IslandDefender90 Posted March 30, 2022 Share Posted March 30, 2022 Have a puzzling one. The thermostat gauge in my 1987 300TDI 90 started to go full throttle! Swings all the way past the H aka “breaking the needle”. I have checked everything. Cooling system is in good shape. No leaks. No air bubbles in the revisor. Water pump is functional. Even changed the thermostat housing. Removed the thermostat (on a island where it isn’t needed). Changed the thermostat sender. Replaced the gauge with a new gauge. Still the gauge reads all the way to the right. Engine is cold and there is no smoke or coolant leaks at all. It happens within 10 seconds of startup as well so no way it has anything to do with the actual temperature. Any ideas what’s happening? It’s driving me to over heat! 😤 Quote Link to comment Share on other sites More sharing options...
miketomcat Posted March 30, 2022 Share Posted March 30, 2022 Sounds like the cable has rubbed through and is shorting out somewhere. The gauge works by connecting to earth (engine block) via a variable resistor (the resistor being the sender) so if the cable shorts to earth the gauge reads hot. Equally if the cable breaks or gets disconnected it doesn't read at all. Mike 1 Quote Link to comment Share on other sites More sharing options...
IslandDefender90 Posted March 30, 2022 Author Share Posted March 30, 2022 (edited) 5 hours ago, miketomcat said: Sounds like the cable has rubbed through and is shorting out somewhere. The gauge works by connecting to earth (engine block) via a variable resistor (the resistor being the sender) so if the cable shorts to earth the gauge reads hot. Equally if the cable breaks or gets disconnected it doesn't read at all. Mike Thanks Mike! So just a simple electrical circuit issue possibly. I checked the wire already and it looked ok but didn’t check the ground. Edited March 30, 2022 by IslandDefender90 Quote Link to comment Share on other sites More sharing options...
steve b Posted March 30, 2022 Share Posted March 30, 2022 Disconnect the sender and see if it still goes full deflection . The engine loom on the 300 is routed under the airfilter mounting plate and it's often that section that rubs through or gets hardened and cracks. While you are there check the main (thick brown) alternator live connection is in good condition . Other thing to consider is that the gauge should be matched to a Defender sender for the 300 . I think I mentioned this on another of your posts ? Steve 1 Quote Link to comment Share on other sites More sharing options...
IslandDefender90 Posted March 31, 2022 Author Share Posted March 31, 2022 (edited) 14 hours ago, steve b said: Disconnect the sender and see if it still goes full deflection . The engine loom on the 300 is routed under the airfilter mounting plate and it's often that section that rubs through or gets hardened and cracks. While you are there check the main (thick brown) alternator live connection is in good condition . Other thing to consider is that the gauge should be matched to a Defender sender for the 300 . I think I mentioned this on another of your posts ? Steve Thanks Steve. I don’t think so. My gauge and sender are fitting for the 300. But will check the lines under the filter and the trick with the sender is a great idea. I think ultimately I need a full electrical wiring overhaul. The tricky part with caribbean island rovers is they have been maintained with what is available and the wiring on this beauty is a rats nest! After checking the ground today I noticed a that the gauge will go full send even when the ignition is just half turned. So narrowed it to an electrical issue for sure. Edited March 31, 2022 by IslandDefender90 Quote Link to comment Share on other sites More sharing options...
Junglie Posted March 31, 2022 Share Posted March 31, 2022 11 hours ago, IslandDefender90 said: ... The tricky part with caribbean island rovers is they have been maintained with what is available and the wiring on this beauty is a rats nest! ... Looking out at the leaden skies and snow (yes, really!) here... I'm struggling to be sympathetic with you being in The Grenadines. Especially as most Landies everywhere appear to have a nest of snakes for wiring... Quote Link to comment Share on other sites More sharing options...
western Posted March 31, 2022 Share Posted March 31, 2022 to eliminate the existing wiring just run a new wire from sender to gauge signal input then see what happens. Quote Link to comment Share on other sites More sharing options...
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