<VV> Thermister Swap?

Jim Simpson simpsonj at verizon.net
Mon Sep 8 12:12:21 EDT 2014 

No debate that there are much higher quality temperature sensor systems
(gauge & probe) than the stock GM ones.  If I were doing bench testing of
engine designs or perhaps racing, I'd look at a new system.  But I'm
looking for a "stock" solution if possible.  That is, use the GM gauge and
find a substitute for the original AC thermistors.

I don't think the AC thermistor uses a nichrome wire; I think they used a
metal-oxide ceramic more typical of a standard thermistor.  If nothing
else, they didn't need the fast response that a air mass sensor provides
for your EFI system.

Jim Simpson
Group Corvair

On Mon, Sep 8, 2014 at 10:52 AM, mike & bev mann <vair65 at sisna.com> wrote:

> A couple of points to remember
> 1-Gauge and thermister was designed and built in the 60's and the quality
> is probably no better then +-20% which is a large error at 500 degrees.
> 2-thermister probably uses a nichrome wire wire which as the heat increses
> would cause the resistance in the wire to increase and thus the reading in
> the guage would rise. The air mass sensor in my efi works on the same
> principle as more air goes by the "hot wire" the resistance is increased
> causing the ecu to send more fuel to the engine.
> 3-You are dealing with a varable resistor!
> 4-I happen to use westech guages and sendors and they have a sender that
> is a direct bolt in for a Corvair but both the guage and sender must be
> used to get correct reading. These are aircraft approved instruments so the
> quality is much better than the stock guages.
> Mike Mann
> On Sun, Sep 7, 2014 at 10:03 PM, Sethracer--- via VirtualVairs <
> virtualvairs at corvair.org> wrote:
>
>> A couple of things struck me, reading the last few posts on the
>> thermistor.
>>  I have a copy of Herb's characteristic curve for the sender. Herb isn't
>> around  to ask his sampling methods, but it would be interesting to know
>> the
>> original design curve for this unit, not just a single sample. At least a
>> few more units through test would provide more confidence that this
>> represents  the designed goal. The same is really true in the instrument
>> area. In
>> addition  to the input voltage - and it would be nice to see what the real
>> world provides  at a Corsa dash plug under running conditions - the level
>> of
>> grounding  of the instrument panel assembly (a Corvair 'issue" over the
>> years),
>> would be  nice to instrument. This is all good info. - Seth
>>
>>
>> In a message dated 9/7/2014 6:03:10 P.M. Pacific Daylight Time,
>> virtualvairs at corvair.org writes:
>>
>> I've  done some more research on the Corsa thermistor, stock gauge and
>> the
>> Rochester thermistor and I may have been WRONG in my earlier posting
>> saying
>> that the Rochester thermistor with a fixed resistor wouldn't  work.
>>
>> The major problem I have is that there's little data on the stock  GM/AC
>> Delco thermistor.  There's one plot made by Herb Berkman back in  1974 of
>> resistance versus temperature.  There's no data I can find  regarding the
>> Corsa dash gauge; I think we've all assumed that GM designed  the gauge to
>> match the thermistor characteristic curve.
>>
>> Since there  was little information on the thermistor, and I don't have a
>> handful to  make new measurements from, I decided to back into the problem
>> and see what  the Corsa gauge "expected" to see at various temperature
>> readings.  At  least I had three gauges to check and see if they were
>> consistent.   Basically I hooked up a Corsa head temp gauge and
>> substituted
>> a variable  resistor and measured the the resistance needed to make the
>> gauge indicate  various temperatures.
>>
>> I was shocked by my results!  If Mr.  Berkman's data are correct, the
>> stock
>> Corsa setup is about correct at only  200 degs F cylinder head temp.
>> After
>> that, it reads progressively  LOW.  Based upon my measurements of the
>> gauge
>> response and Mr.  Berkman's data, I'd estimate that at a real head temp of
>> 500 F, the Corsa  gauge is indicating about 350 F.  That's a scary error!
>> I'd rather it  read too high than too low!  Basically the stock Corsa
>> gauge
>> and the  AC/Delco thermistor don't match each.
>>
>> Regarding the Rochester  thermistor, the only specs I can find were
>> published in a Robinson  Helicopter instruments maintenance manual.  (I
>> can't find any specs on  the Rochester web page; perhaps I just don't know
>> where to look...)   If the Robinson data is correct, then adding a 60 ohm
>> resistor in series  with the Rochester thermistor would make the stock
>> Corsa
>> gauge work pretty  well.  It would be just about right on at 500 degrees
>> and
>> track quite  well at lower temperatures -- probably well within your
>> ability
>> to read it  accurately.  (Above 500 degrees, the Corsa gauge with
>> Rochester
>> thermistor + 60 ohm resistor would again read lower than the  actual temp.
>> But I would be backing off by the time my car's heads hit 500  F...)
>>
>> So I now need to get the real specs on the Rochester thermistor  or
>> actually
>> buy one and measure it's resistance vs temperature  curve.
>>
>> If anyone want's to see my data and analysis, drop me a note and  I'll
>> forward a copy of the spreadsheet with graphs.
>>
>> Jim  Simpson
>> Group Corvair
>>
>> On Fri, Sep 5, 2014 at 4:33 PM, MarK Durham  <62vair at gmail.com> wrote:
>>
>> > Jim is exactly right. It would be  accurate at one point, and less
>> accurate
>> > progressively along the scale  going up or down from that point. That is
>> why
>> > I said on the EFI, it  took an approximate reading to fool the computer
>> in
>> > switching to  normal temp mode form cold start mode and a resistor did
>> that.
>> > The  computer only needed to see an approximate value within a  range.
>> >
>> > A guage needs to be more accurate than  that.
>> >
>> > Guys, its this type of thing that is happening to our  aging cars that
>> > prompted me to suggest installing a new movement in  the dash to replace
>> the
>> > old one, (and you keep the original look) and  to install a matching new
>> > thermistor in the engine, so you have  correct reading gauges. If its
>> CHT,
>> > VDO makes a 90 degree sweep guage  that could be a direct replacement
>> with a
>> > VDO probe. Then your CHT  would be good for longer than most of us would
>> > have the cars! VDO also  does transmission temp or oil temp gauges.
>> >  Regards
>> >
>> >
>> >
>> > Mark Durham
>> > Hauser,  Idaho
>> > 62 Monza coupe Red/Red 4 speed
>> >
>> >
>> > On Fri,  Sep 5, 2014 at 12:32 PM, Jim Simpson via VirtualVairs <
>> >  virtualvairs at corvair.org> wrote:
>> >
>> >> The short answer is  no, it won't work.  The longer answer is that yes,
>> you
>> >> can  put a suitable resistor in series with the Rochester thermistor
>> and
>> >> get
>> >> a correct reading at one (1)  temperature.  Pick the temperature you
>> want
>> >> to
>> >> be  "accurate" and you could find a resistor that would give you that
>> one
>> >> reading.
>> >>
>> >> The problem is that the  Rochester and GM thermistors have different
>> >> temperature vs  resistance curves.  Even with the addition of a
>> resistor,
>> >> the  curves will not overlay.   All you can do is make them cross  each
>> >> other
>> >> at some point.
>> >>
>> >> If  you want to use the Rochester thermistor, you will have to make
>> (or
>> >> have
>> >> made) a black box to translate from it's  response curve to the gauge
>> >> response curve.  In principle,  not a hard thing to do.  But it would
>> take
>> >> a
>> >>  little time and tinkering.
>> >>
>> >> I've been thinking about it  a little and may give it a try in my spare
>> >> time
>> >> in  order to refresh my electronics skills.  But don't hold your
>> breath
>> >> waiting for me to get it done...
>> >>
>> >>  Jim Simpson
>> >> Group Corvair
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