Coolant Level Sensor

UPDATE 2017: This webste has been around for a while in one form or another, and information can become out-of-date, or no longer relevant. A few updates seem worth noting:

  • The Genuine Parts sensor (Made by VDO) now costs significantly more than the £30 it was when I built my setup.
  • The Genuine Parts sensor contains a series resistance of around 680 Ohms (presumably to protect the reed relay contacts). Because of this, most mechanical relays will not work correctly, though the electronic solution will.
  • Various 'pattern part' versions of the sensor are now on the market. At least some of those (Bearmach for one), does not have a series resistance, and should work with a small relay with no problems.

One of the weaknesses most often claimed against the 300Tdi engine is head gasket failure. After experiencing a sudden overheating in the Ibex, resulting in a precautionary replacement head gasket, the root cause was found to be a leaking water pump gasket, causing a gradual loss of coolant.

Shortly afterwards, a near miss on another cook-up was caused by a split expansion bottle. While neither of these failures is unknown on the 300Tdi, there is a design 'feature' which makes them especially serious. The water pump is mounted unusually high up on the engine block. What this means is that only a small loss of coolant is required before the water pump cannot 'pick-up' and catastrophic failure results. This combined with the fact that the 300Tdi is not very robust to overheating means that coolant level is even more important than for most other engines.

I decided that some form of early warning was an essential addition - 300Tdi Defenders have a temperature gauge, but no temperature warning light, and no coolant level warning lamp. In keeping with the long tradition of not redesigning the wheel though, Land Rover have a cap for the expansion bottle which fits, and which contains a float switch to detect low coolant level. PRC7925 is fitted to diesel classic Range Rovers with VM or 200Tdi engines. Quite why it isn't fitted to 300Tdi models (or to Defenders or Discoverys) is a mystery. Retail price is £30 plus the dreaded, which while not cheap, is certainly cheaper than replacing the head gasket (or the head!).

One slight problem is that the sensor is intended to work with an interface, and is designed to fail safe (indicating coolant loss in the event of a wiring problem). Since Defender doesn't have the dashboard electronics required, I used an electronic switch to control a warning lamp. The sensor is closed circuit ('ON') when sufficient water is present, and I used this signal to switch OFF a warning lamp placed in one of the spare lamp positions on the Defender warning lamp module. I didn't use a relay because the relays I had to hand draw up to 0.5A which would be a little high for the tiny magnetic reed switch inside the sensor. However for those not so comfortable messing with electronics, RS sells a relay (part no. 349-2121) which has a low current draw, and spade terminals for easy connection. The only thing it lacks is a mounting bracket, but you could always get creative with the tiger seal ;-)

coolant_sensor_circuit.jpg This circuit should work with low resistance aftermarket sensor caps (e.g. Bearmach) but will not work with the Genuine Parts VDO sensor. Use a relay with a high resistance coil to reduce the risk of damaging the sensor.

I used the 'transmission temperature' warning lamp in the module, but there are several unused positions in the module that can be pressed into service instead.

For about £35 (if you use a new sensor) and a couple of hours work, this must represent good value in terms of the peace of mind it offers.