1980 temp gauge internal wiring question

[theo]

To all you menders of instruments..

I'm presently going through the gauge/sender system on the vette, to make sure it's indicating the right temp.
I've been looking through the wiring diagrams, and although I understand pretty well how the system works in principle (ie, sender resistance gets lower as coolant warms up, giving a bigger voltage drop across the gauge), I'm still a bit unsure of what exactly is going on inside the gauge itself. To me, it would seem logical for the temp gauge to have two terminals.. one for power in, one for the wire to the sender, but looking at the diagrams it seems the gauge uses a third, ground, terminal. What's happening in there?!

This is all purely out of interest, I'm just letting my curiosity get the better of me, really. The main point of the excercise was to see how accurate the gauge is against a k-type thermocouple, but this question has made me go off on a tangent!

Cheers
Theo

 

[JohnZ]

The GM temperature gauge uses the same basic electrical architecture as the fuel gauge - variable resistance to ground and two coils (see diagram below). There are two coils - the limiting coil on the left, and the operating coil on the right, each with different resistance. The pivoting needle has a counterweight on it that holds the needle at one end of travel unless magnetic attraction from the operating coil moves it. The limiting coil gets 12 volts from the ignition switch, and passes it to the operating coil, which is grounded through the gauge case. Where the diagram says "Tank Unit" (a wirewound resistor), in this case that will be the temperature sending unit (a thermistor).

When the key is "on", current is supplied to the limiting coil and to the operating coil through their common connection. From here, the current can go two ways - through the operating coil to ground, or through the sender wire to the variable resistor (thermistor) in the sending unit to ground.

When the engine is cold, the thermistor is at very high resistance, which forces more current through the operating coil to ground, which attracts the gauge needle to the "cold" side of the gauge. When the engine warms up, the resistance to ground through the thermistor is reduced, so more current from the gauge will pass through the sending unit to ground while less current will pass through the operating coil, causing the needle to move toward "hot".

That's why the gauge is grounded - the system must be able to see ground at both ends of the circuit. This is a simple resistance circuit, dependent only on power to the gauge, variable resistance in the sending unit, and good grounds at both ends.

 

[theo]

JohnZ,
You are a genius, that's exactly what I'm after, thanks!
I've only had time to scan through your reply, and will get a chance to have a good look at it later.
For the moment though I have two questions - firstly, which of the coils does the calibrating resistor (the colour coded ceramic one on the back of the unit) connect to? I seem to recall that one end of this ceramic resistor goes to the case ground of the dials, suggesting it connects between the end of the operating coil and ground??
Also, where did you take the attached diagram from? Is that a GM publication? There is no such detail in the electrical test section of my 1980 shop manual...

Thanks again
Theo

 

[JohnZ]

I don't recall how the calibrating resistor is connected - haven't had one apart in decades. The diagram is from the 1957 passenger car shop manual - they haven't bothered to show how that circuit operates in the shop manuals in over 40 years.

 

[theo]

Hello John,
I didn't make the connection between your name over here and on the NCRS board! (Was wondering if I was talking to late Mr DeLorean :L)

Well, that's fair enough, you've already served up some really good info! I think by process of elimination, I'm fairly convinced that the operating coil is the one that connects in series to gound with the ceramic calibration resistor, it has to be this way if the operating coil goes to the case ground (as in your description and pic), because I know from previous tinkerng that the colour coded resistor goes from one gauge post to this ground too. Kinda makes sense really doesn't it?

So, is the needle position a balance of the two "pulls" from both coils, or is it just the operating coil that acts magnetically on the needle?

Is there supposed to be some damping of the needle? I notice my fuel gauge sweeps up quite smoothly at key-on, but the temp gauge "pings" into position with a lot of bounce. If that's not the way it's supposed to be, it's a bit of a job taking it apart just to fix that, so I think I can live with it!

One thing that's often made we wonder, is how the gauge stays in the last hot position when you switch off - I guess the needle is totally free to pivot (apart from any damping) and therefore just stays where it's left when the coils de-energise? If so, that would make it like the late model electric tacho.
I don't suppose you have any other useful scans of this section of the service manual in your PC library, do you? One can never have too much information...

I've since had the sender out and calibrated it for T vs Ohms in a hot oil bath - it's obviously got loads of hysteresis and takes several seconds to respond as the heat goes on, so the gauge is never going to be very accurate in the warmup, and looks to lag the real temp by quite a bit. So the best I can hope for is a good steady state reading. Going to stick a thermocouple in the block and check that this weekend. Once again I've got too bloody engrossed in what was going to be simple exercise!

Thanks again for the explanation John, I was beginning to think I was only going to get the answer from pulling the gauge, which is just too much trouble, and pretty pointless while it still works ok.

Cheers!
Theo

 

[Ken Anderson]

There are no "coils" in your gage and the dampening is from heavy silicone grease which dries/runs out after several years. Your temp gage is not operating correctly, but can be "relubed" with a special tool. You are correct in assuming that your gage is like a late model tach as they both use the same design. My point is not to flame anyone, but just to keep the info correct and keep us all up to date.

 

[theo]

Ken, thanks for the reply.
I appreciate you providing up to date information. Hopefully JohnZ will respond here soon. I'm not sure what you mean about there being no coils in the gauge, but that the gauge operates on the same principal as the tacho - because the *are* two coils in the air-core movement of the tach.. Sorry, am I just reading your response wrongly?
Could you elaborate a little more on what is actually going on inside the gauge? As mentioned above, I could sort this all out by stripping the thing, but while it actually reads a temperature (albeit perhaps a wrong one due to the calibration) I don't want to start tearing into it.

Cheers!
Theo

 

[Ken Anderson]

Your gage is not meant to be serviced, it is a pitch item. It uses an air core meter movement which is a coil & bobbin winding. The pointer shaft is attached to a permanent disc magnet that "floats" in a silicone grease cup.
You cannot "strip it" as it is riveted together. When I say late model tach, I am referring to the 80's. Current design analog GM gauges use a stepper motor.

 

[theo]

Ken
Naturally, it's not meant to be serviced, but then there are a few folks out there who seem to do pretty well at rebuilding these "unserviceable" parts .
The damping aside, the actual pointer position should not be influenced by the "lack" of damping, i would imagine? Damping being a resistance to movement, rather than to position.
I think we're talking about the same vintage of tacho movements, the tach in my 80 uses a two-coil air core movement (but this has no damping), and yeah, I think pretty much everything uses a stepper motor for all gauges/speedo these days.
Does re-greasing involve injecting the grease into the unit from the outside, then?
You've obviously had more hands on experience of this than I, but I can't see how you couldn't remove and subsequently replace the two rivets that hold the face together, or perhaps even use small black-oxide machine screws to re-assemble??
Given that it's an air core type movement like the tacho, are the coils internally connected in the same way as the diagram JohnZ provided? Please forgive the barrage of questions, I'm not totally sure what you meant by "coil and bobbin" and I'm eager to learn

Thanks
Theo

 

[Ken Anderson]

Ken
Naturally, it's not meant to be serviced, but then there are a few folks out there who seem to do pretty well at rebuilding these "unserviceable" parts .
The damping aside, the actual pointer position should not be influenced by the "lack" of damping, i would imagine? Damping being a resistance to movement, rather than to position. Yes it is. If not dampened, then it flops all over.
I think we're talking about the same vintage of tacho movements, the tach in my 80 uses a two-coil air core movement (but this has no damping Yes, it does), and yeah, I think pretty much everything uses a stepper motor for all gauges/speedo these days.
Does re-greasing involve injecting the grease into the unit from the outside, then? Correct
You've obviously had more hands on experience of this than I, but I can't see how you couldn't remove and subsequently replace the two rivets that hold the face together, or perhaps even use small black-oxide machine screws to re-assemble?? It can be done
Given that it's an air core type movement like the tacho, are the coils internally connected in the same way as the diagram JohnZ provided? No Please forgive the barrage of questions, I'm not totally sure what you meant by "coil and bobbin" and I'm eager to learn

Thanks
Theo

See answers to your questions.

 

[JohnZ]

I'm sure Ken is correct - I'm not a gauge expert, only had the ones on my '57 apart once, send out my C2 gauges. I just know they won't work without the case being grounded with either the C1 or C2-up design.

 

[theo]

Thanks, Ken. Nothing if not concise.
And thanks John, I appreciate you taking the time to relay all the information!

I've since run the car up to temp with thermocouples installed in the block, and it looks like I don't have much to worry about - the gauge is over-reading by a lot. With my new fans running at idle, I measure about 87C/188F in the block and top hose, with about a 40C/104F drop across the radiator! That's pretty damned effective, and I can't complain about that.
It doesn't look like it climbs much under road load either, so I'm overall pretty pleased with the setup. Just need to recalibrate the sender or gauge at some point, (which of course was what the original post was about) but now I know how the reading relates to the real temp, I can wait a while before playing with that.
Cheers!

Theo