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Temp Idiot light on Perkins

terry_thatcher

Terence Thatcher
I have a temp gauge and an idiot light for high temperature. I have become concerned after a mistaken wiring of my oil pressure idiot light (now fixed) that my temp warning light may be miswired. No way to check it really without over-heating the engine. It is a simple two wire lamp, one wire coming from one of the temp sensors on the engine. Where does the other wire go? To ground or to positive DC outlet? I would have to check, but I think, if I recall correctly, my oil pressure light goes to the sending gauge and to a positive DC terminal. When the oil pressure is low (e.g., before you start the engine), the sending unit provides a ground and the light illuminates. Any help gladly accepted.
 
Terry, I'll take a look at my spare Perkins and see what / where those wires go. The harness, engine, etc. are all on a dolly so I can backtrack it fairly easy.... do you have a wiring schematic in a Morgan owners manual by chance?
 
Terry
The sensors and idiot lights usually work as switches on the negative side. That means that that a positive "+" wire goes to the sensor, say the high temp sensor, and when the engine temp rises above a certain set point, the switch (sensor) closes and completes a path back to the negative side of the battery via the engine block itself, and either sounds an alarm or lights an idiot light. Thats why there is only one wire that leads to the sensor. I hope that make sense. To do a test to see if a sensor is working just take a short piece of wire and touch the wire that leads to the sensor and "ground" the other end to the engine block. That should bypass the switch (sensor) and activate the alarm or idiot light.

Jim
 
One way to find out where the wire goes is to momentarily touch the end of the wire that goes to the sensor, to the engine block. Observe if the idiot light goes on or if the needle on the analog gauge moves. For the gauges, the inside of the sensor is a variable resistor, one sude of the resistor goes to ground( the engine block) the other side goes to one side of the gauge, the other terminal on the gauge goes to 12v.
 
Hi Terry, search “senders”. I post pictures 2 years ago. Just can’t figure out how to move them.
 
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