Coolant temperature gauge does not work

CopperNick

G-Body Guru
Feb 20, 2018
613
93
Canada
Okay, it could be possible that the gauge is faulty. But right now I am wondering if you have tried to drive the car with that new sending unit plugged in and wired in to see what the gauge does when the motor has a chance to get hot enough that the thermostat can open versus it just sitting there. Where I am going with this is that if the thermostat hadn't opened during the short time that the vehicle had been idling then what you may have run into was a hot spot or bubble in the coolant because it wasn't circulating through the rad, only internally in the block. I have watched gauges head for the boiling point only for the needle to drop like a rock when the t-stat pops open and the coolant gets to cycle though the rad. Possibly take an around the block test drive and them come back and check your coolant level to see if it has gone down because the t-stat has opened and allowed the coolant to return back to the rad.

WARNING!

If you are possible thinking of doing that coolant check at the radiator, Do It With The Engine Running!! if you try to unscrew the rad cap with the engine turned off the internal coolant pressure will blow the coolant out of the rad tank through the filller neck and into your face and body. This equals instant burns to face, hands, eyes, and upper body, and possibly internal poisoning if you ingest the hot liquid. (Don't laugh, you would be surprised how many people tend to stand there with their mouth open while they try to get eyeball close to the radiator coolant filler neck so that they can see what is going on inside the tank.)


Nick
 

fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
Thank you for the quick reply and safety briefs! Nobody wants hot coolant in their face. I had the no-spill funnel hooked up and let it run until the thermostat opened (lots of bubbles in the funnel and upper rad hose got hot), the needle was at exactly the 3 o'clock position at that time and did not continue to rise or fall after that
 
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fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
Side question, does the coolant temp sensor with the 2 terminals and plastic connector control the light and the gauge using the same sensor?
 

CopperNick

G-Body Guru
Feb 20, 2018
613
93
Canada
Side answer, No. Referring back to an answer I posted on the previous page, basically that two terminal sensor is doing one specific job. That is to cause the gauge to show a reading as the coolant in the engine warms up. As the coolant absorbs heat it transfers it to the sensor. The sensor reacts by decreasing its internal resistance, allowing more current to flow and the needle on the gauge to rise or move.

As for an ECM or EEE or similar module, if present the place to look for it is behind the kick panel on the passenger's side of the car. The panel is that plastic or cardboard cover that closes up the A pillar where it comes down beside the hinges to meet the footwell.

Rather than deal with them when they failed, the easiest solution was often to try to decommission them by removing the power wire from the large stud on the starter solenoid and then trying to identify any wires or harnesses that led to them and either disconnecting them or deleting them altogether. The primary complication to all this is that both the carburetor and distributor took and fed input to that module and neither would work right afterwards unless they were modified to do so.

it is a possibility that the gauge is actually either looking for feedback from that old school ECM or trying to feed input to it and if so, its efforts are being thwarted by a dead or open circuit so it won't/doesn't work right itself.

Be very aware that all this is conjecture on my part. The problem could just as easily be a bad engine to body ground; from the factory there were several woven copper wires that were bolted to heads at the back and from there to the firewall. They created a direct ground circuit to the dash that didn't have to depend on going through the frame and taking the long way around. It might be a useful exercise to visit all the grounds to make sure they are clean at the point of connection. Old age and corrosion have a bad habit of consuming them to the point where they might look okay but be totally destroyed internally and only the insulation holding things together.


Nick
 

fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
If the sensor controls the gauge only, what controls the light? Another sensor in a different location?
 

pagrunt

Geezer
Sep 14, 2014
6,296
113
Elderton, Pa
If the sensor controls the gauge only, what controls the light? Another sensor in a different location?
If ther car has only the warning light it's switch would be in the same location as a gauge car's sensor. The warning light switch is set up to complete the electical circuit at a specific temp to turn on the light. The gauge sensor uses a bi-metal set up that reacts to the coolant heat to increase/decrease the ohms rating to move the needed.
 

fleming442

Captain Tenneal
Supporting Member
Dec 26, 2013
12,493
113
In my experience, the 2nd wire usually goes to the polarity the "gauge" side needs to see and isolates the sender from the engine. A 1 wire sensor grounds to the engine. YMMV
 

fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
The car has the gauges and the lights. The one wire seems to operate the gauge and the other wire turns the temp light on when I ground it. If there is only one sensor how does it control both the light and the gauge? Also, what do you mean by polarity on the gauge side? Both wires are at the sensor location. Car has for sure been worked on before so it is difficult to see what some things are supposed to be
 

CopperNick

G-Body Guru
Feb 20, 2018
613
93
Canada
At this point are you absolutely certain that the gauge and that light are wired into the same circuit? To me having a gauge and a light doing essentially the same thing is a belt and suspenders approach to the issue and is not what the factory would have done. Depending on how the car was originally ordered/built, idiot lights on the dash were the default base model approach because they were cheap to install. Gauges usually showed up as either an accessory cost-plus item for some models or as the default installation in what were considered to be high? performance models.

As an example, my first Monte Carlo, a 100.00 mile plus 78 model, was about a basic as it could get and all that showed up on the dash was a vertical stack of lights to the right of the steering wheel beside the cluster. There was no tach, that spot held the clock. The only light that I knew worked for sure was the oil pressure light because as soon as the engine fired it came on and stayed on, even in -30 degree weather.

My latest Monte, my 85 SS, came with a full set of gauges installed in the dash instrument pod. The few idiot lights that were included were for things like the high beam indicator on the head lights. Apparently there was a more basic version of this car that did come with just the essential items, speedometer and fuel gauge, and had lights for most of the rest of the functions, as I managed to harvest the entire pod and cluster from another candidate that had this version installed in its dash. (Didn't need the gauges, was looking to score the housing for a spare.)

All this is making me wonder if a previous owner got frisky and chose to score and install a year correct instrument pod for your car that had instrument gauges instead of lights? This is time consuming and takes a lot of patience but it can be done. The gauges themselves are individual items that can be removed from the base platform on which they sit so even using the non-gauge platform and subbing in the actual gauges is a relatively straightforward exercise; just bring lots of patience and stay sober until done.

What I am suggesting here is that there may be another sensor located somewhere else on the block or in one of the heads that is tied to that light and that it is acting independently of the gauge although they are both performing the same function, more or less. Without being hands on and getting dirty this is about as close as I can come to reality by way of possible solutions.


Nick
 

fly_25

Not-quite-so-new-guy
Thread starter
Oct 20, 2020
31
8
All of that makes sense, what has me confused is that the gauge doesn`t move at all when both wires are hooked up to the "correct" style of sensor, but the gauge does move when only one wire (the one that maxes out the gauge when grounded) is connected to the single blade style sensor, leaving the other wire (the one that turns the light on when grounded) disconnected. I am also getting 12v at each wire coming from the car. The coolant light has all the wiring hooked up and all, and it does operate when that wire is grounded. If one wire is for the gauge and the other wire is for the light (in the case of there being another sensor somewhere), why does the sensor have 2 terminals? I don`t see a third wire anywhere for the second gauge sensor terminal
 
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