Temp Gauge Again


New Member
So I changed the sensor on the 3.9 the one in the head I changed the thermostat and I took the whole gauge cluster to a shop and had it checked out. All the gauges pegged so its not the gauge. When I put the cluster back in I had the battery unhooked an when I hooked it back up an turned the key to on they pegged so that's all working but the gauge still wont move when the car is running. I got heat in the car an the cooling fan is kicking in. I put a 190 thermostat in but when the car is running an the stat is open its only running around 130 140 at the stat housing. What I was wondering if I need to burp it again would an air lock cause the gauge not to work. The cylinder head sensor is supposed to run the gauge I just don't know im running out of things to try . Any body got any ideas Thanks Dirk


Profile Violation
There should be an engine coolant temp sensor as well. The one in the
head is for safety reasons. You get a separate code for for the cylinder
head over temperature; P1285 Cylinder Head Over Temperature Condition.

If you were over-heating, and that sensor is good, that is code you will
get. The main coolant sensor should produce; P0218 is an under temperature
code. P0217 is over heat condition. IF you are not getting any codes, then
you should be running fine. That's not a science, but generally speaking.

Also, you can't measure the temperature of the coolant through the metal t-stat
housing. The metal itself is dissipating heat, and will be lower than what's inside

According to CJ Pony Parts: Coolant Sensor gets mounted above the fuel rail,
and gets threaded into the coolant crossover tube.

My son kept getting the P1285 when his thermostat stuck closed. The radiator did not even
get hot. Thanks to that head sensor, it tripped the code. Even the hose from the T-stat
housing wasn't getting hot, at all, simply because it was not circulating. It was fortunate
that there was that head temperature sensor.


New Member
Ok 08 mustang im not getting any codes and the Ford Dealer told me there is know coolant sensor in the 3.9 2004 mustang. So is there a spot on the cross over pipe to put one or do u have to drill an tap. Fors told me the cy head sensor did it all but it don't even screw into the coolant. it goes in the back of the drivers head. So if theres a coolant sensor where is it located Thanks Dirk


Active Member
From the original Ford manual:

Instrument cluster has two plugs, C250 and C251
Pin 18 in C251 is the "Coolant indicator"
Looking at the schematic, pin 18 goes from ground, through a "Coolant switch #C137", then to something "solid state" (computer) in the instrument cluster.

Going the other way from how the ECU reads temp, the "Engine coolant temp sensor #C178" goes to pin 38 on the ECU. So it is clearly a different sensor. This is for a 2000 mustang though.

The schematic references page 151-2 but that doesn't seem to line up, unless I'm looking at it wrong.

But from rockauto, this is the coolant sensor:
While this is the cylinder temp sensor:
Perhaps the cylinder temp sensor is what actually goes to the gauge. I can't find any mention of it in the book. But judging by the schematics, both "sensors" should take two wires. The ground for the "cylinder temp" looks to be the same ground used by the instrument cluster pin 20 which goes to "G203"

But either way, you have the pin numbers now so you know exactly which wire to look at/test.



Active Member
I've read you're other posts. Looks like the 2004 model might be different, the part numbers for the cylinder head sensor are different at least. But as far as I know, the ECU and instrument clusters are all compatible between 1999-2004 so the pin numbers should all line up. Get a multi meter and measure the ohms from ground, to pin 18 on the cluster, and from ground to pin 38 on the ECU. If they both use the same sensor, it will read the same.

But the other thing is, after nearly 20 years, these clusters just start to crap out. Usually the digital odometer stops displaying. Never really heard of gauges not working though.

Either way, if your cluster is bad, make sure you take it apart and get us some photos of why some people's fuel gauge mysteriously glows blue: www.stangnet.com/mustang-forums/threads/crazy-blue-light-flashing-in-my-gauges :D
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New Member
Ok so everybody keeps showing and telling me about a coolant sensor. But the Ford Dealer told me there is know coolant sensor in the 3.9. There is in the 3.8 up by the T stat housing. The 3.9 came out in 2004 and nobody knows why. Can some one send a pic of where to drill an tap the cross over and I will put a mechinca gauge in under my dash. It is not a ground problem because the gauge was tested and worked it even trip over when I plugged it back in. If everybody says theres a coolant sensor why cant someone say where its located Thanks Dirk


Profile Violation
The 3.9 is the same engine as the 3.9, with a slightly longer stroke, 88mm -vs 86mm.

I have looked, and all I can see is the sensor at the back of the driver side head, yes.
ANSWER.FORD site says all you got is the sensor in the head, yep, that's dumb...

Also, you keep saying "there is know coolant sensor", it's "no" as it none, not "know"
as in knowledge of.

You can drill and tap a sensor anywhere in the cooling system it will fit, after the
thermostat, as that is the hot water from the motor; and run a gauge pod...
That will work. Once the engine is HOT, as long as the radiator has heat, it
should be working. You will get a P1285 from the cylinder head should it actually
be overheating. Hell, see if one will thread into that bleeder bolt location, so you
don't have to drill and tap anything. You can loosen the sensor to bleed the air.

You said you got 140 degrees at the thermostat, so you are heating up.

Also, the gauge sweep/test at the cluster only tells you the gauges wors, that does
not mean it's getting a reading.


New Member
I don't no if u can get a pod small enough the fit in the bleeder tube might have to come up with some different fittings Thanks Dirk


Profile Violation
SENSOR in the threads, not a gauge pod, the gauge pod goes inside the car...

You do not need a FORD OEM one, there are all kinds of sized thread
coolant sensors.




TONS of options, just have to find the right one for YOU.