Coolant temp drops to 70 for a few sec, then 90 again

[8Jb9]

During a long trip I noticed a fault that was previously concealed by how short lived it is compared to how often I look at the instrument panel (sampling frequency).

Temperature was stable at 90*C, when it would suddenly drop to 70*C before returning to 90*C just as suddenly. Sometimes it would stay at 70*C for a few seconds, other times it would drop, and go back up again immediately. No warning lights lit up, and no message was displayed on the driving computer. No change was felt in the way the car drove, which at the time I first noticed it was stable motorway driving on cruise control.

Clearly something about the cooling temp sensor must be failing, but I'm not sure if it's the sensor itself, or perhaps the connections to it. I won't have a mechanic look at it until it is going in for service in half a year (unless it gets worse), but if there's anything I can do now - perhaps cleaning the sensor contacts - then advice is very welcome. Also, if you could please mention any ill effects that you are aware of that may develop because of-, or go undetected because of this sensor fault it will help me judge the seriousness of this fault.

D9 saloon, ES9 J4, manual transmission. Next planned service: july 2013.

[pugmiester406]

hi mate, unplug the coolant temp sensor and inside the plug u will c 2 pins. short these out in the plug end then switch ignition on.... the temp gauge should read
max and go st8 into the red. (u may also get a warning on the display but ignore that)... then i would watch for ten mins or so to see if the gauge moves down at all. it should not. if it remains full in red then i would suspect the sendor is at fault. be sure that the next time u drive it and it does it to quickly pull over and check the rad fans are not running wen they shouldnt be. hope this helps. lee

[8Jb9]

Hi Lee, thank you for your suggestions.

I don't think that the temperature drop corresponds to the fans going on or off - the change is simply too fast (both up and down), and mirror the experience I have from the summer poorly. In the summer the fans turn on and off all the time when standing still, even if the cooling water is (relatively) cold, and I see no immediate effect from it on the temperature gauge. I am guessing that this is happening because the A/C is dumping heat into a similar system which is also allowed to control the fans.

In addition, you say that a very good connection (short) corresponds to it being very hot, and a poor connection (extra resistance) as being cold. The symptom then matches e.g. a wire or the thermocouple/thermistor/?? going bad.

Could you try to explain where exactly this particular sensor is on the V6? Does it require the coolant to be dumped to replace, or is it possible to replace it as-is? What price should I expect for an OEM or OEM-quality replacement?

[jonsowman]

There are no fewer than three (!) water temperature sensors on the ES9J4 engine according to ServiceBox:



Number 9 has a blue connector, 10 is brown, and 11 is green. Hopefully someone will be along with a wiring diagram so we can work out which one controls the fans!

[patapouf72]

Hey,

I had similar problem, temp will be at 90 then drop to 60 and back up, was like watching the rev counter.

I change the coolant and thermostat, give it a good flush first. All fine now. Very easy to do, 20 mns job.

[8Jb9]

I just read on the coupe-forum that this is a common problem on the ES9J4 (apparently not ES9J4S?), and it can be solved by changing the BLUE sensor. More details here:
http://www.406coupeclub.org/phpBB3/view ... 83&t=40126