Pulldown resistors for TPS MAP

T Hergen thergen at svn.net
Mon Aug 3 18:18:11 GMT 1998


I haven't tested the following on an ecm.  It's based on
input from other list members.  Comments, corrections,
etc. invited.
 
Another member (Peter) has indicated the a number of gm ecms
switch in different pullups on the temperature sensor(s).
The advantage of this is to give the ecm better temperature
measuring resolution over a particular temperature range.
Compare this to switching to the 2 volt scale on your
digital multimeter when trying to measure the O2 sensor
voltage (wouldn't make sense use the 200 volt scale).
 
The stock temperature sensor is a two wire device where
one end is connected to ground and the other to the ecm
temperature input.
 
(Use fixed width font to view)
 
Stock temp sensor:
 
     ----- to ecm CTS or MAT input
     |
     /
     \
     /  resistance varies with  temperature
     \
     /
     \
     |
     |
 gnd--
 
 
How to hook up a temperature sensor 'simulator'
 
 
  nc--  (nc = not connected)
     |
     /
     \
     /<---- to ecm CTS or MAT input
  ?k \
     /
     \
     |
     |
 gnd--
 
The ecm will switch in a different pullup when it determines
that the temperature has entered a certain range.  This
'switch point' will correspond to a particular resistance
of the temperature sensor (there's probably some hysteresis
built into the switching algorithum).  After the switch
happens, the voltage across the sensor (or pot) changes.
Even though the voltage has changed, the resistance hasn't
and this still corresponds to the same temperature as
before the switch.

The value of the resistance range of the pot should be
large enough to cover the resistance range of the
sensor it is replacing.  I don't have a table for the
CTS or MAT sensor available at the moment, anyone want
to help with this?
 
The problem with wiring CTS/MAT like a TPS is that the
voltage before and after the switch will not change the
same way as it would if only two terminals of the pot
were used.
 
 +5---
     |    DON'T USE THIS!
     /
     \
     /<---- to ecm CTS or MAT input
  5k \
     /
     \
     |
     |
 gnd--
 
The Thevenin equivalent of the TPS style hookup is a
single resistor connected to some voltage between +5 and
ground which won't give the same results as a resistor
to ground.  Math left to the user:)
 
 
Tom


On Mon, 3 Aug 1998, Bruce Plecan wrote:

> Now, that we know there is pull down resistors, what do I do with
> the 5K pots that I've been using for TPS and MAP signal inputs?.
> I'm over my head here so I need a item by item explaination.
> ie., a lot more specific than saying, buffing the output with an op-amp.
> Thanks
> Bruce
> 




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