Needle Is Too High On Temp Gauge

[inline6]

I do not know what my oil pressure is.  I have not hooked up a mechanical gauge.  

Today, I noticed that when I set the key to the on position, both the temp gauge and the oil gauge needles move from their at rest positions.  I never noticed that before. 

The temp gauge needles moves from pointing directly at the 2 in 120 to just a tad bit closer to the 0 in 120 (my laser gun indicated 70 degrees at the temp switch).  And the oil pressure gauge needle moved from below 0 to pretty much right at 0.  

When I start the car, the oil pressure as indicated on the gauge goes up to almost max (90).  When idling at a stop light, it is barely off of 0.  

I messed with the bi-metallic strip in the temp gauge today.  I was able to get only slight improvement.  Feels wrong to attempt to mess with it more.  I think the bi-metallic strip would have to be drastically messed with to get the gauge to read correctly. 

I drove the car around for a few minutes to get the engine up to normal temp.  I shut it off and took this pic after quickly returning the key to the on position.  Temp needle is a bit lower than it what it was before messing with the bi-metallic strip - oil reads at 0 (it reads below zero with key off).  

 

My laser temp gun aimed at the sender switch showed about 146 degrees fahrenheit with the temp needle in this position in the gauge. 

I'd like to set up some bench testing.  I'd like to be able to wire in a temp sender and apply heat to it, while at the same time measuring the resistance in the temp switch and the temperature of the heat switch... and viewing the position of the needle on the gauge.  Would you be able to help me figure out how to wire that up @SteveJ?  

It would be nice if I could get the needle to show 120 when the temp switch is at 120, and to get the needle on the gauge to show 250 when the temp switch is at 250.  

Edited February 5 by inline6

[Captain Obvious]

12 hours ago, inline6 said:

I noticed that when I set the key to the on position, both the temp gauge and the oil gauge needles move from their at rest positions. The temp gauge needles moves from pointing directly at the 2 in 120 to just a tad bit closer to the 0 in 120 (my laser gun indicated 70 degrees at the temp switch).  And the oil pressure gauge needle moved from below 0 to pretty much right at 0.

That all sounds normal. The gauges should move off their seats when you turn the key to ON.

Bench testing should be relatively easy. Only complication is that you will not be able to directly measure the resistance of the sender unit while the sender is being used in the circuit. I would suggest connecting everything up, heating the sender to temp, note the gauge reading, and then quickly disconnect the sender and measure it's resistance before it has time to change temp dramatically.

[inline6]

Wires for the temp portion of the gauge appear to be 

YR - yellow with red stripe (to power/voltage)
Y - yellow (to temperature sender switch)
B - black (to ground)
 

• If I hook up black and yellow (without the temperature sender), and yellow with red stripe to positive of a battery, I think I should see the gauge needle peg.
• If I hook up black and yellow (with the temperature sender in line), and yellow with red stripe to positive of a battery, I think I should see the gauge needle move, but movement will be dependent on the temperature of the sender.

Because both the temp and oil pressure share the voltage regulation, will I need to hook up wires to it (the oil pressure gauge) also to perform my bench testing?

Edited February 5 by inline6

[Captain Obvious]

The wiring diagram says the sender units connect to a yellow/white wire (not straight yellow), but that may be on the chassis side of the gauge connector. Sometimes they changed wire colors at the connector right there at the gauge. So if it's as simple as Y on the gauge side and Y/W on the dash harness side, then yes... Your bench test connection plans should work out fine.

No, you do not have to have the sender unit (either of them) hooked up. It won't cause any damage, but the needle will just rest way down at the bottom. So you can let the oil pressure connection hang open while you are messing with the temp side testing.

Can you snap a couple pics of the gauge? Some of the connector leading to the gauge, and maybe something of the corresponding connector on the dash side. So we can confirm the color change there at that connector?

Also, can you snap a pic of the inside of your gauge so I can see the wiring and the adjuster (if there is one). The pics I've been posting are from 280 cars. I assume the 240 stuff is the same, but I've never been inside the early 240 gauges to confirm.

[inline6]

Dash harness wires: 

gauge wires:

Inside gauge:
    

Connections on gauge:

Static needle positions:

 

I didn't see the white strip before.  I just glanced at the factory 1971 workshop manual and got the color codes:

  

 

Funny how that has the ammeter and water temp in one gauge and the fuel and oil in the other!

I was able to bench test just briefly yesterday.  I pegged the gauge successfully.  But, I couldn't juggle heating the temp sensor and getting a laser temp reading and seeing the needle position all at once.  I am thinking about putting the sensor in a pot of boiling water to control the temp while connecting the body of the temp switch and ground of the gauge to a negative terminal of a battery... and the yellow/white wire to the positive terminal of a battery.  I am thinking that will give me a fairly steady state for the heat, and I can also measure the resistance of the temp switch at the same time.  Where the needle sits at that point will, in theory, be 212 degrees or thereabout.  Then, I can add the "pot" that I bought for adjusting the resistance.  

I note that the gauge itself has some resistance as well.  I will check that again and put what I find here.

 

[Captain Obvious]

14 hours ago, inline6 said:

Funny how that has the ammeter and water temp in one gauge and the fuel and oil in the other!

Yeah, there are a bunch of mistakes in the early wiring diagram. Another example is that the wire colors on your car are not the same as the diagram. The diagram indicates that both TEMP and OIL use yellow/white, but your car uses yellow/black for one of them.

So a while ago I worked up (what I think is) a better version of the diagram and fixed some mistakes as well as cleaned things up a bunch. I've not uploaded my latest version, but here's the thread where we talked about it:
https://www.classiczcars.com/forums/topic/69026-1970-wiring-diagram/

Which of the gauges uses the Y/B wire? Does that go to the TEMP, or the OIL? I'll change that on my diagram.

And let us know how the bench testing goes.

 

[Captain Obvious]

I looked at an early car here and the oil pressure sender uses the Y/B.

So the Y/W must go to the temp sender. I'll update my diagram.

[inline6]

Y/B goes to the oil pressure.

I opted for the easy way out today - instead of bench testing like I described previously, I looked up how to wire the "trimmer potentiometer" that @SteveJ put me onto. 
https://www.digikey.com/en/products/detail/tt-electronics-bi/93PR2KLF/2408753

 

I didn't know how to wire it up, but found something online that said to hook the middle pin to the wire of what I wanted to attenuate.  I left the dash wiring alone.  Instead, I removed the spade corresponding to the Y/W wire from the 4 spaded plastic connector.  Then I took a new piece of insulated wire and put an identical spade on one end and put that into vacated spot in the 4 spade connector.  I stripped a bit of the insulation from the other end of that wire and soldered it to the center pin of the trimmer.  Then I soldered the original male spade attached to the Y/W wire which is bolted to the gauge to one of the other pins in the trimmer.  I didn't snag pics, but I will.  

I found that when I used a small screw driver to adjust the trimmer to "0", the temp gauge worked as before.  When I just barely moved the adjustment to just off of zero, the needle on the gauge dropped.  The needle was super sensitive to any adjustment, but I got lucky on my 4th try to find something just off of "0".  I drove around the neighborhood and watched as the needle came up to the line near the middle of the sweep of range and sat there.  🙂  

I replaced all of the gauge bulbs with led bulbs while I had the center dash panel out again.  So, now I just need to button up things. 

Edited February 9 by inline6

[Captain Obvious]

Awesome. I love it when a plan comes together.

If you get energetic and want easier trimming of the gauge position, use a 200 Ohm pot instead of 2K. It would be less sensitive and easier to adjust.

[inline6]

So this one - if anyone else wants to do this.  
https://www.digikey.com/en/products/detail/tt-electronics-bi/93PR200LF/5957450

[cgsheen1]

It's a pain (well, certainly can be), but I typically use the gauge adjustment ports of the back of the gauge to alter needle position.