From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Hey Neon
Date: Sat, 19 May 2001 22:03:40 -0400

Chris Hill wrote:
>
> On Sat, 19 May 2001 04:57:28 -0400, bill horne <reddog@rye.net> wrote:
>
> >There seems to be a Transmission Fluid Temp (TFT) sensor in my trans
> >consisting of nothing more than a thermistor whose characteristics are
> >listed in the service manual. Have you perhaps come up with a simple way
> >to get this into a digital readout of temp by simply tapping onto the
> >wire that leads from the thermistor to the computer?
> >
> >What we're talking about here is a 96 Chevy, 5.7L, 4L60-E, and a driver
> >who is a bit reluctant to tinker with the inputs to a computer of which
> >he knows nothing useful about its resistance to smoke leaks.
> >
> >However, one problem I might see is that the closer you get to 200-210
> >F, the flatter the resistance curve gets.
>
> The trouble is that if you merely tap into the wire, won't the
> resistance be different because of the tapping or the connection of
> two devices to the wire?

Not if done properly.  The ECU sends some semblance of a constant
current through these thermisters and monitors the voltage drop.
One can simply tap in with another high impedance voltage monitor
and read the same signal without altering it.

> Since your vehicle is a '96 or newer, you
> might consider and obd II scanning tool.  If you have a laptop, the
> autotap (www.autotap.com) would be one to try.  I have one and it
> shows trani' temp on my Ford, and even allows me to log temps so they
> can be checked later.

Probably a good way to do it.  Bill, sorry I didn't reply sooner.
I'm not aware of an automotive-related instrument that can read the
temperature sensor which is usually a 10kohm thermister. The
thermister response is very non-linear so any candidate instrument
would have to have linearization routines built in.  Summit Racing
and others sell digital temperature gauges but they require their
own sensor.  I imagine that with a little PCB work, one could
disable the current feed to the sensor, then parallel the meter with
the ECU and monitor it that way.  I just don't know of an
off-the-shelf instrument.  If you can't do the scantool solution or
don't want the scan tool laying around all the time, a digital oil
temperature gauge might be the solution even if it requires mounting
a separate sensor.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Hey Neon
Date: Mon, 21 May 2001 18:52:01 -0400

bill horne wrote:

> Thanks, John. If you should ever cobble up a device to convert the
> voltage or current for that sensor to temp numbers, I'm a potential
> customer. Meanwhile, I think I'll just hook up a DVM to determine the
> voltage at 'normal' operating temp. Maybe I can use that as a "Hey, the
> voltage says You're somewhere above the normal temp" indicator.

My quickie solution to one-off nonlinear measurements like this is
to use an analog meter.  Include a high impedance buffer amp if
necessary.  Then simply make a non-linear dial face for the meter.
Trivially easy to do with either a laser or ink jet printer and a
scanner.  I just scan the old face in, edit out the numbers and
graduations, if necessary and then add in the new ones.  Print it
out and paste the new face on the BACK side of the old one.  The
back side is usually bare metal.  With the old face still on the
meter, simply input known values (putting the sensor in a water bath
will do) and make tick marks on the dial with a pen.  When you scan
the dial, you have your fiducal marks already in place and scaled
properly.

Even simpler would be to put the sensor in oil at the high alarm
point and make a mark on the meter dial with a felt tipped pen.
When the pointer approaches the mark, you need to do something.

One other thing you might consider is an alarm unit.  An alarm light
(aka idiot lite) is much more likely to get your attention than a
gauge.  TimeMark Corp, among others, makes a simple little module
that looks like an octal base plug-in relay that closes a contact
when a setpoint voltage is reached.  I'm using one on my automatic
battery discharge tester (discharges a battery to a pre-set voltage
while recording the amp-hours).  The one I'm using is PN
98A00433-05, "2601-12vdc Voltage Sensor"  This one has a span of 10
to 14 volts.  They make 'em for many different voltages.  They have
a web site.  Their address is 11440 E. Pine St, Tulsa, OK 74116.

These voltage sensors are cheap and are handy for a wide variety of
things on a vehicle.  I have another one that will trigger my
battery disconnect relay in the event the power gets disconnected to
the MH while parked and the battery runs down to 11 volts.  It could
also be used to monitor, say, the oil pressure sender signal and
trip an idiot light when the oil pressure is low.