Jason 1979

How accurate are our 40 year old temp gauges and senders?

The reason I ask it I recently installed a Dewitt’s and dual spal fans. The trigger for the fans is 195 degrees. The fans do not come on until the temp gauge shows 230ish.

I know I need to go get a digital thermometer and get the real answer, I will do that as soon as the stores open. But assuming that the spall temp sender is reading correctly, how do I fix the gauge in the car?

just got back from the store, gauge reads 230, water neck is 190, radiator was about 180 at the top and 160 at the bottom.

Redvette2

Check Willcox site... http://repairs.willcoxcorvette.com/c...re-gauge-info/

Redvette2

Jason 1979

Thanks for reminding me about Willcox. So check me here, I should...
1) disconnect wire from sender.
2) connect multimeter to sender with one lead and a good ground with the other lead.
3)start the car and check resistance about every 10 degrees and make a table.
4) compare table to the resistance listed above.

if resistance is correct for the temperature I have a gauge problem. If resistance is incorrect for the temperature I have a sender problem.

Does that sound like it would work?

Redvette2

Sounds right to me but I am not an expert.

Redvette2

Jason 1979

That procedure worked like a charm.

Sender is reading about 20% too high ohms across the board. So the gauge should be showing cooler than the engine is. I am confused.

Water neck never went above 177 after idling for 20 or 30 minutes.

jackson

AFAIK ... as coolant temp increases, gauge sender's resistance (ohms) decreases ... inverse relationship.

be clear & consistant; each time you mention sender, specify if Spal sender or gauge sender.

Jason 1979

The gauge sender is reading higher ohms than the info from Willcox at every point I could measure. Therefore the gauge should be reading cooler than the engine actually is.

The gauge consistently reads 220-230 after the car has been running for awhile.

i bought a temp gun and the car after idling for 20 minutes was showing 180ish at the water neck, 170ish top of radiator and 160ish bottom of radiator.

at 180 the sender was at 160 ohms iirc, which should be showing way cooler than 220 degrees.

Rescue Rogers

What year is your car? If you have a consistently high reading buy Willcoxs adjustable resistor. I t will correct the reading as long as your readings are consistent

Redvette2

Yep make sense...adding some series resistance in the signal line should make the reading go down. But still it seems something is out of whack. Looking at my 1968 year schematic, it shows a ground, the signal and a switched ign. + voltage connection. On my car a 180° F engine temperature shows a reading on the gauge at the exact location in the Willcox picture you attached. Good news is that your car is not overheating.

Redvette2

Jason 1979

I have to pull the dash apart to replace the wiper switch anyway. I think I will check resistance on the resistor on the back of the gauge. If it is reading too low, that might cause the issue.

something is causing the resistance to the gauge to be lower than what it actually is.

0Willcox Corvette

If the car has a factory gauge in it, the simple solution would be to replace the stock resistor with the adjustable one on the back of the gauge. The adjustable resistors can scale the gauge to the sender.

The ohms readings in the picture posted above were taken from multiple 1979 stock original NOS gauges back when they were available from GM. We've used these charts to calibrate these gauges (for setting the needle) for over 25 years. I still have the test charts in an old file in the gauge room here. So if you are not matching up with the chart then I'd look at that but the correction is simpler by just changing the resistor. I posted the link below and I've also posted the installation video.


https://willcoxcorvette.com/corvette...resistor-77-82

Jason 1979

Ordered!

thanks for the feedback and for your store and tech section. Have used both fixing this car.

0Willcox Corvette

It really is the best way to solve the issue.

Ernie