Overheating Bad PCM???

[Kokomo]

Re: Overheating Bad PCM???

I remember reading in other threads that the temp gauge is not analog. It works like an idiot light, either normal, or overheated. It is driven by some computer that decides what it should show.

[Bond #578]

Re: Overheating Bad PCM???

The temperature sensor (ECT) uses a thermistor (Negative temperature coefient- high resistance 30Kohms at 25 deg C and low resistance at high temp, 1Kohms at 120 deg C). The PCM sends out a reference voltage 5V and locks for a signal return. An open circuit would result in the PCM thinking the engine was cold whereas a short circuit the PCM would think the engine was overheating; of course there are expexted limits in PCM calibration. A quick test is to unplug ECT sensor, the PCM should go to high idle then stall; If you place 700 ohm resistor in connector while unplug the coolant fan should come on.
Have you reflashed your PCM. It may have a mixed signal. If you disconnect battery greater than 20 minutes the computer should reset to base calibration.
Good luck .

[Bond #578]

Re: Overheating Bad PCM???

Correction. Did some more checking and the TBird does not use ECT sensor but instead CHT (clyinder head temperature). The CHT is not in a water jacket. I am unawre of the base technology, postive or negative thermistor, reed switch or whatever.
Did confirm that the CHT is controlled by PCM and that the PCM is hardwired using SCP network (prioritized,shared multiple signals) to instrument cluster. I do not have access to PCM/Emissions Diagnosis (PE/ED) manual therefore can not tell you pinouts for CHT, SCP.
Does anyone have the Emission manual?
Still think you have a corrupt PCM and should be reflashed.

[robbie199]

Re: Overheating Bad PCM???

Some manufacturers utilize two coolant temperature sensors. One measures coolant temperature and when hot enough it turns on the fan through a relay. The other coolant sensor sends a signal for the gauge. The PCM would use a signal to determine fuel air ratio. You stated that the temperature dropped when you manually engaged the cooling fan. I would look for another coolant sensor and check the relay and fuse. Hope this helps.

[Lonestar]

Re: Overheating Bad PCM???

I remember reading in other threads that the temp gauge is not analog. It works like an idiot light, either normal, or overheated. It is driven by some computer that decides what it should show.

Your comment is accurate for Ford vehicles of recent vintages – very useless. The guage will set on the center until the high temp threshold is it and then it will jump to H. However timmaddog stated that the coolant got so hot that the recovery bottle was impacted. The gauge should have pegged long before that occurred.

If the engine has gotten that hot I fear that the heads might have become warped or damaged the plastic intake manifold or both.

Typically the fans will come on at around 210 degrees F and stay on until the coolant drops to around 195 degrees F. Also, the fans should kick on if the AC compressor is turned on for you have to draw air through the AC condenser or the AC will not cool.

I agree that disconnecting the battery for 20 to 30 minutes is worth a shot.

[timmaddog]

Re: Overheating Bad PCM???

First let me thank everyone for their great advice. Forums like these are invaluable when it comes to resolving a problem.

I know the original problem started with the thermostat. When my son removed it, he found it was broken. I also tested the thermostat and it failed to open in boiling water.

The coolant bottle leaking was caused by the enigine overheating. THis happened after the themostat was changed and the temprature gauge did not give any warning that the car was overheating. The guage stayed in the middle.

I did disconnect the temprature sensor located in the manifold while I was testing the car. The check engine light came on and the car went into limp mode. So that part of the circuitry works.

I am able to control the fan by jumping accross the connector. This allows me to keep the temprature under control. Therefore, even though it is a new fan, it is not able to come on automatically due a signal problem. I am going to try the computer reset first. If that doesnt work, I will check continuity on the wire going to the computer. Then, I'll cough up the doe and pay the dealer and see what they come up with.

Again thanks. I will everyone informed as to the progress.

[steelblue5505]

Re: Overheating Bad PCM???

There is another temp. sensor. It is the oil temp. sensor and it is located along with the oil pressure sensor down on the oil filter adaptor(mounting that filter threads onto.

I do not know if this has anything to do with your Bird's problem, but earlier on in this thread someone mentioned that there maybe more than one temp. sensor. I could not find anywhere in the shop manual a second coolant sensor other than the cylinder head temperature sensor. There is a caution in the manual that this sensor can not be re used after removal and must be replaced. No mention as to why.

I'm wondering if the coolant control valve isn't the culprit?
Or if one of the new parts you already replaced wasn't faulty also.

[robbie199]

Re: Overheating Bad PCM???

I quickly read this thread and see no mention of checking the water pump. Make sure the water pump is moving the coolant.