<VV> Thermister Swap?

Mon Sep 8 12:43:20 EDT 2014

Seth, I agree that having more samples of the AC thermistor would be
great.  I did get an offer of a few used ones to check and may give a try.

The issue is a high temperature oil bath; I'd want to use silicone oil (Dow
Corning 550 seems like a good fluid).  I'd hate to have an oil fire at the
high end of the temperature run.

Does anyone in VV have access to a high temp oil bath?  Something that
would go to 500 0r 550 F?

I agree that the "as installed" data would be good as well.  In other
words, how does that common poor grounding of the Corsa instrument panel
affect things.  As I noted in another reply, the gauge seems to be pretty
much immune from supply voltage fluctuations.  But the gauge is really a
current meter, so if the ground or supply lines are badly corroded and
limit the supply current, all bets are off.

My tests were on a bench with direct connections to the power supply and
instruments.

Jim Simpson
Group Corvair

On Mon, Sep 8, 2014 at 12:03 AM, <Sethracer at aol.com> 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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