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Unread 11-21-2008   #1 (permalink)
spatula is offline Rookie


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Question 1998 Mustang GT 4.6 engine coolant temperature sensor questions

A few days ago I started having problems getting the engine to stay started and idle when cold. Checked out the ECT sensor and found it was reading about 250 ohms when cold (when it should have been around 30-40k Ohm if I read correctly). Also found that rather than 4 volts on the yellow-green wire connector, I was getting about 9. Not sure if that's a major problem or expected. Mind you, that was with the key on but the engine not running.

Replaced the sensor and disconnected the battery for a few seconds to cycle the computer. The new sensor reads 37.5kOhm when the engine is at outdoor temperature (50-60 degrees). I haven't yet verified that the resistance goes down after the engine has been running for a while, as I just noticed the temperature gauge reading cold on my way down to work today. (I do plan to check that once I get home with the engine still warm.)

Post-replacement, I notice that the temperature gauge on the dash is always reading totally cold (it actually goes backwards after I turn the key on). So a few questions:

1. Is there a different sensor for the gauge on the dash, or does it read off the same sensor? What I changed was immediately to the left of the alternator standing straight up.
2. Could the bad sensor I had have been masking a broken thermostat? (I do get warm air off the car heater, but I wouldn't call it "hot")
3. Is there any way I could have changed the wrong thing (ie, if the answer to 1 is 'yes' and the two sensors look otherwise alike)?
4. Is there anything one has to do to "recalibrate" the computer after changing a sensor or anything like that?
5. Anything else I should be checking?
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Unread 11-22-2008   #2 (permalink)
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Quote:
Originally Posted by spatula View Post
A few days ago I started having problems getting the engine to stay started and idle when cold. Checked out the ECT sensor and found it was reading about 250 ohms when cold (when it should have been around 30-40k Ohm if I read correctly). Also found that rather than 4 volts on the yellow-green wire connector, I was getting about 9. Not sure if that's a major problem or expected. Mind you, that was with the key on but the engine not running.

Replaced the sensor and disconnected the battery for a few seconds to cycle the computer. The new sensor reads 37.5kOhm when the engine is at outdoor temperature (50-60 degrees). I haven't yet verified that the resistance goes down after the engine has been running for a while, as I just noticed the temperature gauge reading cold on my way down to work today. (I do plan to check that once I get home with the engine still warm.)

Post-replacement, I notice that the temperature gauge on the dash is always reading totally cold (it actually goes backwards after I turn the key on). So a few questions:

1. Is there a different sensor for the gauge on the dash, or does it read off the same sensor? What I changed was immediately to the left of the alternator standing straight up.
2. Could the bad sensor I had have been masking a broken thermostat? (I do get warm air off the car heater, but I wouldn't call it "hot")
3. Is there any way I could have changed the wrong thing (ie, if the answer to 1 is 'yes' and the two sensors look otherwise alike)?
4. Is there anything one has to do to "recalibrate" the computer after changing a sensor or anything like that?
5. Anything else I should be checking?
If i remember correctly on my 97. the one temp sensor to the left, looking at the car, was the temp sensor for the computer and the one on the right was for the temp gauge. Should be the same for your 98 if it has two sensors. a haynes or chilton manual will tell you. or call ford. After your car has been started and idling for about 10 mins, feel the coolant hose, carefully, and see if it's hot. if it's still cold, the thermostat is most likely stuck and you will need a new one. For changing sensors like that, i've never heard of anyone resetting the computer, just for that. but with somethings, most of the time, you want to disconnect the battery. also you need to let it sit for over 10 mins for it to truly reset. it was either 10 or 12 mins, can't remember.
Usually if your thermostat gets stuck though, your temps will rise. Check the readings again with a mulitmeter and make sure they are correct. Also the temp gauges, they aren't that accurate. i like aftermarket gauges from autometer. Good luck man
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Unread 11-22-2008   #3 (permalink)
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Unhappy hoboy!

Thanks for the follow-up. To my dismay, when I popped the hood to check on it, it looks like the housing the sensor screws into developed a long crack, possibly when I screwed in the new sensor. I didn't notice it at the time, maybe because the car was cold. It's pretty obvious now, what with the coolant spewing out. Lucky thing I got home!

So now I have to decide if it's worth the time, effort, and money to fix that problem, or if this is a good weekend for car shopping...

But again, many thanks just the same for the info!
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Unread 12-16-2008   #4 (permalink)
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The sensor with the wire comming from the passenger side is for the gauge. Driver's side is for the ecu.

The proper way to see if the sensor is working is to hook it up to a OBDII scanner and see what temperature the computer is seeing at varying conditions.
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