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Re: [Quantum Owners] Re: Low Reading Temperature Gauge Jim Hearne Wed Mar 14 14:00:41 2012

The rectangular one should be connected to the gauge, the larger roundish one to the ecu.

Is it possible these are mis-connected ?

If you are getting these strange readings from the gauge sensor it could be why the gauge is reading low.

My guess would be that there is a dirty contact inside the sensor which is changing it's resistance once it goes past a certain temperature.
You may find that banging it on something would also change it's reading.
These sensors just have a small cylinder of semiconducting material inside, because you can't solder to it the connections are usually just made by 2 spring contact.

It's possible that on an oldish ecu that the temperature sensor would be just a cold/hot input but on the Zetec it's proper analog input.


On 14/03/2012 20:13, Hamish Freeman wrote:
Hi Jim,

Thanks for the resistance vs. temp figures.

Sensor positions - now that is interesting!  I have two sensors on the stat
housing, one underneath with the rectangular socket and one on top with a
roundish plug on a "not round" socket.  Long ago when I built my H4 I mapped
both the Fiesta and Escort harnesses for Old Q for the H4 "Kit-in-a-box" to
allow for using the Escort 1800 Zetec  engine attached to a modified Fiesta
loom.  Whether the paperwork for this has survived the fire is unknown at
present - I have the folder but it is covered in black soot which permeated
everything in its path!  Eddy might well have copies as these were supplied
to Old Q and I saw these at the Sampson establishment.

It is the underside one with rectangular socket that gives this rapid
resistance change at ~ 40°C with the hysteresis whilst the top one gives a
steady change (even if logarithmic) with temperature.  My understanding was
that the ECU sensor effectively acts like a flip-flop going from high to low
at the changeover point at ~40°C - depends upon how it is connected as to
which state is a 1 or 0 - which then flips between maps on the ECU.  That is
how we set up the diesel ECU systems at work.

With any luck I might have some small time tomorrow to try shorting the pins
on the rectangular socket.  I recollect that it was when I removed the top
plug that the gauge went to zero - but then I am getting old..........!

-----Original Message-----
On Behalf Of Jim Hearne
Sent: 14 March 2012 15:01
Subject: Re: [Quantum Owners] Re: Low Reading Temperature Gauge

What happens if you link together the 2 pins of the temp gauge sensors ?, it
should go up to max.
This is the one with the smaller rectangular plug on the front of the stat
housing, not the larger one with the round connector on the side.


From: "Hamish Freeman"<[EMAIL PROTECTED]>
Sent: Wednesday, March 14, 2012 2:46 PM
Subject: RE: [Quantum Owners] Re: Low Reading Temperature Gauge

Yes that is what I thought so I tested each of the sensors to see what
happened with temperature.  The gauge sensor is, as stated, a
"straight forward" thermistor but the ECU sensor is designed to switch
from high resistance to low resistance at ~ 40°C to ensure that the
ECU switches from the "Cold Map" to the "Hot Map" at the earliest
opportunity to save both on economy and the generation of HC.  This
device also has a high level of hysteresis built in to ensure that the
system does not accidentally switch back to the "Cold Map" should the
engine undergo some freak overcooling - I had to immerse the switch in
cold water to get it to switch back after heating whilst the
transition from high to low resistance takes place over about 5°C
around 40°C.  I was surprised at this response so I checked this three
times and the above characteristics were repeated each time.

Whilst the car is not suffering from poor performance,  I will check
the inlet manifold temperature sensor as I had forgotten that!  Thanks
for the tip!

This really is a matter of having everything working properly and
giving readings that are believable.  A low reading temperature gauge
is more of a distraction than anything else as it is not calibrated in
any scale but if it is not indication in the right region I start to
wonder if anything is wrong.

The worst thing in that respect is the speedo on Trish's Micra, which
reads ~9.8% high.  We complained to Nissan shortly after we bought it
to receive the reply  "If it is within 10% then it is legal and we are
not going to change it!".  Since the speedo is driven by the ECU and
recalibration is simply a number change in the ECU via the device they
use to reset the service interval, I was not impressed.  After last
year when they charged us 50% more for the last service done under the
guarantee and only completed half of it as well as cross threading the
sump plug (for which they tried to put the blame on me whilst they are
the only people to have touched it) and not finding the air-con pipe
that had worked its way loose, we will not be returning to that
garage, even to keep the computer record up to date.


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