Re: C3 temperature gauge, internal wiring question

[theo law]

Hello Jack,
Thanks for the explanation, and sorry it's taken me so long to reply - I've been feverishly getting various loose ends tied up before going on holiday.
I've since run the car up to temp with thermocouples installed in the block, and it looks like I don't have much to worry about - the gauge is over-reading by a lot. With my new fans running at idle, I measure about 87C/188F in the block and top hose, with about a 40C/104F drop across the radiator! That's pretty damned effective, and I can't complain about that.
It doesn't look like it climbs much under road load either, so I'm overall pretty pleased with the setup. Just need to recalibrate the sender or gauge at some point, (which of course was what the original post was about) but now I know how the reading relates to the real temp, I can wait a while before playing with that.

Thanks all!

Theo

[Joe C.]

Re: C3 temperature gauge, internal wiring question

Theo,

If you add a variable "trim resistor", in series with the fixed electromagnet, you can dial-in the gauge to a more accurate reading. I get mine at Radio Shack.

Joe

[Jack H.]

I wouldn't do that...

add a 'triming' resistor. Essentially, you're squeezing on a baloon to FORCE the temp system's mid-range reporting to be accurate at the expense of accuracy elsewhere on the dial. The temp vs. resistance profile of the factory original temp sender is NOT a straight line and by adding a fixed resistance (either in series or in parallel), you're changing overall transfer function of the temp sender.

The system was originally designed to be VERY accurate at the top end of the dial (engine about to overheat--shut 'er DOWN) with roughly 12F of built-in inaccuracy in gauge reading at the temperature center band (even more slop the lower the actual temperature). Designers knew there'd be component to component (sender + guage) tolerance differences and accepted that SO LONG as the temp reporting system was almost dead nuts on at the high end of the dial.

Now, we compound this problem in two ways:

(1) Individual gauges are removed and perhaps 'tweaked' by human touch and/or are sent out for cosmetic refurbishment. You can't re-silk screen the face of the dial without removing the pointer needle and when you do that, kiss the factory original gauge calibration bye-bye...

(2) Most of the service replacement temp senders differ slighly from factory original in terms of their transfer function (resistance vs. temp) in order to serve a wider target market of vehicles. Therefore, they can be off even more at the gauge mid-range.

You solve #1 starting the car and reving the engine until the battery is re-charged and the electrical system is running at normal charge rate. Now, with the generator/alternator delivering charge as it would under normal crusing condition, substitutie a precision fixed resistor for the temp sender (50-55 ohm, 1/2 watt does the trick). Your temp guage ought to PEG to 240F.

If it doesn't, the guage's calibration is off and needs to be fixed. The factory calibrated these gauges using a 'simulator' that re-created bona fide engine over heat conditions and the assy operator pushed the pointer needle onto the drive shaft making the gauge read properly.

You solve #2 by once you've done #1, hand sort through various senders until you get one you're happy with its mid-dial reading.

We're so used to today's precision electronics that we 'panic' when we see our classic Corvettes mis-reporting temperature at the dial's mid-range. But, hey, that's the way it really was!

Chevy's alternative to the temp guage was ye olde 'idiot light' that came on to warn you of a temp problem and SHUT DOWN NOW! The analog system used in these older Corvettes mirrored the idiot light being highly accurate ONLY at the top end of the dial with some designed in 'slop' at the dial's mid-range.

The worst thing you could do is to 'jury rig' an existing system to force it to report accurately under normal driving conditions without understanding what you've unintentionally done to the temp system's high end reporting. Wouldn't it be a )((*&*^ to drive a car modified that way, see the temp guage climb and keep on driving because the needle hasn't gone into the dange zone while you melt down the motor?

[John L.]

Re: Jack - How did you get so smart?

Jack what type of engineer are you? I have never had such a detailed explanation as you just gave. I'm constantly amazed at the depth of knowledge that is here on this board.
Best Regards,
John Liner

[Joe C.]

Re: I wouldn't do that...

Jack,

The driver should never rely on these temp gauges as accurate instruments, because, as you correctly point out, the transfer function of resistance-to-indicated temperature is decidedly NOT linear. The gauge is therefore only accurate within a limited range of readings. Generally, if the reading rises too far from what is considered "nominal", then that's an indication that there is an overtemp condition, and caution is warranted. If a driver is so foolish as to wait for the needle to reach 240 degrees (indicated) before taking action, then he (she) might as well have been looking at an "idiot" light! Since these gauges only provide accuracy within a limited range, then I'd rather have them read accurately within the zone where they spend the most time...........the range between 160 and 200 degrees F. This allows the driver to much better monitor temperature variations during the widest range of driving conditions.

I generally move the indicated reading by no more than about 15 angular degrees,(roughly 10 degrees F., at that point of the scale), to cause an indicated 180 degrees to correspond to a measured 180 degree temp at the thermostat housing. In many cases the adjustment actually makes the gauges read more accurately at the top end of the scale, rather than less.

This t******* technique is used by more than a few members of our "family of service providers", with good results. I must respectfully disagree with your line of reasoning, although you are entitled to your opinion.

Joe

[Jack H.]

Re: I wouldn't do that...

We split hairs when we talk about linearity... Actually, the temp reporting system IS a linear analog system as the definition of linearity is based on whether or not a given system obeys the associative and commutive laws of mathematics [e.g. (A + B) + C = D is the same as A + (B + C) = D]. A non-linear system doesn't (such as switching functions).

The transfer function for the temp sender is not a straight line, it's a decaying exponential. But, that IS a linear function. But, it's not worth debate time....

The fact that the temp sender has a decaying aspect is compensated for in the guage with its two opposing electro magnets pull in the needle point AND by how the face of the gauge is graduated. If you look at the gauge face, you'll see there's not much intermediate information there... It's got cold, normal and hot gradicules with a complete lack of info regarding what the intermediate positions correspond to in terms of temperature. That's an intentional design aspect.

Yes, you can FORCE the gauge to read accurately at its mid range point, if you like, by adding 'triming' resistance to the temp sender. In my book, that's OK if you understand what you're doing and the consequence MIGHT BE that you made the system inaccurate elsewhere.

But, why not do it right and check/verify the calibration of the guage itself (per the thumbnail I gave) and then go get a correct matching sender? Personally, I'd rather take that approach vs. using a 'big hammer' Bubba technique that could backfire somewhere/some time.

Last, on why would one continue to drive when the temp needle starts to climb, well it's human nature. Few, want to shut down in the middle of nowhere and the temptation to see if you can make the next exit of the freeway or get to 'civilization' before you stop is STRONG.

Bottom line temp reporting accuracy is variable in the system with precision increasing with absolute temperature. That's just the nature of the beast. It was designed to give the driver a 'reasonable' indication of engine temperature TREND under normal driving conditions and be darn close to being on the money in a bona fide overheat threat situation.

Jury rigging it to be more than it was initially designed to be is a futile effort, in my book...

[theo law]

Re: I wouldn't do that...

Gentlemen..

A damn good discussion, but one that I'm as yet not able to properly digest, owing to my being on a very enjoyable holiday in the islands of Thailand ;-) Just got into a bar to check my emails and I find all this information! I'll look forward to having a proper read in a couple of weeks and get back to you all then, so thanks much for the input so far - thought it was worth letting you know I was still listening, but you'll forgive me if I don't worry too much about the vette for the moment!

Cheers again,
Theo