E30 318i: Correct Temperature Sensor fitted (Brown Plug) + wiring/loom issue

The temperature gauge in the dash has not worked since I bought the E30, which was a little worrying on my 250 mile drive home, but the car does not overheat. Oh, it has some issues with the cooling-system, like the heater-matrix pipes fitted incorrectly and an air-lock at the back of the head, but hey it doesn't overheat. Still though, I thought it best to get the bottom of the faulty temp. gauge for peace of mind, particularly with summer coming / just about here.

The M40 has two separate temperature sensors that are independent from one another. The temperature gauge works from the 'Brown plug' sensor to the rear of the head [right in pic]. The forward sensor is the 'Blue plug' [left in pic] and that connects only the DME (ECU) to tell the car if the engine is cool or warm to help with cold-start procedures. A single sensor cannot be used for both purposes on these older engines as the resistance value ranges required for each function differ, the DME being a digital circuit and the temp. gauge still working in analog. [You can read more about this on the E30 Zone Wiki HERE].

MULTIMETER TESTING:

The temp. sensors can be easily tested with a multimeter set to 20k ohms resistance. 

The Blue plug is a two-pin sensor, so test across both terminals with the multimeter and you are looking for a reading of 4-4.5k ohms for a working sensor. This sensor and plug can be accessed easily without removing any parts from the engine.

The Brown plug though will require removal of the lower inlet-manifold to access the sensor for testing or replacing. This is a single pin sensor, so place one probe of the meter to the terminal and the other to a ground-point in the engine bay or against the block/head. Expect a reading of between 1k and 1.5k ohms for a working sensor. To test the Brown plug sensor without removing any parts from the engine, you can apply the multimeter to Pin 4 of the C101 connector (main wiring loom plug in engine bay) which is easy enough to get at, or at Pin 26 of the blue connector to the right side of the instrument binnacle, though the binnacle will need removing to do this.

WRONG SENSOR / CONFUSING WIRING LOOM:

All M40 engine wiring looms use a single pin temp. sensor on the brown plug that earths through the engine block and have only a single wire in use leading to it. Some looms, using components destined for other BMW models, are known to have a second wire leading to the Brown plug temp. sensor which is redundant on the M40 motor, though this is rare and seen more often on 6-cyl. M20 looms. My car has one such loom, however and I realise that this is what has caused confusion in the past and led to the wrong sensor in fact being fitted...

Upon inspection, the rear temp. sensor fitted to my E30 is black, not brown and has two terminals. The second pin means that the sensor body earths back to the loom and not through the block so, if the redundant earth wire in my loom is not connected to a ground-point then the sensor was open circuit. I guessed that someone had ordered a second Blue plug sensor by mistake and fitted that, but it turns out the part number doesn't match and the sensor is for a completely different model engine entirely. The reading from the multimeter was over 11k ohms, something like 7.5 times what it should be! That sensor was never going to work with the E30 temp. gauge circuit whether it was earthed or not.

I ordered the correct single-pin sensor with the brown base from mr-wiper on eBay, who also supplied my Bosch spark-plugs. It was only £7.69 delivered. My temp. gauge now works, but this may also be due in part to removal of an air-lock at the back of the head due to a heater-matrix problem so coolant is now flowing properly round the area of the sensor. 

A BIT MORE ON WIRING LOOMS:

Brown / Violet, brown being the main colour, denotes that the power source provided from the temp. gauge circuit is earthing through this wire and does not need to be a closed circuit back to the gauge. The other wire is either not wired in to the loom or goes to a body-earth anyway, so earthing the sensor through the block will make no difference. This colour coding is the same for all wires in all BMW looms, I would imagine most cars are the same.