This then allows C1 to charge via VR1 and R1. When the ignition source is initially turned on pin 2 is triggered via a short pulse from the C2 and R2 combination. It works using a basic 555ic timer circuit. This circuit is setup to provide an adjustable delay from the sensor located in the header tank when the level drops. He put together a simple and robust little circuit from fairly commonly available components that achieves our objective i.e., a short (adjustable) delay before the alarm is triggered to prevent false signals when off road.įirstly, here’s a couple of photos of the completed circuit:įinally, here’s Pete’s description of the circuit for those of you who wish to replicate it: At this point my fellow Land Rover suffering mate, Pete, came to the fore. Obviously what is needed is some sort of time delay. This would inevitably lead to false alarms as the coolant sloshes around in the tank giving the sensor a hard time. The sensor is just a switch, and as it is it’d work fine in an on-road vehicle, but our vehicles may occasionally spend some time at a slight angle, or bouncing around a bit. Next we come to the slightly more complicated part. Here it is installed in my 200tdi Defender: Installing the switch was easy, simply drill a 22mm hole in the side of the plastic expansion tank, screw in the sensor and wire it up: Here’s the coolant level sensor, a straightforward level switch, easy to install with minimal chance of gremlins: After a quick phone call I confirmed that the sensors were for sale separately (about $25 IIRC), and a plan was formed in my head. Enter the simple and effective float switch I discovered on This company sells the Little Black Box system which is very comprehensive, but I was only looking for the coolant level sensor. However, the Rangie cap sensor proved to be quite temperamental, so I searched for a more reliable solution. I decided that the expansion tank is the most convenient of the correct places to install a sensor and initially went down the route of using one of those Range Rover caps that incorporated a level sensor. The only example that comes to mind is the 12valve 2.4 Nissan engine used in the '90's in Pintaras and Ford Corsairs. In this system coolant level is ultra critical because any low level is carried in the engine (at cylinder head level) and the radiator will always remain full whilst the pump is in operation. A very few have a reverse coolant flow where the thermostat is mounted below the water pump, and the pump pushes coolant down into the bottom radiator hose, up through the radiator and into the engine. Only when the level is so low that it falls below the level of the water pump (causing the pump to cavitate) will the engine not be full of coolant whilst the pump is operating.Īs I said above, this is the system for the majority of engines. The low level is carried in the radiator, not in the engine. This is in fact done deliberately by most manufacturers to ensure that the engine receives its full level of coolant whilst in operation, even if the system is a little low. It would only be when the pump was stopped that the low coolant level would be equal in both the expansion tank, thermostat housing and radiator. If the coolant level was low it will show up in both the radiator and the expansion tank, before a low level was experienced in the thermostat housing or top radiator hose (whilst the water pump is operating). In our Land Rover engines, the water pump draws coolant in through the bottom radiator hose and pushes the coolant from this hose up through the engine and out through the thermostat housing into the top radiator hose. When the engine is running, these are the two worst places to be looking for low coolant level because of the operation of the water pump. I’ve always had an issue with trying to detect a low coolant state either in the thermostat housing or the top radiator hose. Some time ago I fitted a low coolant alarm to my Defender, and since my version is slightly different to some, I thought I’d share the ideas and reasoning.
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