Jump to content

Low Coolant Alarm

Recommended Posts

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.

I’ve always had an issue with trying to detect a low coolant state either in the thermostat housing or 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. 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. 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). 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. 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. The low level is carried in the radiator, not in 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.

As I said above, this is the system for the majority of engines. 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. 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. The only example that comes to mind is the 12valve 2.4 Nissan engine used in the '90's in Pintaras and Ford Corsairs.

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. However, the Rangie cap sensor proved to be quite temperamental, so I searched for a more reliable solution. Enter the simple and effective float switch I discovered on www.4x4gadgets.com.au. This company sells the Little Black Box system which is very comprehensive, but I was only looking for the coolant level sensor. 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.

Here’s the coolant level sensor, a straightforward level switch, easy to install with minimal chance of gremlins:


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 it is installed in my 200tdi Defender:


Here’s one installed in a TD5:


And another one in a Series II Disco:


Next we come to the slightly more complicated part. 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. :ph34r::lol: This would inevitably lead to false alarms as the coolant sloshes around in the tank giving the sensor a hard time. Obviously what is needed is some sort of time delay. At this point my fellow Land Rover suffering mate, Pete, came to the fore. 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.

Firstly, here’s a couple of photos of the completed circuit:



Now, here’s the circuit diagram:


Finally, here’s Pete’s description of the circuit for those of you who wish to replicate it:

This circuit is setup to provide an adjustable delay from the sensor located in the header tank when the level drops.

It works using a basic 555ic timer circuit. When the ignition source is initially turned on pin 2 is triggered via a short pulse from the C2 and R2 combination. This then allows C1 to charge via VR1 and R1. If the tank switch is closed (the tank is full) C1 is held to ground and a small current flows through R1,VR1 and the tank switch.

This will remain like this until the tank switch is opened or the ignition is turned off.

In the event that the tank switch opens C1 will begin to charge through VR1 and R1. When C1 gets to 60% of the ignition voltage pin 3 on the 555ic will be grounded causing Rly1 to turn on and sound the alarm.

I also had to add a suppressor inline with the supply to prevent false alarms when heavy load equipment is started and stopped. This was typical fans and windscreen wipers.

My setup was set for about 8 seconds of delay. The components I used were what I had laying around but seem to give a good range and do not draw much current through the tank switch.

Component list

R1 – 33k

R2 – 33k

VR1 - 1M

C1 - 4.7 uf

C2 - Green cap unsure but not that critical

D1- IN4001

Rly1 - Standard 12volt relay

Now Pete is a member of this forum and goes by the username of Peterla, so if you have any particularly techo type questions, either PM him direct, or PM me and I’ll pass them on.

I guess the actual alarm is the final part of the puzzle. I simply used the output from the circuit to trigger a standard relay, which is in turn connected to a piezo buzzer and a light. The buzzer is tucked behind the dash, but is easily loud enough to be heard over the racket made by a 200tdi Defender at 110 kph. The light is mounted on the centre dash thus:


There you have it. Simple, reliable and effective and it's been running in my Defender for some time now without issue.

Paul :)

Link to comment
Share on other sites

I like the idea..... (But don't have the ability for the electrickery...)

Anyone on the forums prepared to make and sell these? (Maybe make a contribution to the Gin fund as well ?)

Sign me up for one......


Link to comment
Share on other sites

We fit about two 'little black boxes' a week. This consists of exactly the same float as shown above, and a black box that contains all the electrickery. It also gives an alarm on battery charge, battery level, coolant temp (programmable). We do export if someone in the uk wants one. Check out our website in my signature. The kit comes with installation instructions and is easy to fit.

Here is the url to the workshop page:


Link to comment
Share on other sites

I Looked into this a while ago and decided to fit the Range Rover Coolant Sensor Cap (Even though it was relatively expensive) as when you look into the Specs of the Float switches they are not really suitable for The Pressure / Temperature iof the coolant system.


Link to comment
Share on other sites

  • 10 months later...
Just wondering... does anybody know why is there a diode running parallel to the relay? I'm just interested in how the circuit works.

Yes, its to stop the back emf from the relay coil damaging the chip............. the back emf can potentially be many hundreds of volts, but in the form of a spike at very low current. the diode is often incorrectly referred to as a fly back suppressor circuit.

All switched inductive circuits driven from solid state devices should have a protection diode.



Link to comment
Share on other sites

Yes, its to stop the back emf from the relay coil damaging the chip............. the back emf can potentially be many hundreds of volts, but in the form of a spike at very low current. the diode is often incorrectly referred to as a fly back suppressor circuit.

All switched inductive circuits driven from solid state devices should have a protection diode.



Ah i see, always useful to know stuff like that, thanks Ian. Thats pretty impressive at 12.30 am :D

Link to comment
Share on other sites

My (limited) experience with the Range Rover caps is that they're rather unreliable. That's why I went off looking for a better solution.

In what way did the RR cap fail? Did it fail open or closed?

I've had one for about 8 months, linked to a Madman. No problems as yet.

Link to comment
Share on other sites

In what way did the RR cap fail? Did it fail open or closed?

I've had one for about 8 months, linked to a Madman. No problems as yet.

The RR cap takes water into the base of the unit. Eventually the wire breaks leaving no connection.

At least it fails safe. Well mine did.

I did have a picture somewhere ??


Link to comment
Share on other sites

  • 1 year later...
  • 7 months later...

In my experience float sensors involve major surgery into the expansion tank and have a high early failure rate due to the high temperatures within the tank.

The tank also has to be a "proper" pressurized tank that is a intregral part of the circulating system..Not just a expansion tank which stores expanded coolant and then draws it back on cooling. (you can lose all engine coolant and the level in the tank probably won't drop)

The plastics involved in the floats construction need to be Glass filled nylon to withstand the temps/pressures involved....most floats are not made of this as they are designed to operate in a cold fluid enviroment (water tanks/waste tanks etc.

Link to comment
Share on other sites


I've had an EMS (http://www.littleblackbox.co.za/html/little_black_box.html) fitted to my TD5 110 for about six years now and it's probably saved me a serious amount of cash already on four occasions. It picks up coolant leaks very quickly (even when a radiator hose or the blanking plug at the back of the oil cooler fails) and shouts loudly. No problems with the float or expansion tank at all.


Link to comment
Share on other sites

I read somewhere recently, that someone offering aftermarket coolant level sensors have recalled/withdrawn them due to them not working properly. IIRC, a film of water could remain on the switch when the coolant level drops, thus not triggering a warning.

Am looking for a solution for my TD5 tank at the moment...

Link to comment
Share on other sites

  • 5 years later...

The Madman system I have just uses two stainless steel self tappers screwed into the expansion tank about an inch apart, has worked for 3 years so far without any issues and the warning light did come on the one time it was needed to !

Link to comment
Share on other sites

  • 1 month later...
  • 11 months later...
  • 4 years later...

Hi, I have recently bought an ex MOD defender 110CSW 2002 and have already decided to add a coolant level switch. Was initially going to use the range rover switch, and looked at some level switches. 

I have decided to go down this route instead:

A replacement range rover sport coolant tank, with built in switch available on ebay, link below.

I decided this option, as the whole unit is £23.90, so less than the cost of some switches, and my defender expansion tank is really dirty to the extent that it is impossible to actually see the water level in it!

Doing this will solve all my issues in one go. Just have to blank off one of the hose connectors and need to work out a fixing method to install it in place.

I will be doing this when the weather gets warmer and just thought I would share this as another option too 😎


Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


  • Create New...

Important Information

We use cookies to ensure you get the best experience. By using our website you agree to our Cookie Policy