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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     Johns46coupe 05-10-2010 @ 1:43 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 Hello. My 46 Ford's temp gauge pegs over to max after the car runs only a short time, about 15 minutes of light driving. Using a mechanical thermometer, the max coolant temp is 165 degrees idling after one of these jaunts. I suspect a bad sender but don't know how to tell which one. Is there a way? The right sender is grounded well and measures 14 ohms to ground when engine is warm, infinity when hot. On the left side, both long and short tabs measure infinity to ground warm. All measurements are with wires disconnected. At the same time as all this, my gas gauge quit working but don't know if that's related. The car has a 6V alternator. Just prior to this, I had the radiator boiled out and pressure tested (He doesn't do flow tests but said all is well). I also had the carb rebuilt, I rebuilt the distributor (set it by measuring), installed new Drake waterpumps, new 180 degree thermostats, new hoses. The heater valve is open and I have a 4 pound radiator cap on. Belts are tight and there are no coolant leaks or overflows. The car runs strong, lots of power, no smoke or steam. I have not attempted to adjust the vacuum brake yet. Any ideas what's going on? Thanks for reading this long missive! G Tosel 05-10-2010 @ 2:12 PM reply profile send p.m. Member Posts: 92 Joined: Oct 2009 According to the 1941- 1948 Complete Service Bulletins No.10883 March 18, 1948 the temperature dash gauge is in the hot position when the ignition is off. When the ignition is on, the gauge will move toward the cold position before going to the measured temperature. Is that your set-up? In addition the right unit is grounded in the cold condition. If you can get a copy of the Service Bulletins it will answer your questions to diagnose the problem. Stroker 05-10-2010 @ 2:15 PM reply profile send p.m. Senior Posts: 1460 Joined: Oct 2009 Somewhere in my ageing memory, I recall that Ford used a overheat ignition cut-off on one cylinder head. This was a tar-filled sensor that was not intended to be a heat gage sender, but rather an ignition cut-off. Is it possible that you have your temp gage connected to one of these? The idea was: when the engine became really hot, the tar would melt and interrupt a set of contacts which were connected in-line with the coil primary, and essentially shut the engine off. Ford of course would imply that you then needed to correct the problem, and install a new cut-off. Economically challenged dirt farmers would simply take the unit into the kitchen and cook it upside down in a double boiler until it re-set. I have no idea if this is related to your problem, but I have not seen "temp sending units" used on both heads except for after-market applications. What say the Experts? Johns46coupe 05-10-2010 @ 2:23 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 When turned off, the gauge is max to the right as you note. When I turn it on, the needle moves slowly left to the measured temp. I'll look for the service bulletin. Thanks Johns46coupe 05-10-2010 @ 2:27 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 Stroker, As I understand it, both are sensors on my car. On the left side, there are two tabs, the short one is for the wire to the gauge, the long one for the wire to the other sensor on the right. In any case, the ignition seems fine and it starts easily. Other thoughts? G Tosel 05-10-2010 @ 2:49 PM reply profile send p.m. Member Posts: 92 Joined: Oct 2009 You are correct both are sensors and they are wired in series. Alternate heat created from ignition current on gauge and water heat from sensors causes these sensors to make and break contacts to match water temperature. Service Bulletin Subject 14999-Z shows procedures to troubleshoot sensors and gauge. Thanks Johns46coupe 05-10-2010 @ 2:54 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 G Tosel. Thanks again. Any idea if service bulletins are available online? 51f1 05-10-2010 @ 5:41 PM reply profile send p.m. Senior Posts: 573 Joined: Oct 2009 I wrote instructions for checking the temperature gauge and sending units some time ago. I'll add it below. You can probably skip some of the gauge testing and go straight to the sending units. The sending units and the thermal switch in the block are connected in series so that either switch will cause the gauge to read hot, but only the sender with one terminal actually reads the temperature (in the right cylinder bank only). The thermal switch with two terminals is only an on-off switch. It turns off when the cylinder bank overheats at about 212 degrees F. Probably either the sending unit or the thermal switch is opening (turning off) below 212 degrees which gives you a "HOT" indication on the gauge. You have two units because the two cylinder banks, unlike modern v-engines, are not internally connected. Instructions For Testing V-8 Temperature Gauges: With the ignition switch on and the engine cold, if the gauge is reading "cold," it is working, and you can skip this step. If not, with a voltmeter, check the voltage at the gauge with the ignition switch on. Touch the terminal on the gauge that is connected to the power supply with the negative volt meter lead and connect the other lead to a good ground. If you don't get six volts you need to determine why. If you have six volts to the gauge, proceed. With the ignition switch on, ground the terminal on the gauge that is connected to the sending unit and the thermal switch. The gauge should read "cold." If it doesn't it's bad. There is a temperature sending unit (one terminal) in the engine block on the right-hand cylinder head looking from the driver's seat. The other unit is a thermal switch (two terminals) located in the left-hand head. These are connected in series. If either of these switches is stuck "open," it will cause the gauge to read "hot." With the ignition switch on, disconnect the wire on the thermal switch (left head) that goes to the gauge and ground it. (You can ground it by touching it to the body of the thermal switch.) The gauge should read "cold." If it doesn't, and you have determined that the gauge is working, there could be an open circuit (broken or disconnected wire from the sending unit to the gauge or even a bad engine ground). If the gauge is reading "cold," reconnect the wire. Disconnect the wire at the right hand unit that runs between the two units and connect it to a good ground (the body of the unit). If the gauge does not read "cold," the thermal switch (left head) is probably bad. Replace it. If it does read "cold," the sending unit (right head) is probably bad. Replace it. Richard Johns46coupe 05-10-2010 @ 6:47 PM reply profile send p.m. Member Posts: 68 Joined: Apr 2010 Thanks for the procedure Richard. I'll jump on that as soon as it cools down. I bought the book of service bulletins this afternoon but this will get me going. I'll let you all know how it goes! Thanks all, John 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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