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gkd000

Outside Temperature Sensor - P38 DSE?

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The HEVAC on my 98 DSE is showing an external Temp of -40 degrees.  This means that the electric fans mounted in front of the radiator don't kick in so when sitting in traffic the car gets hot (doesn't overheat but gets near the red).  Once moving again all is well.

I think my external temp sensor is the likely culprit (given the reading).

I can't locate it (RAVE is confusing me as to the actual location - behind bumper / near wipers?)

Any ideas / suggestions.  Or am I likely mis-diagnosing the problem.

(The car is now MOT'd, running OK [apart from above] and all advisories on the MOT have also been sorted.  Only taken me since January to get this far!).  I'm getting there slowly......

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3 hours ago, gkd000 said:

... I think my external temp sensor is the likely culprit (given the reading).
I can't locate it (RAVE is confusing me as to the actual location - behind bumper / near wipers?)

I'm not sure where the RAVE confusion is arising, RAVE clearly defines the position of the ambient air temperature when manufactured.
20 years later it's always possible that a PO has moved the sensor from the earlier position to the later position, however, the dividing point at manufacture is set out in the air conditioning section of the Workshop Manual, where it states quite clearly:
'Ambient air temperature sensor.
The ambient temperature sensor provides the ATC ECU with an input of external air temperature.
Up to VIN 381430, the sensor is installed in the LH air inlet housing.
From VIN 381431, the sensor is installed in a bracket attached to the LH chassis rail, behind the front bumper and immediately in front of the condenser.'

What is your VIN?

However, I recall that the twin fans are switched on via two alternative sources.
One is the ATC ECU, the other is from the engine ECU.

You are concerned about engine temperature, so I think you should be showing more interest in an engine temperature sensor.

I recall there being two temperature sensors on the engine, one is for the gauge in the instrument panel, the other is involved in running the engine.
To see the details for the latter you need to look in the ETM (Electrical Troubleshooting Manual) (Circuit, section A6-Diesel).

Engine Coolant Temperature Sensor (X126)
This sensor is a ‘thermistor’ (a temperature dependent resistor) where the voltage output varies in proportion to coolant temperature.
The ECM (Z132) uses this information in many strategies, i.e. to correct the injected fuel quantity and timing (especially during cold starts), length of glow plug timing, etc.
The sensor is located in the top of the engine block.
In case of a failure, the warning lamp is not activated and the ECM (Z132) selects a substitute value of 50C for glow plug and ignition timing and uses the fuel temperature to correct the
fuel quantity, glow plug timing will not be correct, possibly resulting in long crank times in cold weather.

Correction.
After re-reading the ETM sections involved, it seems that it is only the Petrol ECM that has the facility to switch on the condenser fans to aid engine cooling.
My direct experience was 15+ years ago, so it's possible I have mis-remembered.

Regards.

 

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Thanks for the comprehensive reply.  387167 are last 6 of VIN so should be located where I have been looking  but can't see it at the moment (hence my confusion).  I'll look again...... 

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-40° is the default value for maximum resistance or open circuit. So it could be the sensor really is missing. Did you ever get a normal reading for the temperature?

I ca also confirm the diesel engine can't have the fans switched on by the engine ECM, only by the A/C pressure switches. So your overheating problem has a different cause. If it happens in traffic, it could be the viscous fan that slips too much to provide enough airflow at low rpm. But could also be the water pump not giving enough flow or the radiator that has lost too much efficiency to cope with the lower air and coolant flow when idling. At least head gasket failure is unlikely, as that would be most obvious under load.

Filip

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Thanks for the reply Filip.  No never had a proper reading always been at -40 since I got it in January, just been sorting out all the other (many) jobs and this is one of the last I need to sort.  I have had another look and can't see the sensor on the chassis, so it may well be that it is missing, though I can't see any cut wires either.  I'll have to strip the bumper etc off and have a better look when I have time.      

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There should be a 2-wire black connector behind the bumper, under the A/C fans, where the sensor connects to. From there the wiring goes to a big 14-wire white connector in the LH footwell. There are pics in Rave to help you.

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Dug round further behind the bumper.  Sensor was not on the bracket and wedged loose inside the bottom of the bumper and the wires torn.  I assume that whoever took the bumper off last has caught it when refitting the bumper and not noticed (or not bothered) to fix it.  I have repaired the wires refitted the sensor and now have a working outside temperature.  Still have the HEVAC warning book on the display though, I revisit that at another time.  Thanks for the help.

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Good you got that sorted! The HVAC will need to have the faults cleared, but it should work if there aren't any major physical problems.

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1 hour ago, Escape said:

... The HVAC will need to have the faults cleared, but it should work if there aren't any major physical problems.

My recollection about the HEVAC was that it rechecked its status every time it was switched on, so if all the faults were cleared, a simple switch off and switch on banished the book and exclamation symbol.

The trick bit is finding what exactly is still faulty!!.

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Most faults are indeed cleared at switch on, at least until the problem occurs again. Something like a disconnected sensor or A/C compressor might be bad enough to throw a hard fault. Which still wouldn't stop the other functions form operating as expected.

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