you the wrong reading. IMHO If you get pictures of the wiring some of the electronic gurus around here will help you figure it out and get it working just right.
Jake

[72Highboy]

No No No the guage works fines the temp give me good reading because before I hooked up the wires the needle would never move. Then when I placed the first wire on the manifold it moved the slightest bit. Then when I placed the second wire on the Thermostat the needle moved between C and H. I guess I am bad at explainging things I was just letting you guys know what worked for me. But here is the connection all I did was were the plug is I moved down about 4 inch and cut all the wires and connected new ones to the old ones.

[fordman]

something isnt right. the thermostat sender shouldn't be there on a stock 72 truck. it may be that the one sender was bad and soemone added the other sender because the first one wasnt working right. but they hooked up both of them? try uplugging only one of them at a tiem and see how it read with each one unplugged. try unplugging both at different times to see which one is working and which one isnt. if it takes both to work then get rid of both of them and get a new one for the intake only.

[Dragon]

There is something seriously wrong. The hole in the Thermostat neck is for a vacuum control. It is were the Vacuum retard gets it's signal from and on some the charcoal canister.

The one on top of the intake is the correct place for the sensor. If it takes both to get a reading the one on the block is bad and the other one is less bad. Since it takes two to get a reading both are bad or one bad and one is not for our gauges.

[Thunderfoot]

as the others stated...
That wire shouldn't split from one wire into 2 like that going to 2 sensors...
The way all the factory gauges work is like this: they have power going into the gauge (this is power inside the cab at the instrument cluster/gauge) then the sensor wire to the sensor. The sensor is a variable resistor to ground, this means that the sensor changes how easy it is to make a ground connection (for temp the hotter it gets the less resistance it has = easier to make/see ground connection) then the gauge reads this connection to ground and displays it as a proportion (less grounding = cold, more grounding = hot, in between = normal) . The gauge is calibrated to this variable resistance range if you take and short the wire to the sensor to the engine then the gauge should go to full HOT and with it not shorted (or connected to the sensors) then full COLD the readings in-between are where it has been calibrated with the correct sensor to the gauge for the engine temps ranges.

With what you have with 2 sensors connected together this variable resistance is less (closer to ground connection) then it should be with only 1 sensor (this is how electronics works with parallel circuits) so as the gauge "seems" to be working with both hooked up you either have 2 sensors that are not correct (wrong resistance range) or they are faulty, or your engine is running cooler then you think.
Having 2 sensors to operate the gauge is not the norm for this gauge on our trucks (don't know of anything that it is), even though it seems to work I wouldn't trust it, as you don't know which sensor is doing what... You would be better with a fixed resistance in parallel with 1 sensor so that you knew what the 1 sensor was doing.

So to some up:
You need to wire it up for 1 wire to 1 sensor. That Red/white striped wire to the sensor.
Get new sensor or confirm the operation of the sensors you have, this can be done with an Ohm gauge, thermometer, boiling water, and finding the specs on the sensor as to what the Ohms are at what temp (I think Robroy posted this on the site here)
After that it could be the gauge itself is not reading correctly, if correct sensor or confirmed sensor still doesn't read correct.

I tried to explain this without getting toooooo technical, hope everyone followed along ok

[Dragon]

It looks okay to me Thunderfoot, and I see you have an electronic background with the parallel explanation. I do have an extensive electronic background from the Navy which allowed me to build radical FEs when off work and fix electronic electrical problems when at work.

Remove the one sensor on the Thermostat housing and plug the hole, it will never give correct readings there, test it as shown above. If it responds correctly replace the intake manifold one with it. If it doesn't test the intake one and if it fails by the above test get one new one and remove the splices and repair the wire with a touch of non corrosive silicone wrapped with tape.

[72Highboy]

Ok I will get on that as soon as I can, It will have to be after I get back from my springbreak trip. Were going to Tenn and Ohio. Driving Ill have a camera to take pictures of any bumps out that way. Ill be heading into some of your guys area I am guessing.

Remove the one sensor on the Thermostat housing and plug the hole, it will never give correct readings there.

Question: How would I plug it, are there plugs I can buy or do I fill it with something or buy a new Thermo Neck?

[fordman]

if you remove the one from the thermostat housing just put a pipe plug in its place. but you dont have to remove it just unhook it.

[george worley]

Make sure you get a sensor for a Temp. guage not a temp. light. I was given the one for a light and my guage only worked when the engine over heated.

[Dragon]

It should be a 1/4 pipe plug.

[mrsnicks]

Also I would suggest not using butt connectors on the end of the sensor, but the round/circular/ring type connectors that are on your alternator and bolt them down.

Patrick