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It is a well documented fact that the E46 generation of BMW 3-series vehicles is prone to a variety of cooling related issues. The frequent occurrence of these issues has forced many enthusiasts to preventively replace nearly all cooling related components at ~60K mile intervals. The true severity of the problem lies in the fact that the engine may overheat without any obvious signals, which ultimately leads to costly repairs.

The E46 Thermostat

Conventional thermostats are very basic devices. They are built to mechanically alter the path of coolant traveling through them depending on the temperature of the coolant. The E46 thermostat, however, is a bit more sophisticated. For the E46 models, BMW uses what is known as a “Characteristic Map Thermostat”. This means that the thermostat is built to allow, to some extent, the DME to alter the open/close temperature range. This approach is used mainly to conserve fuel efficiency. Under partial loads, better fuel economy may be achieved if the coolant temperature is raised up to 110 degrees C. Under heavy loads, this temperature may cause engine knock and, as a result, the DME lowers the opening temperature of the thermostat via an electrical connection, allowing cooler running temperatures. By BMW's specifications, the normal operating range for the engines fitted on the E46 is between 75 - 113 degrees C.

The E46 Temperature Gauge

Ever wonder how is it that the coolant temperature on the E46 dashboard manages to stay at the same mark no matter how hot or cold the ambient temperature is, or how hard you may have been driving the car? This is because the temperature reference on the E46 models is "fake"... well, to some degree (no pun intended!). Because BMW utilizes the Characteristic Map Thermostat described above, the actual coolant temperature may vary by as much as 40 degrees under normal driving conditions. As a result, a true gauge would look somewhat sporadic, which could cause an excessive number of calls to the service department. To eliminate this problem, the gauges on the E46 dashboard have been given some "intelligence". For temperatures up to 75 degrees C, the gauge displays the "real" temperature. When it displays the actual read out, the 1st notch after the blue bar indicates roughly 65 degrees C. When the coolant temperature reaches 75 degrees C, the needle is moves to the upright 12 o'clock position. The needle will remain at this position until the coolant reaches a temrperature of 113 degrees C . At this point the needle will start to move upwards, reaching the 3/4 notch at ~120 degrees C.

While this may seem like a reasonable approach, it has one major downside. If the coolant temperature in the car is rising due to an actual malfunction, the driver has absolutely no indication of this until 113 degrees C is reached. Furthermore, subtle movements of the needle are hard to notice, so if the temperature is climbing quickly, it doesn’t take much time at all to reach the red zone.


We have been working on a solution to help E46 enthusiasts to save their engines by providing a preemptive warning before the temperatures are too high. Meet project CoolantSnitch(tm). This simple device is built to operate with the existing BMW coolant temperature sensor and provides constant monitoring of the input signal. When the coolant temperature reaches a critical mark of ~117 degress C, CoolantSnitch(tm) will sound an alarm to help the driver identify the problem while the temperature is still within a non-critical range.

CoolantSnitch(tm) is connected to the DME using 3 T-Tap connectors. The 2 connectors supply +12V and Ground, while the 3rd one is connected to the coolant signal line. The device has very high impedance (~1.5 TOhms) which makes is invisible to the DME and causes no interfiereince with the operation of the stock subsystem.

In the video below, the OBC function 7 is unlocked with the coolant temperature readout displayed in degrees C (the digital output just above the ambient temperature display on the dashboard). The electric fan on the car has been disabled, allowing the temperature to rise above normal. At 2:40 the temperature reaches the triggering point of the device.

CoolantSnitch is a compact device that is tucked away in your DME box next to the driver's side shock tower.

CoolantSnitch has been tested and approved for the following BMW models.
DME VersionModelProduction date
MS45.1E46 330i/Ci/Cic03/2003 and up
E46 325i/Ci/Cic03/2003 and up
MS43E46 330i/Ci/Cicbefore 03/2003
E46 325i/Ci/Cicbefore 03/2003
E46 330XiAll
E46 325XiAll
MS42E46 323i/Ci/CicAll
E46 328i/CiAll

CoolantSnitch Installation Instructions

CoolantSnitch - $89.95 (Includes Shipping in US and Canada, others please contact.)

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