Ford's CHT (cylinder head temp) sensor

[D60]

Does anyone know exactly how the PCM uses this? Specifically, I noticed on an '06 Expedition that after start-up the V(oltage) will read about 0.5 for anywhere from a minute to a couple minutes, even when you're up to temp. The F(arenheit) reading will say, for instance, 198* but show 0.5V, which wouldn't correlate.

This is a 5VREF so 1/2 volt is not ~198F.

But at some undetermined time the Voltage will jump to something more sensical, usually around 3.6V for the 198-201F range

I suspect the PCM is not supposed to actually look at the sensor immediately after start-up, potentially because of heat soak and I have to believe there's some latency in a "dry" sensor screwed into the head vs a sensor immersed in water flow.

If not, then maybe I should be chasing an electrical issue, but the PCM (and gauge) seem unconcerned during the brief periods of 0.5V readings after start-up.

Thoughts?

Here's the PIDs I was monitoring the whole time, although I didn't take the shot during a 0.5V timeframe:

 

[mattd]

What’s the problem you’re trying to solve?

 

[Trav]

Does anyone know exactly how the PCM uses this? Specifically, I noticed on an '06 Expedition that after start-up the V(oltage) will read about 0.5 for anywhere from a minute to a couple minutes, even when you're up to temp. The F(arenheit) reading will say, for instance, 198* but show 0.5V, which wouldn't correlate.

This is a 5VREF so 1/2 volt is not ~198F.

But at some undetermined time the Voltage will jump to something more sensical, usually around 3.6V for the 198-201F range

I suspect the PCM is not supposed to actually look at the sensor immediately after start-up, potentially because of heat soak and I have to believe there's some latency in a "dry" sensor screwed into the head vs a sensor immersed in water flow.

If not, then maybe I should be chasing an electrical issue, but the PCM (and gauge) seem unconcerned during the brief periods of 0.5V readings after start-up.

Thoughts?

Here's the PIDs I was monitoring the whole time, although I didn't take the shot during a 0.5V timeframe:View attachment 298790

I don't think the sensor in the head is a dry sensor, it is still in coolant. The other sensor is probably in a hose connector or the block, I really don't have any idea its been a long time since I worked on anything domestic but IIRC didn't Ford have a phony temp dash gauge that always showed close to the middle that used a different sensor? Does this have something to do with that?

 

[mattd]

I don't think the sensor in the head is a dry sensor, it is still in coolant. The other sensor is probably in a hose connector or the block, I really don't have any idea its been a long time since I worked on anything domestic but IIRC didn't Ford have a phony temp dash gauge that always showed close to the middle that used a different sensor? Does this have something to do with that?

The engine should have a dry sensor. Most CHT on fords are dry sensors .

 

[Nukeman7]

^^^^ Not sure about the V8 in the '06 Expedition, but Cylinder Head Temperature sensor in my '03 Ranger with the Duratec 2.3 liter was essentially a dry sensor.

 

[D60]

It's dry, I've replaced it before.

Ford did some annoying stuff on the modulars like moving the sensor around a bit and I think some of the earlier years (ie bubble bodies) had two: one for PCM, one for the gauge. But AFAIK '06 just used the CHT here for both engine mgmt and the gauge.

I think I'll fire up my wife's '08 F150 and see if the 0.5V behavior occurs. Curious to know.

 

[bdcardinal]

Back in the day, the 10884 base number sensors were for the gauge since the cluster base number is 10849. The 12A648 was for the PCM since a PCM is 12A650, at least that was how I remembered it when people called. Now I just have to remember to sell intake manifold gaskets with a CHT on V engines.

 

[D60]

Back in the day, the 10884 base number sensors were for the gauge since the cluster base number is 10849. The 12A648 was for the PCM since a PCM is 12A650, at least that was how I remembered it when people called. Now I just have to remember to sell intake manifold gaskets with a CHT on V engines

Back when I replaced this one I cheated it under the intake. Not too bad really but you do have to have a flex head ratcheting wrench in 3/4" or 19

 

[D60]

So yeah, the wife's '08 F150 displays the same behavior. At some point V goes from 0.5 - 0.59 to something in the 3.00's.
This seems to happen around thermostat temp but it's not a hard over/under, ie above or below 195* (I think factory may technically be 192*) is not a guarantee of being in the 0.5x or 3.xx range

Anyway I'm sure the behavior is normal but still curious exactly what the criteria/programming are/is