E39 540i coolant temp sensor

[yng_750 247]

So i have an e39 540i 1998

I have a recurring issue that is slowly getting more prevalant.

Occasionally the coolant gauge will quickly flick all the way to the red end of the gauge.

Using the hidden dash functions i can see this is sending a signal of -128'c which is what you get all the time with an open circuit (plug off the sensor)

This usually will sit here for a couple of mins then drop back to a normal signal.

used to do it once a month or so and is now 4-5 days a week

The car has had a new thermostat and i have just installed a new coolant temp sensor but it hasnt changed anything.

the sensor has 4 wires, presumably two for gauge and two for ecu?

does the gauge function run through a control unit or just straight to the dash?

im narrowing it down to either a half broken wire somewhere or the dash playing up.

wiggling wires etc doenst change it and turning the car off then on again usually fixes it. this makes it sound like a computer or dash issue but im a little lost.

not sure if it matters but it usually plays up when coolant is around 60'c and comes back at around 75'c not always but common.

[yng_750 247]

bump any ideas

[nath 134]

Is the sensor you replaced, the one that the cluster displays? My Audi has 4 coolant temp sensors, one fuel trim, one climate control, one cluster needle and one aux fan.....

[yng_750 247]

my coolant sensor has 4 wires, when the gauge plays up the motor runs fine, when i unplugged the sensor all together while running the gauge died (went to hot and red -128'c as usual) but the motor surged a little before returning to a slightly higher idle.

judging by this i would say that the one sensor has two circuits in it, one for gauge and one for DME, much like an e36 does.

it then has a separate sensor in the radiator to run the fan

[allan 295]

http://www.bmw-planet.com/diagrams/release...39old/index.htm any help.