[Jtab]

Application is in my signature.

Looking through a recent datalog, I have quite a few instances of my ECT reading 254°. I see 254° listed as the top end of temp-related tables; does 254° indicate a direct short? I imagine I would have more apparent engine problems if ECT was actually that hot. Did I repin the ECU connector incorrectly? What should I be looking at?

[jsa]

Measure the temperature at the sensor and compare it to the ecu's value.

Additionally it could be the wrong sensor type for your EEC hardware.

[Jtab]

Is an IR gun sufficient to measure temp near the sensor?

Rockauto lists the same part numbers for ECT sensors for a 1993 F250 and a 1995 5.0 Mustang. Do you have reason to believe the two have different resistance ranges? Do you know what type/range thermistor they use so I can compare resistance to temperature reading?

[jsa]

A good IR gun can work well for the task.

I believe there are a couple different sensor types, so just check the right one is fitted.

No I do not know the temperature - resistance curve for your application.

[cgrey8]

I've never looked it up, but if I had to guess the temp sensors are Thermistors. However there are different "type" thermistors that while similar, have slightly different transfer curves similar to how similarly sized MAFs have similar curve, but not identical curves. The most popular thermistors I am aware of are the Precon 10k@77F Type2 and Type3 sensors. Here's a link to Precon's curves:
Precon Thermistor Curves

I have no clue if Ford used 10k, 5k, or some other thermistor. Although it wouldn't be terribly difficult to figure it out. Put the sensor on an ice-cube (32F), measure the resistance. Measure again at room temp (preferably somewhere very near 77F), and again at some really high temp that you measured with your infrared temp sensor. That should be enough datapoints to get you a rough idea of about what the sensor is. Just keep in mind that thermistors change value very fast, so you will need to measure the sensor while it is in contact with whatever is the temp you have it measuring. You can literally watch the resistance change in real time just by holding the sensor with your fingers.

For engine management, close enough is good enough. The EEC/engine will care about differences like 50 degrees, 150 degrees, and 200 degrees. I highly doubt a stock engine on a stockish tune is going to be affected by a sensor that is off by a +/-5 degrees. There'll likely be way more things that affect engine behavior than lack of hyper-accuracy in the ECT sensor.

[tvrfan]

Back to original question ....

Yes, I would guess that 254 degrees DOES indicate zero volts across the ECT sensor, which probably does mean there is a short somewhere, either in the wiring or in the sensor itself. If you have a multimeter, check its resistance (tag to engine block) when engine is cold. As above, resistance may be one of several values, depending upon type, but I expect it will be greater than 60 ohms. If it reads, very low, like 0.5 ohms, it's broke.

If that looks OK, check resistance of the wire connector to earth, which should be several K ohms as it feeds into EEC unit. If that is 0.5 ohms (more or less) then your short is in the wiring somewhere. EEC boxes are pretty robust, it's possible, but not likely, that the EEC itself has a short inside.

Extra info - Thermistors work 'backwards', high resistance is cold, low resistance is hot. So an open circuit (= 5 volts inside EEC) would read very cold (something like -40 degrees) and a short (= 0v in eec) is very hot.

[Jtab]

Thanks for all responses. I work with temperature controls and PLC's daily, so I know about thermistors (also why I know Alt+0176 makes the degree symbol. The sensors and controllers we use read 327° if shorted, -327° if open, and 1000° if the controller's input capacitors fail.

The main reason for my question is whether I'm looking at a failed sensor, failed input, or something else. What is the most likely cause of false high readings?

Anyone know where to source a new Motorcraft temp sensor for my application? If not, what's a reliable aftermarket brand?

Thanks again.
- Joe

[cgrey8]

There are certain components that you often want to pay a little more for (i.e. go to Napa or the dealer for). But other components are fine for AutoZone/Advance. I would say a temp sensor is definitely a get it from wherever you can.

I can't say the same for wax-operated thermostats. I've had thermostats fail closed before. When that happens, the only way to get cooling on the engine is to turn on the heater and hope the heater core is big enough to keep the engine from overheating while you hang your head out the window during the summer! To avoid that unpleasantness, I tried 2 of the more-expensive fail-open sensors (from AutoZone). What a gimmick. Both locked themselves wide open under normal operating conditions. Some of that might've been they were 160s and normal operation for my engine does get the engine temp way above that at idle in the summer. But regardless, that got annoying to keep removing them and unlocking them, so I just opted for a regular sensor, but this time from Napa. It hasn't failed yet...nearly 10 years of daily driving (less the significant lack of driving during 2020). In all fairness, I've had other standard thermostats sourced from AutoZone that also haven't failed either. So this may be more of a rant than useful info.

But belts are definitely one of those products I pay extra for the Napa version of. Napa's belts are made by Gates, a highly respected belt mfg in the industrial world. I have no clue who makes the AutoZone belts. But I recall YEARS ago that you could hold a Napa and AutoZone V-belt side-by-side and the quality difference was unmistakable. I will admit there may be less difference in serpentines...I don't know. But if I ever own another car with a timing belt, I won't chance it and will always go to Napa for them.

[Jtab]

cgrey8 - Nothing wrong with rants! On a different note, your username always makes me think of Chad Gray from Mudvayne.

The ECT sensor in my vehicle is the second new one I've tried. The first failed open and made it run so rich it would stall at idle. Ironically, I replaced the sensor as preventive maintenance, and it only created problems! That is why I'm suspicious of the oreilly's sensor that's in there now, and I'd like to find a new motorcraft sensor. I have a Motorcraft sensor from my 1990 parts engine that I will install next time I feel like playing with coolant.

[AuroraGirl]

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[Jtab]

sensorvalues.jpg

Thanks for the info. Does that ECT resistance chart apply to all vehicles from '84-'96?

[AuroraGirl]

it should

[AuroraGirl]

here is 1993 truck and 1995 5.0 mustang

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