Howdy,
A quick question. I'm trying to fill in a blank on my limited understanding of wiring.

A temperature gauge (on my 65 for example) has 2 wires going into it. Green coming from the temp sender, and pink/black that is common to the temp gauge, fuel gauge, backup light, and parking brake light

I know the temp sender sends a signal based on Ohms of resistance, to the temp gauge.

Is the Temp gauge basically measuring current through the Pink/Black wire, to Ground via the Green wire?

I ask because a friend is having problems with the temp gauge in his own 65. He removed the wires from the back of the gauge and then measured a solid connection to ground, through the pink/black wire. This doesn't make sense to me.

I did read the tech article on BarryK's web site but it doesn't shed any light on the role of the pink/black wire (I assumed it was a hot wire) versus my friend's measurement of a solid ground.

Thank you,
Jeff

 

To quickly check the operation of the gauge, just ground the green wire at the sensor, then remove it from the ground. The gauge should make a full deflection. The sensor simply varies the resistance to ground to swing the gauge.

 

Hi Jeff. I don't have my manual here but I should be able to wing it.

The pink/black wire should be a hot wire with the ignition on and the green wire just runs from the gauge through the temp. sender res. to ground.

Did he check the pink wire for voltage with the ignition on??

Or did he read its res. to ground??

Being it is a supply wire with the ign. on I have no clue what it may read to gnd. with the ign. off, so don't be concerened with that reading.

 

Ok good, that's what I believed to be true based on the wiring diagram and my previous experiences with blowing fuses on that circuit for a shorted fuel sender.

I think he just unplugged the wire, and measured solid connection to ground through it with the key off.

I'll dig into it when I go over to research the problem.

Thanks for the replies.

 

Jeff is helping me with trying to fix my temp gauge. I unplugged the gauge and with the battery connected and ignition " on " checked the black/pink wire of the plug to ground and the needle on my ohm meter swung fully to the right. It should be noted that my gauge needle is fully pegged to the right well past the 240 degree reading and will not move back to the standard cold start setting of 100 degrees on the gauge. In addition I also checked the green sending wire with the same test at the plug end and my ohm meter did not swing at all. I have already sent this gauge off to one of the reputable gauge companies and he told me that the gauge worked perfect when he received it and when he shipped it back. He said that he tested it several times. So I am really puzzled and Jeff is the kind of guy that will always help a friend with a problem and right now I am that friend. Thanks again Jeff.

Jay

 

You cant use ohm meters on powered circuits. Just hook everything up like it is supposed to be and alternately ground and open the green wire.

 

Yes, I plan to do that for sure. I just wanted to better understand how the gauge works. I figured it was simply measuring resistance to ground (through the temp sender) and reporting it via the dash gauge.

But I couldn't resolve how that could be if the gauge is already grounded to the cluster, and the "hot" wire also seemed to have a direct short in it according to Jay's test.

 

Yes, it is. The black/pink wire feeds a constant 12V to a coil in the gauge, and the other end of the coil is connected through the green wire to the temp sender. The temp sender contains a thermistor (variable-resistance element) whose resistance to ground changes with temperature. As the temperature goes up, the resistance goes down, increasing current flow through the coil to ground, which moves the needle to the right.

With the key in "on" or "accessory", disconnect the green wire at the sender - the gauge should peg to full cold (infinite resistance). Then ground the sender wire connector to the engine, and the gauge should peg to hot (zero resistance).

The green wire goes through the multiple connector on the engine side of the fuse block, and those terminals frequently become corroded, which will screw up the gauge reading.

 

Thanks for everyone's replies.

Why is the gauge grounded to the cluster, if it's reading the ground through the temp sending unit?

 

Because there are two coils in the gauge, and both ends of the circuit require a ground reference. The diagram below is for C1 or C3 fuel gauges, but the circuit is the same as that used in the temp gauge - 12V to the gauge, and variable resistance to ground at the sender end. The explanation of how the circuit works below is from my CE article on fuel gauge diagnostics, but it also applies to how the temp gauge works:

Dash Gauge: The dash fuel gauge has two coils in it – 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 “empty” 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.

Tank Sending Unit: The sending unit contains a wirewound variable resistor, and a contact wiper arm connected to the float rod and to ground; when the tank is empty, the wiper contact is at the beginning of the resistance, providing zero ohms to ground, and when the tank is full, the wiper contact is at the end of the resistance, providing 30 ohms (C1) or 90 ohms (C3) of resistance to ground.

How It Works: 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 tan wire to the variable resistor in the sending unit to ground.

When the tank is empty, the contact wiper in the sending unit cuts out the variable resistor entirely, so most of the current from the gauge will pass to the sending unit and directly to ground while very little current will pass through the operating coil in the gauge, causing the gauge to show “empty”.

When the tank is full, the contact wiper in the sending unit is at the other end of the variable resistor, placing more resistance between the tan wire and ground; this forces more current through the operating coil in the gauge to ground, which attracts the gauge needle and moves it to the “full” side of the gauge. This is a simple resistance circuit, dependent only on power to the gauge, variable resistance in the tank unit, and good grounds at both ends.

 

Jeff, I'm wondering if he has the gauge spade terminals in the wrong place. I think it's possible to do this. My gauge is not installed, so I will take a look.

Here is a pic of what it should look like.





Also, instead of working on my car, I put this together:

 

Very cool Buns, as is typical of your posts.

Thanks for the details. I had seen your photo before, but I'd never seen the technical data below it before. That's great. Is that from John's CE article?

 

John,
Thank you for your detailed reply. I'll be bookmarking it for future reference!

Jeff

 

I checked my temperature gauge, and yes, that brown insulator can go on 4 different ways, so I think maybe you could stick a camera up there and snap a pic of the connections.

 

also the resistor that is under that fiber insulator has to be installed correctly as well,it is pictured above and goes from the 9 o clock to the 3 o clock ,they're 2 washers that if not put on to space that resistor it doesn't make contact.
There is also a fiber gasket that is not shown above.

Jason

 

Nice work everyone! Can't get enough of good tech info like this at these prices!!!!

I updated the file based on the earlier thread with the latest info posted above by Buns and JohnZ.

Buns (Tim) great gauge drawings, etc.! John if I made an mistakes transcribing your figure into PowerPoint please let me know.

Thanks everyone!

DZ