Coolant temperature gauge does not work

fly_25

Not-quite-so-new-guy
Oct 20, 2020
31
8
3.8L Buick v6 in a 1985 Oldsmobile cutlass supreme. Recently bought the car with the coolant temp gauge not working, there are 2 wires going to the same coolant sensor in front of the engine, which looks like it has been replaced before. When the wire from the gauge is grounded the temp gauge goes to full hot but never comes back down to cold or lower than full hot. Bought a single blade sensor and grounded the sensor with the wire connected and the needle did not move, the resistance from the blade to the body of the new sensor is 3000ohms at cold (room temperature) but still has no affect on the gauge. Anybody have any experience with this? Any input is appreciated, thank you!
 

5spdCab

G-Body Guru
Dec 29, 2019
834
93
Tukwila, Wa.
3.8L Buick v6 in a 1985 Oldsmobile cutlass supreme. Recently bought the car with the coolant temp gauge not working, there are 2 wires going to the same coolant sensor in front of the engine, which looks like it has been replaced before. When the wire from the gauge is grounded the temp gauge goes to full hot but never comes back down to cold or lower than full hot. Bought a single blade sensor and grounded the sensor with the wire connected and the needle did not move, the resistance from the blade to the body of the new sensor is 3000ohms at cold (room temperature) but still has no affect on the gauge. Anybody have any experience with this? Any input is appreciated, thank you!
Someone else asked a similar question recently. It seems to me that someone else remarked that there are two different sender units, one that gives info to the ECM, and one that goes to the gauge. The single blade sender should screw into the side of the head, either between 1& 3 on the driver side, or 6 & 8 on the passenger side, depending on your wiring harness (oops, forgot that you have the v6, but should still be in the head).
Search other threads for a more definitive answer.
The two wire style is mounted at/near the water neck.
 

fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
I have 2 wires going to the sensor near the water neck, if I ground one wire then the gauge goes to max, if I ground the other wire then the coolant temp light comes on. I dont think my car has an ecu (anymore anyways) as I can't find it and my car has a permanent check engine light and nothing happens when I try to check the codes with the pins
 

5spdCab

G-Body Guru
Dec 29, 2019
834
93
Tukwila, Wa.
I have 2 wires going to the sensor near the water neck, if I ground one wire then the gauge goes to max, if I ground the other wire then the coolant temp light comes on. I dont think my car has an ecu (anymore anyways) as I can't find it and my car has a permanent check engine light and nothing happens when I try to check the codes with the pins
I honestly don't know much about engine computers. My car is a '78, before computers, and I graduated H.S. before my car was even built. That said, about the only thing I can think of is that the sender is defective, or hooked up wrong? Not sure what else might be the problem. Did previous owner try doing any wiring "fixes"?
 

gnvair

G-Body Guru
Sep 1, 2018
858
93
Southern New Jersey near Philly
Buick V6 should have a single blade sensor at the front of the intake. To check the gauge. Turn the ignition on with engine off. Take the connector off the sensor and touch the connector to ground. The gauge should pin the Hot side. If it doesn't then it's a gauge or wiring issue. If it doesn't then it's the sensor.
 
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fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
This is what I have currently. 2 wires from the car going to this location. Ground one wire the gauge goes full hot, ground the other the light comes on. If it's supposed to be a single blade sensor for the gauge, where does the sensor for the light go?
IMG_20201030_122048.jpg
 

5spdCab

G-Body Guru
Dec 29, 2019
834
93
Tukwila, Wa.
This is what I have currently. 2 wires from the car going to this location. Ground one wire the gauge goes full hot, ground the other the light comes on. If it's supposed to be a single blade sensor for the gauge, where does the sensor for the light go?
View attachment 160693
Maybe I am asking a stupid question, but did you try reversing the two wires?, and what is that 3rd wire on that other connection?
 

fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
That third wire goes to a temp sensor for the heater grid inside the carb (turns a relay on and off). Nothing happens when I reverse the wires but when I touch both wires together the gauge goes down.
 

CopperNick

G-Body Guru
Feb 20, 2018
613
93
Canada
Basically your sending unit is a bi-metallic unit. Power comes in on one side and, as the temp of the coolant warms up, the be-metallic strip in the sending unit reacts and causes power to flow through that produces a reading on the gauge. Any grounding that the sending unit might need is done through the block. If you want to test the sending unit, remove it from the block and fab up a test circuit for it consisting of the unit itself, a couple of wires with mini alligator clips connected to them that you can attach to the terminal ends, a 12V power source, a gauge or light for load, a toggle switch, and a cup or tin of hot water. You connect one lead from the sending unit to the gauge using one of the gater end equipped leads and then ground the gauge using a second lead. Your ground can be the neg terminal on the battery. The other terminal on the sending unit also gets a lead that goes to the toggle switch and then from it to the positive side of the battery. Put the sending unit in the cup or tin and fill it with hot water; just enough to cover the internal end of the unit as far as the threads but not enough to submerge the terminal ends. Flip the toggle switch on. If the sending unit is functioning it ought to respond by generating a reading that will appear on the dial of the gauge.

Some sending units can possess multiple terminals because they act as mechanical relays for power being sent to multiple circuits. One example would be a sending unit that delivers a reading to a gauge but also feeds power to an electric choke so that as the engine warms up the choke is encouraged to relax or open and allow the engine to slow down to a warm idle speed.
 
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fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
Just installed a single blade sensor (meant for a gauge) and at cold with the ignition on the needle was at about 100, maybe a bit less, after about 5 minutes of idling the needle maxed out (see picture). Is it possible that the gauge isn't calibrated correctly?
 

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