Went to Radio Shack and bought $6 worth of stuff to dial in a temp gauge using a WT 203 sender. The original gauge has a resistor across the back that is called a "90" ohm resistor but most are around 85 ohms.This harness will take the place of the original "90" ohm resisitor.
The 203 sender needs aprox. 95 ohms to make it read accurately. Radio Shack only has 1 potentiometer that will work and its 25 ohms,so I combined a couple resistors to come up with 78-113 ohms. This a prototype using easily obtainable parts. I'm working a more refined smaller less bulky harness that will attach permanently to the back of the gauge and extend from beneath the dash for easy tweaking of the gauge.


This pic shows the original "90" ohm resistor removed and the adjustable harness installed.

With this length of wire the pot can hang out below while tweaking or it can be run out above through the wiper switch opening. Run the engine , check the engine with a temp gun, now just turn the pot until the gauge reads the temp of the temp gun.

Here is a gauge accuracy tester I put together that when attached to the sender wire will make the gauge read 210 degrees if the gauge is accurate. Its 79 ohms.

This is the much smaller pot I'm working on now.

 

Whoa.....this is MacGyver level stuff.........I like it and appreciate the work. Just tell me in plain English plz.

 

I'm just learning this myself so thats about as plain as I think I can put it.
Here how about this. Put this thing in the pic on the back of your temp gauge and set the needle where you want.

 

Good job Roger. Keep it coming! By the way, did you get a chance to take an ohms reading of a temp gauge - ohms wire to ground. I would be interested in the results.
Thanks

 

I run a calibration lab here in Florida...
we could use a guy like you down here.
Nice work!

 

Roger, great posts on this subject.
Not boring at all.

 

Now when your car runs hot, you can just turn the pot and bam its cool again. Good to Go.

 

Excellent work as ever Roger. So this will work with any sender once callibrated and the guage will read correct across the scale and not in just one place?

When shooting with the IR gun, Where is the best place to take a reading? I take it the lower rad hose is going to be hotter than the upper when all warmed up.

 

Jim I just checked the 68 car and it showed 214. I had another guy stop by with a 71 and it showed 207.

 

Thanks ..... You know , I am a snowbird !!!

Thanks its a lot of fun.

 

Thanks,
The readings I took (posted in the other thread) with a WT203 by gun seemed to be very accurate from around 180 and up. I didn't focus on the cold side of the gauge at this point. I'm reasonably confident that any sender within reason will be accurate from 180 and up. Partly because Willcox told me it should be using this method.
I have been able to dial in the Lectric Limited sender also.
The LL sender takes aprox 125 ohms on the back of the gauge where the WT203 takes aprox 95 ohms and the original Delcos were around 85 ohms.
As you can see very little differences in ohms changes on the back of the gauges make a big difference in the accuracy.

I have been using the temp gun on top of the thermostat housing and at the base of the housing on the drivers side of the intake.

EDIT - here is the other thread and some temp readings.
http://forums.corvetteforum.com/c3-t...e-success.html

 

I read the other post the other day. I like all this kind of stuff! All bookmarked and added to my list of things to do and check! lol.

 

I did a little more testing today using different input signals gathered from different senders and changing the back of the gauge resistor. These are just approximations based on limited senders to get a gauge to read correctly using boiling water as a test temperature.
Sender........................resistor needed on back of gauge
Original AC Delco.............86 ohms
BWD WT203...................95 ohms
Lectric Limited...............125 ohms
Autozone TU5...............126 ohms

Keep in mind all these senders vary somewhat. In general you can put a fixed resistor across the back and be done but it will take some testing to figure out the exact fixed resistor you need. Thats why I think building or buying an adjustable harness would be the easiest for most guys.
Some people set their alarm clock ahead 10 min. , with the adjustable harness you could set the temperature to your comfort level ,for some dead nuts ,for others 10 degrees hotter than the engine actually is.
Unless some EEs speak up and say the harness I built will melt in time, I think I have it tweaked close enough to approach Willcox and see what he thinks about "him" building these in "his house" and selling them.
Any thoughts ???

Anyone care to test one of my prototypes ? If not maybe they can be Christmas gifts this year.

 

Here is my latest way to make the gauge adjustable. This uses the original wire wound resistor which eliminated almost all the heat in the mini pot.

 

This is really great research work!

Have you ever looked at the standard replacement sensors for Vortec heads? I've tried a bunch (not doing any research, but just install and try) and finally gave up and started drilling/tapping the heads for a standard sensor.

 

If they look like a regular sender , I would guess they could be dialed in the same way.
Where do they put the needle on a C3 , first hash mark ?

 

Hi Roger,
I'd be glad to test one for you. Theres a big corvette car show & weekend here in eureka springs ark on sept 29th thru oct 3rd. This would be a great place to test this prototype harness. I'll put at least 300 to 400 miles 0n the car during the weekend driving thru all the ozark mountains, hills and dales. I'll get a better test on the new wt203 sensor I put in yesterday but it looks like its running 20 to 22 degrees warmer then my mechanical gauge I have installed, but is far better temp reading then the old sender I had in there. Going to a neat little car show tomorrow early morning (weather permitting) and a cruisin in the afternoon, so I should get better readings from the gauges to compare them.
I also do not think this is boring or a boring subject so keep up the good work and thanks for sharing this information.

 

Thanks Greg , I'll send you one that starts about 73 ohms so if you need to make it read a little cooler you'll have that option.

 

Roger

was the original resistor across both the terminals of the guage? If I remember all the work on guages (had to test and build a multimeter) there was a resistor across the back of the meter movment to limit tha amount of current through the meter movement. The one I was working with was rated at 30-50 micro-amps

 

Where did you get the smaller pot from?