I found information pertaining to this issue on this site and on Zhome.com.  However, rather than bringing back a thread last contributed to in 2013, I will start a new one and hope I can fix the problem I am having.  

The Needle on my temperature gauge is definitely going to high.  I know however, the engine is not overheating because I have checked it with a laser temperature gun.  Here is the info I have so far regarding the problem. 

When I first bought the car, I got the engine running for a brief amount of time before taking the engine out for rebuilding.  I ran it up to temp in my garage (car was inoperative) and noted that the needle on the temp gauge was a bit high.  Here is a slightly blurry screenshot from a video I took at the time.  The engine was fully warmed up:



Fast forward about six and half years and with the car fully restored, the same temperature sender put the needle (same gauge) on the base of the last stroke of the "M" of TEMP.  That is not all that far away from the red zone of the gauge - near the "250".  So, I figured to fix this, I would just need to buy a new temperature sender.  When I changed to the new one, an aftermarket unit, I saw essentially the same thing, but the needle goes to the left side of the "P".  

I read in one forum thread that buying an OEM temperature sender fixed the issue.  That seemed pretty promising.  So, I sourced an original "Sankei" sender from a seller on Ebay.  I switched to that one... and again found the needle goes to the left side of the "P".  

As I have continued to look for information about solving this problem, I have come across this article on Zhome.com which was referenced in an old thread on this site: 

In the Zhome.com article, there is mention of "adjusting the voltage regulator".  From the context in the write up, it seems this is a different voltage regulator than the one for the alternator.  Instead, this seems to be something, perhaps inside the gauge itself.  However, the author is working with a 280ZX, so I am uncertain of the applicability of the information to my 1971 gauge.  

I did consult with @SteveJ back in November about using a "pot" like this person described in their post in the first of the mentioned threads above.  I purchased this one per his recommendation:
https://www.mouser.com/ProductDetail/BI-Technologies-TT-Electronics/93PR50LF?qs=UjOdgN%2FjCTVtZlza%2Bf%2Fo7g%3D%3D

So, that is what I have so far.  My plan is to check the three temperature senders that I have for differences if any, compare them to the specs provided in the Zhome article, to learn anything relevant about the voltage regulator and try to adjust it if possible, or to wire in the "pot" to ultimately resolve this issue if that is what it comes to.

 

Edited January 24Jan 24 by inline6

Why not try to find a known good gauge that someone possibly here on the forum for testing purposes. It won’t go to waste having an extra for future use. If it doesn’t pan out.

First, have a look at this thread that details how the gauges work:

Then, in this pic... You see the little screw? That's an adjustment screw for the gauge. Make the contacts tighter, it should read higher. Make the contacts looser, it should read lower:

I got back to this today.  I took my gauge out and the cover off.  I found that my gauge does not have a screw.  I should have taken some pictures, but I did not.  After just now reading the above post you shared @Captain Obvious, I believe my gauge my be uncompensated?  I only recall seeing the one U shaped bi-metallic strip with the white wire coiled around only one leg of it.  It seems like to try to adjust it, I will need to bend part of the U?

I think the complication may be that the compensation section is shared by both the gauges in the housing. it's in there, but not obviously attached to the TEMP gauge.

Those pics I posted above are from the fuel gauge which shares the housing with the ammeter (or voltmeter depending on the year). And since the operation(s) of the ammeter/voltmeter are very different than the other gauges (fuel/temp/oil), they do not use a compensator stage.

Here's a pic of the guts for the TEMP/OIL gauge. Note that there is only one compensator stage located down by the OIL gauge section. (in the lower right). That compensator is shared between both TEMP and OIL:

So the bottom line is that if you adjust the compensator, it will affect both TEMP and OIL. If you're OK with that, then that's where you should focus your effort. If you DON'T want to do that (because you like where your OIL gauge reads and don't want it to change), then yes... I'd just tweak the bimetallic strip on the TEMP gauge and call it a day.

Is it possible that heating and cooling of the bi-metallic strip over the years... thousands of times could cause it to harden, can change it's original action?  

I also find myself wondering why the same gauge only read a little  bit higher than the mark in the middle before the restoration, and now is so much higher.  I can rule out the sender, I believe.  I used the original one, and two additional ones with no real change to the high position of the needle now.

Maybe cleaner connections now. So more voltage reaches the gauge. So more heating?

Ah, but does the heating make the needle go up... or down?

I would think up. The default position is down; no power. When you short out a gauge sensor, full current-> full heat, it rises quickly. So I would expect lower resistance on a gauge circuit would make it read higher.

Maybe one of the "sparkies" will chime in

1 hour ago, Patcon said:

I would expect lower resistance on a gauge circuit would make it read higher.

That is correct. The lower resistance, the higher the gauge reads.

So yeah... One possible explanation is that maybe with the work that has been done to the car, including cleaning connections, may result in a higher reading than before that work had been done.

21 hours ago, inline6 said:

Is it possible that heating and cooling of the bi-metallic strip over the years... thousands of times could cause it to harden, can change it's original action?  

I also find myself wondering why the same gauge only read a little  bit higher than the mark in the middle before the restoration, and now is so much higher.

I don't know if the heating and cooling over the years has an effect. The change in needle location before and after the work you did is a little puzzling other than the possibility that you cleaned a bunch of connections between the two ends of the system.

So do you know if your oil pressure gauge is anywhere near accurate? Ever put a mechanical gauge on it? If the oil pressure gauge IS correct, it's still puzzling. But if your oil pressure gauge is reading inaccurately high, you're back to adjusting the gauge. And... It would be good to know if you've got lower oil pressure than you thought you did.

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 5Feb 5 by inline6