dinodon51

I would appreciate a sanity check on my sensor to temperature gauge "epiphany."

My '80 was indicating it was running on the hot side close to the red mark on a 90 plus degree day. But when I stopped and checked all was good under the hood with no indication of overheating. I've got the right sensor, a BWD 359, the radiator has been flushed and the gauge itself was roughly in calibration with the Wilcox help paper info for the model year. The key here is "roughly."

When I put an ohmmeter to the green lead and grounded it, the resistance read 70 ohms. Yet per Wilcox the resistor on the back of the gauge is rated at 90 ohms. A light went on in my used naval officer brain; the ambient system resistance must be 20 ohms too low. This 20ohm difference is trivial at low temps but at high temps will move you entire mark over from indicating warm but sane (as it should be at 95 degrees and 80 mph) to the borderline of the red death zone. I added 20 ohms of resistance to the green lead to restore the designed 90 ohm resistance, ran it on the interstate at speed and performed some full throttle accelleration runs and the gauge never moved past 220 degrees.

Does this easy fix make sense? Can a gauge savvy tech verify my logic?

DFS

0Willcox Corvette

You're close.. Now you have it working and that is great.

The info below is for others that maybe wish to go to the extreme and for those with the opposite issue you have.

More resistance does mean a lower reading.... But the change should be made on the gauge and not on the green wire. If you make the change at the gauge you'll have an accurate change across the entire field. If you add resistance to the green input wire you are accurate at one fixed point.

Issue: With the current production of bogus sending units out there (and I’ve tested them ALL) the issue is just the opposite… Showing cooler then what the car actually is. Most new sending units will only go to about 100 ohms resistance when at 212 degrees which is not enough to make most dash gauges read properly and adding resistance will only make it read cooler.

The issue is how much change is needed to go either way… Lower or higher.. For me the only way to match calibrate is to put an adjustable pot on the back of the gauge in place of the resistor and make tiny changes until they match the output of the sender being used. Once I have the change dialed in I can then add that resistance to the back of the gauge.

Induce that change (+/-) on the back of the gauge and you'll have consistent non linear change... and an accurate gauge in relation to the sending unit in the car. This is why I tell people to know what the sender output is at what temp and establish a sender output scale. Make sense?

Anyway.. my2cents… IMHO,

Willcox

dinodon51

I don't understand "one fixed point." Adding 20 ohms of resistance to the sender wire is going to add a constant 20 ohms irrespective of temperature to the variable resistance of the sensor sending unit. Since the ohm to degree indicated reading is an inverse square relationship that constant 20 ohm addition (just like the ambient 70 ohms resistance I found earlier) has a much greater impact at higher gauge temperature indications. That would at least make the gauge reading consistent with its factory design specs.

Now assuming (a very dangerous word) the sensor is okay the system should be more accurate across the board. At least I only have one component (the sensor) off.

I fully agree the ideal technical solution is to piece match the gauge to the sensor being installed. But I almost always do more damage pulling the dash apart than the original fix I intended.

...Roger...

You haven't mentioned whether you have verified the temp with a gun.

dinodon51

Tested with a themocouple between the sensor and thermostat housing at the front of the manifold. 195 degrees by the thermocouple read just a shade over the 200 hash mark (call it 201 or 202) on the gauge. This is probably still 5 ohms off (needing more resistance) but is close enough for me; I'd rather err 5 to the hot.

...Roger...

Did the thermocouple readings follow closely to the gauge readings at lower temps ?

dinodon51

Yes, the readings were consistent at around 150 and 175 degrees on the thermocouple vs the gauge. I checked at the 1st hash mark and half way to the 200. I tried to get the engine hotter but it is a cool day. The ohm requirement for the guage at the first hash mark is 219 ohms so a 20 ohm adjustment isn't going to move the needle alot.

...Roger...

Sounds like you have a winner,good job.
Thank goodness your gauge was reading high.

Budman68

So who makes the best sensors? My 68 is way off and I think I purchased it from ZIP. Moves the gauge about 1/8 to 1/4 inch.

0Willcox Corvette

Yes Roger.. you are correct.. very unusual for sure and correctable.

0Willcox Corvette

Dino- If using a temp gun to establish the temperature your change will only be accurate at a fixed point. Why.. because the actual gauge is non linear. If the gauge was a linear gauge then your change would be correct across the scale. Now this is not to say that the dash unit is not close at other fixed points...

The issue is that the hotter (lower ohms) means a drastic change in the scale vs. the cooler readings.

Again.. you've got your gauge close and that is what matters and my post was only to let others know that you can dial in the dash unit to closely match the sender across the scale.

IMHO,

Willcox

0Willcox Corvette

NO ONE.... Point blank and period!

Bud.. I've test them all.. every brand you can imagine! I don't care who is making them they are not correct. The thermister in the sender is not correct nor are the componets inside! (and I've cut all of them open seeking a solution to no avail).

again.. IMHO....

Ernie

...Roger...

As Willcox said none are 100% correct,I've had pretty good luck out of the WT203 from Advanced but even they have 2 different 203s listed.
I bring several home ,set them up in rapidly boiling water and get as close as I can to 79 ohms. This usually produces good results,if I need it to be closer I install one of my variable resistors across the back of the gauge and then dial it in.

Budman68

I'll try one of those. Since I don't have a overheating issue, I haven't worried about it. But it would be nice to know it was close.

Strokemyaxe

That's what I'm using in my '77. Gauge is reading 15-20 degrees higher when full operating temp (195) is reached. I have to pull my center cluster this weekend to install an AFR gauge so I might dive into the gauge with the fix Wilcox gave above.

-Stroke

Strokemyaxe

Thanks! Do you have a suggestion on the pot to use?

-Stroke

carriljc

Strokemyaxe-- I REALLY, REALLY, recommend just getting the adjustable resistor from Willcox.... I put one in mine and it is just fantastic. See my story below. It's easy too (well, I've had dash apart a million times in 30+ years), but it's wonderful:

https://www.corvetteforum.com/forums...-question.html

Link to willcox adjustable resistors below---I believe you have a 77 per yoru profile:
https://willcoxcorvette.com/corvette...resistor-77-82

http://repairs.willcoxcorvette.com/?...ature+resistor

theandies

Found you problem..........