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EFV-8 Club Forum / General Ford Discussion / False Overheat Indication

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Posted By Discussion Topic: False Overheat Indication -- page: 1 2     TomO 05-16-2010 @ 5:20 PM reply profile send p.m. Senior Posts: 7261 Joined: Oct 2009 John, we have all been led down the primrose path by faulty test equipment. Go to your local hardware store and buy a candy thermometer. They come with a hand clip that allows you to clip it to a pan. You can test it by clipping it to a pan of water and bringing the water to a boil. Tom Johns46coupe 05-16-2010 @ 12:13 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 Well, assuming my right sensor was bad and the new one not having arrived yet, I was driving around ignoring the temp gauge when the car started running rough. I pulled over and the coolant was overflowing and the engine really was overheating. When I got it home I was prepared for the worst. I checked compression and it varied from 105 to 120 pounds. Great. I re-read the posts and Super had said use a "reliable" thermometer to check coolant temp which had been 165 deg. I had borrowed it so I compared it with a meat thermometer and found it had been reading 25 to 30 deg low. I then removed the two new thermostats and boiled them along with my old stats. One of the new stats opened but not much and the other one not at all. The old ones opened wide. I've decided to run with no stats and the temp gauge only goes up about a quarter of the way. The moral of the story is be skeptical concerning new parts and use reliable (thanks Super) test equipment. Many, many thanks to everyone who responded with ideas and suggestions. You certainly taught me a lot. Thanks again. John Johns46coupe 05-12-2010 @ 2:49 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 Many thanks you guys. I have a right sensor on order that should be here Fri or Sat. I'll update this discussion with the results. I sure learned a lot from everyone about cooling systems and senders. Thanks, John TomO 05-12-2010 @ 8:01 AM reply profile send p.m. Senior Posts: 7261 Joined: Oct 2009 The right sensor is the one causing the problem. Try removing the sensor, cleaning the threads on the sensor and the block, then re-installing the sensor with a small amount of pipe dope on the first couple of threads. Teflon tape and other sealing compounds may compromise the ground circuit. Tom 51f1 05-11-2010 @ 3:55 PM reply profile send p.m. Senior Posts: 573 Joined: Oct 2009 Sounds like it's the temperature sending unit (one wire) on the right head. The way you describe the symptoms, that makes sense. Richard Johns46coupe 05-11-2010 @ 1:07 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 Thanks Super. I have checked the coolant temp with a mechanical thermometer with the car idling and saw a temp of 165 degrees although the gauge was at max at the time. Also, when turned off, the fins at the top of the radiator were hot and were noticeably cooler at the bottom, as you would expect. I don't think the belts are slipping although the pumps are new Drakes so are a little stiffer than the old ones. Thanks, John Thanks, John supereal 05-11-2010 @ 11:44 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 That says that the gauge and the associated wiring is OK. I assume that you have put a reliable thermometer in the radiator neck to check the actual temperature and confirm that the coolant is not rising to near or past the boiling point. At a temperature in the 180-190 range, the gauge usually reads at, or just above the center of the dial. Check the actual temp of the coolant when the gauge reaches that point, and again when the needle goes over toward the H peg. If the coolant temp is still below the 180-190 region, the sensor is bad. If the coolant is hotter, in the 200-210 range, you have a cooling system problem, which can range from a partially clogged radiator to a slipping water pump belt, to a leaking head gasket. I've even seen dragging brakes cause overheating. Johns46coupe 05-11-2010 @ 10:26 AM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 Thanks for all the help Richard, Tom and Super. Here is where I'm at. When the engine is cold, the gauge reads cold, OK so far. Also, when cold I can read the voltage on the right hand sensor and see the "vibration" Super mentioned. Seemed to be about two pulses per second or so. Went for a drive and the gauge started going up slowly, as normal, held steady around the TE area for a bit, then slowly went to full hot. I put a jumper across the two terminals on the left sensor and the gauge did not change.. Stopped the engine and while still hot, grounded the right sensor wire with the ignition turned on. Gauge immediately went cold. As the car cools off and I monitor the gauge, it slowly moves to cool as you would expect it to. At this point, I suspect the right sensor is not totally failing but is sending incorrect info because the gauge moves gradually and because grounding the right sensor causes an immediate cold reading on the gauge. Am I reading all the smoke signals correctly?? Thanks, John supereal 05-11-2010 @ 8:22 AM reply profile send p.m. Senior Posts: 6819 Joined: Oct 2009 The sensor with the single terminal has an internal heating coil to properly set (bias) the unit. The points in the sensor actually vibrate as they open and close, and the rate is in proportion to the surrounding (ambient) heat. If the coil in the sensor is broken, or otherwise failing, it is likely the gauge reading will behave as you describe. Lack of proper grounding of the body of the sensor will also interfere with the operation. The two terminal sensor is just a thermostatic switch that opens when at or near the boiling point of the coolant. To eliminate it as a cause, simply short the two terminals together. If the problem goes away, that sensor is bad. The only cure for a defective single terminal sensor is replacement. Repro sensors are of suspect quality if not NOS. TomO 05-11-2010 @ 7:37 AM reply profile send p.m. Senior Posts: 7261 Joined: Oct 2009 John, I would also look at the connections at the gauge. When there is no voltage supplied to the gauge, it will move toward the hot side. The gas gauge would move to the empty side if voltage were removed. You can bypass the sender with the 2 terminals by connecting both wires on one terminal. Drive the car and if the indicator does not go to hot, it is the sender with the 2 terminals that is failing. If it does go to hot, it is the sender with 1 terminal failing. I would not try to diagnose the problem with a voltmeter or an ohmmeter, because the conditions change as the engine heats up. The one terminal sender consists of a coil wrapped around a bi-metal strip with contacts at the end. When the contacts close, current flows in the circuit heating the bi-metal strip in the sending unit and in the gauge, causing the gauge needle to move. As the engine heats up, the contacts are further apart and less current flows in the circuit. The ohmmeter would read an open circuit on the single terminal sender, when the engine is warm, because the contacts are open. Tom This message was edited by TomO on 5-11-10 @ 8:58 AM

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