Engine temp sending unit backwards?
 

***EDIT*** I had my multimeter on the wrong setting so I uploaded new pictures and retyped the post to avoid confusion.

My sending unit is showing 128 ohms when the engine is 228 degrees. Davepl posted a good chart showing that at that temp I should be getting 65 or 70 ohms. What ohm rating are you getting when fully warmed up and if your sender is working well, where did you get it from?

Re: Engine temp sending unit backwards?
 

The resistance of the sending unit should go down as the engine temperature increases. The wire needs to be disconnected from the sender when measuring the sender resistance. If you leave the wire attached, then the resistance measurement will not be valid.

Re: Engine temp sending unit backwards?
 

And your guage is upside too! :lol:

Re: Engine temp sending unit backwards?
 

Is the sender for a gauge or a light

Re: Engine temp sending unit backwards?
 

Good luck getting a gauge/sender combo to work. I've spent many hours on mine, and finally got it close after playing with several different shunt resistors across the top terminal and ground. The pointer doesn't start moving until ~150-160 degrees. Then it points close to straight up at around 200, and really starts moving fast after that. So it could be worse.

One of these days I'm gonna try an Autometer gauge/sender combo if I can make one fit the instrument cluster. This may be of some help, but all temp senders are not equal. https://67-72chevytrucks.com/vboard/...d.php?t=689065

Until then, here's a gauge I can depend on.;)

Re: Engine temp sending unit backwards?
 

I have a mechanical Autometer Phantom® mechanical water temp gauge. I hung that one under the dash because I didn't want to stand on my head to take the factory gauge out. The needle was going everywhere. Probably that resistor?

Re: Engine temp sending unit backwards?
 

Yeah, it's always fun to pull out the panel without removing the seat. If only I was 4" shorter and 30 lbs lighter!

Re: Engine temp sending unit backwards?
 

Oh heck, I hope there aren't 2 different types of sending units. I'll check on mine and see which it is for, thanks.

Re: Engine temp sending unit backwards?
 

Lol, I guess I could wire up the multimeter and and watch to make sure I don't go lower than 120 ohms.

Re: Engine temp sending unit backwards?
 

There are two kinds of sensors. When testing the warning light senders they will show as open below the designated temperature of the sender and zero resistance above that temperature. They are simply a temperature controlled switch.

And as stated above remove the wire before checking with a meter.

One thing that can affect how the guage reads is how the needle is positioned on the shaft. Over the 50 plus years since new many times the needle has slipped on the shaft and that affects the needle position at normal running temperature.

Re: Engine temp sending unit backwards?
 

Okay, my sender is not a light sender then because mine moves from 783 ohms when the engine is cold, to 128 ohms when the engine is warm. Yeah, I can only check the sending unit if I remove the sending unit wire because the wire covers the sending unit connection.

Re: Engine temp sending unit backwards?
 

All I know is you can buy a three of them, and all would be a little different. And probably none exactly like your original. Even the originals from the factory may have varied and were paired with the appropriate shunt resistor for the gauge.

Re: Engine temp sending unit backwards?
 

Hey can anyone here connect a multimeter to a ground and their sending unit to see how many ohms they get when their engine is warm?

Re: Engine temp sending unit backwards?
 

Just an observation, in your first post the picture shows 128K ohms, not 128 ohms.
davepl gave us a chart of the exact readings from his temp sending in this post: https://67-72chevytrucks.com/vboard/...53#post7680753
This is about where the temperature gauge sits when fully warm
https://67-72chevytrucks.com/vboard/...0&d=1559952213

Re: Engine temp sending unit backwards?
 

Thanks, I see he was getting about 70 ohms at 200 degrees. Graphs I've seen show 80 to 90 ohms at 200. My gauge doesn't start moving until I hit 120 ohms so it makes sense that my gauge just reads cold all the time. I wonder if I can change out the resistor plate on the back of my gauge.

Are you sure about the 128,000 ohms? My multimeter is showing 0.128 kiloohms which is 128 ohms. But, I'm new to all of this so I may be wrong.

Re: Engine temp sending unit backwards?
 

My mistake, I didn’t see the decimal point.

Re: Engine temp sending unit backwards?
 

I bought a potentiometer so I could test the LMC gauge. My numbers are coming up close to what Davepl showed in his GM chart so my gauge is good but my sending unit is not showing numbers anywhere near what Dave found when bench tested a sending unit with a multimeter.

My gauge is showing that my engine is 170 degrees but a temp gun shows the engine is 220 degrees. My sending unit is not a good match for the reproduction stock gauges and isn't in line with what GM shows on their charts.

I'll have to see if I can get a better sending unit or replace the resistor on the back of my gauge with a lower ohm resistor.

Re: Engine temp sending unit backwards?
 

The common currently available (last 20 years) sending units are trash. Only somewhat accurate units are old NOS AC or Lectric Limited branded sending units.

Re: Engine temp sending unit backwards?
 

https://67-72chevytrucks.com/vboard/...1&d=1766513069

That's close to how mine works now with an aftermarket sender. Mine's either AC Delco or whatever NAPA sold me. I can't remember the shunt resistor value.

My truck's operating temp is typically in the 190-200 range according to my engine compartment thermometer. So if the pointer is between 11:30 and 12:30, all is well. Still want to check out an AutoMeter gauge someday to see if I can make it fit. I've used the Sport Comp series electric gauges on two vehicles, and they work well.

Re: Engine temp sending unit backwards?
 

This is the one I use, https://www.amazon.com/ACDelco-G1852...dp/B000C9SM22/, and I had it turned and re-threaded to 3/8 NPT by forum member A1971Blazer, details here in this thread, https://67-72chevytrucks.com/vboard/...d.php?t=703388

Re: Engine temp sending unit backwards?
 

If you have the new sending unit and reproduction gauge. I do not think the resistor on gauge is required. Maybe try that.

Re: Engine temp sending unit backwards?
 

Same here.
I re-engined my '71 GMC Jimmy K/1500 in 1999 with a Mr. Goodwrench 350 crate. The new electric Water Temp sender [from LMC -- I think] didn't work. I was told I needed to add some Resistors inline with the wire. That didn't work, so went with a mechanical AutoMeter gauge in an underdash kneescraper mount. Works great.
:gmc2:

Re: Engine temp sending unit backwards?
 

^ Yep, Bourdon tube is pretty reliable tech.