diagnostic light

gradeafailure · Post by **gradeafailure** » 13 Feb 2004, 15:38

so, i was driving home from work last night, contemplating the weekend of tinkering on the ebay car, when the orange engine diagnostic light comes on on the dash - happened when doing some dual-carriageway foot-to-the-floor overtaking... have discovered that it only comes on when the accelerator is actually on the floor, and about 5secs after lifting off the pedal completely, it goes out. anyone know what that light actually signifies, and if there are any common causes?
cheers guys!

alan s · Post by **alan s** » 13 Feb 2004, 16:14

The best description of this light's operation I was told was that it basically is the engine management system saying "Hey Dude; something's malfunctioned." Nothing sinister necessarily, just to let you know that something has been playing up.
We have a BX TZi that used to do it occasionally. We tracked it down to crap getting on the Lambda (O2) sensor if the car didn't start right the first time. Our suspicions were that the car was about 12 years old with 90K klms on the speedo & almost all of it in City driving hence the sensor possibly coated with crap.
When it did it, we switched the motor off, restarted & it went OK.
Eventually, we did a battery disconnect which wiped the memory & we have lived happily ever after.
It is usually a matter of it letting you know something is on teh way out although at times, as in outr case, it can just be a case of a hiccup in the system & it letting you know about it.
If you disconnect the battery for a couple of hours & then reconnect it can often sort it. Just be sure that you know any codes you may require first before you follow that course of action.
Alan S [:D]

gradeafailure · Post by **gradeafailure** » 13 Feb 2004, 17:29

cheers alan, shall give that a try tomorrow along with all the other stuff i'm doing as well... engine seems to be rather thirsty as well, along with an idle that hunts slightly, so shall give it a service and try and have a look at the lambda sensor as well...

AndersDK · Post by **AndersDK** » 13 Feb 2004, 19:54

A '91 XU engine would have the 2pole engine temp sensor for the ECU - working as the "auto-choke".
If this fails - or it's connector - the idle will be erratic - tending to engine stall.
As this sensor is a NTC resistor with HI resistance at cold engine - a disrupted sensor or cable (= HI resistance) signals the ECU - that a hot engine is still "cold" - thus causing constant "choke" - detoriating mpg values & causing random error lamp.
Likewise - the air temp sensor (ATS) is the same type of sensor - and may cause the ECU to control too rich mix.

gradeafailure · Post by **gradeafailure** » 13 Feb 2004, 20:15

anders, many thanks for that - you are indeed a font of great knowledge! where are the sensors located on the engine, do you know?

AndersDK · Post by **AndersDK** » 13 Feb 2004, 20:38

Engine temp sensor (CTS) to be found at or near the thermostat housing. Either green or blue plastic memberpiece with 2pole contact.
Air temp sensor (ATS) to be found on throttle body - usually right above the throttle flap - seen as a tiny device reaching into the airstream.
Both NTC sensors reach up in 10K ohms range if very cold - and down to 500 ohms range when hot. It's not the exact ohm's reading that counts - but the huge ohms variation over temp.
On Gabor's BX DIY site you have some very fine diagnostic help on these engine systems - valid for all PSA engines the same type as found in the BX'es :
http://www.tramontana.co.hu/citroen/fuel/test.html

gradeafailure · Post by **gradeafailure** » 13 Feb 2004, 20:42

looks like i'll be taking the dvm home tonight as well then!