Pulldown resistors for TPS MAP

[T Hergen]

Here's some possible values...

Peter indicated the that some gm ecms switch in a 348 Ohm or 4 KOhm pullup
(see his 7/31/98 post).  Looking at the temp vs resistance tables for a
CTS and IAT, there is quite a range to cover.  Over 100K for very low
temps and about 177 Ohm around 100C.  The 348 Ohm resistor would likely be
switched in at the higher temps.

Given this range, a 100 KOhm log taper potentiometer would work.  Use only
two terminals, the wiper and the one that shows 100 KOhm at one end and
much less than 50 KOhm when the shaft is in the center position.

With a 100 KOhm pot it will be difficult to adjust the high temperature
end of the scale with any precision.  For this reason, it would be nice to
be able to switch in a 5 KOhm to 1 KOhm pot (again using only two
terminals) to cover this range. This "user" switching of pots is
independent of when the ecm decides to switch in a different pullup.

Adding a 100 Ohm resistor in series with the wire to the ecm temperature
input would reduce the peak current that could be drawn and reduce the
chance of damaging the ecm.

Does anyone know if the 730 or 165 ecms are ones that really do switch the
pullups?

Thanks,
Tom


On Mon, 3 Aug 1998, T Hergen wrote:

> 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
> > 
>