<VV> Thermister Swap?

[Jim Simpson]

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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