<VV> Thermister Swap?

[MarK Durham]

Jim, that's excellent news. And I am not surprised that the gauges were off
by that much.

Mark Durham
Hauser, Idaho
62 Monza coupe Red/Red 4 speed

On Sun, Sep 7, 2014 at 5:38 PM, Jim Simpson <simpsonj at verizon.net> wrote:

> 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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>>
>