Which Coolant Temp Sending Unit Do I Need?

[formula jg]

'79 w/gauges, originally 301 now stroked 461 ('73 block with 6X). I can't recall if I transposed the sending unit from the 301 to the 461 (many years ago) but it has the nub connector and hasn't been accurate for as long as I can recall.  I replaced the sending unit with a WVE 1T1049 which had a spade connector but now the gauge doesn't move at all.  I tested the new unit with multimeter and it works as the ohms drop and the temp goes up, I'm the original owner of this car and if I had to guess I would say the gauge is OK.

Do I have the wrong sending unit?

[firebirdparts]

We used to have a thread about this, didn't we?  The good news is, GM senders were very standardized for a long time.  The bad news is, people keep complaining that the available parts (plentiful) are inaccurate.  You can find compensators all over the internet, and people replacing the factory resistor on the back of the gauge.

It looks like on Rockauto the fitment on these senders starts in 1979.  So was 1979 the break year?

Online, corvette people tend to compare these brands, but of course parts come and go.  My Redbird seems to run real cool which I presume to be "right", I should look at the ohms on that one.  I seem to recall the older calibration was about 100 ohms at boiling and hundreds, like 500 ohms, to take it off the low peg.

[formula jg]

The replacement was 550ish ohms with eng. cold en and approx. 320ish ohms at 170-180F (I'll have to recheck because I didn't write it down).

[Wallington]

Don't use too much thread tape either. It needs to seal but also to ground the body.

[formula jg]

I installed the sender without thread tape.

[Grand73Am]

Of course, be sure to get the sender for a guage and not a sender for a light.

[formula jg]

I have tried 2 new senders each of which are from different mfg. and ordered for a gauge. When I ground the sender wire there is no needle movement at the gauge, is there any other testing I can do to double check whether the gauge is actually faulty?

[formula jg]

Update.
I removed the temp gauge and tested it with jumper wires from gauge to batt. +/- and the needle jumped to full hot so presumed it was good. Then took a closer look at the original printed circuit board and it appeared to have a break in one of the lines leading to the temp gauge so I replaced the board, reinstalled everything and when I retested with the green wire connected directly to batt ground there was no movement in the gauge needle

Does the green sending unit wire run directly to the multi-pin harness that plugs into the rear of gauge cluster or does it connect to something else somewhere under the dash or in the fuse box?

Is there a way to test for any breaks in the green line?

[chief poncho]

Actually I just did some of this.  You can test continuity between the wire where it attaches at the sending unit and inside the car where it comes through the harness.  Just short the wire to ground on the engine or chassis somewhere, then check continuity from the connector at the instrument cluster.  Assuming they didn't change it over the years, it should be the third pin up from the bottom on the right side.  If not, just follow the trace from the temp gauge to the post on the right side.

You can also connect the wire to the sending unit and confirm the resistance when checking it to ground inside the car is similar. 

And to answer your question, yes the green wire goes through the firewall junction box, so there is an engine bay run and an underdash run of wire.

[formula jg]

Can you school me on checking continuity, what setting is used on the multimeter and where the blk and red multimeter wires are connected (one at the gauge harness pin and the other?)?

Also, what number on the meter represents a break in continuity?

So does the green wire pass right through the firewall junction box or is there a connection at the connection box?

[chief poncho]

Hopefully, I'll be able to answer your questions.

1.  To check continuity using a multimeter the easiest way is to set it to OHMs, and choose the lowest setting.  It could be 10 ohms, or 100 ohms or even some other range.  But choose the setting that will give you single digit resistance readings.  If it has a continuity setting it will simply beep when you have continuity.  For continuity testing there is no polarity, so it doesn't matter what side of the wires of the multimeter (red or black) you use. Simply connect one end of the ohm meter (could be red or black, doesn't matter) to the pin on the connector, and connect/touch the other end to a good chassis ground point such as a screw on the frame etc (choose a good unpainted surface or scratch through any paint).  I like to use the door switch on my car since I know it has a good connection to the body.  You'll be looking for a very low reading usually less than 10 ohms.  It really should be less than 5 ohms.  Basically you're doing the same thing as touching the wires of the ohm meter together (which should read zero, but with cheap harbor freight meters I've seen it read as high as 3 or 4), except that your running it through the harness and the body of the car.  A break in continuity is when the meter is going to read a very high number or be out of range.  An open or broken wire will be out of range regardless of the multi-meter setting.  Remember you'll want to run a jumper wire from the connection under the hood to a good ground point (again, a fastener or some area that isn't painted).  If you do get continuity with that wire grounded, then remove the ground under the hood and recheck inside the car to make sure the wire also isn't shorted to ground somewhere.  When you disconnect from outside you should get an out of range reading.  Another way to check this is to connect it to the sensor and then simply measure the resistance between the instrument cluster connector and chassis ground.  With the engine cold you should get a reading in 500-3000 ohm range, with it warm it should drop into the low 100's.

2.  The green wire has a connection at the firewall junction box.  It does not pass through.  If the harnesses have never been removed from the car, its very doubtful that will be a problem.  The engine harness side bolts to the car side and typically doesn't go bad unless it's damaged or very corroded.  If there is a break, it'd most likely be under the hood somewhere or under the dash. 

Here's a wiki that might help too.
https://www.wikihow.com/Test-Continuity-with-a-Multimeter

[formula jg]

That's one of the best detailed explanations of any procedure I have every read, it's so comprehensive I don't even have any further questions, thank you.

As far as the firewall junction box goes I'm the original owner so there is no doubt that it ever had been messed with.

[chief poncho]

No worries, I guess it's just one of the habits of being an engineer.  Hopefully you'll be able figure this out and get it working. 

[formula jg]

Update.
Traced the issued right back to the original spade connector at the temp sending unit. There was a break within the green wire sheathing, it looked all in tact on the outside so I was stumped for a while. 

After repairing the connector the gauge would go full hot with the green wire grounded on the engine side but when I reconnected the green wire to the sending unit and checked cold resistance the multimeter would give a brief ohm reading and then the screen would go blank. This would happen when I cold tested resistance directly at the sensor and cold tested sensor resistance at the gauge cluster wire harness. The brief reading would be 1700ish ohm prior to going blank, is this a normal behavior when checking resistance for a  temp sensor?

[firebirdparts]

Well, you can't have it plugged up.  It needs to be unplugged. 

[formula jg]

firebirdparts, please clarify what needs to be ‘unplugged’?

[firebirdparts]

The sender.  If you want to check ohms through the sender, it has to be unplugged. 

The sender is a resistor, so it made a ton of sense to me that you were trying to measure the ohms of the sender itself.

[formula jg]

Got it, thank you.