Coolant Temperature Sensor not getting power - Ford Powerstroke Diesel Forum
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post #1 of 12 Old 05-30-2017, 08:03 AM Thread Starter
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Coolant Temperature Sensor not getting power

I have had my 2002 Ford F250 7.3L Automatic w/186k miles for about 2 weeks. One of the only issues I am trying to solve is the temp sensor is always on "cold".

When I turn on the engine, the gauge goes up slightly...but never even up to the bottom line. When I disconnected the batteries and then reconnected and turned the key over the first time, the gauge went all the way to the left and right along with the other gauges, so i assume that the gauge is not defective.

I read a lot about others with similar issues, and heard that aftermarket sensors from Autozone or elsewhere look and fit the same, but don't have the same resistance values. Therefore, I followed the advice I found on the message board and got the OEM Motorcraft sensor. It made no difference.

I tried shorting the connector with a resistor to see if the gauge would move, and it didn't. Finally (I should have done this FIRST, but didn't) I checked the voltage of the connector...nothing! So it appears that the sensor isn't getting any power. I also noticed the tan wire going to the sensor has some exposed wire, but it doesn't look like it is shorting out to anything.

I haven't been able to visually find the other end of the wires. I couldn't tell if it is part of a larger wiring harness (I assume) or if it is more of a stand-alone.

I have a Haynes manual but it wasn't much help. I am trying to find a Factory service manual or wiring diagram, but haven't found one yet.

Has anybody experienced the coolant temp sensor not getting power at the plug/connector? Is there a stand-alone grounding wire just for this sensor, as there isn't anything else that seems to be wrong (other than the OBD2 port getting power but not communicating with the PCM)? If it was just a ground, I assumed that I could use the voltmeter and ground to the chassis instead of the plug and I should see voltage, but I didn't.

Also, does the engine use this coolant temp sensor to control engine parameters or a separate sensor? I believe what I found on the message boards was that the automatic used the oil temp for the PCM, while the manual transmission used the coolant temp sensor.

Thanks for your help and any guidance/ideas would be greatly appreciated!
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post #2 of 12 Old 05-30-2017, 08:04 AM Thread Starter
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OBD2 Port Powered but can't connect to ANYTHING

I have had my 2002 F250 7.3 for about 3 weeks, and absolutely love it! I have worked on cars for years, but this is a whole new experience and the learning curve is steep. I have searched the message boards extensively and haven't found an answer to this yet (and have tried whatever I can from the posts I have found), so if there is a post that describes this and I overlooked it, my apologies!

My F250 has about 186,000 miles on it, almost no blow-by, and is in overall great shape. It was dealer maintained until 140K miles (from CarFax), so overall it is well taken care of. The only two things "wrong" with it that I haven't been able to fix are relatively minor (water temp sensor, and OBD2 port). I will post on the issue with the water temp sensor separately.

When I was checking out the truck, I brought my OBD2 scanner, which works on all of my other vehicles. It is an Action Autoscanner Plus (CP9580A). It powers on when connected, tries to communicate with it, but then eventually it says that it can't connect and to check the connection. I have a cheaper OBD2 scanner that I tried as well, and got the same result. I also have a ScanGauge II that I tried, and it doesn't have its own power (no batteries), so it powers on when connected and therefore I can confirm that the OBD port does have power. I also went to Autozone and tried their higher-end Actron scanner...had the same issue...powers on but can't communicate with it.

I also purchased and used an Edge Evolution 15000 programmer. It says it tries to communicate with the PCM, and then eventually times out and says to disconnect then reconnect. I know that this engine (and similar diesels) aren't technically OBD2 in the traditional sense, and some normal OBD2 protocols don't apply. If this was the problem, then I assume the Edge Evolution 15000 programmer (which is also a scanner) is specific to the 7.3L and it would work...but it didn't.

Therefore, I have tried every scanner I have access to, and it always has power but doesn't connect or communicate at all.

From everything I saw on the forums, the go-to solution was the fuse box. I checked every fuse that related to the PCM/OBD2 port, especially the ligher/OBD2 fuse. I also swapped the fuses around just to see if the fuses were bad but not visually evident that they were. It made no difference. I also used DeOxit D5 on the OBD2 port to see if it was corroded. As another step, I disconnected the batteries (to re-set the PCM if this was the issue) and cleaned the terminals. It made no difference at all.

I have no check engine codes or any other engine issues other than the coolant temp sensor not having power going to the connector and always reading cold.

Is there anything that I am missing? Has anybody experienced something similar? I would really like to have the OBD2 port work so I could use a programmer and use for diagnostics. Is it a messed up PCM potentially? If so, I assume I would have lots of error codes. Is it possible the previous owner programmed it and this is causing an issue with interfacing with the PCM?

Sorry for the long message and thanks for your help! I am loving my PowerStroke and looking forward to contributing to the message board as I get more experience with my 7.3L!
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post #3 of 12 Old 05-30-2017, 01:45 PM
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What happens when you put the sensor wire directly to ground?

Gunga galunga...gunga -- gunga galunga.

97 F-250 CC 4x4 7.3L PSD E4OD 203K all original
99 F-250 CC 4x4 7.3L PSD ZF6 253K all original
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post #4 of 12 Old 05-30-2017, 04:49 PM Thread Starter
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Originally Posted by greenskeeper View Post
What happens when you put the sensor wire directly to ground?
When I short the two wires, or put the power wire to ground, the gauge behaves the same....stays just slightly below the lowest temperature line but a little below the line when the truck is off. When I start the car it moves slightly up, but it does that even if the sensor is disconnected. Also, when I use the voltmeter and connect it to the power and ground wire, or to the power wire and directly to the chassis, it behaves the same.

I checked with the dealership about the wiring harness that is associated with the coolant temps sensor...it was about $800! I really don't want to go down that path unless absolutely necessary. As long as the PCM/ECU doesn't use this info from this sensor for engine management, I would try to install an aftermarket sensor. At the same time, I would love to have the OEM one work in the gauge. I got the harness, wiring diagram from the dealer and have been trying to interpret where the terminal/connector "9278" is. If I interpreted it correctly, "10884" is for the coolant temperature sensor.
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post #5 of 12 Old 05-31-2017, 01:28 PM
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What year truck? My E99 has a single wire for the coolant temp sensor.

The PCM uses oil temp, not coolant temp for engine management

Gunga galunga...gunga -- gunga galunga.

97 F-250 CC 4x4 7.3L PSD E4OD 203K all original
99 F-250 CC 4x4 7.3L PSD ZF6 253K all original
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post #6 of 12 Old 05-31-2017, 02:22 PM Thread Starter
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Quote:
Originally Posted by greenskeeper View Post
What year truck? My E99 has a single wire for the coolant temp sensor.

The PCM uses oil temp, not coolant temp for engine management
Mine is an early 2002 (build date 1/02). From what I had read, the manuals used the coolant temp sensor and the auto's used the oil temp, exactly like you said. This is a relief given the current situation!

My sensor definitely has two wires, and the sensor I replaced it with had matching connectors (two of them)

The previous owner thought it was just a sensor issue and was using the transmission temp gauge as an approximation of the coolant/engine temperature. How valid of an approach is that until I figure out the coolant temperature issue? Does the transmission temperature track with the coolant temperature to a certain degree? Fortunately, my Transmission temp gauge in the cluster (as well as the others, with the exception of the coolant temp) are working fine.
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post #7 of 12 Old 06-01-2017, 01:49 PM
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I don't believe that the transmission determines what the PCM references....I always thought it was oil temp for everything since any scanner will give you bogus readings for coolant temp.

My coolant temp sensor is at the top of the water pump adjacent to the hose going to the heater core - single wire. If I ground that wire out the gauge will peg. This is a simple resistance scaled sensor.

The transmission temp "should" be much cooler than actual coolant temp, on my 97 the transmission temp generally is ambient + 100 degrees, while coolant temp is around 220F or so.

Gunga galunga...gunga -- gunga galunga.

97 F-250 CC 4x4 7.3L PSD E4OD 203K all original
99 F-250 CC 4x4 7.3L PSD ZF6 253K all original
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post #8 of 12 Old 06-04-2017, 11:55 AM Thread Starter
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Quote:
Originally Posted by greenskeeper View Post
I don't believe that the transmission determines what the PCM references....I always thought it was oil temp for everything since any scanner will give you bogus readings for coolant temp.

My coolant temp sensor is at the top of the water pump adjacent to the hose going to the heater core - single wire. If I ground that wire out the gauge will peg. This is a simple resistance scaled sensor.

The transmission temp "should" be much cooler than actual coolant temp, on my 97 the transmission temp generally is ambient + 100 degrees, while coolant temp is around 220F or so.
Thanks! That is very helpful. I suppose that if the coolant is getting too hot the trans temp would go up, but probably when it started to affect trans temp that much you would be SOL.

I can access the oil temp from the PCM I suppose, which is probably a better secondary measure than the trans temp, given they are both essentially encasing/flowing through the block. I REALLY want to figure this out! It makes me nervous.

As a backup/"giving up" plan, I assume I could just install an aftermarket sensor into the location of the current sensor and wire a gauge into the cab? I found a few on Amazon but it wasn't clear if they had the same thread pattern.

I have tried grounding and testing the sensor every way I can think of, and it seems to be dead-in-the-water. No voltage to it, no reaction when grounding, nothing.

With regards to the manual/automatic difference I mentioned, I really didn't know, but I read the following info on a thread elsewhere that I found on the web:
-------------------------------------------
This is listed for 03

Engine Coolant Temperature Sensor (ECT) - With Manual Transmission Only

The Engine Coolant Sensor is used as the primary input to the Electronic Control System to enable adaptive cooling. This provides a means of providing adequate cooling in severe engine temperature conditions. When ECT is greater than 107C (225F) , the fueling rate of the engine is modified to provide cooling protection and prevent engine damage due to overheating.

Engine Coolant Temperature Sensor - With Manual Transmission Only (front of engine)

The Engine Coolant Temperature (ECT) sensor is a thermistor device in which resistance changes with temperature. The electrical resistance of a thermistor decreases as the temperature increases, and increases as the temperature decreases. The varying resistance affects the voltage drop across the sensor terminals and provides electrical signals to the PCM corresponding to temperature.

If the PCM receives a high engine temperature signal from the ECT, it will adjust fueling rates to protect the engine from damage due to overheating.


Engine Oil Temperature Sensor

The Engine Oil Temperature sensor is a thermistor mounted to the oil reservoir whose resistance decreases as engine oil temperature increases. The Engine Oil Temperature signal is used by the PCM to calculate fuel quantity, injection timing, glow plug operation and exhaust back-pressure.

At low ambient air temperatures, and oil temperature below 50C (122F) , low idle is increased to a maximum of 1300 rpm to increase engine warm-up. Fuel quantity and timing is controlled throughout the total operating range to provide adequate torque and power.

An Engine Oil Temperature signal detected out of range, high or low, by the PCM will cause the PCM to assume an engine oil temperature of 20C (68F) for starting purposes and 100C (212F) for operating purposes. The Malfunction Indicator Lamp in the instrument cluster will be illuminated as long as the fault condition exists.
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post #9 of 12 Old 06-04-2017, 04:43 PM
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I wouldn't trust the OEM gauge sensor even if it was working.

Get a quality electric gauge and you can monitor down to the exact degree.

Most likely you'll need some pipe fitting adapters to make the sending unit fit.

Where is your oem sensor located?

Gunga galunga...gunga -- gunga galunga.

97 F-250 CC 4x4 7.3L PSD E4OD 203K all original
99 F-250 CC 4x4 7.3L PSD ZF6 253K all original
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post #10 of 12 Old 06-04-2017, 06:14 PM Thread Starter
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Quote:
Originally Posted by greenskeeper View Post
I wouldn't trust the OEM gauge sensor even if it was working.

Get a quality electric gauge and you can monitor down to the exact degree.

Most likely you'll need some pipe fitting adapters to make the sending unit fit.

Where is your oem sensor located?
That makes sense! Are all of the OEM sensors less than reliable on this truck or just the coolant temp sensor on this truck? Are there any specific units that have been used? I will also do some searching on the message board. Has this swap to an aftermarket sensor been done before, or does this require starting from scratch?

The coolant temp sensor is just to the left of the thermostat and radiator hose (see attached pictures). The blue hose is the coolant bypass filter hose for the system I installed.

Thanks!
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