Just installed a new temp sender I bought from a major Corvette parts vender. They said it is the correct sender for a 1970, 350/300, with a straight pin connector. After installation, the temp was still not correct. I have an IR temp gun. I measured the resistance of the sender while installed and the green wire disconnected. At ambient temp, about 85*, it measured 909 ohms. Am I doing something wrong? I tested it at least three times with the same results. Do I have a defective sender?

Just for grins I tested the resistance in the green wire to ground. Ignition off I got 56 ohms and with the ignition on, 0 ohms.

I need to get this ironed out before moving to the gauge. I’m going to buy a potentiometer this afternoon.

 

The only way IMO to test the sender is put it in boiling water and check the ohms reading.

 

I checked the sender that I removed the same way, there is about a 400 ohm difference between them.

 

Not sure what 400 ohms difference will give us but bottom line you need the new sender to be about 80 ohms in boiling water.
212 degrees = 78-82 ohms

Mind me asking who the new sender came from ?

 

put a resistor in line or on the back side of gauge to dial it in?

 

I got it from Zip. They're good folks but I think the part is set wrong or defective. Just performed the boiling test. Water temp 209 and it read 130 ohms.

 

That won't get it , your probably registering cold all the time,common problem.

 

HAve you done the standard gauge test ?
Key on
sender wire off - needle goes to cold
ground sender wire-needle goes to hot
If you have done this , I would suggest you go to Advanced Auto or Orileys and pick up a BWD #WT203 , bring it home and test it in boiling water. If it ohms out around 82 put it in your car and try it.

 

Do you by any chance have the original AC sender ?

 

Ironically, I found a sender that matches almost identical to the original. It came out of a cheap Equus water temp gauge I picked up from Autozone. With this sender, the gauge is almost dead on from 190-200 degrees as confirmed by an infared thermometer.

I agree with Roger on the WT203. It reads a tad high but overall it's one of the closest matching ones out there.

 

Yes, I did the test. I even got a potentiometer from Radio Shack, the gauge read 210 and I measured 83 ohms on the pot. I decided to get the sender you recommended from Auto Zone. It seems to read closer than it ever has. Still reads a little low, but well within my standards. Thanks everyone.

 

I'll have to go back and check BUT Autozone usually crosses the WT203 to a TU5 , and the TU5 was not a good match.

The BWD WT 203 from Advanced and Orileys were the closest matches.
This is why I always recommend that you put the sender in boiling water and make sure the ohms reading is around 80 ohms.
To me the boiling water standard is easy to achieve because you don't even need a thermometer ,just an ohm meter.

 

Just to confirm.
If you mean "a little high" in "ohms" which will make the gauge read a little "low" temperature wise then you are correct.

FACT:
THE HIGHER THE OHMS READING FROM THE SENDER,THE LOWER THE TEMP READINGS ON THE GAUGE.

 

Purchased last year Orileys and pick up a BWD #WT203 ($6.95), (after trying the unit w/Zip @ $22), and today's first run of the season, gauge reads ~230, with IR gun on the sender ~230, with IR gun on the stat house 180-185. I think you can't ask for better than that. Good Luck

 

In most cases you will not find a current production sending unit to work with your car. I have tested to my knowledge all brands with no consistent luck. Sad but true... There are the rare times when you'll get lucky and find one that will work but to me the odds are very slim.

Testing the senders in boiling water before installing is the best way and knowing what input is required by the gauge to read each temperature is also a must.

When we did the testing for each gauge we used NOS gauges in an attempt to find out an approximate scale for our shop. The issue is that not all dash gauges are in the same shape and over time the dampening oil will also deteriorate.

It is always best to either find an original sending unit or repair the one you removed from the car if it functioned properly.

Roger did a thread on here a while back showing how you can cut the top out of one of the senders. In most cases the original sender is corroded inside and by taking it apart you can clean them and make them work. Is this practical.. Nope. You have to cut through the black plastic top and then epoxy it back together.

Inserting resistance inline is not an option and will only give you 1 correct reading on the gauge.
and adding resistance gives you a cooler face reading. But even then some senders will just go nuts on you and read way high.

I think the best way is to either Add or Subtract resistance to the back of the gauge (see the 90 ohm resistor in the pic below) to make a the gauge function with the output of the sending unit. To do this you'll need to know the output of the new sending unit at the various temperatures and then change the 90 ohm resistor to make the correction. This will give you a linear change to the non linear gauge. (in other words it will work correctly too)

Where a problem comes in with this is that most new sending units will not achieve below 100 ohms and they usually reach this when they are around 200-210 degrees.. some will run up in a normal fashion and the shoot high. As you can see below a stock gauge requires 79 ohms to reach 210.

My advice is to always use an original or a repaired original if possible.

IMHO

(ignore the test codes to the right of the chart, they are the settings on my tester)

 

interestingly I can't remember who I was speaking with on this topic a few years ago, mybe Summit or maybe Wilcox, and they advised me that most of the new sensors will not read true so I was best to not put in a new one. Mine reads high, not low so I could probably use a resistor. I think mine is from Summit from the looks of it.

 

I made a typo above.. you may want to re-read this.

You can change the resistance on the back of the gauge to make a correction either plus or minus, just don't do it inline unless you only care if the gauge is accurate at a fixed point.

The senders have been messed up for years... and I'm wondering now if we should not maybe offer a match service with gauge. Do it on exchange.. Something like we pick a year gauge, match the sender to it and include both.

Not sure about this yet.

Willcox