wiring the guages: "Heat Gauge additional resistor" ??

[maytag]

Hey all!
I'm nearing the end of my rewire on the car. Right now, I'm just working-through the instrumentation. I've got a question about the water gauge.
In my wiring diagram (pictured below) it shows a "Heat Gauge additional resistor", labeled as #17, middle/left of photo.



So as I read the circuit, I've got power to one side of the gauge. the other side is that green wire, which goes direct to the temp gauge sending unit (labeled #18), and through that to ground. But jumpered into that wire is a resistor and then the temp "switch" (labeled #14) which pegs the gauge when it closes to ground.

So my question: what's that resistor? its specs and function?

[majicwrench]

Can I guess??
The Temp "Switch" if it goes to ground when closed might be too little resistance for the gauge, and could burn out the gauge. With a resistor in the curcuit you would reduce the amount of current flowing thru the gauge.

Tis interesting, a temp sending unit and a switch. Do you have the switch in your hands?? At what temp does it close/open??

[maytag]

Can I guess??
The Temp "Switch" if it goes to ground when closed might be too little resistance for the gauge, and could burn out the gauge. With a resistor in the curcuit you would reduce the amount of current flowing thru the gauge.

Tis interesting, a temp sending unit and a switch. Do you have the switch in your hands?? At what temp does it close/open??

Yeah, that probably makes sense.
So I'm with ya: WHY?

only thing I can figure is that for some reason there's a temp where the gauge quits climbing, and that would coincide with when the switch kicks-in?
I've already installed the switch in the head, so I don't know at what temp it closes. Anybody know anything more about that?

But what I AM thinking about is wiring the switch to a light, as opposed to being sort of redundant in the gauge.

[RRoller123]

Just my guess: Usually in instrumentation these things are added to calibrate between gauge and sensor so they are reasonably accurate. Different datum points for each device, from different manufacturers, so the Resistor probably pulls them together.

[spider2081]

I think there is one sender for the gauge to display the temp. The other sender is to make sure, even if the first sender is not working properly that your meter pegs into the red zone if the car is overheating. Both senders are in the head between the spark plugs.

[manoa matt]

First and foremost: http://www.artigue.com/fiat/wiring-diagrams

The temp gauge sending unit in the cylinder head works via resistance. As the temp rises the sending unit increses in electrical resistance. The increased electrical resistance is measured by the dash gauge and corresponds to the movement of the needle. The wiring for the circuit is as follows: Temp gauge sending unit is wired in with the temp gauge sensor that pegs the needle. Both sensors are then wired to a "temperature gauge resistor" which is located up under the dash in what looks like a fuse holder. The wiring then goes to the dash gauge.

The senders and gauge rely on resistance for the reading at the gauge, therefore any additional resistance in the wiring circuit will result in a higher than actual temp reading at the dash gauge. Corrosion at the terminals or hard cracked wiring near the cylinder head sensors will add resistance. To correct the situation and make the gauge read a more accurate reading you need to clean all the electrical contacts in the circuit and replace any wiring that is corroded or exhibits hard cracked insulation. You also need to gently clean the contact surfaces in the temperature gauge resistor. When you open the holder you will see a ceramic core with a tiny copper wire wound around it with caps on each end. You only need to clean the caps at the ends and the corresponding contact points inside the resistor holder. Do not break the copper wire. Sorry don't know the specifications of the resistor but it can easily be checked with a multimeter.

Once complete you can verify the results with a non contact laser thermometer. You want to point the laser at the metal cylinder head between the sensors. Its the metal temp you are looking for not the water temp.

[maytag]

Matt, maybe it's just me, but I have two fundamental problems with Brad's wiring diagrams:
1) he doesn't seem to have one that covers the '76
2) I've been reading wiring diagrams for 30 years, and his are by far the most difficult to follow of ANY wiring diagrams I've ever seen. I can't follow him through any of it. Again: probably just me.

Both sensors are then wired to a "temperature gauge resistor" which is located up under the dash in what looks like a fuse holder. The wiring then goes to the dash gauge.

Are you saying my diagram (above) is wrong then? Because it clearly shows the gauge-sender NOT circuited through the resistor. Which makes sense, since the sender IS a resistor. And my understanding is that as the temp goes up, the resistance is DECREASED, not INCREASED. That's what allows the needle to swing. you'll note that grounding the gauge without a resistor will peg the needle. And that's why I don;t understand the purpose of a resistor in the "switch" (which is shown here as a switch, not as a resistance-sensitive sender).

I like your idea of comparing the temp gauge msrmnt to a real measurement taken with a thermal imager. I'll do it!

[manoa matt]

Brad's diagrams are copied exactly from the original Fiat wiring diagrams that were in the factory service manuals. They are vector based CAD drawings in PDF therefore they can be blown up to the size of a wall poster without loosing image quality. I have a copy of an orignial Fiat 1976 wiring diagram on my work computer and can send it to you when I get back from vacation. The 1975 and 1976 diagrams are virtually the same with the exception of one or two minor components/circuits IIRC.

I do see that the diagram you have is a bit easier to read, and relates the sensors to the spark plug locations.

I think one of the most disputed items in the wiring diagrams regarding the sensor locations is the actual locations of those two sensors in the cylinder head. Some say the gauge sensor is more forward, some say the thermo switch is more forward. I know on my 78 the length of the wires is different and due to the colors the gauge sensor is more forward and the switch that pegs the needle is more rearward, however your diagram shows the opposite. Brad's diagram is semi ambiguous with regards to the locations unless you go by the wire colors.

It doesn't matter which is more forward as long as the correct wires go to the correct sensor/switch.

The reason I say the sensor increases resistance with heat is due to the fact that once you clean all the electrical contacts in the circuit the gauge reads a lower temp. If the connections are dirty and wiring corroded and brittle, the gauge reads a higer temp. This is just what I've experienced. I guess if you think about it, as the resistance in the circuit goes up, less voltage makes it to the dash gauge. As less voltage makes it to the dash gauge the needle reads a higher temp.

You were right about how its not all wired in series, rather parallel. I copied that text from a post I made a while ago as it seams the question about the temp gauge resistor comes up about once a month here. (I can't remember all that and didn't have the time to type it all) The good thing is you second guessed my reply and now everyone can benifit from this extra rock that was turned over revealing more information about the circuit and how it works.

[maytag]

All good stuff, Matt. And thanks for your input.

I'm still hung-up on this:

.....As less voltage makes it to the dash gauge the needle reads a higher temp.

Following that logic, the less resistance, the lower the reading, but If you ground the gauge, without the sensor, just straight to ground, the gauge pegs hot.

I've got some various size resistors here. I'll try a couple and see what happens. it'd be good to know, so we can trouble-shoot accurately.

[spider2081]

I did substitute resistors for the sensor with my 81 Spider.

50 ohm resistor and gauge reads 260
150 ohm resistor and gauge reads 190
750 ohm resistor and gauge reads 120

[maytag]

I did substitute resistors for the sensor with my 81 Spider.

50 ohm resistor and gauge reads 260
150 ohm resistor and gauge reads 190
750 ohm resistor and gauge reads 120

so more resistance = lower reading.

[RRoller123]

I believe these are calibration resistors. Probably when the designs were chosen from each vendor, it was determined thenwhat the proper resistance was to get the gauge to accurately reflect the sensor reading. (?)

[spider2081]

Sorry if I caused some confusion. I wanted to know what value of resistance a coolant sensor would have at certain temperatures. I tested 3 temperature gauges and found the results in my previous post. For the gauge to display 120 degrees the senders resistance should measure close to 750 ohms. For the gauge to display 190 degrees the senders resistance is close to 150 ohms and for the gauge to display 260 degrees the senders resistance is close to 50 ohms.

The resistances stated are not installed in the car they are the values I found that made the 3 gauges display the corresponding temperatures.

The gauge resistor installed in the car is a current limiting resistor for the over heat circuit that pegs the gauge into the red area when the car is too hot. I have not tested the over heat switch to see what temperature it closes at to peg the gauge. I would expect it to be something between 230 and 260 degrees.