92 GT Coolant sensors - Ford Mustang Forum
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post #1 of 15 (permalink) Old 12-22-2019 Thread Starter
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92 GT Coolant sensors - Solved

Hi all. great forum here! I appreciate all the great knowledge.

Picked up a 92 GT Convertible from a reliable buddy of mine. He replaced the heater core before I picked it up. It has a Low Coolant Level light on (not sure if it was on prior to heater core change), and the coolant temp gauge doesn't work. Runs terrific tho.

On the coolant temp gauge, I replaced the sensor with a new one before and gauge still doesn't work. Will check wires and gauge itself soon.

The Coolant level sensor... The light is on when plugged in, but goes out when unplugged which I read is normal. I verified the 3 wires... red/yellow, green and black. red/yellow and green both have 12v, and the black tests good for ground. I jumped the two metal tabs at the bottom of the sensor (read on a forum that some people do this) and light is still on. I put in a new coolant level sensor and still get a light on the dash. fluid level is full.

Is there a way to test the coolant level sensor? Could a bad temp gauge affect the low coolant system somehow?


Last edited by kasteer; 01-15-2020 at 10:12 AM.
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post #2 of 15 (permalink) Old 12-22-2019
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Did you replace the temp sender or sensor?
Sender for the gauge- https://www.cjponyparts.com/temperat...SABEgJspfD_BwE

Sensor- for the computer
https://lmr.com/item/LRS-12648A/83-9...erature-Sensor

The low coolant is just the deal in the overflow bottle. It wouldn’t affect the gauge reading I don’t think.

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post #3 of 15 (permalink) Old 12-26-2019 Thread Starter
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Yeah. I changed out the temp sensor (2 wires) at first. Just replaced the temp switch (1 sire) and temp gauge is working again!

The low coolant light is still on (with a new sensor and with plenty of coolant in the overflow and with 12v being supplied to it and with a good ground)... could the new coolant level sensor be defective? Not sure what else it could be.
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post #4 of 15 (permalink) Old 12-28-2019 Thread Starter
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Contacted LMR and they're sending a replacement Low Coolant Sensor...
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post #5 of 15 (permalink) Old 12-28-2019
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Quote:
Originally Posted by kasteer View Post
Contacted LMR and they're sending a replacement Low Coolant Sensor...
Latemodel, CJ Pony Parts, American Muscle, etc all have fairly decent customer service. But, if you ever need any sensors such o2 sensors, or an idle air control valve/motor, or anything else like that the parts stores are always cheaper.
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post #6 of 15 (permalink) Old 01-01-2020
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I believe the submersion of the metal prongs @ the bottom of the low coolant sensor is what closes the contacts on the sensor & keeps the light off,but once the coolant level drops below those prongs,this turns the light back on again.I believe disconnecting the harness will shut the light off.If so,& if the 2nd sensor from LMR doesn't shut the light off,I would just disconnect the harness and be done with it,unless you just cant live without the light functioning as designed.If you wanna try a few tests for it,a test procedure & wiring diagram is listed in the images below.

** EDITED INFO **
Test diagrams listed in post 6 may only be valid for 87-89 models that use the sensor with a float.The float is shown in post 15.I edited this info in after the fact.So be aware of this when testing your sensor.90-95 models did not come with a float.
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1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's

Last edited by wbrockstar; 01-15-2020 at 11:04 PM.
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post #7 of 15 (permalink) Old 01-01-2020 Thread Starter
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Thanks guys!

Yeah, I figured the coolant completes the circuit between the two prongs at the bottom of the sensor. I tried connecting those two prongs with a wire to simulate the coolant, but light was still on.

Thanks for the diagram and test procedures. Will print those and see what I get.

Looking forward to seeing how the new one does. Like I said, I'd like for it to work since I have it... but if I can't figure it out I'll just unplug it.

I'll follow up on here when I get the new one.

TY!
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post #8 of 15 (permalink) Old 01-03-2020
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I've been playing around with that circuit myself, because I'm installing a new instrument panel. I believe there is a little electronic module potted into the bottom of the reservoir cap, (I can see a small well filled with potting compound) and that's why it needs the 3 wires. I'm working with a '91 5L but the circuit looks the same as the one wbrockstar posted above. The red/yel is +12, the black is ground, and the bulb in the dash has one side connected to 12V in the instrument panel. The bottom of the indicator bulb runs forward to the sensor as a blue wire. So grounding the blue wire should turn on the light. That explains why it goes out when you unplug the sensor. Sounds like your sensor is shorted blue to black, and your instrument panel and wiring are good. I'm assuming that there's a little amplifier built into the cap, and it senses a resistance across the two electrodes at the bottom of the tube and shorts blue to black (ground) when it senses a little leakage current. Antifreeze isn't conductive enough to put enough current to light a bulb, so they probably put a little amplifier in the cap. The red wire probably just powers the amplifier. So if the amplifier in the cap is shot, shorting the two contacts at the end may not do anything. Hopefully a new sensor does the trick. I would try it right out of the box, don't try shorting the sense points with a wire. Good Luck!
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post #9 of 15 (permalink) Old 01-05-2020
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Quote:
Originally Posted by 67 Car Guy View Post
I've been playing around with that circuit myself, because I'm installing a new instrument panel. I believe there is a little electronic module potted into the bottom of the reservoir cap, (I can see a small well filled with potting compound) and that's why it needs the 3 wires. I'm working with a '91 5L but the circuit looks the same as the one wbrockstar posted above. The red/yel is +12, the black is ground, and the bulb in the dash has one side connected to 12V in the instrument panel. The bottom of the indicator bulb runs forward to the sensor as a blue wire. So grounding the blue wire should turn on the light. That explains why it goes out when you unplug the sensor. Sounds like your sensor is shorted blue to black, and your instrument panel and wiring are good. I'm assuming that there's a little amplifier built into the cap, and it senses a resistance across the two electrodes at the bottom of the tube and shorts blue to black (ground) when it senses a little leakage current. Antifreeze isn't conductive enough to put enough current to light a bulb, so they probably put a little amplifier in the cap. The red wire probably just powers the amplifier. So if the amplifier in the cap is shot, shorting the two contacts at the end may not do anything. Hopefully a new sensor does the trick. I would try it right out of the box, don't try shorting the sense points with a wire. Good Luck!


Good theory/explanation especially about why the light comes on when submerged vs when shorted by a jumper wire.

1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's
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post #10 of 15 (permalink) Old 01-08-2020 Thread Starter
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Quote:
Originally Posted by wbrockstar View Post
I believe the submersion of the metal prongs @ the bottom of the low coolant sensor is what closes the contacts on the sensor & keeps the light off,but once the coolant level drops below those prongs,this turns the light back on again.I believe disconnecting the harness will shut the light off.If so,& if the 2nd sensor from LMR doesn't shut the light off,I would just disconnect the harness and be done with it,unless you just cant live without the light functioning as designed.If you wanna try a few tests for it,a test procedure & wiring diagram is listed in the images below.
What book is the diagram and testing procedures from? I ordered a Haynes book and there's nothing on instrument lights or Low Coolant light/sensor in this book....

P.S. expecting the new Low Coolant sensor tomorrow...

1992 Mustang GT Convertible
Vortech T-Trim Supercharger
Trick Flow Twisted Wedge Aluminum Heads
X303 Camshaft
Pro M 70mm Mass Air Meter, Throttle Body
GT40 Upper & Lower Intake
Paxton Fuel System, 42lb Injectors
Moroso Deep Sump Oil Pan
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Quote:
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What book is the diagram and testing procedures from? I ordered a Haynes book and there's nothing on instrument lights or Low Coolant light/sensor in this book....

P.S. expecting the new Low Coolant sensor tomorrow...

A member posted those diagrams in a thread on another site & thats where they originated from,but the member didnt list the source of the diagrams.I know they're not from a Chiltons manual because I just checked mine & you clarified they're not from a Haynes manual,so they're most likely from the factory Ford service manual (EVTM).Its available on Helminc,Ebay,etc,but its $195 on Helmsinc.You can get a manual in pdf format from Alldata through Autozone.Its not free though.Autozone use to have some free great online repair guides for the Foxbody & other Ford models,but when Ford got wind of it,they made AZ stop offering them for every Ford model built.Other manufacturer guides are available @ Autozone.com though .(Chevy,Dodge,etc) Heres a link to the Helminc site.


Owner Manuals, Service Manuals, Wiring Diagrams, Service Bulletins - Helm Incorporated

1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's
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The Ford manuals are also available in digital form through Bishko. That's where I got the Service Manual and EVTM for my '95, which shows an almost identical diagram for that circuit. Almost.

The fuse number is different on my '95, as are the lamp pin number and ground point numbers- minor differences which I would expect to change between generations. But the troubleshooting procedures are written up a little differently, and the chart in post #6 mentions a "float" that doesn't sound consistent with the 2-prong sensor you have. Your 2-prong sensor sounds much more like like the one on my '95, which looks identical to LMR's "1990-95" compatible sensor, neither of which use a float.

I'd feel better if we could make sure that we're looking at the correct info here. The photos in post #6 certainly appear to be from an official Ford manual, but it would be nice to confirm the year and model.

Who knows, maybe the whole "float" thing is just a typo. But there's another thing that sticks out to me here- a note in my 1995 diagram pointing to the low engine coolant switch that says:
Quote:
When closed, supplies ground to turn on the coolant indicator lamp for several seconds, when the ignition switch is turned to start or run continuously when the coolant level drops 2 inches below the full cold mark on the side of the recovery bottle.
This function detail is absent from the manual pics shown in post #6. Was the brief initial illumination (bulb test) feature an updated design for SN95 cars? Could it be that the Foxbodies did in fact use a float-type sensor which could be tested via simple continuity check, and that the slightly more complex two-prong piece with its little potted module is a backwards-compatible replacement?

Anyway, getting back on track here, it sounds like your sensor was probably shorted. Looking forward to hearing if the new one fixes the problem...

1995 Mustang GT (5.0), daily driver
1995 GMC C2500 (5.7), alternate daily driver
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post #13 of 15 (permalink) Old 01-15-2020 Thread Starter
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Sorry for the delay in responding! Had dental work done, got an infection... long, painful story...

Got the new sensor and installed it... and guess what... it worked! So the initial sensor I received from LMR was bad out of the box!

@Urambo Tauro ... My sensor (1992) does not use a float. I noticed that too in the photo and just assumed it was a generational thing as everything else seemed correct.
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1992 Mustang GT Convertible
Vortech T-Trim Supercharger
Trick Flow Twisted Wedge Aluminum Heads
X303 Camshaft
Pro M 70mm Mass Air Meter, Throttle Body
GT40 Upper & Lower Intake
Paxton Fuel System, 42lb Injectors
Moroso Deep Sump Oil Pan
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post #14 of 15 (permalink) Old 01-15-2020 Thread Starter
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Sorry for the delay in responding! Had dental work done, got an infection... long, painful story...

Got the new sensor and installed it... and guess what... it worked! So the initial sensor I received from LMR was bad out of the box!

@Urambo Tauro ... My sensor (1992) does not use a float. I noticed that too in the photo and just assumed it was a generational thing as everything else seemed correct.

1992 Mustang GT Convertible
Vortech T-Trim Supercharger
Trick Flow Twisted Wedge Aluminum Heads
X303 Camshaft
Pro M 70mm Mass Air Meter, Throttle Body
GT40 Upper & Lower Intake
Paxton Fuel System, 42lb Injectors
Moroso Deep Sump Oil Pan
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**UPDATE**


Wanted to update some info for future reference.The diagrams for the test procedure in post number 6 may only be relevant for 87-89 models as these were the only two model years to come with the sensor w/ float assembly,mentioned in the test info in diagram #1 of post 6.The picture below shows the float mentioned in the tests.This sensor w/ float,& the low fluid module display at the bottom right of the dash cluster,only came on 87-89 GT's,but not the LX's.The 90-95 models came with the sensor that has the long skinny tube and two prongs at the bottom.So this should help clear things up.
Im editing a note into post 6 to not cause confusion.

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1991 Mustang lx
D.S.S. 342ci {10.5:1}
TFS 190cc + track heat
Lunati 61011 .549".565"
Pro-M 76mm maf
Jetronic 30lb inj
QA1 Coilovers
17" Weld ProStar XP's
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